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Air pollution in towns adjacent to and downwind of large cities can be similar or even higher than in the city itself. In the case of towns constituting the greater Bogotá area, with more than half a million inhabitants and strong industrial activity, little is known about the factors that affect their poor air quality. This work investigated the incremental excess of the composition and source contribution to PM2.5 in two sites near Bogotá (Soacha: 4°35'4.59" N, 74°13'11.62" W; and Mosquera: 4°42'9.75" N, 74°13'54.94" W), using the Chemical Mass Balance receptor model with organic molecular markers, and back trajectory analysis. Simultaneous sample collection was carried out for two-months. Organic matter was the major component of the PM2.5 mass (66 ± 14% and 61 ± 12%), while secondary inorganic ions (sulfate, nitrate, and ammonium) constituted 13 ± 8 % and 10 ± 2 %. The main anthropogenic sources contributing to PM2.5 at Soacha were wood combustion (23%), diesel vehicles (19%), and coal combustion at small facilities (11%). At Mosquera, they were gasoline vehicles (26%), diesel vehicles (19%), and coal combustion at small inefficient facilities (15%). The contribution of regional Secondary Organic Aerosol to PM2.5 was significant (19% and 15%), arriving mostly from the Orinoco basin but higher with air masses arriving from the Amazon rainforest. The regional contribution to secondary inorganic aerosols was higher with winds from the Magdalena Valley. The methods presented in this manuscript will be useful in other megacities and large cities to better manage impacts of local and regional air pollution sources.
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