Atmospheric Environment (v.36, #23)

Development of a comprehensive, multiscale “one-atmosphere” modeling system: application to the Southern Appalachian Mountains by James W. Boylan; Mehmet T. Odman; James G. Wilkinson; Armistead G. Russell; Kevin G. Doty; William B. Norris; Richard T. McNider (3721-3734).
A comprehensive three-dimensional Eulerian photochemical model (URM-1ATM) was developed that simulates urban and regional gas and size-resolved aerosol concentrations of pollutants in the atmosphere and both wet and dry deposition. In this study, RAMS and EMS-95 are used to generate meteorological and emission input files, respectively. The modeling system is then applied to simulate the evolution, transport, and removal of atmospheric pollutants over the Eastern US for the 11–19 July 1995 episode. Performance statistics are calculated for ozone, speciated fine particles, and acid deposition mass fluxes.
Keywords: Mathematical modeling; Ozone; Aerosols; Wet deposition;

Four years of continuous total gaseous mercury (TGM) measurements at sites in Ontario, Canada by P Blanchard; F.A Froude; J.B Martin; H Dryfhout-Clark; J.T Woods (3735-3743).
Total gaseous mercury (TGM) measurements have been conducted around the Great Lakes since 1997. At two of these sites (Egbert and Burnt Island), TGM concentrations presented significant seasonal variations. Possible explanations for the larger winter–spring/smaller summer–fall concentrations included seasonal meteorological differences, a northern hemispherical increase in coal combustion for wintertime heating, seasonal cycles of atmospheric oxidants and overall hemispherical source–sink relationship. The impact of populated/industrialized areas on the TGM concentrations at rural sites was demonstrated using pollution roses. Trend analyses for Egbert and Point Petre indicated relatively stable TGM concentrations between 1997 and 2000. Principal component analysis of TGM and trace metals confirmed the influence of the industrialized/populated area of Southwestern Ontario on both Egbert and Point Petre as well as metal recovery activities from Northern Ontario. For one episode of significant TGM concentrations, only a general geographical area could be implied as the source of atmospheric mercury.
Keywords: TGM; Atmospheric mercury; Trend; PCA; Great lakes;

Aircraft measurements of the Sacramento pollutant plume were obtained over the western Sierra Nevada on 7 days in the summers of 1995 and 1996. These measurements, as well as those documenting the meteorological setting and evolution of surface conditions, are summarized. High concentrations of residual ozone above the surface layer in the early morning hours appear to be necessary to produce afternoon concentrations in excess of 125 ppbv. The moderately polluted days were characterized by low residual pollutant concentrations aloft in the early morning and moderate to weak vertical stratification during the day. The cleanest days had good ventilation with deep layers of near neutral stratification and moderate to strong winds. Horizontal spatial variability of ozone concentrations within the mixed layer was often significant at scales ⩽5 km. The maximum ozone concentrations were frequently observed in the afternoon, 40–80 km downwind of the city, but these decreased to about one-half those values at distances 120 km downwind. The measured oxides of nitrogen were generally just above detection limits (∼1 ppbv), except in the surface layer in the early morning hours.
Keywords: Air pollution transport; Urban plume; Aircraft measurements; Complex topography;

Epiphytic lichens collected from trees within northeastern North America (ca. 500,000 km2) have been used as natural filters of airborne lead in order to document the spatial distribution of this metal in the atmosphere and identify its principal sources. Lichens record an integrated signal over a few years of atmospheric fall-out and thus minimise any signals due to variable (seasonal) atmospheric circulation patterns, and biases related to discrete events focusing atmospheric pollution from a single point source. The Pb/Al ratios measured in lichens are 20–600 times higher than that of upper crustal rocks, indicating that most of their Pb is of anthropogenic origin. Hence, their Pb isotopic composition is directly related to that of the pollution sources. The 206Pb/207Pb values for the lichens vary from 1.146 to 1.206 and these are interpreted as results of mixing between Canadian and USA industrial lead. Pb isotope ratios of lichens define a coherent geographic distribution since samples from the US have high 206Pb/207Pb, typical of US industrial Pb, whereas lichens sampled in northern Québec have the lowest 206Pb/207Pb, typical of Canadian industrial Pb. Lichens sampled along the St. Lawrence Valley have intermediate isotopic composition, which is interpreted as a result of mixing between USA and Canadian sources. USA industrial Pb contributes 75–35% of the total atmospheric Pb in this area and the contribution decreases from Montréal towards the Gulf of St. Lawrence. Overall, geographic Pb isotope gradients may be sharp and indicate a strong control by both sources and prevailing meteorological conditions. From these results, it is possible to draw the first Pb isotopic distribution map of atmospheric Pb pollution over northeastern North America.
Keywords: Atmosphere; Lead; Pollution; Lichens; North America;

Changes in regional climate can alter conditions that control the transport, chemical transformation, and eventual deposition of atmospheric pollutants. In Pennsylvania, climate change is projected to increase mean annual temperatures by 4°C and annual precipitation amounts by about 5% through 2100. The objective of this study was to determine how increases in temperature and precipitation would affect concentrations and wet deposition of SO4 2−, NO3 , NH4 +, and H+ ions at a National Atmospheric Deposition Program site (NADP, PA15) in central Pennsylvania. Event-based wet deposition data were obtained from the Multi-State Atmospheric Power Production Pollution Study (MAP3S) monitoring program for the period 1976–1989. Forward stepwise regression was used to predict log-normal concentrations per event using mean temperature, precipitation, Julian Date, and interactions of temperature and precipitation with Julian Date as predictor variables. Julian Date was included to adjust for time trends in precipitation chemistry and climate data. Results were categorized by annual periods, growing season/dormant season, and synoptic climate types. Significant positive effects of temperature on concentration and deposition were found for SO4 2−, NH4 +, and H+ but not for NO3 . Precipitation increases reduced the concentration of each ion due to dilution, but the lower concentration only minimally offset the increase in wet deposition due to the increased precipitation. The effects of climate change during the growing season (April–September) were projected to cause greater increases in the magnitude of SO4 2−, NO3 , and H+ concentrations and wet deposition than in the dormant season (October–March). Ammonium (NH4 +) increases were greatest during the dormant season. Climate change effects on wet deposition were generally similar among synoptic climate types with the greatest effects occurring for cold fronts and warm fronts. The impacts of climate change for this central PA site could increase overall annual wet deposition of SO4 2− by up to 29%, NO3 by up to 6%, NH4 + by up to 23%, and H+ by up to 17% through the year 2100, assuming that the statistical relationships hold true and that no changes in the emission of these ions occur.
Keywords: Pennsylvania; Precipitation chemistry; Temperature; Precipitation dilution; MAP3S; Regression models;

Air pollutant emissions associated with forest, grassland, and agricultural burning in Texas by Ann Dennis; Matthew Fraser; Stephen Anderson; David Allen (3779-3792).
Outdoor fires, such as wildfires and prescribed burns, can emit substantial amounts of particulate matter and other pollutants into the atmosphere. In Texas, an inventory of forest, grassland and agricultural burning activities revealed that fires consumed vegetation on 1.6 and 1.7 million acres of land, in 1996 and 1997, respectively. Emissions from the fires were estimated based on survey and field data on acres burned and land cover and literature data on fuel consumption and emission factors. Fire data were allocated spatially by county and temporally by month. While fire events can cause high transient air pollutant concentrations, for most criteria pollutants, the fire emissions were a relatively small fraction of the annual emission inventory for the State. For fine particulate matter, however, the annual emission estimates were 40,000 tons/yr, which is likely to represent a significant fraction of the State's emission inventory, especially in the counties where the emissions are concentrated.
Keywords: Fire; Texas; Emission inventory; Land cover; Particulate matter; Criteria pollutants;

Biogenic volatile organic compound emissions from a lowland tropical wet forest in Costa Rica by Chris Geron; Alex Guenther; Jim Greenberg; Henry W Loescher; Deborah Clark; Brad Baker (3793-3802).
Twenty common plant species were screened for emissions of biogenic volatile organic compounds (BVOCs) at a lowland tropical wet forest site in Costa Rica. Ten of the species examined emitted substantial quantities of isoprene. These species accounted for 35–50% of the total basal area of old-growth forest on the major edaphic site types, indicating that a high proportion of the canopy leaf area is a source of isoprene. A limited number of canopy-level BVOC flux measurements were also collected by relaxed eddy accumulation (REA). These measurements verify that the forest canopy in this region is indeed a significant source of isoprene. In addition, REA fluxes of methanol and especially acetone were also significant, exceeding model estimates and warranting future investigation at this site. Leaf monoterpene emissions were non-detectable or very low from the species surveyed, and ambient concentrations and REA fluxes likewise were very low. Although the isoprene emission rates reported here are largely consistent with phylogenetic relations found in other studies (at the family, genus, and species levels), two species in the family Mimosaceae, a group previously found to consist largely of non-isoprene emitters, emitted significant quantities of isoprene. One of these, Pentaclethra macroloba (Willd.) Kuntze, is by far the most abundant canopy tree species in the forests of this area, composing 30–40% of the total basal area. The other, Zygia longifolia (Humb. & Bonpl.) Britton & Rose is a common riparian species. Our results suggest that the source strength of BVOCs is important not only to tropical atmospheric chemistry, but also may be important in determining net ecosystem carbon exchange.
Keywords: Isoprene; Biogenic volatile organic compounds; Relaxed eddy accumulation; Pentaclethra macroloba; Palmae; La selva biological station;

Anthropogenic and natural levels of arsenic in PM10 in Central and Northern Chile by L. Gidhagen; H. Kahelin; P. Schmidt-Thomé; C. Johansson (3803-3817).
A few copper and gold smelters in Chile are behind a large fraction of global arsenic emissions, raising concerns for increased concentrations of arsenic in PM10 in Central and Northern Chile. This concern is amplified by the fact that Northern Chile soils and rivers in general are characterized by a high arsenic content. A monitoring and modeling study has been performed to quantify the regional impact of the smelter emissions. Measured atmospheric arsenic concentrations from 2.4 to 30.7 ng m−3 were found at seven rural stations, located tens to hundreds of kilometers away from the nearest smelter. Analyses of topsoil and subsoil samples taken from PM10 monitoring stations revealed levels up to 291 mg kg−1, the highest values found in the northern Atacama desert in Chile. An absolute principal component analysis of selected trace elements in PM10 shows that the regional impact of anthropogenic smelter emissions on airborne arsenic concentrations is more important than the effect of soil dust resuspension. The dominance of the smelter emissions is larger in Central Chile than in the northern parts. The impact of resuspended soil dust on airborne arsenic levels in rural areas was estimated not to exceed 5 ng m−3. The model calculations support the dominant role of anthropogenic emissions and give spatial and temporal variations in atmospheric concentrations consistent with the monitored levels at five of the seven stations. At two of the northernmost stations indications were found of unidentified sources other than the smelters and the resuspended soil dust, contributing to about 5 ng m−3 of total arsenic levels. The study confirms that a strong control or elimination of arsenic emissions from the smelters would lead to arsenic in PM10 levels in Northern and Central Chile comparable to non-polluted areas in other countries.
Keywords: Soil resuspension; Smelter emissions; Enrichment factors; Receptor modeling; Regional dispersion modeling;

Regional dispersion of oxidized sulfur in Central Chile by G Olivares; L Gallardo; J Langner; B Aarhus (3819-3828).
Chile has a long tradition of exploiting mineral resources, particularly copper (Cu). One of the largest Cu smelters, Caletones, located some 150 km south of the country's capital, Santiago, in Central Chile, is responsible for about 0.4% of about 70 Tg S/yr oxidized sulfur (SO x ) emitted by anthropogenic sources worldwide. Santiago, a megacity with 5 million inhabitants, stands for about 5 Gg S/yr. The average meteorological conditions are unfavorable for the dispersion of pollutants in this area. All this poses risks for human health and vegetation. Also, downwind from these polluted areas there may be large-scale impacts on cloud properties and on oxidative cycles. Here, we present the first attempt to assess the regional distribution of SO x in Central Chile using a dispersion model (MATCH) driven with data from a limited area weather forecast model (HIRLAM). Emphasis has been given to the impact of Cu smelters upon urban air quality, particularly that of Santiago. Six 1-month long periods were simulated for the years 1997, 1998 and 1999. These periods span over a broad range of typical meteorological conditions in the area including El Niño and La Niña years. Estimates of the regional dispersion and deposition patterns were calculated. The emissions from the large Cu smelters dominate the distribution of SO x . A budget of SO x over an area of 200×200 km2 around Santiago is presented. There is too low a number of monitoring stations to perform a detailed evaluation of MATCH. Nevertheless, the model reproduces consistently all the regional-scale characteristics that can be derived from the available observations.
Keywords: Oxidized sulfur; Copper smelters; Central Chile; Regional modeling;

Coastal lows and sulfur air pollution in Central Chile by L Gallardo; G Olivares; J Langner; B Aarhus (3829-3841).
Air pollutants in Santiago (33.5°S, 70.8°W, 500 m a.s.l.), a city with 5 million inhabitants, located in a basin in Central Chile surrounded by the high Andes, frequently exceed air quality standards. This affects human health and it stresses vegetation. The most extreme winter and fall pollution events occur when the subsident regime of the Pacific high is further enhanced by coastal lows (CLs), which bring down the base of the subsidence inversion. Under these conditions, the air quality worsens significantly giving rise to acute air pollution episodes. We assess the ability of a regional transport/chemistry/deposition model (MATCH) coupled to a meteorological model (High Resolution Limited Area Model—HIRLAM) to simulate the evolution of oxidized sulfur (SO x ) in connection with intensive CLs. We focus on SO x since it is an environmental issue of concern, and the emissions and concentrations of SO x have been regularly monitored making it easier to bracket model outputs for SO x than for other pollutants. Furthermore, the SO x emissions in the area are very large, i.e., about 0.4% of the global anthropogenic sources. Comparisons with observations indicate that the combination of HIRLAM and MATCH is a suitable tool for describing the regional patterns of dispersion associated with CLs. However, the low number and the limited geographical coverage of reliable air quality data preclude a complete evaluation of the model. Nevertheless, we show evidence of an enhanced contribution of the largest copper smelter in the area, i.e., Caletones, to the burden of SO x in the Santiago basin, especially in the form of sulfate associated to fine particles (diameters <2.5 μm), during CLs. Further, we speculate that the Caletones plume may trigger or promote secondary aerosol formation during CLs in the Santiago basin.
Keywords: Oxidized sulfur; PM2.5; Coastal lows; Regional modeling;

Concentrations of benzene and toluene in the atmosphere of the southwestern area at the Mexico City Metropolitan Zone by Humberto Bravo; Rodolfo Sosa; Pablo Sánchez; Emma Bueno; Laura González (3843-3849).
The Mexico City Metropolitan Zone (MCMZ) presents important emissions of hazardous air pollutants. It is well documented that the MCMZ suffers a critical air pollution problem due to high ozone and particulate matter concentrations. However, toxic air pollutants such as benzene and toluene have not been considered. Benzene has accumulated sufficient evidence as a human carcinogen, and the ratio benzene/toluene is an excellent indicator to evaluate control strategies efficiency.In order to evaluate the levels of these two air toxic pollutants in the MCMZ, ambient air samples were collected in canisters and analyzed with a gas chromatograph with a flame ionization detector, according to procedures described in the United States Environmental Protection Agency (USEPA) method TO-15. Quality assurance was performed collecting duplicate samples which were analyzed in replicate to quantify the precision of air-quality measurements.Three different sites located in the Southwestern area in the MCMZ were selected for the sampling: the University campus, a gas station, and a vertical condominium area, in the same neighborhood, which presents different activities. At these sites, grab air samples were collected during the morning hours (7–8 a.m.), while for the University area, 24 h integrated air samples were collected simultaneously, with grab samples.Benzene concentrations (24 h sampling) in the atmosphere around the University campus have similar present levels as in other cities of North America. Mean values in this site were about 1.7 ppb.A significant variation exists between the benzene and toluene concentrations in the studied sites, being the more critical values than those registered at the gas station (an average of 25.8 ppb and a maximum of 141 ppb of benzene). There is a fuel regulation for gasoline in Mexico, which allows a maximum of 1 percent of benzene. However, since more than 60 percent of vehicles do not have catalytic converters (models before 1991) it is expected that most of this benzene be emitted through exhaust pipe. Another strategy being implemented is the use of vapor recovery systems at the gas stations. Vehicles emission control technology must be matched with adequate fuel characteristics in the problem area where it will be implemented, to achieve maximum emission reductions.
Keywords: Benzene; Toluene; Mexico city; Volatile organic compounds; Air quality;

The composition and sources of PM2.5 organic aerosol in two urban areas of Chile by Manolis Tsapakis; Evaggelia Lagoudaki; Euripides G Stephanou; Ilias G Kavouras; Petros Koutrakis; Pedro Oyola; Dietrich von Baer (3851-3863).
Fine particle (PM2.5) samples were collected, using a charcoal diffusion denuder, in two urban areas of Chile, Santiago and Temuco, during the winter and spring season of 1998. Molecular markers of the organic aerosol were determined using GC/MS. Diagnostic ratios and molecular tracers were used to investigate the origin of carbonaceous aerosols. As main sources, road and non-road engine emissions in Santiago, and wood burning in Temuco were identified. Cluster analysis was used to compare the chemical characteristics of carbonaceous aerosols between the two urban environments. Distinct differences between Santiago and Temuco samples were observed. High concentrations of isoprenoid (30–69 ng m−3) and unresolved complex mixture (UCM) of hydrocarbons (839–1369 ng m−3) were found in Santiago. High concentrations of polynuclear aromatic hydrocarbons (751±304 ng m−3) and their oxygenated derivatives (4±2 ng m−3), and of n-alk-1-enes (16±13 ng m−3) were observed in Temuco.
Keywords: PM2.5; Organic aerosol; Polynuclear aromatic hydrocarbons; Oxygenated polynuclear aromatic compounds; n-alk-1-enes; Charcoal denuder; Cluster analysis;

Corrigendum to “PM10 measurements at McMurdo Station, Antarctica” by David M Mazzera; Douglas H Lowenthal; Jiduth C Chow; John G Watson; Vanda Grubı̆sı́c (3865).