Regional and global distributions and lifetimes of sulfate aerosols from Mexico City and southeast China

To learn what possible influence a megacity and an industrial region, both of which have poor air quality, have on the global scale, sulfate aerosol derived from Mexico City and from southeast China is examined. It is found that the highest values of sulfate from Mexico City mostly travel westward and northward. Sulfate from southeast China travels eastward, and its distribution encircles the Earth in summer and autumn. Mexico City, which emits ∼ 1% of the global anthropogenic sulfur emissions, contributes ∼ 1% to the global sulfate burden. Southeast China, which emits 11.6% of the global anthropogenic sulfur emissions, contributes 9% to the global sulfate burden, a nonlinear response between emissions and burden. When the anthropogenic sulfur emissions are doubled in the two regions, the sulfate burden derived from Mexico City approximately doubles, but the sulfate burden from southeast China more than doubles. An examination of the sulfate budget indicates that for sulfate derived from southeast China more sulfate is produced via gas-phase reaction, thus allowing the sulfate to be less susceptible to wet deposition. We qualitatively analyze black carbon aerosol from Mexico City and southeast China and determine that hydrophilic black carbon has a lifetime similar to sulfate aerosol when the emitted black carbon is assumed to be of hydrophilic form. When the emitted black carbon is hydrophobic and its transfer rate to the hydrophilic form is 1.9 days, which is the same as the global sulfur dioxide lifetime, the lifetime of hydrophilic black carbon is 3.5 days longer than sulfate. The difference between the two types of aerosol is attributed to the aqueous production pathway of sulfate and consequently its availability to rain out. If the transfer rate is 5 days, then hydrophilic black carbon is found in regions with little cloud activity thus increasing its lifetime to 10 days or more.