Although the source of these mixed-metal particles can't be determined with complete certainty from the data collected in this study, the pattern matched that found by a previous study of fly ash from incinerators burning municipal waste. Emissions from smelting, another possible source of metal aerosols, tend not to include chloride or phosphorous, both of which were mixed in with the morning metal aerosols. An absence of metals in the air on weekends and during a holiday, when industrial incinerators were closed, bolsters the case for pinning metals in the air onto burning waste.
"If you were to burn electronic waste, you would get particles very similar to these," said Ryan Moffet, lead author on both papers and a former graduate student at UC San Diego. Moffet now studies atmospheric aerosols at the Lawrence Berkeley National Laboratory.
The source of smoke particles is more clear. Satellite images taken during the experiment show a swath of fires burning in the hills surrounding Mexico City. By matching the types of smog bits they captured to the wind patterns each day, the team was able to attribute the afternoon influx of freshly burned biomass to these distant fires. Winds picked up each morning after 11:00 am, blowing the industrial waste away and bringing smoke from the fires to the south, they report in a forthcoming issue of the journal Atmospheric Chemistry and Physics.
Fuel burned by street vendors also contributes to smoke in the city, the scientists say. "A lot of people use charcoal on the streets to cook their food,
|Contact: Susan Brown|
University of California - San Diego