[1] The presence of regional pollution in cities, while required theoretically, has only been recognized recently. This pollution forms a background that can limit efforts to reduce air pollution in cities. Its concentrations are difficult to quantify, however, even with transport models. This paper describes a novel technique for quantifying regional aerosol solely from a series of fast-response measurements of total aerosol at a single point within a city. It uses Beijing as an example and utilizes the strong asymmetric ''sawtooth cycles'' of aerosol there, which contain a smoothly increasing baseline, dominated by secondary inorganic aerosol, with daily cycles superimposed. The sawtooths have average durations of 5 days and are controlled by synoptic cycles, specifically the passage of cold fronts. During a sawtooth, the concentration of aerosol increases rapidly while back trajectories rotate from the northwest to the west and south, the air becomes more humid, and the particle size of the aerosol decreases. Ancillary measurements around Beijing show that the baselines represent regional aerosol, while the daily peaks represent local aerosol. For Beijing, the regional component averages about 50% and can range from 10%-20% during northwesterly flow to 70% or so during southerly flow. A preliminary error analysis shows that the uncertainties of the concentrations of regional aerosol can be up to 50% for a single day but <10% when totaled over a sawtooth. This technique should be applicable to a wide range of locales because sawtooths have also been observed in other places in northeast China and over much of eastern North America. Sawtooths should be a general feature of populated midlatitude areas crossed regularly by the polar front. They are also seen for trace gases and should yield analogous local and regional components.

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Aerosols could play a critical role in many processes which impact on our lives either indirectly (e.g., climate) or directly (e.g., health). However, our ability to assess these possible impacts is constrained by our limited knowledge of the physical and chemical properties of aerosols, both anthropogenic and natural. This deficiency is attributable in part to the fact that aerosols are the end product of a vast array of chemical and physical processes. Consequently, the properties of the aerosol can exhibit a great deal of variability in both time and space. Furthermore, most aerosol studies have focused on measurements of a single aerosol characteristic such as composition or size distribution. Such information is generally not useful for the assessment of impacts because the degree of impact may depend on the integral properties of the aerosol, for example, the aerosol composition as a function of particle size. In this overview we discuss recent work on atmospheric aerosols that illustrates the complex nature of the aerosol chemical and physical system, and we suggest strategies for future research. A major conclusion is that man has had a great impact on the global budgets of certain species, especially sulfur and nitrogen, that play a dominant role in the atmospheric aerosol system. These changes could conceivably affect climate. Large-scale impacts are implied because it has recently been demonstrated that natural and pollutant aerosol episodes can be propagated over great distances. However, at present there is no evidence linking anthropogenic activities with a persistent increase in aerosol concentrations on a global scale. A major problem in assessing man's impact on the atmospheric aerosol system and on global budgets is the absence of aerosol measurements in remote marine and continental areas. This dearth is especially acute for the southern hemisphere, where we could expect natural sources to predominate because of the relatively low level of industrial development and energy utilization.

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