Person:

Wofsy, Steven

Model calculations are conducted to investigate the production of carboxylic acids from photochemical decomposition of isoprene, one of the main natural hydrocarbons emitted from vegetation. Both gas-phase and aqueous-phase chemical reaction pathways are examined. A simple dynamical model is proposed to simulate the boundary layer of the Amazon rain forest, and model predictions are compared to measurements made in that region in July 1985. It is found that formic acid, methacrylic acid, and pyruvic acid can be produced in significant quantities by gas-phase decomposition of isoprene. In the Amazon basin, this source may yield concentrations of these acids in the order of 1 ppb, 0.1 ppb, and 0.02 ppb, respectively. Production of formic acid in cloud by aqueous-phase oxidation of CH2O does not greatly increase the formic acid concentration predicted from the gas-phase mechanism; cloud droplets with pH > 4 are actually expected to constitute net sinks for formic acid. No significant production of acetic acid is expected from the photochemical decomposition of isoprene. Comparisons of model predictions with field data indicates that isoprene could be a major source of formic acid and pyruvic acid observed in the gas phase and in rainwater; however, acetic acid must originate from another source.

The boundary layer chemistry over the Amazon forest during the dry season is simulated with a photochemical model. Results are in good agreement with measurements of isoprene, NO, ozone, and organic acids. Photochemical reactions of biogenic isoprene and NO can supply most of the ozone observed in the boundary layer. Production of ozone is very sensitive to the availability of NO, but is insensitive to the isoprene source strength. High concentrations of total odd nitrogen (NO,) are predicted for the planetary boundary layer, about 1 ppb in the mixed layer and 0.75 ppb in the convective cloud layer. Most of the odd nitrogen (≈70%) is present as PAN‐type species, which are removed by dry deposition to the forest. The observed daytime variations of isoprene are explained by a strong dependence of the isoprene emission flux on sun angle. Nighttime losses of isoprene exceed rates of reaction with NO and O and appear to reflect dry‐deposition processes. The 24‐hour averaged isoprene emission flux is calculated to be 38 mg m d. Photooxidation of isoprene could account for a large fraction of the CO enrichment observed in the boundary layer under unpolluted conditions and could constitute an important atmospheric source of formic acid, methacrylic acid, and pyruvic acid.

Biomass-burning plumes and haze layers were observed during the ABLE 2A flights in July/August 1985 over the central Amazon Basin. The haze layers occurred at altitudes between 1000 and 4000 m and were usually only some 100 to 300-m thick but extended horizontally over several 100 km. They could be traced by satellite imaging and trajectory studies to biomass burning at the southern perimeter of the Amazon Basin, with transport times estimated to be 1–2 days. These layers strongly influenced the chemical and optical characteristics of the atmosphere over the eastern Amazon Basin. The concentrations of CO, CO2, O3, and NO were significantly elevated in the plumes and haze layers relative to the regional background. The NO/CO ratio in fresh plumes was much higher than in the aged haze layers, suggesting that more than 80% of the NOx in the haze layers had been converted to nitrate and organic nitrogen species subsequent to emission. The haze aerosol was composed predominantly of organic material, NH4+, K+, NO3−, SO4=, and anionic organic species (formate, acetate, and oxalate). While the concentrations of most aerosol ions were substantially higher in the haze layers than in the regional background aerosol, the ratios between the aerosol ions in the haze layer aerosols were very similar to those in the boundary layer aerosol over the central Amazon region. Simultaneous measurements of trace gas and aerosol species in the haze layers made it possible to derive emission ratios for CO, NOx, NH3, sulfur oxides, and aerosol constituents relative to CO2. Regional and global emission estimates based on these ratios indicate that biomass burning is an important contributor in the global and regional cycles of carbon, sulfur, and nitrogen species. Similar considerations suggest that photochemical ozone production in the biomass-burning plumes contributes significantly to the regional ozone budget.

A three-dimensional chemical tracer model for the troposphere is used to simulate the global distribution of 85Kr, a long-lived radioisotope released at northern mid-latitudes by nuclear industry. Simulated distributions for the period 1980–1983 are in excellent agreement with data from six latitudinal profiles measured over the Atlantic. High concentrations of 85Kr are predicted over the Arctic in winter, advected from European sources, and somewhat smaller enhancements arising from the same sources are predicted over the tropical Atlantic in summer. Latitudinal gradients are steepest in the northern tropics, with distinctly different seasonal variations over the Pacific, as compared to the Atlantic. The global inventory of 85Kr is reconstructed for the period 1980–1983 by combining the concentrations measured over the Atlantic with the global distributions predicted by the model. The magnitude of the Soviet source is derived. The interhemispheric exchange time is calculated as 1.1 years, with little seasonal dependence.

We describe a method for simulating photochemical production of O in a continental‐scale tropospheric model with only six independent chemical variables representing tracers transported in the model. The tracers are two primary hydrocarbon families, CO, NO, peroxyacylnitrates, and odd oxygen (O). A chemical module is developed to compute the production and loss rates of tracers over a model time step of 4 hours, using only information on the tracer concentrations input to the module at the beginning of the time step. Test simulations for summertime conditions at mid‐latitudes indicate little loss in accuracy compared to detailed model simulations of chemistry with high time resolution. Minimization of the number of tracers and use of a long time step reduces computer time and storage requirements. In addition, parameterization of the chemical computation is facilitated, allowing further savings in computer time.