Atmospheric Environment (v.35, #19)

In 1997, the United States National Ambient Air Quality Standard (NAAQS) for ozone was revised from a 1-h average of 0.12 parts per million (ppm) to an 8-h average of 0.08 ppm. Analysis of ozone data for the ensemble of the contiguous United States and for the period 1980–1998 shows that the average number of summer days per year in exceedance of the new standard is in the range 8–24 in the Northeast and in Texas, and 12–73 in southern California. The probability of exceedance increases with temperature and exceeds 20% in the Northeast for daily maximum temperatures above 305 K. We present the results of several different approaches to analyzing the long-term trends in the old and new standards over the continental United States from 1980 to 1998. Daily temperature data are used to resolve meteorological variability and isolate the effects of changes in anthropogenic emissions. Significant negative trends are found in the Northeast urban corridor, in the Los Angeles Basin and on the western bank of Lake Michigan. Temperature segregation enhances the detection of negative trends. Positive trends occur at isolated sites, mostly in the Southeast; a strong positive trend is found in Nashville (Tennessee). There is some evidence that, except in the Southwest, air quality improvements from the 1980s to the 1990s have leveled off in the past decade.
Keywords: Air quality; Air quality standard; Ozone; Ozone trends; Ground-level ozone; Surface ozone;

Real-time measurements of the chemical composition of size-resolved particles during a Santa Ana wind episode, California USA by Sergio A Guazzotti; Jeffery R Whiteaker; David Suess; Keith R Coffee; Kimberly A Prather (3229-3240).
Size-resolved particle composition, mass and number concentrations, aerosol scattering coefficients, and prevailing meteorological conditions were measured at the Ellen Browning Scripps Memorial Pier located in La Jolla, California on 15 December 1998. Aerosol particles were sampled using a field transportable aerosol time-of-flight mass spectrometer, allowing for the continuous detection and characterization of single particles from a polydisperse sample. An extensive and rapid change in the chemical composition of aerosol particles with aerodynamic diameters between 1.0 and 2.5 μm has been observed during the onset of a Santa Ana Winds condition. Coincident with the observed change in meteorological conditions, a substantial decrease in sea salt particles corresponds to an increase in dust and carbon-containing particles. This paper examines observations of the rapid changes occurring in the chemical composition of single aerosol particles and demonstrates the new types of information that can be obtained by measuring single particle size and composition with high temporal resolution.
Keywords: Aerosol composition; Sea salt; Single particle analysis; Coastal aerosol; Dust;

Estimates of the atmospheric deposition to Galveston Bay of polycyclic aromatic hydrocarbons (PAHs) are made using precipitation and meteorological data that were collected continuously from 2 February 1995 to 6 August 1996 at Seabrook, TX, USA. Particulate and vapor phase PAHs in ambient air and particulate and dissolved phases in rain samples were collected and analyzed. More than 95% of atmospheric PAHs were in the vapor phase and about 73% of PAHs in the rain were in the dissolved phase. Phenanthrene and napthalene were the dominant compounds in air vapor and rain dissolved phases, respectively, while 5 and 6 ring PAH were predominant in the particulate phase of both air and rain samples. Total PAH concentrations ranged from 4 to 161 ng m−3 in air samples and from 50 to 312 ng l−1 in rain samples. Temporal variability in total PAH air concentrations were observed, with lower concentrations in the spring and fall (4–34 ng m  −3) compared to the summer and winter (37–161 ng m−3). PAHs in the air near Galveston Bay are derived from both combustion and petroleum vaporization. Gas exchange from the atmosphere to the surface water is estimated to be the major deposition process for PAHs (1211 μg m− 2  yr− 1), relative to wet deposition (130 μg m−2  yr− 1) and dry deposition (99 μg m−2  yr− 1). Annual deposition of PAHs directly to Galveston Bay from the atmosphere is estimated as 2  t yr−1.
Keywords: Polycyclic aromatic hydrocarbons (PAHs); Wet deposition; Dry deposition; Air–sea gas exchange; Galveston Bay.;

Lidar-assisted measurement of PM10 emissions from agricultural tilling in California's San Joaquin Valley – Part I: lidar by Britt A Holmén; Teresa A James; Lowell L Ashbaugh; Robert G Flocchini (3251-3264).
Vertical profiling with point samplers is an accepted method for quantifying the fluxes of PM10 from non-point fugitive dust sources, but is limited by uncertainty in estimates of the actual height of the dust plume, especially for plumes that exceed the highest sampling height. Agricultural land preparation operations in the San Joaquin Valley were monitored using upwind–downwind vertical PM10 profiles and data collected during the first successful experiment to include light detection and ranging (lidar), in 1998, were analyzed to provide modeling criteria for the 1996 and 1997 data. A series of six comprehensive PM10 tests with concurrent lidar data was examined to: (a) develop a framework for analyzing upwind–downwind point PM10 concentration profiles of land preparation operations (disking, listing, root cutting, and ripping) and (b) identify conditions under which the field sampling strategies affect the reproducibility of PM10 concentration measurements. Lidar data were used to verify that the plume heights and shapes extrapolated from the point sampler vertical profiles adequately described the plumes. The shortcomings of the vertical profiling technique and lidar methods are discussed in the light of developing efficient robust methods for accurate PM10 emissions quantification from complex non-point sources.
Keywords: PM10, Lidar; Plume height; Agricultural dust; Nonpoint sources;

Emission factors for agricultural operations are needed in order to develop reliable PM10 emissions inventories and air quality models for air basins with significant agricultural land use. A framework was developed to analyze the PM10 vertical profiles collected downwind of tilling operations in the San Joaquin Valley. The methods calculate emission factors on the basis of profile shape and assign quality ratings to each land preparation test. Uncertainties in the calculated emission factors and plume heights were used as one criterion for evaluating the relative quality of the reported emission factor. Other quality ratings were based on the magnitude of the difference in measured up- and downwind concentrations, wind direction, whether the tests were conducted near the edges of the field, and how well the proposed model fit the profile data. The emission factors from different operations were compared taking the quality of the emission factor into account. Plume heights and emission factors for 24 valid test profiles ranged from 2 to 20 m (mean=9.8; SD=3.6; median=9.8) and zero to 800 mg m−2 (mean=152; SD=240; median=43), respectively. Key environmental properties governing PM10 emission from these operations include relative humidity, soil moisture and vertical temperature gradient. Surprisingly, no discernable relationships were found between implement type or wind speed and the measured emission factors.
Keywords: PM10; Emission factor; Quality rating; Agricultural dust;

A one-year field study was conducted to investigate the control factors of the monoterpene emissions from slash and loblolly pine saplings at the Austin Cary Forest site in Florida. The α-pinene, camphene, β-pinene, myrcene, d-limonene, and β-phellandrene were identified in the emission samples collected from native pine trees. The α-pinene was the principal (>60%) monoterpene emitted by both slash and loblolly pine saplings. Terpene emission rates in spring were the highest and most volatile for slash pine trees, possibly due to the influences of bud formation and elongation. Loblolly pine emissions, under a similar environmental temperature range, revealed different seasonal patterns of emissions when compared to those for slash pines. Emission rates of monoterpenes from slash and loblolly pine trees were found to depend on temperature, season's change (e.g., bud emissions), tree age, needle surface wetness, and rough handling. It is suggested that the emission control factors besides the environmental temperature should also be taken into account in assessing regional biogenic emissions for compling a worldwide hydrocarbon emission inventory. It is also found that monoterpene emission rates could easily change over a long period of time (e.g., years), and so it is desirable to analyze the emission data based on the short term (e.g., season, month) for reasonable temperature-emission algorithm.
Keywords: Monoterpene; Slash and loblolly pines; Emission control factor; Bud emission; Season’s change;

The effect of black carbon (BC) on climate forcing is potentially important, but its estimates have large uncertainties due to a lack of sufficient observational data. The BC mass concentration in the southeastern US was measured at a regionally representative site, Mount Gibbes (35.78°N, 82.29°W, 2006 m MSL). The air mass origin was determined using 48-h back trajectories obtained from the hybrid single-particle Lagrangian integrated trajectory model. The highest average concentration is seen in polluted continental air masses and the lowest in marine air masses. During the winter, the overall average BC value was 74.1 ng m−3, whereas the overall summer mean BC value is higher by a factor of 3. The main reason for the seasonal difference may be enhanced thermal convection during summer, which increases transport of air pollutants from the planetary boundary layer of the surrounding urban area to this rural site. In the spring of 1998, abnormally high BC concentrations from the continental sector were measured. These concentrations were originating from a biomass burning plume in Mexico. This was confirmed by the observations of the Earth probe total ozone mapping spectrometer. The BC average concentrations of air masses transported from the polluted continental sector during summer are low on Sunday to Tuesday with a minimum value of 256 ng m−3 occurring on Monday, and high on Wednesday to Friday with a maximum value of 379 ng m−3 occurring on Friday. The net aerosol radiative forcing (scattering effects plus absorption effects) per unit vertical depth at 2006 m MSL is calculated to be −1.38×10−3  W m−3 for the southeastern US. The magnitude of direct radiative forcing by aerosol scattering is reduced by 15±7% due to the BC absorption.
Keywords: Aerosol; Black carbon; Absorption; Direct radiative climate forcing; Temporal patterns;

Use of stable sulfur isotopes to identify sources of sulfate in Rocky Mountain snowpacks by M.Alisa Mast; John T Turk; George P Ingersoll; David W Clow; Cynthia L Kester (3303-3313).
Stable sulfur isotope ratios and major ions in bulk snowpack samples were monitored at a network of 52 high-elevation sites along and near the Continental Divide from 1993 to 1999. This information was collected to better define atmospheric deposition to remote areas of the Rocky Mountains and to help identify the major source regions of sulfate in winter deposition. Average annual δ 34S values at individual sites ranged from +4.0 to +8.2‰ and standard deviations ranged from 0.4 to 1.6‰. The chemical composition of all samples was extremely dilute and slightly acidic; average sulfate concentrations ranged from 2.4 to 12.2 μeq l−1 and pH ranged from 4.82 to 5.70. The range of δ 34S values measured in this study indicated that snowpack sulfur in the Rocky Mountains is primarily derived from anthropogenic sources. A nearly linear relation between δ 34S and latitude was observed for sites in New Mexico, Colorado, and southern Wyoming, which indicates that snowpack sulfate in the southern part of the network was derived from two isotopically distinct source regions. Because the major point sources of SO2 in the region are coal-fired powerplants, this pattern may reflect variations in the isotopic composition of coals burned by the plants. The geographic pattern in δ 34S for sites farther to the north in Wyoming and Montana was much less distinct, perhaps reflecting the paucity of major point sources of SO2 in the northern part of the network.
Keywords: Snowpack chemistry; Rocky Mountains; Sulfur isotopes; Sulfate; Acid deposition; High-elevation ecosystems;

Atmospheric deposition of organochlorine contaminants to Galveston Bay, Texas by June-Soo Park; Terry L Wade; Stephen Sweet (3315-3324).
Atmospheric monitoring of PCBs and chlorinated pesticides (e.g., HCHs, chlordanes, and DDTs) in Galveston Bay was conducted at Seabrook, Texas. Air and wet deposition samples were collected from 2 February 1995 and continued through 6 August 1996. Vapor total PCB (tPCB) concentrations in air ranged from 0.21 to 4.78 ng m−3 with a dominance of tri-chlorinated PCBs. Dissolved tPCBs in rain ranged from 0.08 to 3.34 ng l−1, with tetra-chlorinated PCBs predominating. The predominant isomers found in air and rain were α- and γ-HCH, α- and γ-chlordanes, 4,4′-DDT, and dieldrin. The concentrations of PCBs and pesticides in the air and rain revealed no clear seasonal trend. Elevated levels of PCBs in the air occurred when temperatures were high and wind came from urban and industrialized areas (S, SW, NW, and W of the site). Concentrations of HCHs were elevated in April, May, and October, perhaps due to local and/or regional applications of γ-HCH (lindane). Other pesticides showed no notable temporal variation. When winds originated from the Gulf of Mexico (southeasterly), lower concentrations of organochlorines were detected in the air. The direct deposition rate (wet+dry) of PCBs to Galveston Bay (6.40 μg m−2  yr−1) was significantly higher than that of pesticides by a factor of 5–10. The net flux from gas exchange estimated for PCBs was from Galveston Bay water to the atmosphere (78 μg m−2  yr−1). Gas exchange of PCBs from bay water to the atmosphere was the dominant flux.
Keywords: Polychlorinated biphenyls (PCBs); Pesticides; Dry deposition; Wet deposition; Air–water gas exchange; Galveston Bay;

Atmospheric polychlorinated biphenyl concentrations and apparent degradation in coastal New Jersey by Paul A. Brunciak; Jordi Dachs; Cari L. Gigliotti; Eric D. Nelson; Steven J. Eisenreich (3325-3339).
To characterize the atmospheric dynamics and behavior of organic compounds in the NY–NJ Harbor Estuary, atmospheric concentrations of polychlorinated biphenyls (PCBs) were measured at coastal, suburban and urban sites in New Jersey in 1997–1999. ∑PCB concentrations at the suburban site varied from 86 to 2300 pg m−3 and from 84 to 1100 pg m−3 at the coastal site. Although the temporal trends of total concentrations were significantly different at the three sites (p<0.01), PCB congener profiles revealed similar patterns (r 2>0.90, p<0.001) implicating a dominant emission type and/or process. Temperature explained >50% of the total variability in ln[PCB] at both sites. Atmospheric concentrations at the suburban site increased when winds blew from an eastnortheast vector, while increased wind speeds led to a slight dilution. Wind speed and direction were not significantly correlated with the concentrations measured at the coastal site. Temporal changes in congener distribution at the suburban site are consistent with the preferential atmospheric removal of 3–5 Cl-biphenyls by hydroxyl radical attack with estimated half-lives of 0.7–1.8 years.
Keywords: PCBs; Sources; Atmosphere; Degradation;

Isoprene emission capacity for US tree species by Chris Geron; Peter Harley; Alex Guenther (3341-3352).
Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) species and species from six other genera previously found to emit significant quantities of isoprene. Sampling was conducted at physiographically diverse locations in North Carolina, Central California, and Northern Oregon. Emissions from several sun leaves of each species were measured at or near standard conditions (leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m−2  s−1) using environmentally controlled cuvette systems and gas chromatography with reduction gas detectors. Species mean emission capacity ranged from 39 to 158 μg C g−1  h−1 (mean of 86), or 22 to 79 nmol m−2  s−1 (mean of 44). These rates are 2–28 times higher than those previously reported from the same species, which were summarized in a recent study where isoprene emission rates were assigned based on published data and taxonomy. These discrepancies were attributed to differences in leaf environment during development, measurement technique (branch or plant enclosure versus leaf enclosure), and lack of environmental measurements associated with some of the earlier branch enclosure measurements. Mass-based emission capacities for 15 of 18 oak species, sweetgum (Liquidambar styraciflua), and poplars (Populus trichocarpa and P. deltoides) were within ranges used in current biogenic volatile organic compound (BVOC) emission models, while measured rates for the remaining three oak species, Nyssa sylvatica, Platanus occidentalis, Robinia pseudoacacia, Salix nigra, and Populus hybrids (Populus trichocarpa × P. deltoides) were considerably higher. In addition, mean specific leaf mass of the oak species was 30% higher than assumed in current emission models. Emission rates reported here and in other recent studies support recent conclusions that isoprene emission capacities for sun leaves of high emitting species may be better represented by a value of 100±50 μg C g−1  h−1 during hot summer conditions. We also find that intermediate isoprene emission rates previously suggested for some tree species may not represent their true emission capacities, and that broadleaf plant species may have either low (<1.0 μg C g−1  h−1) or very high (∼100 μg C g−1  h−1) genetic capacity to emit isoprene when mature foliage is exposed to a high ambient temperature and light environment.
Keywords: Liquidambar; Nyssa; Populus; Quercus; Robinia; Salix; Oak; Emission factor; Isoprene capacity; Leaf temperature; Photosynthetically active radiation;

Impact of recent urbanization on formic and acetic acid concentrations in coastal North Carolina rainwater by G.Brooks Avery; Yan Tang; Robert J. Kieber; Joan D. Willey (3353-3359).
Concentrations of formic and acetic acids in Wilmington, North Carolina, USA, rainwater collected between 1996 and 1998 have increased dramatically since an earlier study conducted at the same site between 1987 and 1989. The current volume-weighted concentrations of acetic acid are within the range of values reported for urban locations whereas values from the earlier study at this site were similar to those obtained for rural locations. The ratios of formic to acetic acids (F : A) in the current study (approximately 1 : 1) are considerably lower than those previously reported (approximately 2.7 : 1). Current F : A's are similar to F : A's from direct automobile emissions. Increases in formic and acetic acid concentrations and the shift in formic to acetic acid ratios likely reflect the impact of extensive population growth in the surrounding region. Assuming increases in formic and acetic acid concentrations result from increased anthropogenic sources, we estimate at least 1/2 of the formic and at least 2/3 of the acetic acid in Wilmington, NC growing season rainwater results from anthropogenic sources.
Keywords: Formic acid; Acetic acid; Rainwater; Anthropogenic;

Concentrations of a suite of trace elements (Al, Ag, As, Cd, Co, Cr, Cu, Fe, Ni, Pb, Sr, V, Zn) were measured in aerosol and precipitation samples collected at a coastal site in New Castle, NH, from August 1996, through July 1997. Metal concentrations in aerosol and precipitation exhibit a high degree of temporal variability over the annual cycle, varying by approximately one order of magnitude or less for aerosol metals and by ∼2–3 orders of magnitude in precipitation. Estimates of the total annual atmospheric deposition of metals to the Gulf of Maine range from ∼103  kg yr−1 for Ag, ∼104–105  kg yr−1 for the majority of metals, and ∼106  kg yr−1 for the crustal elements Al and Fe.
Keywords: Trace elements; Aerosol; Precipitation;