|dc.description.abstract||This work collected atmospheric aerosols at Chiang Mai (1,536 m a.s.l., Thailand) in the northern part of Indochina Peninsula and Mt. Lulin Atmospheric Background Station (LABS, 2,862 m a.s.l., Taiwan), respectively, during the springtime of 2013. The objectives of this study were to investigate aerosol characteristics in the near-source biomass burning (BB) area and transported and aged aerosols at a background site. In addition, aerosol characteristics of Indochina Peninsula observed during the period of 2010-2013 and that of LABS during 2003-2013 were also summarized to explore middle- and long-term aerosol chemical characteristics and source contributions.
The results showed that PM2.5 dominated PM10 at Chiang Mai in 2013. For BB events, sulfate and ammonium ions were the major species in PM2.5 water-soluble ions (WSIs); while OC3 and EC1-OP were significant in aerosol carbonaceous components. The fraction of water-soluble organic carbon in organic carbon (OC) was 61% and oxalate was the dominant component of dicarboxylates. As for anhydrous sugars, levoglucosan was undoubtedly the most significant component. The BB tracers such as nss-K+, OC3, EC1-OP, and levoglucosan were correlated well with each other (with the coefficient of determination R2≧0.6, N=38), which confirmed that aerosols were affected by BB in the area. Inferred from the characteristics of specific aerosol component ratios, PM2.5 aerosol was contributed from open forest burning from a mix of softwood and hardwood and the combustion state was smoldering. The yearly component fractions of PM2.5 from near-source BB during 2010-2013 were close to each other. The PM2.5 fraction of cations, anions, OC, elemental carbon were 5.7±0.6%, 12.2±1.3%, 40.9±4.1%, 7.4±1.0%, respectively. The resulted fractions of PM2.5 should be typical for Indochina BB aerosol.
The PM2.5 aerosol observations at LABS showed that PM2.5 mass, WSIs, and carbonaceous content from BB backward trajectory group were all higher than that of other trajectory groups especially for BB tracers such as nss-K+, NO3-, OC3, EC1-OP and levoglucosan as summarized from 2003 to 2013. In contrast, the mass fractions of PM2.5 WSIs of non-BB (NBB) were higher than that of BB, an indication of more anthropogenic influence. By employing prevailing carbonaceous component, char-EC/soot-EC, and OC/EC ratios, aerosols from BB backward trajectory group were confirmed with BB characteristics and that from NBB trajectory group were more toward to vehicle emissions.
This work compared PM2.5 component ratios at the upwind Chiang Mai Thailand (Mt. Suthep in 2010 and Mt. Ang Khang in 2013) with the corresponding values at the downwind LABS to find that the ratios of nss-K+, OC3, and EC1-OP were quite stable. It implies that these three PM2.5 components are appropriate for acting as BB aerosol tracers through long-range transport. As a result, aerosol aging characteristics were investigated by using stable nss-K+ to show the enhancements of NH4+, NO3 , SO42-, OC1, OP, and dicarboxylates and the degradation of anhydrosugars during transport.
Six source types were resolved from Positive Matrix Factorization (PMF) for PM2.5 aerosols classified into BB backward trajectory group at LABS from 2003 to 2013. The resolved source types in source contributions from high to low are BB mixing secondary aerosol (28.9%), BB (26.3%), BB mixing sea salt (15.0%), BB mixing Dicarboxylates (14.7%), BB mixing soil dust (9.1%), and Vehicle emissions (5.9%). PMF source apportionment indicated that 94% of PM2.5 mass concentration was contributed from BB plume mixed with other source contributions in the path of long-range transport. In contrast, four source types were resolved from PMF and secondary aerosol contributed predominantly to 70.2% of PM2.5 mass concentration during the NBB period. The NBB air masses were mostly originated from ocean (including Pacific Ocean and South China Sea) but few with a mix of marine contributions and anthropogenic pollutants from China coastline and transported through lower atmosphere to LABS.