V. Stroud

Distributions of brominated organic compounds in the troposphere and lower stratosphere

A comprehensive suite of brominated organic compounds was measured from whole air samples collected during the 1996 NASA Stratospheric Tracers of Atmospheric Transport aircraft campaign and the 1996 NASA Global Tropospheric Experiment Pacific Exploratory Mission-Tropics aircraft campaign. Measurements of individual species and total organic bromine were utilized to describe latitudinal and vertical distributions in the troposphere and lower stratosphere, fractional contributions to total organic bromine by individual species, fractional dissociation of the long-lived species relative to CFC-11, and the Ozone Depletion Potential of the halons and CH3Br. Spatial differences in the various organic brominated compounds were related to their respective sources and chemical lifetimes. The difference between tropospheric mixing ratios in the Northern and Southern Hemispheres for halons was approximately equivalent to their annual tropospheric growth rates, while the interhemispheric ratio of CH3Br was 1.18. The shorter-lived brominated organic species showed larger tropospheric mixing ratios in the tropics relative to midlatitudes, which may reflect marine biogenic sources. Significant vertical gradients in the troposphere were observed for the short-lived species with upper troposphere values 40–70% of the lower troposphere values. Much smaller vertical gradients (3–14%) were observed for CH3Br, and no significant vertical gradients were observed for the halons. Above the tropopause, the decrease in organic bromine compounds was found to have some seasonal and latitudinal differences. The combined losses of the individual compounds resulted in a loss of total organic bromine between the tropopause and 20 km of 38–40% in the tropics and 75–85% in midlatitudes. The fractional dissociation of the halons and CH3Br relative to CFC-11 showed latitudinal differences, with larger values in the tropics.

Oceanic bromoform sources for the tropical atmosphere

Oceanic bromoform (CHBr3) is the major source of organic Br to the atmosphere and may be significant for ozone depletion through the contribution of reactive bromine to the upper troposphere and lower stratosphere of the midlatitudes and tropics. We report the first analyses of boundary layer air, surface and deep ocean waters from the tropical Atlantic. The data provide evidence of a source of CHBr3 throughout the tropical open ocean associated with the deep chlorophyll maximum within the tropical thermocline. Equatorial upwelling carries the CHBr3 to the surface, adding to increased concentrations in the equatorial mixed layer and driving oceanic emissions that support locally elevated atmospheric concentrations. In air masses that had crossed the coastal upwelling region off NW Africa even higher atmospheric mixing ratios were measured. The observations suggest a link between climate, wind-driven upwelling, and the supply of Br to the upper atmosphere of the tropics.

Measurements of bromine containing organic compounds at the tropical tropopause

The amount of bromine entering the stratosphere from organic source gases is a primary factor involved in determining the magnitude of bromine catalyzed loss of ozone. Thirty two whole air samples were collected at the tropical tropopause during the NASA STRAT Campaign in Feb., Aug., and Dec., 1996 and were analyzed for brominated organic compounds. Total organic bromine was 17.4±0.9 ppt with 55% from methyl bromide, 38% from the Halons, 6% from dibromomethane, and 0.8% from bromochloromethane and dichlorobromomethane. One flight showed the presence of 0.42 ppt of additional organic bromine from bromoform and dibromochloromethane.

Methyl bromide, other brominated methanes, and methyl iodide in polar firn air

We report measurements of brominated, bromochlorinated, and iodinated methanes in air extracted from deep firn at three polar locations (two Antarctic and one Arctic). Using a firn diffusion model, we are able to reconstruct a consistent temporal trend for methyl bromide from the two Antarctic sites. This indicates a steady increase by about 2 ppt from the midtwentieth century to 8 ppt today. The Arctic firn, however, contained extremely high levels of methyl bromide as well as numerous other organic gases, which are evidently produced in situ. The other brominated species (dibromomethane, bromochloromethane, bromodichloromethane, dibromochloromethane, and bromoform) showed little or no long-term trend in Antarctic firn and therefore are evidently of entirely natural origin in the Southern Hemisphere. A clear seasonal trend was observed in the upper firn for the shortest-lived halocarbons (notably bromoform and methyl iodide). The same species were present at lower abundance at the higher altitude and more inland Antarctic site, possibly due to their origin from more distant oceanic sources.

Multiscale simulations of tropospheric chemistry in the eastern Pacific and on the U.S. West Coast during spring 2002

[ 1] Regional modeling analysis for the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment over the eastern Pacific and U. S. West Coast is performed using a multiscale modeling system, including the regional tracer model Chemical Weather Forecasting System (CFORS), the Sulfur Transport and Emissions Model 2003 (STEM-2K3) regional chemical transport model, and an off-line coupling with the Model of Ozone and Related Chemical Tracers ( MOZART) global chemical transport model. CO regional tracers calculated online in the CFORS model are used to identify aircraft measurement periods with Asian influences. Asian-influenced air masses measured by the National Oceanic and Atmospheric Administration (NOAA) WP-3 aircraft in this experiment are found to have lower DeltaAcetone/DeltaCO, DeltaMethanol/DeltaCO, and DeltaPropane/DeltaEthyne ratios than air masses influenced by U. S. emissions, reflecting differences in regional emission signals. The Asian air masses in the eastern Pacific are found to usually be well aged (> 5 days), to be highly diffused, and to have low NOy levels. Chemical budget analysis is performed for two flights, and the O-3 net chemical budgets are found to be negative ( net destructive) in the places dominated by Asian influences or clear sites and positive in polluted American air masses. During the trans-Pacific transport, part of gaseous HNO3 was converted to nitrate particle, and this conversion was attributed to NOy decline. Without the aerosol consideration, the model tends to overestimate HNO3 background concentration along the coast region. At the measurement site of Trinidad Head, northern California, high-concentration pollutants are usually associated with calm wind scenarios, implying that the accumulation of local pollutants leads to the high concentration. Seasonal variations are also discussed from April to May for this site. A high-resolution nesting simulation with 12-km horizontal resolution is used to study the WP-3 flight over Los Angeles and surrounding areas. This nested simulation significantly improved the predictions for emitted and secondary generated species. The difference of photochemical behavior between the coarse (60-km) and nesting simulations is discussed and compared with the observation.

A long‐term record of carbonyl sulfide (COS) in two hemispheres from firn air measurements

Carbonyl sulfide (COS) was measured in polar firn air from one Arctic and two Antarctic locations. The air samples represent atmospheric composition from the early to mid-20th century up to the present day. This provides the longest record to date of atmospheric COS. Southern Hemispheric (SH) concentrations appear to have been almost constant at 482±13 ppt over this period, apart from a slight rise in the earliest part of the record. Northern Hemispheric (NH) concentrations also showed relatively little variation with a mean of 525±17 ppt. Over the last ten years, however, NH concentrations appear to have declined by about 8±5%. Such a decline might be due to decreased carbon disulfide (CS2) emissions by the viscose-rayon industry. The absence of any large trend in COS concentrations over the last fifty or more years argues against COS being the origin of reported increases in stratospheric sulfate aerosol.

Observations of APAN during TexAQS 2000

Measurements of peroxycarboxylic nitric anhydrides (PANs) made in Houston, Texas during TexAQS (Texas Air Quality Study) 2000 showed a relatively abundant PAN compound that had not been identified in previous studies in North America [cf Williams et al., 2000]. This compound was hypothesized to be peroxyacrylic nitric anhydride {CH 2 =CH-C(O)OONO 2 , APAN} based on the work of Tanimoto and Akimoto, [2001]. APAN was synthesized and characterized on one of the two GC systems used to make those measurements, subsequent to the TexAQS 2000 field study, confirming that APAN was observed during TexAQS 2000, both on the ground and in airborne measurements. Mixing ratios of APAN were estimated from the response of the system to PAN and PPN and ranged up to 502 pptv, which was 30% of PAN. High APAN values were associated with the precursor species 1,3-butadiene and acrolein, which had local petrochemical sources. The presence of APAN at these unprecedented levels demonstrates the impact of these reactive VOC species, and may have associated health effects.

Alkyl nitrate measurements during STERAO 1996 and NARE 1997: Intercomparison and survey of results

Alkyl nitrates were measured over the northeastern plains of Colorado during the Stratospheric Tropospheric Exchange: Radiation, Aerosols and Ozone (STERAO) 1996 campaign and over the North Atlantic during the North Atlantic Regional Experiment (NARE) 1997. Alkyl nitrate measurements were performed by two different laboratories enabling an intercomparison of analytical techniques. A weighted bivariate linear regression analysis of comparable samples (National Oceanic and Atmospheric Administration versus National Center for Atmospheric Research) for the four alkyl nitrates measured by both techniques (2-propyl nitrate, 1-propyl nitrate, 2-butyl nitrate, and 3-pentyl nitrate) suggests there is neither a significant calibration difference nor a systematic offset. The precision of the measurements is indicated by the scatter in the correlations. Good precision is observed for the 2-butyl nitrate measurements (square of the correlation coefficient, R2 = 0.90). However, relatively small values of R2 and relatively large uncertainties in the determination of the slopes and intercepts (2σ confidence intervals) for 2-propyl nitrate, 1-propyl nitrate, and 3-pentyl nitrate suggest significant imprecision when compared to ambient mixing ratios for at least one of the measurement techniques. Vertical profiles during NARE 1997 show high levels of alkyl nitrates in well-defined photochemical pollution layers of continental origin. A comparison of alkyl nitrate to hydrocarbon ratios with a sequential reaction scheme model suggests our understanding of C4–C5 alkyl nitrate photochemistry is adequate to reproduce the observations. Air masses with larger ratios of alkyl nitrate to hydrocarbons were observed during NARE 1997 than STERAO 1996, consistent with the NARE 1997 measurements being in the more remote North Atlantic. These results suggest that information related to the photochemical age of an air mass can be derived from alkyl nitrate to hydrocarbon ratios.