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C o lo r a d o N IO UN AND 2005 Air Quality Data Report CO NS TI TU TI ON Colorado Department of Public Health and Environment Air Pollution Control Division COLORADO AIR QUALITY DATA REPORT 2005 Air Pollution Control Division APCD-TS-B1 4300 Cherry Creek Drive South Denver, Colorado 80246-1530 (303) 692-3100 September 2006 Cover photo View from the Denver visibility camera on Sunday January 23, 2005 at 11:50 a.m. This is a “Good” visibility day. The Air Pollution Control Division operates a web-based camera that can be viewed by clicking on the “Live Image” tab on the left side of the screen at the Air Pollution Control Division’s web site http://apcd.state.co.us/psi. There is a great deal of other information available from this site in addition to the image at the visibility camera. The Front Range Air Quality Forecast, Air Quality Advisory, Monitoring Reports and Open Burning Forecast are also available. This report is available electronically at http://apcd.state.co.us/documents/techdocs.html Table of Contents 1.0 Purpose of the Annual Data Report .................................................................................................................. 1 1.1 Symbols and Abbreviations.......................................................................................................................... 1 1.2 Description of Monitoring Areas in Colorado.............................................................................................. 1 1.2.1 Eastern Plains Counties......................................................................................................................... 1 1.2.2 Northern Front Range Counties ............................................................................................................ 2 1.2.3 Southern Front Range Counties ............................................................................................................ 2 1.2.4 Mountain Counties ................................................................................................................................ 2 1.2.5 Western Counties .................................................................................................................................. 2 2.0 Criteria Pollutants ............................................................................................................................................. 7 2.0.1 Exceedance Summary Table ................................................................................................................. 8 2.1 Carbon monoxide ......................................................................................................................................... 8 2.1.1 Carbon monoxide – Standards .............................................................................................................. 8 2.1.2 Carbon monoxide – Health Effects ....................................................................................................... 8 2.1.3 Carbon monoxide – Sources.................................................................................................................. 9 2.2 Ozone............................................................................................................................................................ 9 2.2.1 Ozone – Standards............................................................................................................................... 10 2.2.2 Ozone – Health Effects ....................................................................................................................... 10 2.2.3 Ozone – Sources.................................................................................................................................. 10 2.3 Sulfur dioxide ............................................................................................................................................. 11 2.3.1 Sulfur dioxide – Standards .................................................................................................................. 11 2.3.2 Sulfur dioxide – Health Effects ........................................................................................................... 11 2.3.3 Sulfur dioxide – Sources ..................................................................................................................... 11 2.4 Nitrogen dioxide ......................................................................................................................................... 12 2.4.1 Nitrogen dioxide – Standards .............................................................................................................. 12 2.4.2 Nitrogen dioxide – Health Effects....................................................................................................... 12 2.4.3 Nitrogen dioxide – Sources ................................................................................................................. 12 2.5 Particulate Matter – PM10 ........................................................................................................................... 13 2.5.1 Particulate Matter – PM10 – Standards ................................................................................................ 13 2.5.2 Particulate Matter – PM10 – Health Effects ......................................................................................... 14 2.5.3 Particulate Matter – PM10 – Sources ................................................................................................... 14 2.6 Particulate Matter – PM2.5........................................................................................................................... 15 2.6.1 Particulate Matter – PM2.5 – Standards ............................................................................................... 15 2.6.2 Particulate Matter – PM2.5 – Health Effects ........................................................................................ 16 2.6.3 Particulate Matter – PM2.5 – Sources................................................................................................... 16 2.7 Lead ............................................................................................................................................................ 16 2.7.1 Lead – Standards ................................................................................................................................. 16 2.7.2 Lead – Health Effects .......................................................................................................................... 17 2.7.3 Lead – Sources .................................................................................................................................... 17 3.0 Non-Criteria Pollutants ................................................................................................................................... 18 3.1 Visibility ..................................................................................................................................................... 18 3.1.1 Visibility – Standards .......................................................................................................................... 18 3.1.2 Visibility – Health Effects................................................................................................................... 18 3.1.3 Visibility – Sources ............................................................................................................................. 19 3.1.4 Visibility – Monitoring........................................................................................................................ 19 3.1.5 Visibility – Denver Camera................................................................................................................. 19 3.2 Nitric Oxide ................................................................................................................................................ 21 3.3 Total Suspended Particulates...................................................................................................................... 21 3.4 Meteorology................................................................................................................................................ 21 3.5 Air Toxics ................................................................................................................................................... 22 3.6 PM2.5 Chemical Speciation ......................................................................................................................... 22 4.0 Statewide Summaries For Criteria Pollutants ................................................................................................. 23 4.1 Carbon monoxide ....................................................................................................................................... 23 4.2 Ozone.......................................................................................................................................................... 24 4.3 Sulfur Dioxide ............................................................................................................................................ 25 4.4 Nitrogen Dioxide ........................................................................................................................................ 25 4.5 Particulates – PM10 ..................................................................................................................................... 26 4.6 Particulates – PM2.5..................................................................................................................................... 27 4.7 Lead ............................................................................................................................................................ 27 5.0 National Comparisons For Criteria Pollutants ................................................................................................ 29 5.1 Carbon monoxide ....................................................................................................................................... 29 5.2 Ozone.......................................................................................................................................................... 29 5.3 Sulfur Dioxide ............................................................................................................................................ 30 5.4 Nitrogen Dioxide ........................................................................................................................................ 30 5.5 Particulates ................................................................................................................................................. 31 5.6 Lead ............................................................................................................................................................ 32 6.0 Monitoring Results by Area in Colorado ........................................................................................................ 33 6.1 Eastern Plains Counties .............................................................................................................................. 33 6.2 Northern Front Range Counties.................................................................................................................. 36 6.3 Southern Front Range Counties.................................................................................................................. 63 6.4 Mountain Counties ..................................................................................................................................... 70 6.5 Western Counties........................................................................................................................................ 75 References............................................................................................................................................................. 80 Table of Figures Figure 1 – Monitoring Areas in Colorado.............................................................................................. 6 Figure 2 – Changes in National Carbon Monoxide Emissions from 1970 – 2004 ................................ 9 Figure 3 - Changes in National Sulfur Dioxide Emissions from 1970 – 2004 .................................... 12 Figure 4 - Changes in National Oxides of Nitrogen Emissions from 1970 – 2004 ............................. 13 Figure 5 - Changes in National PM10 Emissions from 1970 – 2004.................................................... 15 Figure 6 - Changes in National PM2.5 Emissions from 1970 – 2004 ................................................... 16 Figure 7 - Changes in National Lead Emissions from 1970 – 2004 .................................................... 17 Figure 8 - Best and Worst Visibility Days for 2005 ............................................................................ 20 Figure 9 - Statewide Ambient Trends – Carbon Monoxide................................................................. 23 Figure 10 - Statewide Ambient Trends – Ozone.................................................................................. 24 Figure 11 - Statewide Ambient Trends – PM10 .................................................................................... 27 Figure 12 - Eastern Plains Particulate Graphs ..................................................................................... 34 Figure 13 - Eastern Plains Wind Rose Graph ...................................................................................... 35 Figure 14 - Northern Front Range PM10 Particulate Graphs................................................................ 38 Figure 15 - Northern Front Range PM2.5 Particulate Graphs ............................................................... 40 Figure 16 - Northern Front Range Lead Graphs .................................................................................. 42 Figure 17 - Northern Front Range Carbon Monoxide Graphs............................................................. 44 Figure 18 - Northern Front Range Ozone Graphs................................................................................ 48 Figure 19 - Northern Front Range Nitrogen Dioxide Graphs .............................................................. 50 Figure 20 - Northern Front Range Sulfur Dioxide Graphs .................................................................. 50 Figure 21 - Denver Visibility Data (January 2005 to December 2005) ............................................... 51 Figure 22 - Denver Visibility Comparison (1995 to 2005).................................................................. 52 Figure 23 - Fort Collins Visibility Data (January 2005 to December 2005)........................................ 53 Figure 24 - Fort Collins Visibility Data (1995 to 2005) ...................................................................... 54 Figure 25 - Northern Front Range Wind Roses ................................................................................... 55 Figure 26 - Southern Front Range PM10 Particulate Graphs................................................................ 64 Figure 27 - Southern Front Range PM2.5 Particulate Graphs ............................................................... 66 Figure 28 - Southern Front Range Lead Graph.................................................................................... 67 Figure 29 - Southern Front Range Carbon Monoxide Graphs ............................................................. 67 Figure 30 - Southern Front Range Ozone Graph ................................................................................. 68 Figure 31 - Southern Front Range Wind Rose..................................................................................... 69 Figure 32 - Mountain Counties PM10 Particulate Graphs .................................................................... 71 Figure 33 - Mountain Counties PM2.5 Particulate Graphs.................................................................... 73 Figure 34 - Mountain Counties Lead Graphs....................................................................................... 74 Figure 35 – Mountain Counties Wind Roses ....................................................................................... 74 Figure 36 - Western Counties PM10 Particulate Graphs ...................................................................... 76 Figure 37 - Western Counties PM2.5 Particulate Graph ....................................................................... 78 Figure 38 - Western Counties Carbon Monoxide ................................................................................ 78 Figure 39 - Western Counties Wind Roses .......................................................................................... 79 Table of Tables Table 1 - Statewide Continuous Monitors In Operation For 2005......................................................... 3 Table 2 - Statewide Particulate Monitors In Operation For 2005..................................................... 4 - 5 Table 3 - National Ambient Air Quality Standards ............................................................................... 7 Table 4 - 2004/2005 Exceedance Summaries ........................................................................................ 8 Table 5 - Historical Maximum 1-Hr and 8-Hr Carbon Monoxide Concentrations.............................. 24 Table 6 - Historical Maximum 1-Hour Ozone Concentrations............................................................ 25 Table 7 - Historical Maximum Annual Average Sulfur Dioxide Concentrations................................ 25 Table 8 - Historical Maximum Annual Average Nitrogen Dioxide Concentrations ........................... 26 Table 9 - Historical Maximum 24-Hour PM10 Concentrations............................................................ 26 Table 10 - Historical Maximum Quarterly Lead Concentrations ........................................................ 28 Table 11 - 2005 National Ranking of Carbon Monoxide Monitors by 8-Hr Concentrations in ppm .. 29 Table 12 - 2005 National Ranking of Ozone Monitors by 1-Hr Concentrations in ppm..................... 29 Table 13 - 2005 National Ranking of Ozone Monitors by 8-Hr Concentrations in ppm..................... 30 Table 14 - 2005 National Ranking of SO2 Monitors by 24-Hr Concentrations in ppm....................... 30 Table 15 - 2005 National Ranking of NO2 Monitors by 1-Hr Concentrations in ppm ........................ 31 Table 16 - 2005 National Ranking of PM10 Monitors by 24-Hr Maximum Concentrations in µg/m3 31 Table 17 - 2005 National Ranking of PM2.5 Monitors by 24-Hr Maximum Concentrations in µg/m3 31 Table 18 - 2005 National Ranking of Lead Monitors by 24-Hr Maximum Concentration in µg/m3 .. 32 Table 19 - Eastern Plains Monitors In Operation For 2005 ................................................................. 33 Table 20 - Eastern Plains Particulate Values For 2005........................................................................ 33 Table 21 - Northern Front Range Particulate Monitors In Operation For 2005 .................................. 36 Table 22 - Northern Front Range Particulate Values For 2005 ........................................................... 37 Table 23 - Northern Front Range TSP and Lead Values For 2005...................................................... 41 Table 24 - Northern Front Range Continuous Monitors In Operation For 2005 ................................. 43 Table 25 - Northern Front Range Carbon Monoxide Values for 2005 ................................................ 44 Table 26 - Northern Front Range Ozone Values For 2005.................................................................. 47 Table 27 - Northern Front Range Oxides of Nitrogen and Sulfur Dioxide Values For 2005 .............. 50 Table 28 - Denver Visibility Standard Exceedance Days.................................................................... 51 Table 29 - Fort Collins Visibility Standard Exceedance Days ............................................................ 53 Table 30 - Southern Front Range Monitors In Operation For 2005..................................................... 63 Table 31 - Southern Front Range Maximum Particulate Values For 2005.......................................... 64 Table 32 - Southern Front Range TSP and Lead Values For 2005...................................................... 66 Table 33 - Southern Front Range Carbon Monoxide Values For 2005 ............................................... 67 Table 34 - Southern Front Range Ozone Values For 2005.................................................................. 68 Table 35 - Mountain Counties Monitors In Operation For 2005 ......................................................... 70 Table 36 - Mountain Counties Particulate Values For 2005................................................................ 71 Table 37 - Mountain Counties TSP and Lead Concentrations For 2005 ............................................. 74 Table 38 - Western Counties Monitors In Operation For 2005 ........................................................... 75 Table 39 - Western Counties Particulate Values For 2005 .................................................................. 76 Table 40 - Western Counties Carbon Monoxide Values For 2005...................................................... 78 1.0 Purpose of the Annual Data Report The Colorado Department of Public Health and Environment, Air Pollution Control Division (APCD) publishes the Colorado Air Quality Data Report as a companion document to the Colorado Air Quality Control Commission Report to the Public. The Air Quality Data Report addresses changes in ambient air quality measured by Division monitors. The Report to the Public discusses the policies and programs designed to improve and protect Colorado’s air quality. 1.1 Symbols and Abbreviations The following symbols and abbrevations are used through out this report: CO – Carbon monoxide SO2 – Sulfur dioxide SOX – Sulfur oxides NOX – Nitrogen oxides NO – Nitric oxide NO2 – Nitrogen dioxide O3 – Ozone Met – meteorological measurements, wind speed, wind direction, temperature, relative humidity and standard deviation of horizontal wind direction. TSP – Total suspended particulates PM10 – Particulate matter less than 10 microns PM2.5 – Particulate matter less than 2.5 microns Pb – Lead ppm – parts per million – this is used with gasseous pollutants. µg/m3 – micrograms per cubic meter – this is used with particulate pollutants. 1.2 Description of Monitoring Areas in Colorado The state has been divided into five multicounty areas that are generally based on topography. The areas are: the Eastern Plains; the Northern Front Range; the Southern Front Range; the Mountain Counties and the Western Counties. These divisions are a somewhat arbitrary grouping of monitoring sites with similar characteristics. The Eastern Plains Counties consist of those east of the I-25 corridor. These counties are generally rolling agricultural plains below 6000 feet. The Front Range used in this definition is defined by the counties along or associated with the I25 corridor not by the Continental Divide. A division using the Continental Divide would place Leadville with the same counties as Colorado Springs and Denver. Leadville as the highest city in the U.S. has more in common with Breckenridge and Aspen than Denver or Colorado Springs. The Mountain Counties are those along both sides of the Continental Divide and the Western Counties are the ones adjacent to the Utah border. Other divisions can and have been made, but these five divisions seemed appropriate for this report. Figure 1 shows the boundries of these areas. 1.2.1 Eastern Plains Counties The Air Pollution Control Division has only monitored for particulates and meteorology in the Eastern Plains Counties. The Eastern Plains Counties do not have the pollution sources that can generate health impacting concentrations of the other pollutants. The Division has monitored for particulates in the communities along I-76, I-70 and along US Highway 50. The only monitors still in operation are in Lamar. The other monitors were discontinued after a review of the data showed that levels of particulates were well below the standard and were declining. 1 1.2.2 Northern Front Range Counties The Northern Front Range Counties are those along the urbanized I-25 corridor from the Colorado/Wyoming border to just south of the city of Castle Rock. This area has the majority of the population in the state. It also has the majority of the monitors, with the Denver-metro area being the most heavily monitored. The remaining monitors are located in or near Fort Collins, Greeley, Longmont and Boulder. 1.2.3 Southern Front Range Counties The Southern Front Range Counties are those along the urbanized I-25 corridor from south of the city of Castle Rock to the southern Colorado border. The cities with monitoring in the area include Colorado Springs, Pueblo, Cripple Creek, Cañon City and Alamosa. Colorado Springs is the only city in the area that is monitored for carbon monoxide and ozone; the other cities are only monitored for particulates. In the past the APCD has conducted particulate monitoring in both Walsenburg and Trinidad. The monitoring in those cities was discontinued after a review of the data showed that levels of particulates were below the standard and were declining. 1.2.4 Mountain Counties The Mountain Counties are those counties along the Continental Divide. The cities are usually located in tight mountain valleys where nighttime temperature inversions trap any pollution near the ground. Their primary monitoring concern is with particulate pollution from wood burning and road sanding. These communities range from Steamboat Springs in the north to Telluride in the southwest and include Silverthorne and Breckenridge in the I-70 corridor; Aspen, Leadville, Crested Butte, Mt. Crested Butte, Vail and Gunnison in the central mountains. 1.2.5 Western Counties The Western Counties generally contain smaller towns located in fairly broad river valleys. Grand Junction is the only large city in the area and the only location that monitors for carbon monoxide on the western slope. The other Western Slope monitors are located in the cities of Parachute, Delta, Durango and Pagosa Springs. These locations monitor only for particulates. 2 Table 1 - Statewide Continuous Monitors In Operation For 2005 X - Monitors continued in 2005 A – Monitors added in 2005 D – Monitors discontinued in 2005 County Site Name Location CO SO2 NOX O3 Met Eastern Plains Counties Prowers Lamar - POE Adams Commerce City 7100 Hwy 50 A Northern Front Range Counties 7101 Birch St. th Welby 78 Ave. & Steele St. Arapahoe Highland Res. 8100 S. University Blvd. Boulder Boulder 2150 28th St. X X X X Longmont 440 Main St. Auraria Lot R 12th St. & Auraria Parkway Denver CAMP Denver Carriage Douglas Jefferson Larimer X X X th X Denver NJH 14 Ave. & Albion St. 1901 13th Ave. (Visibility) Firehouse #6 1300 Blake St. Chatfield Res. 11500 N. Roxborough Pk. Rd. th X X 2105 Broadway DESCI Building X X rd 23 Ave. & Julian St. X D 1405½ S. Foothills Hwy. Denver X X X A X X X X X X X X Arvada 9101 W. 57 Ave. NREL 2054 Quaker St. X X Rocky Flats 16600 W. Hwy. 128 X X 11501 Indiana St. D 9901 Indiana St. X 18000 W. Hwy. 72 D 11190 N. Hwy. 93 D Welch 12400 W. Hwy. 285 Fort Collins 708 S. Mason St. X X X X X 300 Remington St. (Visibility) 4407 S. College Ave. Weld Greeley th 905 10 Ave. X X 3101 35th Ave. X Southern Front Range Counties El Paso Colorado Springs I-25 & Uintah St. X USAF Rd. 640 690 W. Hwy. 24 Teller Manitou Springs 101 Banks Pl. Cripple Creek Warren Ave. & 2nd St. X X X X Mountain Counties Routt Steamboat Springs th 137 10 St. Mesa Grand Junction 645 ¼ Pitkin Ave. D Western Counties X X 3 Table 2 - Statewide Particulate Monitors In Operation For 2005 X - Monitors continued in 2005 A – Monitors added in 2005 D – Monitors discontinued in 2005 H – Hourly particulate monitor S – Chemical Speciation County Site Name Location TSP Pb PM10 PM2.5 Eastern Plains Counties Elbert Prowers Elbert Lamar Wright-Ingraham Inst. 100 2nd St. 104 Parmenter St. Adams Brighton Commerce City Globeville Welby Arapahoe Comm. College Longmont Boulder X X X Northern Front Range Counties Arapahoe Boulder Denver Douglas Larimer Weld Denver CAMP Denver Gates Denver NJH Denver Visitor Center Lowry Denver Gates - East Denver Animal Shelter Swansea Elementary Sch. Chatfield Reservoir Fort Collins Greeley Platteville 22 S. 4th Ave. 7101 Birch St. 5400 Washington St. 78th Ave. & Steele St. 6190 S. Santa Fe Dr. 350 Kimbark St. 2440 Pearl St. 2102 Athens St. 2105 Broadway 1050 S. Broadway 14th Ave. & Albion St. 225 W. Colfax Ave. 8100 Lowry Blvd. 305 E Mississippi Ave. 678 S. Jason St. 4650 Columbine St. 11500 Roxborough Rd 251 Edison St. 1516 Hospital Rd. 1004 Main St. X X X X X X X/H/S X/H X X X D X D X/H D A/D A X X A/D A/H X X/H X H X/H H A/D A X X X A/H X X/H X/S Southern Front Range Counties Alamosa Alamosa El Paso Colorado Springs Fremont Pueblo Teller Cañon City Pueblo Cripple Creek Archuleta Gunnison Pagosa Springs Crested Butte Mt. Crested Butte Gunnison Mt. Crested Butte - New Leadville Aspen Steamboat Springs Telluride Breckenridge 359 Poncha Ave. 425 4th St. 3730 Meadowlands 101 W. Costilla St. 128 Main St. 211 D St. 209 Bennett Ave. X X X X X X X X X X X/S X Mountain Counties Lake Pitkin Routt San Miguel Summit 309 Lewis St. Colo.135 & Whiterock 9 Emmons Rd. 221 N. Wisconsin St. 19 Emmons Rd. 510 Harrison St. 120 Mill St. 136 6th St. 333 W Colorado Ave. 501 N. Park Ave. X X X D X A X D X/H X X X D X A X 4 Table 2 - Statewide Particulate Monitors In Operation For 2005 (continued) X - Monitors continued in 2005 A – Monitors added in 2005 D – Monitors discontinued in 2005 H – Hourly particulate monitor S – Chemical Speciation County Site Name Location TSP Pb PM10 PM2.5 X X A A A A A A X X X D X X H X Western Counties Delta Garfield La Plata Delta Parachute Rifle New Castle Silt – Bell Ranch Silt – Daley Ranch Silt – Cox Ranch Glenwood Springs Durango Mesa Grand Junction 560 Dodge St. 100 E. 2nd St. 144 E. 3rd Ave. 402 W. Main St. 512 Owens Dr. 884 County Rd. 327 5933 County Rd 233 109 8th St. 1060 2nd Ave. 56 Davidson Creek Rd. 1235 Camino del Rio 1455 S. Camino del Rio 117 Cutler Dr. 650 South Ave. 645 ¼ Pitkin Ave. X/H/S 5 Figure 1 Monitoring Areas in Colorado The pin symbols on the map show the approximate location of the monitors in Colorado. 6 2.0 Criteria Pollutants The criteria pollutants are those for which the federal government has established ambient air quality standards in the Federal Clean Air Act and its amendments. There are six criteria pollutants. They are carbon monoxide, ozone, sulfur dioxide, nitrogen dioxide, lead and particulate matter. The standards for criteria pollutants are established to protect the most sensitive members of society. These are usually defined as those with respiratory problems, the very young and the infirm. The concentrations of each standard for the criteria pollutants are discussed in each section and a summary is presented in Table 3. Table 3 - National Ambient Air Quality Standards1 Pollutant Carbon Monoxide (CO) Primary Primary Ozone (O3) Primary Secondary Nitrogen Dioxide (NO2) Primary Secondary Sulfur Dioxide (SO2) Primary Primary Secondary Particulate (PM10) Primary Primary Particulate (PM2.5) Primary Primary Lead (Pb) Primary Averaging Time Concentration 1-hour* 8-hour* 35 ppm 9 ppm 8-hour** Same as primary 0.08 ppm Annual arithmetic mean Same as primary 0.053 ppm Annual arithmetic mean 24-hour* 3-hour* 0.03 ppm 0.14 ppm 0.5 ppm Annual arithmetic mean**** 24-hour*** 50 µg/m3 150 µg/m3 Annual arithmetic mean**** 24-hour***** 15 µg/m3 65 µg/m3 Calendar quarter 1.5 µg/m3 * This concentration is not to be exceeded more than once per year. th ** The 8-hour Ozone standard is set at 0.08 ppm as the 3-year average of the annual 4 maximum 8-hour average concentration. *** The 24-hour standard is attained when the expected number of exceedances for each calendar year, averaged over three years, is less than or equal to one. **** The annual arithmetic mean standard is a 3-year average. th ***** The 24-hour PM2.5 standard is based on the three-year average of the 98 percentile. 7 2.0.1 Exceedance Summary Table Table 4 is a summary of the number of exceedances of the ambient air quality standards for Colorado for 2004 and 2005. There were no exceedances of any criteria pollutant at any state operated monitor in 2004. This is only the second time since the APCD began monitoring for criteria pollutants in the early 1970’s that no exceedances were recorded at any state operated monitor. The levels of the standards are listed in Table 3. Table 4 - 2004/2005 Exceedance Summaries 2004* 2005 Location PM10 Mt. Crested Butte Breckenridge * X X There were no exceedances of any National Ambient Air Quality Standard in 2004. 2.1 Carbon monoxide Carbon monoxide is a colorless and odorless gas, formed when carbon in fuel is not burned completely. It is a component of motor vehicle exhaust, which contributes about 60 percent of all carbon monoxide emissions nationwide. Nonroad vehicles account for the remaining carbon monoxide emissions from transportation sources. High concentrations of carbon monoxide generally occur in areas with heavy traffic congestion. In cities, as much as 85 percent of all carbon monoxide emissions may come from automobile exhaust. Other sources of carbon monoxide emissions include industrial processes, nontransportation fuel combustion, and natural sources such as wildfires. Peak carbon monoxide concentrations typically occur during the colder months of the year when carbon monoxide automotive emissions are greater and nighttime inversion conditions (where air pollutants are trapped near the ground beneath a layer of warm air) are more frequent.2 2.1.1 Carbon monoxide – Standards The U.S. Environmental Protection Agency (EPA) has developed two national standards for carbon monoxide. They are 35 ppm averaged over a 1-hour period and 9 ppm averaged over an 8-hour period. These values are not to be exceeded more than once in a given year at any given location. A location will violate the standard with a second exceedance of either standard in a calendar year. The EPA directive requires that comparison with the carbon monoxide standards will be made in integers. Fractions of 0.5 or greater are rounded up, thus, actual concentrations of 9.5 ppm and 35.5 ppm or greater are necessary to exceed the 8-hour and 1-hour standards, respectively.3 2.1.2 Carbon monoxide – Health Effects Carbon monoxide affects the central nervous system by depriving the body of oxygen. It enters the body through the lungs, where it combines with hemoglobin in the red blood cells. Normally, hemoglobin carries oxygen from the lungs to the cells. The oxygen attached to the hemoglobin is exchanged for the carbon dioxide generated by the cell’s metabolism. The carbon dioxide is then carried back to the lungs where it is exhaled from the body. Hemoglobin binds approximately 240 times more readily with carbon monoxide than with oxygen. In the presence of carbon monoxide the distribution of oxygen is reduced throughout the body. Blood laden with carbon monoxide can weaken heart contractions with the result of lowering the volume of blood distributed to the body. It can significantly reduce a healthy person's ability to do manual tasks, such as working, jogging and walking. A life-threatening situation can exist for patients with heart disease when these people are unable to compensate for the oxygen loss by increasing the heart rate.2 The EPA has concluded that the following groups may be particularly sensitive to carbon monoxide exposures: angina patients, individuals with other types of cardiovascular disease, persons with chronic obstructive pulmonary disease, anemic individuals, fetuses and pregnant women. Concern also 8 exists for healthy children because of increased oxygen requirements that result from their higher metabolic rate.3 Carbon monoxide is exhausted from the body at varying rates, depending on physiological and external factors. The general guideline is that 20 to 40 percent is lost from the system after 2 to 3 hours following exposure.3 The severity of health effects depends on both the concentration and the length of exposure because it takes time to remove it from the blood stream. 2.1.3 Carbon monoxide – Sources In Denver, the APCD estimates that 86 percent of the carbon monoxide emissions are from automotive sources. An estimated 3 percent of Denver's carbon monoxide emissions are from woodburning stoves and fireplaces. The remainder originates from aircraft, locomotives, construction equipment, power plants and space heating.4 These numbers are similar to the nationwide emissions.5 The percentage of carbon monoxide emissions contributed from various sources has not changed appreciably since 1970.6 What has changed is the amount of carbon monoxide emitted by these sources (Figure 2). In 1970 the total carbon monoxide emissions were approximately 197 million tons in 2004 this had been reduced to 87 million tons.2 Figure 2 – Changes in National Carbon Monoxide Emissions from 1970 – 20042 Millions of Tons/Year 200 175 150 125 100 75 1970 1975 1980 1985 1990 1995 2000 2004 Year 2.2 Ozone Ozone is a highly reactive form of oxygen. At very high concentrations it is a blue, unstable gas with a characteristic pungent odor often associated with arcing electric motors, lightning storms or other electrical discharges.7 However, at ambient concentrations, ozone is colorless and odorless. Ozone concentrations at remote locations, such as the Western National Air Pollution Background Network, range from 0.02 to 0.04 ppm year-round.8 At ground level, ozone is a pollutant. Although chemically identical, ground level ozone should not be confused with the stratospheric ozone layer. The stratospheric ozone layer is found between 12 and 30 miles above the earth's surface and shields the earth from intense, cancer-causing ultraviolet radiation. Concentrations of ozone in this layer are approximately 10 to 12 ppm or more than 100 times the National Ambient Air Quality Standard for ozone. Occasionally, meteorological conditions result in stratospheric 9 ozone being brought to ground level and this can increase concentrations by 0.05 to 0.10 ppm. This stratospheric intrusion has caused concentrations higher than the 0.12 ppm standard.8 2.2.1 Ozone – Standards In July 1997, the U.S. Environmental Protection Agency established a new ozone standard. The reasons for these changes were: “. . . to provide protections for children and other at-risk populations against a wide range of ozone induced health effects, including decreased lung function (primarily in children active outdoors), increased respiratory symptoms (particularly in highly sensitive individuals), hospital admissions and emergency room visits for respiratory causes (among children and adults with pre-existing respiratory disease such as asthma), inflammation of the lung and possible long-term damage to the lungs.”9 “The 1-hour primary standard of 0.12 ppm was replaced by an 8-hour standard at a level of 0.08 ppm with a form based on the 3-year average of the annual 4th-highest daily maximum 8hour average ozone concentration measured at each monitor within an area.”9 The 8-hour averaging time is more directly associated with health effects of concern at lower ozone concentrations than is the former 1-hour averaging time. Therefore, the 8-hour standard was felt to be more appropriate for a human health-based standard than the 1-hour standard.9 2.2.2 Ozone – Health Effects Exposure to ozone has been linked to a number of health effects, including significant decreases in lung function, inflammation of the airways, and increased respiratory symptoms, such as cough and pain when taking a deep breath. Exposure can also aggravate lung diseases such as asthma, leading to increased medication use and increased hospital admissions and emergency room visits. Active children are the group at highest risk from ozone exposure because they often spend a large part of the summer playing outdoors. Children are also more likely to have asthma, which may be aggravated by ozone exposure. Other at-risk groups include adults who are active outdoors (e.g., some outdoor workers) and individuals with lung diseases such as asthma and chronic obstructive pulmonary disease. In addition, long-term exposure to moderate levels of ozone may cause permanent changes in lung structure, leading to premature aging of the lungs and worsening of chronic lung disease. Ozone also affects vegetation and ecosystems, leading to reductions in agricultural crop and commercial forest yields, reduced growth and survivability of tree seedlings, and increased plant susceptibility to disease, pests, and other environmental stresses (e.g., harsh weather). In long-lived species, these effects may become evident only after several years or even decades and may result in long-term effects on forest ecosystems. Ground level ozone injury to trees and plants can lead to a decrease in the natural beauty of our national parks and recreation areas.10 The recently completed review of the ozone standard (by the EPA and others) also highlighted concerns with ozone effects on vegetation for which the 1-hour ozone standard did not provide adequate protection. These effects can include reduction in agricultural and commercial forest yields, reduced growth and decreased survivability of tree seedlings, increased tree and plant susceptibility to disease, pests and other environmental stresses and potential long-term effects on forests and ecosystems.10 2.2.3 Ozone – Sources Ozone is not emitted directly from a source, as are other pollutants, but forms as a secondary pollutant. Its precursors are certain reactive hydrocarbons and nitrogen oxides, which react chemically in sunlight to form ozone. The main sources for these reactive hydrocarbons are automobile exhaust, gasoline, oil storage and transfer facilities, industrial paint solvents, degreasing agents, cleaning fluids and 10 ink solvents. High temperature combustion combines nitrogen and oxygen in the air to form oxides of nitrogen. Vegetation can also emit reactive hydrocarbons such as terpenes from pine trees, for example.10 Although some ozone is produced all year, the highest concentrations usually occurr in the summer. The stagnant air and intense sunlight on hot, bright summer days provide the conditions for the precursor chemicals to react and form ozone. The ozone produced under these stagnant summer conditions remains as a coherent air mass and can be transported many miles from its point of origin. 2.3 Sulfur dioxide Sulfur dioxide is a colorless gas with a pungent odor. It is detectable by smell at concentrations of about 0.5 to 0.8 ppm.11 It is highly soluble in water. In the atmosphere, sulfur oxides and nitric oxides are converted to “acid rain.” 2.3.1 Sulfur dioxide – Standards There are two primary standards for sulfur dioxide. The first is a long-term, one-year arithmetic average not to exceed 0.03 ppm. The second is a short-term, 24-hour average where concentrations are not to exceed 0.14 ppm more than once per year. The secondary standard is a 3-hour average not to exceed 0.5 ppm more than once per year.11 2.3.2 Sulfur dioxide – Health Effects High concentrations of sulfur dioxide can result in temporary breathing impairment for asthmatic children and adults who are active outdoors. Short-term exposures of asthmatic individuals to elevated sulfur dioxide levels during moderate activity may result in breathing difficulties that can be accompanied by symptoms such as wheezing, chest tightness, or shortness of breath. Other effects that have been associated with longer-term exposures to high concentrations of sulfur dioxide, in conjunction with high levels of particulate mater, include aggravation of existing cardiovascular disease, respiratory illness, and alterations in the lungs’ defenses. The subgroups of the population that may be affected under these conditions include individuals with heart or lung disease, as well as the elderly and children.12 Together, sulfur dioxide and oxides of nitrogen are the major precursors to acidic deposition (acid rain), which is associated with the acidification of soils, lakes, and streams and accelerated corrosion of buildings and monuments. Sulfur dioxide also is a major precursor to PM2.5, which is a significant health concern, and a main contributor to poor visibility.12 2.3.3 Sulfur dioxide – Sources Sulfur dioxide belongs to the family of gases called sulfur oxides, or SOX gases. These gases are formed when fuel containing sulfur (mainly coal and oil) is burned at power plants and during metal smelting and other industrial processes. Most sulfur dioxide monitoring stations are located in urban areas. The highest monitored concentrations of sulfur dioxide are recorded near large industrial facilities. Fuel combustion, largely from electricity generation, accounts for most of the total sulfur dioxide emissions.13 11 Figure 3 - Changes in National Sulfur Dioxide Emissions from 1970 – 200413 35 Tons/Year 30 25 20 15 10 1970 1975 1980 1985 1990 1995 2000 2004 Years 2.4 Nitrogen dioxide In its pure state, nitrogen dioxide is a reddish-brown gas with a characteristic pungent odor. It is corrosive and a strong oxidizing agent. As a pollutant in ambient air, however, it is virtually colorless and odorless. Nitrogen dioxide can be an irritant to the eyes and throat. Oxides of nitrogen (nitric oxide and nitrogen dioxide) are formed when the nitrogen and oxygen in the air are combined in high temperature combustion. 2.4.1 Nitrogen dioxide – Standards The annual standard for nitrogen dioxide is 0.053 ppm expressed as an annual arithmetic mean (average).14 “Los Angeles is the only U.S. city that has recorded exceedances of the nitrogen dioxide annual standard in the past twelve (now sixteen) years.”15 2.4.2 Nitrogen dioxide – Health Effects Elevated concentrations of nitrogen dioxide cause respiratory distress, degradation of vegetation, clothing and visibility, and increased acid deposition. Nitrate aerosols, which result from nitric oxide and nitrogen dioxide combining with water vapor in the air, have been consistently linked to Denver's visibility problems. 2.4.3 Nitrogen dioxide – Sources About 44 percent of the emissions of nitrogen dioxide in the Denver area come from large combustion sources such as power plants. Almost 33 percent comes from motor vehicles, 15 percent from space heating, 3 percent from aircraft and 5 percent from miscellaneous off-road vehicles. Minor sources include fireplaces and woodstoves and high temperature combustion processes used in industrial work.16 12 Figure 4 - Changes in National Oxides of Nitrogen Emissions from 1970 – 200417 30 Tons/Year 27 24 21 18 15 1970 1975 1980 1985 1990 1995 2000 2004 Years 2.5 Particulate Matter – PM10 Particle pollution is a mixture of microscopic solids and liquid droplets suspended in air. This pollution, also known as particulate matter, is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, soil or dust particles, and allergens (such as fragments of pollen or mold spores). The size of particles is directly linked to their potential for causing health problems. Small particles, less than 10 micrometers in diameter, pose the greatest problems. The smallest particles can get deep into your lungs, and some may even get into your bloodstream. Exposure to such particles can affect both your lungs and your heart. Larger particles are of less concern, although they can irritate your eyes, nose, and throat. Small particles of concern include "fine particles" (such as those found in smoke and haze), which are 2.5 micrometers in diameter or less; and "coarse particles" (such as those found in wind-blown dust), which have diameters between 2.5 and 10 micrometers.18 2.5.1 Particulate Matter – PM10 – Standards In July 1987, EPA promulgated National Ambient Air Quality Standards for particulates with an aerodynamic diameter of 10 microns or less (PM10). This is a size that can be inhaled into the bronchial and alveolar regions of the lungs. The standard has two forms, a 24-hour standard of 150 µg/m3 and an annual arithmetic mean standard of 50 µg/m3.19 1. 2. The 24-hour standard is attained when the expected number of exceedances for each calendar year, averaged over three years, is less than or equal to one. The estimated number of exceedances is computed quarterly using available data and adjusting for missing sample days. The annual arithmetic mean standard is attained when the annual mean, averaged over three years is less than or equal to the level of the standard. Each annual mean is computed from the average of each quarter in the year, with adjustments made for missing sample days. 13 3. In both cases, a data recovery of 75 percent is needed for each calendar quarter to be considered a valid quarter of data. The 24-hour standard was modified in by EPA in July 1997, but was subsequently nullified back to this form in May 1999 due to a challenge in the courts. 2.5.2 Particulate Matter – PM10 – Health Effects According to American Lung Association’s paper The Perils of Particulates: “The health risk from an inhaled dose of particulate matter depends on the size and concentration of the particulate. Size determines how deeply the inhaled particulate will penetrate into the respiratory tract where they can persist and cause respiratory damage. Particles less than 10 microns in diameter are easily inhaled deep into the lungs. In this range, larger particles tend to deposit in the tracheobronchial region and smaller ones in the alveolar region. Particulates deposited in the alveolar region can remain in the lungs for long periods because the alveoli have a slow mucociliary clearance system.”20 “Fine particulate pollution does not affect the health of exposed persons with equal severity. Certain subgroups of people potentially exposed to air pollutants can be identified as potentially ‘at risk’ from adverse health effects of air borne pollutants. There is very strong evidence that asthmatics are much more sensitive (i.e., respond with symptoms at relatively low concentrations) to the effects of particulates than the general healthy population. Conversely, little scientific evidence exists that show elderly persons (greater than 65 years old) are particularly sensitive to the effects of particulate matter air pollution”16 The welfare effects of particulate exposure may be the most widespread of all the pollutants. Because of the potential for extremely long-range transport of fine particles and chemical reactions that occur, no place on earth has been spared from the particulate pollution generated by urban and rural sources. The effects of particulates range from visibility degradation to climate changes and vegetation damage. General soiling, commonly thought to be just a nuisance, can have long-term adverse effects on building paints and other materials. Acid deposition as particulates can be detected in the most remote areas of the world. 2.5.3 Particulate Matter – PM10 – Sources Most anthropogenic (manmade) particulates are in the 0.1 to 10 micron diameter range. Particles larger than 10 microns are usually due to “fugitive dust”. Fugitive dust is wind-blown sand and dirt from roadways, fields and construction sites that contain large amounts of silica (sand-like) materials. Anthropogenic particulates are created during the burning of fuels associated with industrial processes or heating. These particulates include fly ash (from power plants), carbon black (from automobiles and diesel engines) and soot (from fireplaces and woodstoves). The PM10 particulates from these sources contain a large percentage of elemental and organic carbon. These types of particles play a role in both visual haze and health issues.21 Figure 5 shows the changes in national particulate emissions from 1970 through 2004. 14 Figure 5 - Changes in National PM10 Emissions from 1970 – 200421 12.0 Tons/Year 10.0 8.0 6.0 4.0 2.0 0.0 1970 1975 1980 1985 1990 1995 2000 2004 Years 2.6 Particulate Matter – PM2.5 According to the Environmental Protection Agency’s Latest Findings on National Air Quality: 2000 Status and Trends, Particulate Matter, “PM2.5 is composed of a mixture of particles directly emitted into the air and particles formed in the air by the chemical transformation of gaseous pollutants. The principle types of secondary pollutants are ammonium sulfate and ammonium nitrate formed in the air from gaseous emissions of SO2 and NOX, reacting with ammonia. The main source of SO2 is combustion of fossil fuels in boilers and the main source of NOX are the combustion of fossil fuels in boilers and mobile sources. Some secondary particles are also formed from semi-volatile organic compounds which are emitted from a wide range of combustion sources.” 2.6.1 Particulate Matter – PM2.5 – Standards In 1997, the EPA added new fine particle standards, PM2.5, to the existing PM10 standards. The numbers, 2.5 and 10 refer to the particle size measured in microns. EPA added an annual PM2.5 standard set at a concentration of 15 micrograms per cubic meter (µg/m3) and a 24-hour PM2.5 standard set at 65 µg/m3. However, a lawsuit by the American Trucking Association questioned the EPA's authority to create the new standard. A US District court ruling blocked implementation of the PM2.5 standard, but the US Supreme court reversed the lower court and unanimously upheld the legality of the EPA and its creation of the PM2.5 standard. The Supreme Court decision was issued on February 27, 2001. The annual component of the standard was set to provide protection against typical day-to-day exposures as well as longer-term exposures, while the daily component protects against more extreme short-term events. Areas will be considered in compliance with the annual PM2.5 standard when the 3-year average of the annual arithmetic mean PM2.5 concentrations, from single or multiple community-oriented monitors, is less than or equal to 15 µg/m3. The 24-hour PM2.5 standard is based on the 98th percentile of 24-hour PM2.5 concentrations in a year (averaged over 3 years). The change to a percentile based standard from a second maximum based standard was designed to eliminate the effect of anomalously high concentrations. In addition this change is an attempt to focus more on the true health effects of the pollutant. 15 2.6.2 Particulate Matter – PM2.5 – Health Effects The health effects of PM2.5 are not just a function of their size, 1/20th the size of a human hair, which allows them to be breathed deeply into the alveoli the lungs, but of their composition. These particles can remain in the lungs for a long time and cause a great deal of damage to the lung tissue. They can reduce lung function as well as cause or aggravate respiratory problems. They can increase the longterm risk of lung cancer or lung diseases such as emphysema or pulmonary fibrosis.22 2.6.3 Particulate Matter – PM2.5 – Sources Figure 6 shows the nationwide changes in emissions of PM2.5 particulates from 1995 through 2004. The primary source of fine particles emitted directly into the air come from crustal materials, ground up rock, carbonaceous material. The carbonaceous material is generated by the incomplete combustion of fossil fuels and other organic compounds.21 Particles less than 2.5 microns in diameter, or PM2.5, are the major contributors to visibility problems because of their ability to scatter or absorb light. In Denver, the effects of this particulate pollution can be seen as the “Brown Cloud” or more appropriately, the “Denver Haze” because it is frequently neither brown nor an actual cloud. Figure 6 - Changes in National PM2.5 Emissions from 1970 – 200422 2.50 Tons/Year 2.25 2.00 1.75 1.50 1970 1975 1980 1985 1990 1995 2000 2004 Years 2.7 Lead Since the late 1980s the most significant sources for atmospheric lead are battery plants and nonferrous smelters. With the near elimination of lead as an additive in gasoline the contribution from that source has been reduced significantly. 2.7.1 Lead – Standards The current federal standard for lead is a calendar quarter (3-month) average concentration not to exceed 1.5 micrograms of lead per cubic meter of air (µg/m3). This standard was established to maintain blood lead concentrations below 30 micrograms per deciliter (µg/dL) due to exposure to atmospheric lead concentrations.23 In the future, the focus on lead monitoring will shift to ensure that stationary sources do not create violations of the standard in localized areas. Colorado has at least one such source in the 16 Denver area that is the subject of monitoring. The Historical Lead Comparison graphs show data back to 1990. The concentrations recorded at most of the monitoring sites are approaching the limits of detection for ambient lead. The last violation of the lead standard in Colorado was the first quarter of 1980. 2.7.2 Lead – Health Effects Exposure to lead occurs mainly through inhalation of air and ingestion of lead in food, water, soil, or dust. It accumulates in the blood, bones, and soft tissues and can adversely affect the kidneys, liver, nervous system, and other organs. Excessive exposure to lead may cause neurological impairments such as seizures, mental retardation, and behavioral disorders. Even at low doses, lead exposure is associated with damage to the nervous systems of fetuses and young children, resulting in learning deficits and lowered IQ. Recent studies also show that lead may be a factor in high blood pressure and subsequent heart disease. Lead can also be deposited on the leaves of plants, presenting a hazard to grazing animals and humans through ingestion.24 2.7.3 Lead – Sources “Because of the phase-out of leaded gasoline, lead emissions and concentrations decreased sharply during the 1980s and early 1990s. Emissions of lead decreased 96 percent over the 24-year period 1980–2004. These large reductions in long-term lead emissions from transportation sources have changed the nature of the ambient lead problem in the United States. Because industrial processes are now responsible for all violations of the lead NAAQS, the lead monitoring strategy currently focuses on emissions from these point sources.”24 Figure 7 shows the decline in lead emissions in the past 34 years. Figure 7 - Changes in National Lead Emissions from 1970 – 200424 0.250 Tons/Year 0.200 0.150 0.100 0.050 0.000 1970 1975 1980 1985 1990 Years 1995 2000 2004 17 3.0 Non-Criteria Pollutants Non-criteria pollutants are those pollutants for which there are no current national ambient air quality standards. These include but are not limited to visibility, total suspended particulates, nitric oxide and air toxics. Meteorological measurements of wind speed, wind direction, temperature and humidity are also included in this group. 3.1 Visibility Visibility is unique among air pollution effects in that it involves human perception and judgment. It has been described as the maximum distance that an object can be perceived against the background sky. Visibility also refers to the clarity with which the form and texture of distant, middle and near details can be seen as well as the sense of the trueness of their apparent coloration. As a result, measures of visibility serve as surrogates of human perception. There are several ways to measure visibility but none of them tell the whole story or completely measure visibility as human beings experience it. 3.1.1 Visibility – Standards The Colorado Air Quality Control Commission established a visibility standard in 1990 for the Front Range cities from Fort Collins to Colorado Springs. The standard, an atmospheric extinction of 0.076 per kilometer, was based on the public's definition of unacceptable amounts of haze as judged from slides of different haze levels taken in the Denver area. At the standard, 7.6 percent of the light in a kilometer of air is blocked. The standard applies from 8 a.m. to 4 p.m. each day, during those hours when the relative humidity is less than 70 percent. Visibility, along with meteorology and concentrations of other pollutants for which National Ambient Air Quality Standards exist, is used to determine the need for mandatory woodburning and voluntary driving restrictions. There is no quantitative visibility standard for Colorado's pristine and scenic rural areas. However, in the 1977 amendments to the Federal Clean Air Act, Congress added Section 169a25 and established a national visibility goal that created a qualitative standard of “the prevention of any future and the remedying of any existing, impairment of visibility in mandatory Class I federal areas which impairment results from manmade air pollution”. The implementation of Section 169a has led to federal requirements to protect visual air quality in large national parks and wilderness areas.26 Colorado has 12 of these Class I areas. Federal and state law prohibits visibility impairment in national parks and wildernesses due to large stationary sources of air pollution. 3.1.2 Visibility – Health Effects Visual air quality is an element of public welfare. Specifically, it is an important aesthetic, natural and economic resource of the state of Colorado. The worth of visibility is difficult to measure; yet good visibility is something that people undeniably value. Impaired visibility can affect the enjoyment of a recreational visit to a scenic mountain area. Similarly, people prefer to have clear views from their homes and offices. These concerns are often reflected in residential property values and office rents. Any loss in visual air quality may contribute to corresponding losses in tourism and usually make an area less attractive to residents, potential newcomers and industry. There is increasing information that shows a correlation between ambient concentrations of particulate matter and respiratory illnesses. Some researchers believe this link may be strongest with concentrations of fine particles, which also contribute to visibility impairment. In July 1997, the EPA developed a National Ambient Air Quality Standard for particulate matter less than 2.5 microns in diameter (PM2.5). See the section 2.6 for more information on PM2.5. Any control strategies to lower ambient concentrations of fine particulate matter for health reasons will also improve visibility. 18 3.1.3 Visibility – Sources The cause of visibility impairment in Colorado is most often fine particles in the 0.1 to 2.5 micrometer size range (one micrometer is a millionth of a meter). Light passing from a vista to an observer is either scattered away from the sight path or absorbed by the atmospheric fine particulate. Sunlight entering the pollution cloud may be scattered into the sight path adding brightness to the view and making it difficult to see elements of the vista. Sulfate, nitrate, elemental carbon and organic carbon are the types of particulate matter most effective at scattering and/or absorbing light. The man-made sources of these particulates include woodburning, electric power generation, industrial combustion of coal or oil, and emissions from cars, trucks and buses. Visibility conditions vary considerably across the state. Usually, visibility in Colorado is among the best in the country. Our prized western vistas exist due to unique combinations of topography and scenic features. Air in much of the West contains low humidity and minimal levels of visibility-degrading pollution. Nevertheless, visibility problems occur periodically throughout the state. Woodburning haze is a concern in several mountain communities each winter. Denver's has its“Brown Cloud.” Even the national parks, monuments, and wilderness areas shows pollution-related visibility impairment on occasion due to regional haze, the interstate or even regional-scale transport of visibility-degrading pollution. The vis