Numerical assessment of PM2.5 and O3 air quality in Continental Southeast Asia: Impacts of future projected anthropogenic emission change and its impacts in combination with potential future climate change impacts

Air quality in future will change in response to anthropogenic emission change as well as climate change. In this study, an advanced online coupled Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) model was applied to the Continental Southeast Asia. The targets were to investigate the impacts of projected change in future anthropogenic emissions and its impacts in combination with climate change impacts on future particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5) and ozone (O3) air quality. Future anthropogenic emissions were produced by scaling Hemispheric Transport of Air Pollution (HTAP) emissions in year 2010 using ratios taken from the Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants (ECLIPSE) emission database in year 2050 and 2010 under current legislation (CLE) scenario. Future climate projections were obtained by implementing a dynamical downscaling method based on pseudo global warming (PGW) technique using future (2046-2055) climate conditions and present (2006-2015) climate conditions provided by the Coupled Model Intercomparison Experiment (CMIP5) Community Earth System Model (CESM) data and the 2014 meteorological conditions. Two Representative Concentration Pathway (RCP) climate scenarios, RCP4.5 and RCP8.5, were considered. Overall, anthropogenic emission change alone appears to cause higher PM2.5 and O3 levels in future period. Across four target countries namely Cambodia, Laos, Thailand, and Vietnam, on average, PM2.5 and O3 concentrations increase by +4.56 μg/m3 (+13.76%) and +5.12 ppb (+13.20%) during the dry season, +3.03 μg/m3 (+27.72%) and +5.89 ppb (+23.97%) during the wet season, respectively. PM2.5 increase is attributed to the emission growth of primary PM2.5 and PM2.5 precursors. Among PM2.5 species, while raising trend was mainly predicted for anthropogenic secondary organic aerosol (PM2.5 ASOA), sulfate ion (PM2.5 SO42−), nitrate ion (PM2.5 NO3−), and ammonium ion (PM2.5 NH4+) concentrations, reducing trend was mainly predicted for element carbon (PM2.5 EC) and primary organic aerosol (PM2.5 POA) concentrations due to the projected primary PM2.5 emission decrease in China. O3 increase is driven by high nitrogen oxides (NOx) emission increase rather than non-methane volatile organic compounds (NMVOCs) emission increase since NOx-limited regime mostly dominates over the region. Both future PM2.5 and O3 increases in Continental Southeast Asia are also contributed by pollutant emission growth in India. Driven by climate change and emission change in combination, the rise in PM2.5 and O3 concentrations is larger in RCP8.5 scenario than that in RCP4.5 scenario. Averagely, under RCP4.5 scenario, PM2.5 and O3 concentrations, across four target countries, were projected to increase by +3.43 μg/m3 (+10.36%) and +4.37 ppb (+11.27%) during the dry season, +2.20 μg/m3 (+20.15%) and +5.20 ppb (+21.16%) during the wet season. Under RCP8.5 scenario, they increase by +7.10 μg/m3 (+21.43%) and +6.35 ppb (+16.38%) during the dry season, +3.08 μg/m3 (+28.15%) and +6.20 ppb (+25.20%) during the wet season, respectively. The simulation results indicated that emission trend is a major factor in the variation in PM2.5 and O3 concentrations, while the climate change also plays an important role. Current legislations and pollution controls as in ECLIPSE CLE scenario are inadequate for better PM2.5 and O3 air quality in the region. A stronger emission control strategy will be required to cope with this PM2.5 and O3 air quality degradation, particularly under RCP8.5 scenario.