Atmospheric Environment (v.35, #23)

Wet deposition of low molecular weight mono- and di-carboxylic acids, aldehydes and inorganic species in Los Angeles by Kimitaka Kawamura; Spencer Steinberg; Lai Ng; Isaac R. Kaplan (3917-3926).
About 60 rainwater samples were collected at west Los Angeles, California in 1981–1984 and were analyzed for C1–C9 monocarboxylic acids (0.33–79 μM, average (av.) 13±15 μM), C2–C10 dicarboxylic acids (2.9–51 μM, av. 7.5±14 μM) and C1–C4 aldehydes (0.85–28 μM, av. 9.2±11 μM). Distributions of monocarboxylic acids show a predominance of formic (average concentration: 6.5 μM) and acetic (av. 5.6 μM) acids followed by propionic acid (av. 0.44 μM). Oxalic acid is the dominant diacid (av. 3.9 μM) followed by succinic acid (av. 1.0 μM). Formaldehyde (av. 6.9 μM) is the dominant aldehyde, with the next most abundant, acetaldehyde, being minor (av. 0.65 μM). For select rain samples described in this paper, were found to comprise monocarboxylic acids 0.9–12.3% (av. 4.4±3.4%), diacids comprise 1.2–9.5% (av. 4.2±3.3%) and aldehydes comprise 0.2–6.2% (av. 2.1±2.2%) of total organic carbon (TOC, 2.0–18.6 mg C l−1; av. 9.8±5.4 mg C l−1). Annual rain fluxes of monocarboxylic acids and aldehydes during 1982–1983 were calculated to be 0.24 and 0.11 g m−2  yr−1, respectively, with an annual estimated wet deposition in the Los Angeles Basin of 3120 and 1430 tons, respectively. These fluxes are equivalent to 2500 times of the acids and 2.5 times of the aldehydes emitted from automobile exhausts in the Los Angeles air basin. This comparison suggests that major portions of the carboxylic acids detected in the rain are not directly emitted from auto-exhausts, but are most likely produced in the atmosphere by gaseous and/or aqueous phase photo-induced reactions.

Diurnal variations in major rainwater components at a coastal site in North Carolina by G Brooks Avery; Joan D Willey; Robert J Kieber (3927-3933).
Concentrations of several major rainwater components were determined in rain events occurring during the early morning hours (12:00 midnight to 6:00 a.m.) and during the afternoon (12:00 noon to 6:00 p.m.) to examine possible diurnal variations. Generally, rainwater components with gas phase origins (H+, NO3 , formaldehyde, H2O2, formic acid, acetic acid, pyruvic acid, oxalic acid, and lactic acid) had higher concentrations during p.m. rain events compared to a.m. events. Although source strengths of both biogenic and anthropogenic rainwater components are generally higher during the daytime, nocturnal removal of a wide variety of components in similar proportions (approximately 2–3× less at night) indicates a physical rather than a chemical process affecting diurnal variations. Rainwater components with aerosol origins (Cl, and SO4 2−) displayed the opposite diurnal pattern or showed no diurnal variation. Possible reasons for these variations include one or both of the following scenarios: (1) the formation of dew at night removes gas phase atmospheric gasses but not aerosols or (2) during the night, a marine air mass containing lower concentrations of all analytes and higher concentrations of Cl is advected into the area.
Keywords: Dew; Rainwater; Diurnal variations;

Individual particle analysis of indoor, outdoor, and community samples from the 1998 Baltimore particulate matter study by Teri L Conner; Gary A Norris; Matthew S Landis; Ronald W Williams (3935-3946).
The United States Environmental Protection Agency (US EPA) recently conducted the 1998 Baltimore Particulate Matter (PM) Epidemiology-Exposure Study of the Elderly. The primary goal of that study was to establish the relationship between outdoor PM concentrations and actual human PM exposures within a susceptible (elderly) sub-population. Personal, indoor, and outdoor sampling of particulate matter was conducted at a retirement center in the Towson area of northern Baltimore County. Concurrent sampling was conducted at a central community site. The main objective of this work was to use computer-controlled scanning electron microscopy (CCSEM) with individual-particle X-ray analysis to measure the chemical and physical characteristics of geological and trace element particles collected at the various sampling locations in and around the retirement facility.The CCSEM results show that the relative abundances of some geological and trace element particle classes identified at the outdoor and community locations differ from each other and from the indoor location. Particle images acquired during the computer-controlled analyses played a key role in the identification of certain particle types. Review of these images was particularly useful in distinguishing spherical particles (usually indicative of combustion) from non-spherical particles of similar chemical composition. Pollens and spores were also identified through a manual review of the particle images.
Keywords: Scanning electron microscopy; Individual particle; Indoor air sources; Indoor–outdoor comparison; Inorganic particulate matter;

The average total (wet plus dry) nitrogen deposition to the Tampa Bay Estuary was 7.3 (±1.3) kg-N ha−1  yr−1 or 760 (±140) metric tons-N yr−1 for August 1996–July 1999, estimated as a direct deposition rate to the 104,000-ha water surface. This nitrogen flux estimate accounted for ammonia exchange at the air–sea interface. The uncertainty estimate was based on measurement error. Wet deposition was 56% of the total nitrogen deposition over this period, with an average 0.78 ratio of dry-to-wet deposition. Wet nitrogen deposition rates varied considerably, from near zero to 1.3 kg-N ha−1  month−1. About 40% of the total nitrogen flux occurred during the summer months of June, July and August when rainfall was the highest, except for 1997–1998 when the El Niño phenomenon brought unseasonal rainfall. Ammonia/ammonium contributed to 58%, and nitric acid/nitrate 42%, of the total nitrogen deposition over the 3-yr period. In one summer as waters of Tampa Bay warmed above 28°C and ammonium concentrations reached 0.03 mg l−1, the estimated net flux of ammonia was from the Bay waters to the atmosphere.
Keywords: Buoy model; Estuary; Annular denuder system; Uncertainty; Net ammonia flux;

Major-ion chemistry of the Rocky Mountain snowpack, USA by John T Turk; Howard E Taylor; George P Ingersoll; Kathy A Tonnessen; David W Clow; M.Alisa Mast; Donald H Campbell; John M Melack (3957-3966).
During 1993–97, samples of the full depth of the Rocky Mountain snowpack were collected at 52 sites from northern New Mexico to Montana and analyzed for major-ion concentrations. Concentrations of acidity, sulfate, nitrate, and calcium increased from north to south along the mountain range. In the northern part of the study area, acidity was most correlated (negatively) with calcium. Acidity was strongly correlated (positively) with nitrate and sulfate in the southern part and for the entire network. Acidity in the south exceeded the maximum acidity measured in snowpack of the Sierra Nevada and Cascade Mountains. Principal component analysis indicates three solute associations we characterize as: (1) acid (acidity, sulfate, and nitrate), (2) soil (calcium, magnesium, and potassium), and (3) salt (sodium, chloride, and ammonium). Concentrations of acid solutes in the snowpack are similar to concentrations in nearby wetfall collectors, whereas, concentrations of soil solutes are much higher in the snowpack than in wetfall. Thus, dryfall of acid solutes during the snow season is negligible, as is gypsum from soils. Snowpack sampling offers a cost-effective complement to sampling of wetfall in areas where wetfall is difficult to sample and where the snowpack accumulates throughout the winter.
Keywords: Snowpack chemistry; National atmospheric deposition program; Rocky Mountains; Acid deposition; Sulfate; Nitrate; Calcium; Wyoming; Colorado; Montana; New Mexico;

In an effort to reduce uncertainties in the quantification of aerosol direct radiative forcing (ADRF) in the southeastern United States (US), a field column experiment was conducted to measure aerosol radiative properties and effects at Mt. Mitchell, North Carolina, and at an adjacent valley site. The experimental period was from June 1995 to mid-December 1995. The aerosol optical properties (single scattering albedo and asymmetry factor) needed to compute ADRF were obtained on the basis of a procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, aerosol optical depth, and diffuse-to-direct solar irradiance ratio. The regional values of ADRF at the surface and top of atmosphere (TOA), and atmospheric aerosol absorption are derived using the obtained aerosol optical properties as inputs to the column radiation model (CRM) of the community climate model (CCM3). The cloud-free instantaneous TOA ADRFs for highly polluted (HP), marine (M) and continental (C) air masses range from 20.3 to −24.8, 1.3 to −10.4, and 1.9 to −13.4 W m−2, respectively. The mean cloud-free 24-h ADRFs at the TOA (at the surface) for HP, M, and C air masses are estimated to be −8±4 (−33±16), −7±4 (−13±8), and −0.14±0.05 (−8±3) W m−2, respectively. On the assumption that the fractional coverage of clouds is 0.61, the annual mean ADRFs at the TOA and the surface are −2±1, and −7±2 W m−2, respectively. This also implies that aerosols currently heat the atmosphere over the southeastern US by 5±3 W m−2 on annual timescales due to the aerosol absorption in the troposphere.
Keywords: Aerosol radiative forcing; Column radiation model; Southeastern US; Observation; Aerosol absorption;

Identification of sources of pollutants in precipitation measured at the mid-Atlantic US coast using potential source contribution function (PSCF) by Derrick Lucey; Lubomir Hadjiiski; Philip K. Hopke; Joseph R. Scudlark; Thomas Church (3979-3986).
Potential source contribution function (PSCF) was employed to study the source receptor relationships for 14 chemical species (Mn, SO4 2−, Zn, Al, Fe, Cu, Cr, Ni, Cd, NO3, NH4 +, K+, Mg2+,and Pb) found in precipitation collected at Lewes, Delaware. This study identified areas of the Eastern United States as possible emission source areas that could have contributed to the 14 element concentrations observed at Lewes. The identified regions in the Eastern United States generally coincide well with known emission source areas. The likely emission sources for these chemical species include oil- and coal-fired power plants, incinerators, motor vehicles, and iron and steel mills.
Keywords: Receptor models; Potential source contribution function; Precipitation; Trajectories; Sulfate; Trace elements;

In order to assess the importance of mercury emissions from naturally enriched sources relative to anthropogenic point sources, data must be collected that characterizes mercury emissions from representative areas and quantifies the influence of various environmental parameters that control emissions. With this information, we will be able to scale up natural source emissions to regional areas. In this study in situ mercury emission measurements were used, along with data from laboratory studies and statistical analysis, to scale up mercury emissions for the naturally enriched Ivanhoe Mining District, Nevada. Results from stepwise multi-variate regression analysis indicated that lithology, soil mercury concentration, and distance from the nearest fault were the most important factors controlling mercury flux. Field and lab experiments demonstrated that light and precipitation enhanced mercury emissions from alluvium with background mercury concentrations. Diel mercury emissions followed a Gaussian distribution. The Gaussian distribution was used to calculate an average daily emission for each lithologic unit, which were then used to calculate an average flux for the entire area of 17.1 ng Hg m−2  h−1. An annual emission of ∼8.7×104  g of mercury to the atmosphere was calculated for the 586 km2 area. The bulk of the Hg released into the atmosphere from the district (∼89%) is from naturally enriched non-point sources and ∼11% is emitted from areas of anthropogenic disturbance where mercury was mined. Mercury emissions from this area exceed the natural emission factor applied to mercury rich belts of the world (1.5 ng m−2  h−1) by an order of magnitude.
Keywords: Mercury; Atmospheric mercury; Mercury emission; Natural sources; Atmosphere-surface exchange;

A trajectory-puff model is used to test the effects of horizontal variability of the transport winds and two different vertical dispersion schemes on the prediction of maximum 30-min-averaged ground-level concentration in the shoreline environment of Cape Canaveral, Florida. Data are used from three dispersion experiments conducted at the Cape Canaveral Air Station as part of the US Air Force's Model Validation Program. A total of 79 half-hour-averaged ground-level concentrations of tracer-gas from 11 near-surface tracer releases during unstable and near-neutral conditions are simulated. A result of this study is that a large difference is not seen in the results between the use of a horizontally uniform wind observed at the tracer release site and horizontally variable wind for puff trajectory calculations. The uniform wind gives a slightly more accurate average result, but only because the uniform wind (observed near the shoreline) has greater speed than the variable wind field. The puff-transport wind speed for the variable wind case would be increased if the effects of vertical wind shear are taken into account. It is shown that for unstable conditions, including near-neutral conditions, the assumption of rapid vertical mixing leads to an under prediction (negative bias) of the maximum ground-level concentrations by about a factor of 2.3 with a typical factor of about 11 scatter between predicted and observed values. If a less rapid vertical mixing is assumed, then the overall predictions equal the overall observations, i.e., near-zero bias with a typical factor of scatter of about 3.6. If the near-neutral data are removed from the evaluation statistics, then the assumption of rapid vertical mixing leads to an overprediction by a factor of about 0.03 with a scatter factor of about 4; the assumption of less rapid mixing leads to an overprediction by a factor of about 0.55 with a scatter factor of about 3. These results are consistent with the fact that on-shore flows are less turbulent than flows over land for the same stability class.
Keywords: Coastal diffusion; Dispersion modeling; Internal boundary layers; Cape Canaveral;

Mercury-bearing material has been placed in municipal landfills from a wide array of sources including fluorescent lights, batteries, electrical switches, thermometers, and general waste. Despite its known volatility, persistence, and toxicity in the environment, the fate of mercury in landfills has not been widely studied. The nature of landfills designed to reduce waste through generation of methane by anaerobic bacteria suggests the possibility that these systems might also serve as bioreactors for the production of methylated mercury compounds. The toxicity of such species mandates the need to determine if they are emitted in municipal landfill gas (LFG). In a previous study, we had measured levels of total gaseous mercury (TGM) in LFG in the μg/m3 range in two Florida landfills, and elevated levels of monomethyl mercury (MMM) were identified in LFG condensate, suggesting the possible existence of gaseous organic Hg compounds in LFG. In the current study, we measured TGM, Hg0, and methylated mercury compounds directly in LFG from another Florida landfill. Again, TGM was in the μg/m3 range, MMM was found in condensate, and this time we positively identified dimethyl mercury (DMM) in the LGF in the ng/m3 range. These results identify landfills as a possible anthropogenic source of DMM emissions to air, and may help explain the reports of MMM in continental rainfall.

Survey of volatile organic compounds associated with automotive emissions in the urban airshed of São Paulo, Brazil by Maribel Colón; Joachim D Pleil; Thomas A Hartlage; M Lucia Guardani; M Helena Martins (4017-4031).
The Metropolitan Region of São Paulo (MRSP), Brazil, is one of the largest metropolitan areas in the world (population 17 million, approx.) and relies heavily on alcohol-based fuels for automobiles. It is estimated that about 40% of the total volume of fuel is ethanol with some vehicles using pure ethanol and others a gasoline/ethanol blend. As such, São Paulo is an excellent example of an oxygenates-dominated airshed of mobile sources and is most likely indicative of the future in heavily populated areas in the US such as Los Angeles where “oxy-fuels” are becoming an important replacement for the conventional pure petroleum-based fuels. In this work, we surveyed the ambient air to identify and quantify the organic compounds associated with the evaporative and exhaust emissions of these fuels and to begin to understand the potential for human exposure. Because this was an initial test without detailed prior knowledge of the airshed of the area, we applied two different air sampling methods for various time periods to assess the ambient concentrations of a variety of polar and nonpolar volatile organic compounds (VOCs). For quality assurance (QA), we collected all the samples in duplicate (whole-air samples in Summa canisters and adsorbent-based samples on Perkin-Elmer Air Toxics tubes) at various flow rates to test performance. All samples were collected over identical time frames, typically for 1-, 2-, and 4-h periods per day at six different locations over a period of 1 week. Overall São Paulo results demonstrate that mean concentrations of single-ring aromatics are 2–3 times higher, volatile aldehydes are 5–10 times higher, and simple alcohols 10–100 times higher as compared to results of a recent study performed by EPA in the Los Angeles basin. C4–C11 n-alkanes were only slightly elevated in São Paulo.
Keywords: Aldehydes; VOCs; Aromatics; Ethanol fuel; Automobile emissions; Summa canisters; Adsorbent tubes;

Chemical composition of fugitive dust emitters in Mexico City by E. Vega; V. Mugica; E. Reyes; G. Sánchez; J.C. Chow; J.G. Watson (4033-4039).
The gravimetric and chemical composition of fugitive dust emitters of Mexico City were analyzed to determine the particulate matter source profiles. Samples of geological material, unpaved and paved roads, agricultural soil, dried lake, asphalt, cement plants, landfill, gravel, and tezontle soil, were collected directly from the ground using a broom and a dustpan. These were dried, sieved and taken through a laboratory resuspension chamber to emulate the natural wind-blown processes of bulk soils and also to provide a uniform deposit on Teflon membrane and quartz fiber filters for further gravimetric and chemical analyses of PM2.5 and PM10 size fractions. Chemical analyses of the filters included X-ray fluorescence for elemental composition, ion chromatography for water soluble anions, atomic absorption for water soluble metals, automated colorimetric analysis for ammonium and thermal/optical reflectance analysis for carbon species. The data show that most fugitive emitters are composed of 20–30% PM2.5, which is relatively less than the reported contribution by fossil fuels and biomass (40–60%).
Keywords: PM10; PM2.5; Source profile; Mexico City; Fugitive dust; Chemical mass balance;

Mexico City air quality: a qualitative review of gas and aerosol measurements (1960–2000) by G.B Raga; D Baumgardner; T Castro; A Martı́nez-Arroyo; R Navarro-González (4041-4058).
Mexico City, one of the largest cities in the world, has a major problem with high levels of anthropogenic gases and aerosols. Some facets of this problem have been studied through measurements made during the past 40  years . These measurements are reviewed and evaluated with respect to physical processes that underlie the primary and secondary formation of gases and aerosols, their spatial and temporal evolution as well as their potential impact on the local and regional environment. Past measurements are heavily biased towards certain locations and time periods, and are of limited use for understanding fundamental processes that govern the formation and evolution of the principal pollutants. Recommendations are made whereby the measurement database could be expanded to better represent the characteristics of Mexico City gases and aerosols and to contribute to mitigation strategies that would lessen the environmental impact of these pollutants.
Keywords: Ambient gas and aerosol measurements; Air pollution; Mexico City;