Annual emissions of air toxics emitted from crop residue open burning in Southeast Asia over the period of 2010-2015

Southeast Asia (SEA) has large agricultural crop production and huge amounts of crop residues generated annually are commonly burned in the field to quickly clear land for the next crop planting. This study developed annual emission inventory for crop residue open burning (CROB) covering 17 emission species/groups for 10 SEA countries during 2010-2015, illustrated with relative contributions by country and by crop type. The fractions of crop residue subjected to open burning (OB), a large source of uncertainty in the activity data, compiled from surveys in SEA were significantly higher than those suggested in international databases. Emission factors for rice and maize residue OB were obtained from field experiments conducted in Thailand. The best estimates of the annual emissions averaged over 6 years, of air toxics were: 32 Gg of polycyclic aromatic hydrocarbons, 0.03 Gg organ-chlorinated pesticides, 292 Gg total chlorines, and 94 g I-TEQ dioxins. Emissions of PM2.5, BC and OC, in Tg yr(-1), were respectively 1.8, 0.08 and 0.8. The coefficients of variation of annual emissions during the period were relatively small (2.6-8.6% depending on species) but still showed an increasing trend that reflected the changes in production of major crops during the period. Regionally, CROB shared 10-43% of the total biomass open burning emissions but varying with country: by far dominant in Vietnam and Philippines, and much less dominant in Indonesia, Myanmar, and Thailand. Rice straw open burning was the most dominant (19-97%) in the total CROB emissions of the 8 considered crops. The spatial distributions of annual emissions (0.1 degrees x 0.1 degrees) showed higher emission intensity over the areas cultivated with rice and sugarcane, while higher monthly emissions coincided with major harvesting periods in the dry season. The obtained EI data can be further used for air quality modeling to assess effects of CROB emission and to promote non-OB alternatives in the region.