The effects of natural hydrocarbons must be considered in order to develop a reliable plan for reducing ozone in the urban atmosphere. Trees can emit significant quantities of hydrocarbons to metropolitan areas such as Atlanta, and model calculations indicate that these natural emissions can significantly affect urban ozone levels. By neglecting these compounds, previous investigators may have overestimated the effectiveness of an ozone abatement strategy based on reducing anthropogenic hydrocarbons.

Three regions of the northern mid-latitudes, the continental-scale metro-agro-plexes, presently dominate global industrial and agricultural productivity. Although these regions cover only 23 percent of the Earth's continents, they account for most of the world's commercial energy consumption, fertilizer use, food-crop production, and food exports. They also account for more than half of the world's atmospheric nitrogen oxide (NOx,) emissions and, as a result, are prone to ground-level ozone (O(3)) pollution during the summer months. On the basis of a global simulation of atmospheric reactive nitrogen compounds, it is estimated that about 10 to 35 percent of the world's grain production may occur in parts of these regions where ozone pollution may reduce crop yields. Exposure to yield-reducing ozone pollution may triple by 2025 if rising anthropogenic NOx emissions are not abated.

concentration were-15%, 5%, and 8%, respectively. Mineral aerosol contributed-16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999.

The aqueous-phase chemistry of deliquescent sea-salt aerosols in the remote marine boundary layer is investigated with a steady state box model. The model simulates the scavenging of soluble and reactive gaseous species by the sea-salt aerosols, the chemical reactions of these species and sea-salt ions in the deliquescent solution, and changes in the aerosol composition that occur as a result of these processes. The calculations indicate that deliquescent sea-salt aerosols are strongly buffered with a pH that remains close to 8 until the amount of acid added to the aerosol solution exceeds the alkalinity of sea salt. The oxidation of chloride by 03 and by free radicals is found to proceed at extremely slow rates, and thus these reactions cannot explain the high-chloride deficits recently observed over the North Atlantic Ocean. On the other hand, the oxidation of dissolved Siv by 03 in sea-salt aerosols is found to proceed at rates approaching 0.1 eq L -1 hr -1 and appears to be sufficiently rapid to qualitatively explain the observations of nss-SO• in sea-salt aerosols over the North Atlantic Ocean. The high rate of Siv oxidation is found to proceed until the amount of nss-SO• generated in the aerosol is sufficiently large to overwhelm the buffering capacity of the deliquescent solution and lower the pH below 6. As a result, the calculations suggest the existence of a removal mechanism for atmospheric S that is largely controlled by the alkalinity of seawater and the flux of this alkalinity into the atmosphere in sea salt. It is estimated that this process will generate about 0.75 neq m -3 of nss-SO• associated with sea salt in the marine boundary layer and ultimately remove about (1-4) x 10 TM moles of SO 2 from the atmosphere annually. Comparison of this loss rate with other elements of the atmospheric S cycle suggests that sea salt may remove a significant amount of S from the marine atmosphere and thereby depress the SO2 concentration in the marine boundary layer and limit the number of cloud condensation nuclei generated from the oxidation of SO2.

Calculations are presented that simulate the free radical chemistries of the gas phase and aqueous phase within a warm cloud during midday. It is demonstrated that in the presence of midday solar fluxes the heterogeneous scavenging of OH and HO2 from the gas phase by cloud droplets can represent a major source of free radicals to cloud water, provided the accommodation or sticking coefficient for these species impinging upon water droplets is ≥10−4. The aqueous‐phase HO2 radicals are found to be converted to H2O2 by aqueous‐phase chemical reactions at a rate that suggests that this mechanism could produce a significant fraction of the H2O2 found in cloud droplets. The rapid oxidation of sulfur species dissolved in cloudwater by this free‐radical‐produced H2O2 as well as by aqueous‐phase OH radicals could conceivably have a significant impact upon the chemical composition of rain.