Atmospheric Environment (v.34, #11)

The national atmospheric deposition program: an overview by Dennis Lamb; Van Bowersox (1661-1663).
The National Atmospheric Deposition Program (NADP) is the organization overseeing the long-term sampling and analysis of precipitation across the United States, Puerto Rico and the Virgin Islands. The three NADP networks that collectively involve routine operations at about 280 sites strive to develop a chemical climatology of atmospheric deposition for the benefit of scientists and agencies concerned with environmental effects. The papers included in the Special Section focused on the uses of NADP data offer some new perspectives on chemical wet deposition.
Keywords: Deposition; Precipitation chemistry; NADP;

Phase I of Title IV of the Clean Air Act Amendments of 1990 (CAAA) was implemented on 1 January 1995. On that date limitations were imposed on sulfur dioxide (SO2) emissions from 110 coal-fired electric utility plants (263 units) in 21 states of which 17 are east of the Mississippi River. Sulfur dioxide emissions at these plants dropped an average 2.86 million tons (38%) from 1995 through 1997, when compared to mean 1993–1994 emissions from these plants. As a result of these reductions, wet sulfate (SO4 2−) deposition decreased an average of 2.4–4.0 kg ha−1 (10–25%) over a large area of the eastern USA. Based on more than 15 years of data, the decrease in wet deposition was unprecedented in magnitude and spatial extent. The largest decrease occurred across the Mid-Appalachian and Northeast regions of the country, the regions specifically targeted by Phase I for reductions in acidic deposition. Although the reductions in wet SO4 2−deposition were generally consistent with observed reductions in SO4 2− concentrations, deviations in precipitation volumes between the pre- and post-Phase I periods did result in some sites recording significantly (p<0.05) lower post-Phase I concentrations but higher wet depositions. Deviations in precipitation within and between regions also influenced the magnitude and spatial distribution of the observed changes in wet SO4 2− deposition. Clearly, deviations in the amount and distribution of precipitation need to be considered along with concentration changes in evaluating the CAAA effectiveness in reducing acidic deposition. This is particularly important in the eastern USA, since many of the most acid-sensitive ecosystems are located in regions, mostly mountainous and forested, that are subject to considerable fluctuations in precipitation volumes and subsequently acidic wet deposition.
Keywords: Acid precipitation; Trend analysis; Atmospheric deposition; Sulfate deposition; Clean Air Act Amendments of 1990; Sulfur dioxide emissions;

The sensitivity of sulfur wet deposition to atmospheric oxidants by Ariel F. Stein; Dennis Lamb (1681-1690).
The wet deposition of sulfate (SO4 2−) in eastern North America is chemically linked to primary emissions of sulfur dioxide (SO2) via atmospheric oxidants and therefore also to the emissions of nitrogen oxides (NO x ) and hydrocarbons (RH) within the region. Despite the major reductions in SO2 sources mandated by the Clean Air Act Amendments of 1990, large sulfate deposition events are still observed in eastern North America. A case study of one such event in June 1996 in central Pennsylvania is used to gain an understanding of the linkages between sulfur deposition and oxidant chemistry and transport. These issues are addressed here using a combination of meteorological and chemical models (HY-SPLIT and OZIPR) in order to interpret air and precipitation chemistry data (AIRMoN) from the Penn State research sites. The large deposition of sulfate resulting from this rain event can be accounted for by the scavenging of ambient sulfate formed prior to the storm. The potential impacts that future NO x controls might have on ambient sulfate, visibility, and acidic deposition depend in part on the resulting changes in total oxidant levels and the competition that naturally exists between the clear-air and in-cloud oxidation pathways. The findings here suggest that lowered levels of NO x , even when they result in reduced production of ozone (O3), may nevertheless lead to enhanced generation of hydrogen peroxide (H2O2) and therefore to the continued occurrence of sulfate episodes and high-deposition events in the future.
Keywords: Acid deposition; Sulfate; Photochemical oxidants; Ozone; Hydrogen peroxide; AIRMoN;

Annual and seasonal trends in mercury deposition in Maryland by R.P Mason; N.M Lawson; G.R Sheu (1691-1701).
Wet deposition collections for total mercury (Hg) and methylmercury (MMHg) have been made at four sites in Maryland. The Chesapeake Biological Laboratory (CBL) site has been in operation since May 1995; the other sites (the Science Center (SC) in urban Baltimore; Stillpond (STP) on the eastern shore of the Chesapeake Bay; and Frostburg (FRB) in western Maryland) have been operation since 1996 or 1997. Annual depositional fluxes for Hg at CBL over the period were variable but a large fraction of the variability could be accounted for by variability in annual and seasonal rainfall. Fluxes were higher for the regional Maryland sites (CBL, STP and FRB) than those in the mid-west USA. The results for CBL do not show a measurable annual trend in Hg deposition with time over this period. Seasonal variation is apparent with higher fluxes in summer, and the lowest fluxes in winter at all sites. The seasonal signal apparent in the total Hg data is not reflected in the MMHg record. Wet depositional fluxes at the urban SC site were 2–3 times higher than the rural sites. This difference was comparable to differences in particulate and reactive gaseous Hg between SC and CBL. The rural sites, two on the Chesapeake Bay shoreline (CBL and STP) and one in western Maryland (FRB), have similar annual fluxes suggesting that these sites represent the average regional Hg depositional signal. The SC site clearly shows the urban influence for Hg deposition.

Annual wet and dry deposition of sulfur and nitrogen in the snowy range, Wyoming by Karl Zeller; Debra Harrington; Al Riebau; Evgeny Donev (1703-1711).
The collocation of three national networked programs NADP, EPA's CASTNET, and the Forest Service's IMPROVE Module A, within a few hundred meters of each other in the pristine Medicine Bow forest of Wyoming has made it possible to assess the total amount of sulfur and nitrogen deposition, both wet and dry for this alpine/subalpine ecosystem. Additional sites within a few kilometers add spatial depth to this study. Wet deposition assessed using NADP data accounts for 1 to a little over 3 kg ha−1  yr−1 for both nitrogen and sulfur; however, annual trends for the two species differ. Dry deposition assessed using both CASTNET (a.k.a. NDDN) and IMPROVE (for sulfur) indicates 1–2 kg ha−1  yr−1 for nitrogen but less than 1 kg ha−1  yr−1 for sulfur. The overall trend of wet plus dry for nitrogen has been downward from 5 kg ha−1  yr−1 in 1989 to 3.6 kg ha−1  yr−1 in 1994, while varying between 2 and under 4 kg ha−1  yr−1 for sulfur. This paper introduces the sites and presents the three programs and the analysis approach. Spatial comparisons between sites are investigated. Weekly data are analyzed from three NADP sites, separated horizontally 6.8 and 2.4 km and vertically 430 and 98 m from the highest elevation site. The site comparisons demonstrate that winter season data requires careful analysis due to the vagaries of inefficient precipitation collection during high winds and snow fall.
Keywords: Nitrogen deposition; Sulfur deposition; Wet deposition; Dry deposition; Atmospheric deposition;

Comparison of precipitation chemistry in the Central Rocky Mountains, Colorado, USA by Kristi Heuer; Kathy A Tonnessen; George P Ingersoll (1713-1722).
Volume-weighted mean concentrations of nitrate (NO3 ), ammonium (NH4 +), and sulfate (SO4 2−) in precipitation were compared at high-elevation sites in Colorado from 1992 to 1997 to evaluate emission source areas to the east and west of the Rocky Mountains. Precipitation chemistry was measured by two sampling methods, the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) and snowpack surveys at maximum accumulation. Concentrations of NO3 and SO4 2− in winter precipitation were greater on the western slope of the Rockies, and concentrations of NO3 and NH4 + in summer precipitation were greater on the eastern slope. Summer concentrations in general were almost twice as high as winter concentrations. Seasonal weather patterns in combination with emission source areas help to explain these differences. This comparison shows that high-elevation ecosystems in Colorado are influenced by air pollution emission sources located on both sides of the Continental Divide. It also suggests that sources of nitrogen and sulfur located east of the Divide have a greater influence on precipitation chemistry in the Colorado Rockies.
Keywords: Precipitation chemistry; Snow; High-elevation ecosystems; Nitrate; Ammonium; Sulfate; Air pollution; National Atmospheric Deposition Program/National Trends Network;

The acidity and conductivity of precipitation at two sites located east of the Continental Divide in the Front Range of Colorado is examined for temporal trends, differences, and relationships to atmospheric circulation patterns for a 14 yr period (1984–1997). The elevationally lower station, Sugarloaf, is about 20 km west of Boulder, CO, and closer to the Denver/Boulder urban corridor than the higher site, Niwot, which lies 11.4 km northwest of Sugarloaf, and 1000 m higher in elevation. Key findings of this study are that Sugarloaf precipitation has the higher pH (less acidic) and lower conductivity of the two sites, and that different circulation patterns are associated with different inter-site precipitation characteristics: chemistry, amount, occurrence, and seasonal differences. Circulation indices account for about one-fourth to one-third of the variability in precipitation chemistry, and synoptic scale circulation patterns are clearly different for extremely low versus extremely high pH conditions at these two sites.
Keywords: Synoptic; Deposition; Indices; pH; Conductivity;

Agricultural operations may contribute to the respirable dust (RD) that affects the air quality of California's Central Valley. In order to minimize RD production, it is necessary to understand the role of two of the more important variables that influence RD production, soil water content and air temperature. Personal cyclone samplers were mounted on agricultural implements used in farming operations over two years to capture ⩽4 μm aerodynamic-diameter RD particles. For cultivation operations under widely different environmental conditions, RD concentration decreased as a power function as soil water content increased between 2 and 14% and increased linearly as air temperature increased. Similar results were found for an experiment with a land plane when samples were collected at 126, 156, 186, 216, 246, and 276 cm above the soil surface with either soil water content or air temperature held constant. For some cultivation operations, RD increased as a power function of air temperature between 7 and 24°C. For a loam surface horizon at one sample site, RD concentration increased 2–5 times at 34°C compared to 18.6°C.
Keywords: Fugitive dust; PM-4; Soil moisture; Air temperature;

Application of SAFER model to the Los Angeles PM10 data by Bong Mann Kim; Ronald C. Henry (1747-1759).
The chemical mass balance (CMB) model has been used widely in the PM10 source apportionment study and the PM10 state implementation plan (SIP) development. In its modeling application, the CMB model requires source composition profiles, which can either be measured by source testing or estimated by a multivariate receptor model such as source apportionment by factors with explicit restrictions (SAFER) from the ambient data only. The SAFER model is a multivariate receptor model utilizing a series of linear programming methods to estimate source compositions with explicit physical constraints. The SAFER model was applied to ambient PM10 data collected in 1986 in the South Coast Air Basin. Source compositions of selected major source categories were estimated. The SAFER model-estimated source profiles were compared with the measured source profiles by checking some important elemental ratios. Estimated source compositions were consistent with the measured profiles. Then the SAFER model-estimated source profiles were used for CMB analysis to estimate source contributions from each source category. Contributions from the roadway source range from 20 to 34 μg m−3, from the secondary source from 17.75 to 31.40 μg m−3, from the marine source from undetectable to 2.50 μg m−3, and from the crustal source from 4.06 to 8.13 μg m−3. Organic carbon seems to be mainly (81%) contributed by the primary roadway source, and sulfates and nitrates are mainly from the secondary source, although 32% of the sulfate is from primary sources such as roadway, crustal, and marine sources.
Keywords: Principal component analysis; SAFER model; Self modeling curve resolution; Additional physical constraints; Stoichiometric constraint; Estimation of source compositions;

A review and synthesis of monoterpene speciation from forests in the United States by Chris Geron; Rei Rasmussen; Robert R. Arnts; Alex Guenther (1761-1781).
The monoterpene composition (emission and tissue internal concentration) of major forest tree species in the United States is discussed. Of the 14 most commonly occurring compounds (α-pinene, β-pinene, Δ3-carene, d-limonene, camphene, myrcene, α-terpinene, β-phellandrene, sabinene, ρ-cymene, ocimene, α-thujene, terpinolene, and γ-terpinene), the first six are usually found to be most abundant. Expected regional variability based on the monoterpene composition fingerprints and corresponding tree species distribution and abundance is examined. In the southeast, α-pinene and β-pinene seem to dominate monoterpene emissions, while in the northern forests emissions are distributed more evenly among the six major compounds. In some parts of western forests, β-pinene and Δ3-carene can be more abundant than α-pinene. Among the other eight compounds, β-phellandrene and sabinene occasionally are significant percentages of expected local monoterpene emissions. Ocimene and ρ-cymene are estimated to be more common in regions dominated by deciduous broadleaf forests, although total emission rates are generally lower for these forests relative to those dominated by conifers. These percentages are compared with monoterpene composition measured in ambient air at various sites. Estimated monoterpene emission composition based on local forest species composition agrees fairly well with ambient measurements for the six major compounds. The past assumption that α-pinene composes approximately 50% of total monoterpene emissions appears reasonable for many areas, except for possibly the northern coniferous forests and some areas in the west dominated by true firs, spruce, and western pines (lodgepole and ponderosa pines). The oxygenated monoterpenes such as camphor, bornyl acetate, and cineole often compose high percentages of the monoterpenes within plant tissues, but are much less abundant in emission samples. Even after adjusting for lower vapor pressures of these compounds, emission rates relative to the hydrocarbon monoterpenes are often lower than would be expected from their internal concentrations. More study is warranted on monoterpene emission rates and composition, especially from the spruces, true firs, hemlocks, cedars, and some deciduous species such as the maples. Non-invasive canopy level and whole ecosystem flux studies are also needed to establish uncertainty estimates for monoterpene emission models.
Keywords: α-pinene; Conifers; Emission model; Biogenic volatile organic compounds; Essential oil;

A fuel-based approach was used to estimate stabilized exhaust carbon monoxide (CO) and volatile organic compound (VOC) emissions in California's South Coast Air Basin (SoCAB). Fleet-average emission factors of 80±7 g l−1 CO and 9.3±1.5 g l−1 VOC were calculated from more than 60,000 infrared remote sensor measurements collected at 35 sites throughout the greater Los Angeles area during summer 1997. Fuel use by SoCAB cars and light/medium-duty trucks was estimated to be 5.9±0.3×107  l day−1 based on statewide gasoline sales during summer 1997. Fuel-based estimates of 4700±500 metric tons day−1 CO and 550±90 metric tons day−1 VOC are higher than stabilized exhaust emissions estimates of California's MVEI 7G model by factors of 2.4±0.2 for CO and 3.5±0.6 for VOC. According to fuel-based inventory estimates, stabilized CO emissions in 1997 were lower by 20% than emissions during summer 1991. Fuel use increased by 8% during this period while the CO emission factor decreased by 26%. The relationship between income level and vehicle emissions was examined using census data resolved to the zip code level. On average, CO and VOC emission factors of vehicles registered in the lowest income areas were double those of vehicles registered in the highest income areas. Differences in vehicle emissions between neighborhoods were due in part to differences in the prevalence of older vehicles, but also resulted because vehicles of the same age had higher emissions in lower-income areas.
Keywords: Remote sensing; SCOS97-NARSTO; Carbon monoxide; Volatile organic compounds; Motor vehicle; Emission inventory;

The history of the atmospheric deposition of major and trace elements over southwestern Quebec, Canada, was reconstructed using multielemental analysis of lacustrine sediments sampled in a small and undisturbed lake located on top of a mountain in the heart of the industrialized St. Lawrence Valley. Acid leachable and residual elements were extracted from a 37-cm long core (1-cm resolution) using clean techniques and analyzed by inductively coupled plasma mass spectrometry. Organic matter and sulfur concentrations were high and played a major role in the low postdepositional diagenetic remobilization of many trace elements. Sulfur, manganese, iron, arsenic, molybdenum and barium displayed a high mobility making it exceedingly difficult to infer unambiguously time-dependent changes in atmospheric deposition for these elements. Atmospheric deposition rates for the less mobile elements (e.g., potassium, vanadium, chromium, cobalt, nickel, copper, zinc, rubidium, cadmium, tin, antimony, mercury, thallium, lead, and bismuth) increased regularly between 1942 and 1960–1975 in the Lake Hertel area and then stabilized for most of these elements, with the exception of nickel, copper, zinc and tin. Lead deposition rate was reduced by about 25% between 1982 and 1995, and a slight decreasing trend was also found for cobalt, mercury, and thallium during the same period. Present-day atmospheric deposition of metals directly on the lake surface represents a small percentage of the sedimentary deposition rates at this location. Deposition followed by surface runoff and outwash of terrestrial organic and inorganic matter most likely is the driving mechanism leading to the non-diagenetic enrichment of metals in Lake Hertel sediments.
Keywords: Major and trace elements; Atmospheric deposition; Lacustrine sediment; Diagenesis; Multielemental analysis;

Single particle analysis of suspended soil dust from Southern California by Philip J Silva; Richard A Carlin; Kimberly A Prather (1811-1820).
Single particle analysis of soil dust has been performed using aerosol time-of-flight mass spectrometry. The presence of crustal elements is observed in the mass spectra of individual particles. Aluminum and iron constitute the two most commonly detected cations. Other common cations observed in the mass spectra of soil particles include sodium, magnesium, potassium, calcium, titanium, and barium, and also lithium and lanthanide oxides which are not detectable by conventional techniques such as X-ray fluorescence. Silicates make up the bulk of detected anions, although the presence of phosphate is also observed. Characteristic peaks in both the positive and negative ion mass spectra enable us to identify individual dust particles in atmospheric samples and track chemically distinct dust particles in the atmosphere with a temporal resolution of 15 min using aerosol time-of-flight mass spectrometry.
Keywords: Mass spectrometry; Aerosols; Trace elements; Minerals; Particulate matter;

Our research group has been continuously measuring aerosol sulfate at Whiteface Mountain, New York, since 1979 and at Mayville, New York, since 1984. Monthly aerosol SO2− 4 concentrations for these sites are reported here through 1997. Seasonal and long-term profiles of SO2− 4 are presented along with SO2 concentrations. As these sites are directly downwind of the high SO2 emission region in the Midwestern United States and the same sampling and analytical protocol has been maintained, they are ideally suited to evaluate long-term trends. The recent reductions in SO2 emissions in response to provisions of the 1990 Clean Air Act provide an excellent opportunity to evaluate the sulfur source–receptor relationship. A linear relationship is demonstrated between aerosol SO2− 4 and total sulfur (ST, sum of S present as SO4 and SO2) concentrations at these sites and SO2 emissions upwind in the Midwest. This is the most direct evidence to date that deposition of sulfur in the Northeast is linearly related to upwind SO2 emissions. The results support the possibility of using the ratio of SO2− 4/SO2 emissions to develop an empirical source–receptor relationship for the region.

Cross-border transport and spatial variability of suspended particles in Mexicali and California's Imperial Valley by Judith C Chow; John G Watson; Mark C. Green; Douglas H Lowenthal; Barbara Bates; William Oslund; Gaspar Torres (1833-1843).
The city of Mexicali, the state capitol of Baja California, Mexico, lies 189 kilometers (km) inland from the Pacific Coast on the California border at the lower end of California's Imperial Valley. Mexicali is adjacent to the California city of Calexico and 20 km south of the Imperial county seat of El Centro. The Mexicali and Calexico PM10 monitors were separated by only 10 km; yet, three measurements at Calexico exceeded 150 μg m−3, while 23 samples exceeded this PM10 standard at the Mexicali site. Air flow through Mexicali is channeled by the Imperial Valley and is usually from the northwest or southeast, with northwesterlies being most frequent. During the study period from March 1992 through August 1993, hourly PM10 concentrations were higher in the border area during southerly than northerly flow. For wind flow patterns in both directions, PM10 initially decreased with wind speed due to improved ventilation, then increased at high wind speeds due to increased suspension of soil particles. Average cross-border transport of PM10 was three times higher for southerly flow from Mexico than for northerly flow from the United States into Mexico. Because northerly winds were more frequent, the time-integrated cross-border transport over the study period was only about one-and-one-half times higher from Mexico. PM10 mass concentrations at the Mexicali site were highest when flow was over the main Calexico–Mexicali urban area (northwesterly flow) but were lower when flow was from less urbanized desert areas (southeasterly flow). Although Mexicali's PM10 concentrations were almost double those measured at Calexico, annual average relative source contributions were similar, with 70% from fugitive dust, 10–15% from motor vehicle exhaust, 4–8% from vegetative burning or cooking, 2–3% from marine aerosol (Gulf of California), 1.5–3% from secondary ammonium sulfate, and 1.5% to 2.5% from secondary ammonium nitrate. Primary industrial source contributions were negligible.
Keywords: Cross-border transport; PM10; CMB source apportionment;

Concentrations of benzene, toluene, ethylbenzene, o-xylene, and m- and p-xylene were measured at an urban sampling site in Las Vegas, NV by sorbent sampling followed by thermal desorption and determination by GC-PID. Simultaneously, measurements of oxalic, malonic, succinic, and adipic acids were made at the same site by collection on quartz filters, extraction, esterification, and determination by GC-FID. For the period from April 7, 1997 to June 11, 1997, 201 sets of hydrocarbon measurements and 99 sets of acid measurements were made. Additional measurements of dicarboxylic acids were made on samples that represented potential direct sources, e.g. green plants and road dust. Correlations between the hydrocarbon and CO concentrations (measured by the Clark County Health District at a nearby site) were highly significant and a strong negative correlation of hydrocarbon concentration with ozone concentration (also from the county site) was observed under quiescent atmospheric conditions. In general, dicarboxylic acid concentrations were well correlated with one another (with the exception of adipic acid) but not well correlated with hydrocarbon, CO, and ozone concentrations. Multiple sources and complex formation processes are indicated for the dicarboxylic acids.
Keywords: Volatile organic compounds; Photochemistry; Gas chromatography; Hydrocarbon oxidation;