Abstract
Abstract. Mainland and maritime Southeast Asia is home to more than 655 million people, representing nearly 10 % of the global population. The dry season in this region is typically associated with intense biomass burning activity, which leads to a significant increase in surface air pollutants that are harmful to human health, including ozone (O3). Latitude-based differences in the dry season and land use distinguish two regional biomass burning regimes: (1) burning on the peninsular mainland peaking in March and (2) burning across Indonesia peaking in September. The type and amount of material burned in each regime impact the emissions of nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs), which combine to produce ozone. Here, we use the nested GEOS-Chem atmospheric chemistry transport model (horizontal resolution of 0.25∘ × 0.3125∘), in combination with satellite observations from the Ozone Monitoring Instrument (OMI) and ground-based observations from Malaysia, to investigate ozone photochemistry over Southeast Asia in 2014. Seasonal cycles of tropospheric ozone columns from OMI and GEOS-Chem peak with biomass burning emissions. Compared to OMI, the model has a mean annual bias of −11 % but tends to overestimate tropospheric ozone near areas of seasonal fire activity. We find that outside these burning areas, the underlying photochemical environment is generally NOx-limited and dominated by anthropogenic NOx and biogenic non-methane VOC emissions. Pyrogenic emissions of NOx play a key role in photochemistry, shifting towards more VOC-limited ozone production and contributing about 30 % of the regional ozone formation potential during both biomass burning seasons. Using the GEOS-Chem model, we find that biomass burning activity coincides with widespread ozone exposure at levels that exceed world public health guidelines, resulting in about 260 premature deaths across Southeast Asia in March 2014 and another 160 deaths in September. Despite a positive model bias, hazardous ozone levels are confirmed by surface observations during both burning seasons.
Highlights
Mainland and maritime Southeast Asia, including Myanmar, Laos, Vietnam, Cambodia, Thailand, Malaysia, Indonesia, Singapore, Brunei, Timor-Leste and the Philippines, is home to more than 655 million people
We find that when Ozone formation potential (OFP) is sorted by the source sector of emitted non-methane VOC (NMVOC), the majority of the OFP is attributed to biogenic NMVOCs, with only about 10 % and 5 % of the total OFP attributed to pyrogenic and anthropogenic NMVOCs, respectively
Some ground sites are located near cities where anthropogenic sources may exert a larger influence on ozone production, we find that exceedances of monthly mean maximum daily 8 h average (MDA8) are not confirmed by observations anywhere in Malaysia in either May or December when regional biomass burning is at a minimum
Summary
Mainland and maritime Southeast Asia, including Myanmar, Laos, Vietnam, Cambodia, Thailand, Malaysia, Indonesia, Singapore, Brunei, Timor-Leste and the Philippines, is home to more than 655 million people. Urban populations are growing at a rate faster than global mean values, and future projections suggest this trend will continue, with massive implications for regional anthropogenic emissions These changes in urbanization occur against the backdrop of widespread seasonal biomass burning and significant biogenic emissions. One-half of the populated area across Southeast Asia is exposed to recurring fire activity every year (Vadrevu et al, 2019), with implications for surface air pollution and human health Over this region, fires originate predominately from land use change (Marlier et al, 2015) and seasonal agricultural practices (Sastry, 2002; Jones, 2006; Korontzi et al, 2006; Herawati and Santoso, 2011).
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