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.