Elaine G. Chapman

Unexpectedly high concentrations of molecular chlorine in coastal air

The fate of many atmospheric trace species, including pollutants such as nitrogen oxides and some volatile organic compounds, is controlled by oxidation reactions. In the daytime troposphere, these reactions are dominated by photochemically produced OH radicals; at night and in polluted environments, NO3 radicals are an important oxidant. Ozone can contribute to the oxidation of atmospheric species during both day and night. In recent years, laboratory investigations, modelling studies, measured Cl deficits in marine aerosols and species-nonspecific observations of gaseous inorganic chlorine compounds other than HCl have suggested that reactive halogen species may contribute significantly to—or even locally dominate—the oxidative capacity of the lower marine troposphere. Here we report night-time observations of molecular chlorine concentrations at a North American coastal site during onshore wind flow conditions that cannot be explained using known chlorine chemistry. The measured Cl2 mixing ratios range from <10 to 150 parts per 1012 (p.p.t.), exceeding those predicted for marine air by more than an order of magnitude. Using the observed chlorine concentrations and a simple photochemical box model, we estimate that a hitherto unrecognized chlorine source must exist that produces up to 330 p.p.t. Cl2 per day. The model also indicates that early-morning photolysis of molecular chlorine can yield sufficiently high concentrations of chlorine atoms to render the oxidation of common gaseous compounds by this species 100 times faster than the analogous oxidation reactions involving the OH radical, thus emphasizing the locally significant effect of chlorine atoms on the concentrations and lifetimes of atmospheric trace species in both the remote marine boundary layer and coastal urban areas.

Organic acids in springtime wisconsin precipitation samples

Abstract A short-term study on organic acids in precipitation was conducted from 15 March to 1 June 1984 at two sites on the Wisconsin Acid Deposition Monitoring Network. Aliquots of collected samples were fixed with tetrachloromercurate (TCM) and analyzed for low molecular weight organic anions via ion-exclusion chromatography (ICE). Unfixed aliquots were subjected to standard network inorganic analyses. Of the 31 samples collected, 30 contained detectable concentrations of formate and acetate ions, with concentrations ranging from

Ozone loss in soot aerosols

The fractal-like structure of atmospheric soot (e.g., elemental carbon) provides a large surface area available for heterogeneous chemistry in the upper troposphere and lower stratosphere [Blake and Kato, 1995]. One potentially important reaction is ozone decomposition on soot. Although extensively studied in the laboratory, a wide range of reaction probabilities have been observed (γ∼10−3 to γ∼10−7) which have been attributed to differences in reactivity between fresh (i.e., nonoxidized) versus aged (i.e., oxidized) soot [Schurath and Naumann, 1998]. The importance in understanding soot-ozone chemistry is particularly important in light of recent nighttime field measurements [Berkowitz et al., 2000] made over Portland, Oregon. The data revealed episodes of an anticorrelation between ozone mixing ratio and aerosol surface area density. During these episodes a single scattering albedo in the range 0.8–0.9 was measured, indicating an increased absorptive component of the aerosol, perhaps due to elemental carbon. In addition, an increase in the concentration of aerosols contained in the small size range of the fine mode (<0.1–0.15 μm) was observed, suggestive of new aerosol formation. In this article we attempt to explain these field observations. One explanation of the field observations is ozone loss occurring on atmospheric soot aerosol. Here we present laboratory results obtained using a static aerosol reactor that indicate that direct ozone loss on soot aerosol is unlikely under ambient conditions in the troposphere. An alternative and more likely explanation of the field data is based on ozone-mediated organic aerosol production. This could occur by either nighttime nitrate radical oxidation or direct ozone oxidation of hydrocarbons as suggested previously [Starn et al., 1998; Griffin et al., 1999; Kamens et al., 1999; Yu et al., 1999; De Gouw and Lovejoy, 1998].

Observations of dimethyl sulfide over the western North Atlantic Ocean using an airborne tandem mass spectrometer

An atmospheric sampling tandem mass spectrometer has been evaluated for aircraft monitoring of dimethyl sulfide (DMS) and then used for DMS monitoring during portions of six flights over the western North Atlantic Ocean. Laboratory evaluations demonstrated that the mass spectrometer is highly selective for DMS, and responds linearly over a wide range of mixing ratios. The detection limit for DMS is 1–2 ppt in dry air. However, the response is suppressed in the presence of water vapor, so the sample air must be dried or the response must be corrected for this effect. There appeared to be no consistent effect of altitude/pressure on instrument response. The mass spectrometer was installed on the Battelle Gulf stream G-1 research aircraft and used to monitor DMS and a number of other chemicals during flights over the western North Atlantic Ocean in August and September, 1992. DMS mixing ratios ranged from <2 ppt to 332 ppt, and were highly variable both horizontally and vertically. Vertical profiles indicated that there are times when the marine boundary layer is stratified by one or more temperature inversions, and that DMS emitted by surface seawater can be confined near the surface within a shallow layer a few hundred meters deep. Under such circumstances, the DMS photooxidation products may be removed rapidly by deposition, lessening the potential for cloud nucleation. The mean DMS mixing ratio in the boundary layer below the lowest observed temperature inversion was 61 ppt, with a range of 7–332 ppt. DMS measurements in the free troposphere were lower than the boundary layer values, but high enough to suggest significant transport from the boundary layer to the free troposphere. Significant horizontal variability was observed during constant altitude flights in the boundary layer. In one case the DMS mixing ratio was observed to vary with the ocean depth under the flight path, with higher mixing ratios observed over the shallower coastal shelf and undersea banks. In several cases we also observed an apparent association between atmospheric DMS mixing ratio at low elevation and sea surface temperature.

BrCl production in NaBr/NaCl/HNO3/O3 solutions representative of sea-salt aerosols in the marine boundary layer

Atomic bromine and chlorine liberated from sea-salt aerosol is thought to play an important role in chemistry of the marine boundary layer. Despite numerous modeling studies, no prior experimental investigations of the oxidation of halide species contained in simulated, or actual, sea-salt solutions have been performed. We present laboratory data that examines chemistry in NaBr/NaCl/HNO 3 /O 3 solutions at 290 K. Ozonation experiments were performed by flowing ozone in air through a nitric acid/salt solution and monitoring pH with time using an ion-sensitive electrode. The rate of oxidation was observed to be first order in ozone concentration and to have a non-first order bromide concentration dependence. Ion chromatography was used to measure both bromide disappearance as well as oxidation products formed during the course of the reactions studied. Our measurements of the oxidation rate versus ion concentration indicate that the high ionic strength present in sea-salt aerosol will possess unique kinetics different from dilute solution behavior. In addition, our results are consistent with the reaction sequence O 3 + H + + Br - → O 2 + HOBr and HOBr + Cl - + H + → BrCl + H 2 O. These observations support the HOBr mediated Cl- oxidation process proposed previously (Vogt et al., 1996).

Continuous airborne measurements of gaseous formic and acetic acids over the western North Atlantic

A tandem mass spectrometer modified for aircraft applications was used to measure gas-phase formic and acetic acid mixing ratios over the western North Atlantic during late summer 1992. The sensitive, specific, and essentially real-time measurements provided by the mass spectrometer allow a true spatial evaluation of these compounds in the study area. Formic and acetic acid mixing ratios showed substantial vertical variation, varying by factors of up to 13 and 6, respectively, within 2-km profiles extending from the boundary layer into the free troposphere. Substantial horizontal variation was also observed at constant altitudes within both the boundary layer and free troposphere. Mixing ratios of the two acids were correlated (r²>0.70) throughout the study region.

Aircraft observations of aerosols, O3 and NOy in a nighttime urban plume

Abstract Nighttime measurements of aerosol surface area, O 3 , NO y and moisture were made downwind of Portland, Oregon, as part of a study to characterize the chemistry in a nocturnal urban plume. Air parcels sampled within the urban plume soon after sunset had positive correlations between O 3 , relative humidity, NO y and aerosol number density. However, the air parcels sampled within the urban plume just before dawn had O 3 mixing ratios that were highly anti-correlated with aerosol number density, NO y and relative humidity. Back-trajectories from a mesoscale model show that both the post-sunset and pre-dawn parcels came from a common maritime source to the northwest of Portland. The pre-dawn parcels with strong anti-correlations passed directly over Portland in contrast to the other parcels that were found to pass west of Portland. Several gas-phase mechanisms and a heterogeneous mechanism involving the loss of O 3 to the aerosol surface, are examined to explain the observed depletion in O 3 within the pre-dawn parcels that had passed over Portland.

DUSTRAN 1.0 User’s Guide: A GIS-Based Atmospheric Dust Dispersion Modeling System

Abstract : The U.S. Department of Energy s Pacific Northwest National Laboratory just completed a multi-year project to develop a fully tested and documented atmospheric dispersion modeling system (DUST TRANsport or DUSTRAN) to assist the U.S. Department of Defense (DoD) in addressing particulate air quality issues at military training and testing ranges. The project was primarily funded by DoD s Strategic Environmental Research and Development Program with additional funding from the U.S. Forest Service and U.S. Environmental Protection Agency (EPA) to address their issues related to the off-target drift of aerially applied pesticides.

Inter-storm comparisons from the OSCAR high density network experiment

Abstract The high density network component of the Oxidation and Scavenging Characteristics of April Rains (OSCAR) experiment combined aircraft, surface and sequential precipitation chemistry measurements to characterize the physicochemical and dynamic features of four storms sampled during an April 1981 field investigation. A surface network of 47 precipitation sampling stations, covering a region roughly 110 km by 110 km, was established in the area surrounding Fort Wayne, Indiana. The network provided temporal and spatial resolution of rainfall chemistry via the use of specially designed automatic sequential bulk precipitation collectors, while aircraft and surface sampling provided measurements of the major aerosols and trace gases in the boundary-layer inflow region. Composite concentration and ion ratio profiles for the events were analyzed to investigate potential pollutant scavenging pathways. This analysis led to the following observations: 1. (i) dryfall deposition during pre-rainfall exposure periods influenced initial sampler stage chemistry; 2. (ii) relative precipitation acidity increased throughout the events; SO 4 2− and NO 3 − were the major contributors to this acidity; 3. (iii) evidence exists for the in-cloud oxidation of SO 2 during Events 3 and 4, while scavenging of HNO 3 and aerosol NO 3 − probably produced precipitation NO 3 − ; 4. (iv) the non-frontal meteorology of Event 3 influenced the precipitation chemistry associated with this storm and led to distinct concentration profiles; 5. (v) an anomalous pattern of NH 4 + concentrations observed during Event 1 cannot be explained by known NH 4 + scavenging behavior or by non-scavenging related influences, such as local source contamination or NH 3 volatilization; 6. (vi) Event 4 is more suitable for analysis by one- and two-dimensional diagnostic wet removal models. Analysis of the other events is complicated by more complex meteorological behavior and, in some cases, a less complete chemistry data set. This paper enlarges on these observations with comparisons of the major meteorological and chemical characteristics of the four events.

Simulation of the tropospheric distribution of carbon monoxide during the 1984 MAPS experiment

Abstract A global, three-dimensional tropospheric chemistry model was used to perform simulations of the tropospheric distribution of carbon monoxide (CO) coinciding with NASAs Measurement of Air Pollution from Satellites (MAPS) experiment which took place during 5–13 October 1984. Archived meteorological data for September and October, 1984, were obtained from the European Centre for Medium-Range Weather Forecasting and used to drive the offline chemical transport model simulations. Base-case CO emissions were generated by applying emission factors to compiled inventories for related or co-emitted trace species. Simulation results from September and October have been compared with a recent re-release of the 1984 MAPS data and with in situ correlative data taken during the MAPS mission. Because of unrealistically large spatial variability in N2O mixing ratios measured concurrently by MAPS, model results were also compared with an adjusted CO data set generated by assuming that errors in N2O measured mixing ratios were correlated with errors in the MAPS CO data. These comparisons, in conjunction with simulations probing model sensitivities, led to the conclusion that biomass burning CO emissions from central and southern Africa may have been larger during September and October, 1984, than our initial best estimate based on the CO2 emissions data of Hao et al. (1990. Fire in the Tropical Biota; Ecosystem Processes and Global Challenges. Springer, Berlin, pp. 440–462; 1994. Global Biogeochemical Cycles 8, 495–503) . This result is in disagreement with recent estimates of biomass burning emissions from Africa ( Scholes et al., 1996, Journal of Geophysical Research 101, 23677–23682) which are smaller than previously thought for emissions from this region. Although unknown model deficiencies cannot be conclusively ruled out, model sensitivity studies indicate that increased CO emissions from central and southern Africa offer the best explanation for reducing observed differences between model results and MAPS data for this time period. Our results, in combination with a disparity in recent CO emission estimates from this region ( Scholes et al., 1996 ; Hao et al., 1996, Journal of Geophysical Research 101, 23577–23584), and in light of recent indications of highly variable biomass burning activities from the tropical western Pacific ( Folkins et al., 1997, Journal of Geophysical Research 102, 13291–13299), seem to suggest that biomass burning emissions exhibit significant year-to-year variability. This large variability of emissions sources makes the accurate simulation of specific time periods very difficult and suggests that biomass burning trace species inventories may have to be developed specifically for each simulated time period, employing satellite-derived information on fire coverage and flame intensity.