Smoke smudges Mexico City's air, chemists identi...( Mexico City once topped lists of pla...)

Mexico City once topped lists of places with the worst air pollution in the world. Although efforts to curb emissions have improved the situation, tiny particles called aerosols still clog the air. Now, atmospheric scientists from UC San Diego and six other institutions have sorted through the pall that hangs over the city to precisely identify aerosols that make up the haze and chart daily patterns of changes to the mix.

This forensic work will help to identify the sources of these persistent pollutants, which plague other megacities in places like China and India as well. With this information, leaders will be better able to develop policies that will effectively clear the air.

Using an instrument that can quickly read the size and chemical fingerprint of individual particles one-by-one in real time, the scientists saw a daily rhythm in the chemical makeup of Mexico City's smog. Metal aerosols spiked in the early morning, contributing up to 73 percent of the particles they measured. By afternoon, shifting winds swept these industrial emissions away but blew in smoke particles from fires set to clear agricultural fields or burning in the hills south of the city. Burned bits of biomass accounted for as much as 76 percent of the smallest particles when strong winds flowed directly from the fires, they found.

"Nobody really knew that these kinds of aerosols were so abundant in the center of the city," said Kimberly Prather, professor of chemistry and biochemistry at UC San Diego and senior author on both papers. "Our instrument brings in a new level of precision by allowing us to identify high levels of specific pollutants that occur in transient peaks. A harmful type that is present in high amounts for just a few hours might be overlooked in a sample collected over the course of an entire day and night. But if you live nearby, you still breathe air with concentrated pollutants.

Aerosols with the chemical signature of inciner
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