Emissions from forest fires near Mexico City

Abstract. The emissions of NO_x (defined as NO (nitric oxide) + NO₂ (nitrogen dioxide)) and hydrogen cyanide (HCN), per unit amount of fuel burned, from fires in the pine forests that dominate the mountains surrounding Mexico City (MC) are about 2 times higher than normally observed for forest burning. The ammonia (NH₃) emissions are about average for forest burning. The upper limit for the mass ratio of NO_x to volatile organic compounds (VOC) for these MC-area mountain fires was ~0.38, which is similar to the NO_x/VOC ratio in the MC urban area emissions inventory of 0.34, but much larger than the NO_x/VOC ratio for tropical forest fires in Brazil (~0.068). The nitrogen enrichment in the fire emissions may be due to deposition of nitrogen-containing pollutants in the outflow from the MC urban area. This effect may occur worldwide wherever biomass burning coexists with large urban areas (e.g. the tropics, southeastern US, Los Angeles Basin). The molar emission ratio of HCN to carbon monoxide (CO) for the mountain fires was 0.012±0.007, which is 2–9 times higher than widely used literature values for biomass burning. The ambient molar ratio HCN/CO in the MC-area outflow is about 0.003±0.0003. Thus, if only mountain fires emit significant amounts of HCN, these fires may be contributing about 25% of the CO production in the MC-area (~98–100 W and 19–20 N). Comparing the PM₁₀/CO and PM_2.5/CO mass ratios in the MC Metropolitan Area emission inventory (0.0115 and 0.0037) to the PM1/CO mass ratio for the mountain fires (0.133) then suggests that these fires could produce as much as ~79–92% of the primary fine particle mass generated in the MC-area. Considering both the uncertainty in the HCN/CO ratios and secondary aerosol formation in the urban and fire emissions implies that about 50±30% of the "aged" fine particle mass in the March 2006 MC-area outflow could be from these fires.