Atmospheric Environment (v.35, #6)

An intensive two-week study of an urban CO2 dome in Phoenix, Arizona, USA by Craig D Idso; Sherwood B Idso; Robert C Balling (995-1000).
Atmospheric CO2 concentrations were measured prior to dawn and in the middle of the afternoon at a height of 2 m above the ground along four transects through the metropolitan area of Phoenix, Arizona on 14 consecutive days in January 2000. The data revealed the existence of a strong but variable urban CO 2 dome, which at one time exhibited a peak CO2 concentration at the center of the city that was 75% greater than that of the surrounding rural area. Mean city-center peak enhancements, however, were considerably lower, averaging 43% on weekdays and 38% on weekends; and averaged over the entire commercial sector of the city, they were lower still, registering 30% on weekdays and 23% on weekends. Over the surrounding residential areas, on the other hand, there are no weekday–weekend differences in boundary-layer CO2 concentration. Furthermore, because of enhanced vertical mixing during the day, near-surface CO2 concentrations in the afternoon are typically reduced from what they are prior to sunrise. This situation is additionally perturbed by the prevailing southwest-to-northeast flow of air at that time of day, which lowers afternoon CO2 concentrations on the southern and western edges of the city still more, as a consequence of the importation of pristine rural air. The southwest-to-northeast flow of air also sometimes totally compensates for the afternoon vertical-mixing-induced loss of CO2 from areas on the northern and eastern sides of the city, as a consequence of the northeastward advection of CO2 emanating from the central, southern and western sectors of the city. Hence, although complex, the nature of the urban CO2 dome of Phoenix, Arizona, is readily understandable in terms of basic meteorological phenomena and their interaction with human activities occurring at the land/air interface.
Keywords: Automobiles; Boundary layer; Carbon dioxide; City climate; Urban environment;

Measurement and analysis of atmospheric concentrations of isoprene and its reaction products in central Texas by Christine Wiedinmyer; Stephen Friedfeld; William Baugh; Jim Greenberg; Alex Guenther; Matthew Fraser; David Allen (1001-1013).
A field experiment was conducted in August 1998 to investigate the concentrations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level concentrations of isoprene ranged from 0.3 (lower limit of detection – LLD) to 10.2 ppbv in rural regions and from 0.3 to 6.0 ppbv in the Austin urban area. Rural ambient formaldehyde levels ranged from 0.4 ppbv (LLD) to 20.0 ppbv for 160 rural samples collected, while the observed range was smaller at Austin (0.4–3.4 ppbv) for a smaller set of samples (37 urban samples collected). Methacrolein levels did not vary as widely, with rural measurements from 0.1 ppbv (LLD) to 3.7 ppbv and urban concentrations varying between 0.2 and 5.7 ppbv. Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agreement with emission estimates based on local ground cover data. Ozone formation attributable to biogenic hydrocarbon oxidation was also calculated. The calculations indicated that if the ozone formation occurred at low VOC/NO x ratios, up to 20 ppbv of ozone formed could be attributable to biogenic photooxidation. In contrast, if the biogenic hydrocarbon reaction products were formed under low NO x conditions, ozone production attributable to biogenics oxidation would be as low as 1 ppbv. This variability in ozone formation potentials implies that biogenic emissions in rural areas will not lead to peak ozone levels in the absence of transport of NO x from urban centers or large rural NO x sources.
Keywords: Biogenic emissions; Isoprene; Isoprene reactions; Ozone formation; Texas;

A comparison of data records in the 1990s, both before (1991–1994) and after (1995–1997) implementation of Phase I of the Clean Air Act Amendments (CAAA) of 1990 for the eastern US, shows a significant reduction in SO2 emissions for most states, except for Texas, North Carolina, Illinois, Florida, and Alabama. However, of the major NO x emitting states, only two eastern states (New York and Pennsylvania) show significant declines in NO x . A pattern of large declines in SO2 emissions (>20%) after CAAA implementation, and large declines in precipitation SO4 2− and H+, as well as air concentrations of SO2 and SO4 2− (components of dry deposition), exists for most regions of the eastern US. In most cases, the emission/concentration relations are close to 1 : 1 when the source region based on 15-h back trajectories is used for the New England region, and source regions based on 9-h back trajectories are used for the six other eastern US regions that were studied. The southern Appalachian Mountain region, an acid-sensitive area receiving high levels of acidic deposition, has not seen an appreciable improvement in precipitation acidity. This area has also shown the least improvement in wet and dry sulfur concentrations, of the areas examined. Precipitation base cations (Ca2+ and Mg2+) show a pattern of either increasing or level concentrations when comparing 1990–1994 to 1995–1998 data, for six of the seven regions examined. Ammonium concentrations have generally changed <10%, except for the Illinois and southern Appalachian Mtn. regions, which increased >15%.
Keywords: Sulfur dioxide; Nitrogen oxides; Acid deposition; Base cations; Dry deposition;

Eastern North American transport climatology during high- and low-ozone days by Bret A. Schichtel; Rudolf B. Husar (1029-1038).
A synoptic scale transport climatology during days with high and low ozone concentrations was established for five summers from 1991 to 1995 over eastern North America. The airmass transport patterns were estimated from source impact regions derived from forward airmass histories. Daily maximum ozone concentrations were used to define locally and regionally high- (90th percentile) and low- (10th percentile) ozone days. Examination of transport during locally high-ozone days revealed that there is poor transport in the central part of the domain, i.e. from Tennessee to northern Indiana, and in the South from Texas to South Carolina due to stagnating or recirculating air masses. However, high ozone concentrations over the western and northern sections of the domain occurred during strong and persistent southerly and westerly winds, respectively. In addition, on average, high ozone concentration surrounding the central part of the domain were associated with transport from this region. These results support the notion that ozone exceedances in the central and Southeastern US are predominately “homegrown” while the western and northern section of the domain are also influenced by regional transport. In contrast, on low-ozone days, the transport was predominantly from outside (e.g., Canada and the Gulf of Mexico) into the Eastern US. The transport conditions during regionally high-ozone days were characterized by slow meandering transport over Kentucky, Tennessee, and West Virginia, with a strong clockwise transport around this region of stagnation.
Keywords: Ozone; Long-range transport; Local source impact; Airmass history; Lagrangian particle model.;

We reconstructed the historical trends in atmospheric deposition of nitrogen to Cape Cod, Massachusetts, from 1910 to 1995 by compiling data from literature sources, and adjusting the data for geographical and methodological differences. The reconstructed data suggest that NO3-N wet deposition to this region increased from a low of 0.9 kg N ha−1  yr−1 in 1925 to a high of approximately 4 kg N ha−1  yr−1 around 1980. The trend in NO3-N deposition has remained since the early 1980s at around 3.6 kg N ha−1  yr−1. In contrast, NH4-N wet deposition decreased from more than 4 kg N ha−1  yr−1 in the mid 1920s to about 1.5 kg N ha−1  yr−1 from the late-1940s until today. Emissions of NO x -N in the Cape Cod airshed increased at a rate of 2.1 kg N ha−1 per decade since 1910, a rate that is an order of magnitude higher than NO3-N deposition. Estimates of NH3 emissions to the northeast United States and Canada have decreased slightly throughout the century, but the decrease in reconstructed N-NH4 + deposition rates does not parallel emissions estimates. The trend in reconstructed total nitrogen deposition suggests an overall increase through the century at a rate of 0.26 kg N ha−1 per decade. This overall increase in deposition may expose coastal forests to rates of nitrogen addition that, if exceeded, could induce nitrogen saturation and increase nitrogen loads to adjoining estuaries.
Keywords: Nitrate emissions; Ammonium emissions; Reconstruction; Eutrophication; Nitrogen saturation;

The Atmospheric Integrated Research and Monitoring Network (AIRMoN) of NOAA is a research program aimed at developing and implementing improved dry and wet deposition monitoring methodologies. For dry deposition, the array is built on the basis of air-surface exchange research stations, originally set up as the “CORE/Satellite Dry Deposition Inferential Method” array under the National Acid Precipitation Assessment Program (NAPAP). For wet deposition, the program is founded on the Multistate Atmospheric Power Production Pollution Study (MAP3S), previously operated by the Department of Energy but now continuing under NOAA sponsorship. AIRMoN-wet is a research subnetwork of the National Atmospheric Deposition Program. In general, AIRMoN sites are located (a) in locations where changes should be most easily detected, (b) at sites where experienced and interested operators are already on hand, and (c) so that research opportunities (such as may result from collocation with other activities) can be maximized. The present analysis concerns the air chemistry data collected as part of the AIRMoN-dry activity. Sulfur data indicate a slow downward trend in air concentrations, at the rate of 3–4% yr−1 over the last 15 yr, doubtlessly partially attributable to the emissions reductions mandated by the Clean Air Act Amendments of 1990. For the same period, nitric acid vapor concentrations in air indicate a slight increase rather than the decrease seen for sulfur.
Keywords: Air quality; Trends; AIRMoN; Dry deposition;

Reductions in human benzene exposure in the California South Coast Air Basin by Scott A Fruin; Michael J.St Denis; Arthur M Winer; Steven D Colome; Frederick W Lurmann (1069-1077).
Benzene typically contributes a significant fraction of the human cancer risk associated with exposure to urban air pollutants. In recent years, concentrations of benzene in ambient air have declined in many urban areas due to the use of reformulated gasolines, lower vehicle emissions, and other control measures. In the California South Coast Air Basin (SoCAB) ambient benzene concentrations have been reduced by more than 70% since 1989. To estimate the resulting effect on human exposures, the Regional Human Exposure (REHEX) model was used to calculate benzene exposures in the SoCAB for the years 1989 and 1997. Benzene concentration distributions in 14 microenvironments (e.g. outdoor, home, vehicle, work) were combined with California time-activity patterns and census data to calculate exposure distributions for 11 demographic groups in the SoCAB. For 1997, the calculated average benzene exposure for nonsmoking adults in the SoCAB was 2 ppb, compared to 6 ppb for 1989. For nonsmokers, about half of the 1997 exposure was due to ambient air concentrations (including their contributions to other microenvironments), but only 4% for smokers. Passive tobacco smoke contributed about one-fourth of all exposure for adult nonsmokers. In-transit microenvironments and attached garages contributed approximately 15 and 10%, respectively. From 1989 to 1997, decreases in passive smoke exposure accounted for about one-sixth of the decrease in exposure for nonsmoking adults, with the remainder due to decreases in ambient concentrations. The reductions in exposure during this time period indicate the effectiveness of reformulated fuels, more stringent emission standards, and smoking restrictions in significantly reducing exposure to benzene.
Keywords: REHEX; Air toxics; Microenvironments; Urban air pollution; Indoor exposure;

The aqueous-phase photoformation of hydroxyl radical ( OH ) and singlet molecular oxygen (O2(1Δg) or 1 O 2 ) was characterized in winter fog waters collected in Davis, California. All of the samples studied formed OH and 1 O 2 upon illumination with simulated sunlight. Nitrite photolysis was a major source of OH in these samples, accounting for 47–100% of OH photoformation. Compared to calculated rates of gas-to-drop partitioning, in situ photoformation was a significant source of OH to all but the smallest fog drops, and was the dominant source of aqueous-phase 1 O 2 . Measured lifetimes of OH in the fog drops ranged from 0.38 to 1.5 μs. These values are significantly shorter than those predicted based on known drop constituents, indicating that uncharacterized compounds – likely organic – are significant sinks for OH in fog waters. Based on measured steady-state concentrations, both OH and 1 O 2 are likely to play significant roles in the transformations of trace species in fog drops. Hydroxyl radical appears to be a relatively significant sink for refractory compounds and a minor sink for reactive trace species. Conversely, 1 O 2 will be a minor sink for refractory compounds but a significant sink for certain electron-rich reactive trace species.
Keywords: Fog chemistry; Cloud chemistry; Oxidant formation; Aqueous phase chemistry; Photochemistry;

Although amino compounds are seemingly ubiquitous in atmospheric particles and deposition, little is known of their fate in the troposphere. We report here on the fate of 21 amino acids and alkyl amines in fog waters from Davis, California, illuminated with simulated sunlight or monochromatic light. In all experiments four amino acids – histidine (His), methionine (Met), tryptophan (Trp), and tyrosine (Tyr) – consistently decayed, with half-lives that ranged from ∼1  h (Met) to ∼23  h (Tyr) in midday, winter-solstice sunlight at Davis, CA (solar zenith angle=62°). Half-lives for the remaining amino compounds examined were typically >45  h in our experiments. Reactions with photoformed hydroxyl radical ( · OH ) and singlet molecular oxygen (O2(1Δg) or 1 O 2 ) accounted for essentially all of the loss of His and Tyr, the less reactive of the four amino acids that consistently decayed, but were minor sinks for the more reactive compounds (Met and Trp). Additional experiments revealed that methionine sulfoxide (MetSO) was formed with a yield of 58–88% during the oxidation of methionine, suggesting that the ratio of MetSO to Met might be a useful chemical marker for the age of atmospheric particles and drops. Other products expected from the transformation of amino compounds include ammonia, organic acids, and possibly mutagenic nitrosoaromatics.To complement our laboratory experiments, we also calculated rates of transformations of amino acids in near-neutral pH fog drops under ambient conditions. These calculations reveal that ozone should be a major sink for amino acids and that half-lives for many amino acids in ambient fog drops will be much shorter than those determined in our photochemistry experiments. Overall, our results indicate that reactions in atmospheric condensed phases will transform amino nitrogen compounds (including free amino acids as well as proteins and peptides) and, consequently, increase the bioavailability of nitrogen in atmospheric deposition.
Keywords: Amine compounds; Nitrogen cycle; Organic nitrogen; Fog chemistry; Oxidation in-cloud;

Micrometeorological measurements and ambient air samples, analyzed for concentrations of NH3, HNO3, NH4 +, and NO3 , were collected at an alpine tundra site on Niwot Ridge, Colorado. The measured concentrations were extremely low and ranged between 5 and 70 ng N m−3. Dry deposition fluxes of these atmospheric species were calculated using the micrometeorological gradient method. The calculated mean flux for NH3 indicates a net deposition to the surface and indicates that NH3 contributed significantly to the total N deposition to the tundra during the August–September measurement period. Our pre-measurement estimate of the compensation point for NH3 in air above the tundra was 100–200 ng N m−3; thus, a net emission of NH3 was expected given the low ambient concentrations of NH3 observed. Based on our results, however, the NH3 compensation point at this alpine tundra site appears to have been at or below about 20 ng N m−3. Large deposition velocities (>2 cm s−1) were determined for nitrate and ammonium and may result from reactions with surface-derived aerosols.
Keywords: Nitrogen; Flux; Compensation point; Deposition velocity; Gradient method;

Variability in ultraviolet total optical depth during the Southern California Ozone Study (SCOS97) by Laurent Vuilleumier; Robert A. Harley; Nancy J. Brown; James R. Slusser; Donald Kolinski; David S. Bigelow (1111-1122).
Formation of photochemical air pollution is governed in part by the solar ultraviolet actinic radiation flux, but wavelength-resolved measurements of UV radiation in polluted urban atmospheres are rarely available. As part of the 1997 Southern California Ozone Study, cosine weighted solar irradiance was measured continuously at seven UV wavelengths (300, 306, 312, 318, 326, 333 and 368 nm) at two sites during the period 1 July to 1 November 1997. The first site was at Riverside (260 m a.s.l.) in the Los Angeles metropolitan area, which frequently experiences severe air pollution episodes. The second site was at Mt Wilson (1725 m a.s.l.), approximately 70 km northwest of Riverside, and located above much of the urban haze layer. Measurements of direct (i.e., total minus diffuse) solar irradiance were used to compute total atmospheric optical depths. At 300 nm, optical depths (mean±1 S.D.) measured over the entire study period were 4.3±0.3 at Riverside and 3.7±0.2 at Mt Wilson. Optical depth decreased with increasing wavelength, falling at 368 nm to values of 0.8±0.2 at Riverside and 0.5±0.1 at Mt Wilson. At all wavelengths, both the mean and the relative standard deviation of optical depths were larger at Riverside than at Mt Wilson. At 300 nm, the difference between the smallest and largest observed optical depths corresponds to over a factor 2 increase in the direct beam irradiance for overhead sun, and over a factor 7 increase for a solar zenith angle of 60°. Principal component analysis was used to reveal underlying factors contributing to variability in optical depths. PCA showed that a single factor (component) was responsible for the major part of the variability. At Riverside, the first component was responsible for 97% of the variability and the second component for 2%. At Mt Wilson, 89% of the variability could be attributed to the first component and 10% to the second. Dependence of the component contributions on wavelength allowed identification of probable physical causes: the first component is linked to light scattering and absorption by atmospheric aerosols, and the second component is linked to light absorption by ozone. These factors are expected to contribute to temporal and spatial variability in solar actinic flux and photodissociation rates of species including ozone, nitrogen dioxide, and formaldehyde.
Keywords: Sun photometry; Optical depth; Variability; Aerosol; Principal component analysis;

Isoprene emission rates of 64 plant species found in California's urban and natural landscapes were measured using a dynamic flow-through chamber enclosure technique. Species were selected to provide data for previously unmeasured species and to test estimates of isoprene emission rates based upon taxonomic relationships developed for compilation of biogenic emission inventories as proposed by . Branch-level isoprene emission rates ranged from undetectable for 47 species, to 54 μg g−1  h−1 for Quercus kelloggii, California black oak. Isoprene emission rate estimates based on taxonomy agreed well with our measurements for species within the same genus, with the exception of the Quercus genus for which a wide range of isoprene emission rates have been reported. As expected, family-level estimates based on taxonomy showed greater deviation from our measured values than did genus-based estimates. The data developed in the present study support use of a taxonomic predictive methodology, especially if previous measurements within specific families, sub-families, and genera are extensive, and the results of such assignment are treated with proper caution. A taxonomic approach may be most useful where plant species in natural and urban landscapes are numerous, such as in California, where no experimental measurements are available for thousands of species.
Keywords: Isoprene; Biogenic hydrocarbons; Isoprene emission rates; Taxonomic methodology; Volatile organic compounds;

Overall elemental dry deposition velocities measured around Lake Michigan by Seung-Muk Yi; Usama Shahin; Jakkris Sivadechathep; Sait C Sofuoglu; Thomas M Holsen (1133-1140).
Overall dry deposition velocities of several elements were determined by dividing measured fluxes by measured airborne concentrations in different particle size ranges. The dry deposition measurements were made with a smooth surrogate surface on an automated dry deposition sampler (Eagle II) and the ambient particle concentrations were measured with a dichotomous sampler. These long-term measurements were made in Chicago, IL, South Haven, MI, and Sleeping Bear Dunes, MI, from December 1993 through October 1995 as part of the Lake Michigan Mass Balance Study. In general, the dry deposition fluxes of elements were highly correlated with coarse particle concentrations, slightly less well correlated with total particle concentrations, and least well correlated with fine particle concentrations. The calculated overall dry deposition velocities obtained using coarse particle concentrations varied from approximately 12 cm s−1 for Mg in Chicago to 0.2 cm s−1 for some primarily anthropogenic metals at the more remote sites. The velocities calculated using total particle concentrations were slightly lower. The crustal elements (Mg, Al, and Mn) had higher deposition velocities than anthropogenic elements (V, Cr, Cu, Zn, Mo, Ba and Pb). For crustal elements, overall dry deposition velocities were higher in Chicago than at the other sites.
Keywords: Dry deposition velocity; Flux; Aerosol concentration; Elemental composition; Surrogate surface;

Trajectory cluster analysis and the potential source contribution function (PSCF) model have been used to investigate the source–receptor relationship for the total gaseous mercury (TGM) measured in the Canadian High Arctic (Alert, 82.5°N, 62.3°W) during 1995. Cluster analysis of 10-day back-trajectories in 1995 shows that the synoptic flows arriving at Alert are dominated by the air masses from the north. Long-range transport only occurs in the cold seasons while summertime flows tend to circulate in the Arctic Ocean. The potential source regions identified by the PSCF modeling include Eurasia and populated areas in the North America and Europe. Based on the modeling results, it is suggested that the elevated TGM concentrations found in the Arctic summer should be of geological origins, mainly from the evasion of volatile Hg0 from earth's surfaces. In the autumn and winter, mercury is transported to the receptor site from remote anthropogenic sources. The preferred sources of TGM in the spring cannot be clearly determined due to the Arctic springtime mercury depletion, which significantly reduces the number of trajectories contributing to PSCF values. Using TGM data of higher temporal resolution improves the sensitivity of the PSCF modeling results.
Keywords: Mercury; Receptor modeling; Trajectory; Cluster analysis; Atmospheric transport; Source identification;