Abstract
Ambient aerosol samples (PM1 and PM2.5) were collected at mountain site in Doi Ang Khang (1536 m, a.s.l.), Chiang Mai Province, Thailand, during 2014 dry season (March to April). Besides 24-h samples, daytime and nighttime samples were collected, and the detailed chemical speciation of the aerosol samples included water-soluble ion species, anhydrosugar, sugar alcohol, carbonaceous species, water-soluble organic carbon (WSOC), along with isoprene-derived SOA molecular markers (2-methyltetrols). Moreover, using EC-tracer method, the contributions of secondary organic carbon (SOC) were estimated from OC. Overall, the average concentration ratios of PM1/PM2.5 were high for most species and good correlation was observed between various species concentrations in PM1 and PM2.5 samples, indicating that most of components were present in submicron particles (PM1) and might have come from similar PM emission sources. Sulfate was the major ion species in ambient aerosols samples, comprising 14.28% and 37.6% of total ion species in PM1 and PM2.5, respectively. Enhanced biomass burning tracers in the collected aerosols, such as non-sea-salt-potassium (nss-K+) and levoglucosan, revealed that the air quality at Doi Ang Khang was significantly influenced by biomass burning activities. The good correlation between levoglucosan and threitol suggested that threitol might be a good biomass burning tracer at the sampling site. The analysis also showed that directly-emitted organic carbon (OC) and formed from secondary processes enhanced the carbonaceous pollution simultaneously. The biomass burning tracers, i.e., levoglucosan and nss-K+, exhibited significant positive correlation with nitrate (NO3-), others anhydrosugar and sugar alcohol (mannosan, galactosan and xylitol), OC and WSOC suggested biomass burning emission as a dominant source. According to marker ratios (Lev/OC, Lev/Man, K+/Lev), the collected biomass burning aerosols might be contributed by wood or agriculture waste burning, and burning was predominantly in the smoldering phase. Using an emission factor obtained from previous chamber studies, the biomass smoke contributions to organic carbon were estimated to 49.41±8.43% and 50.32±9.29% in PM1 and PM2.5 at Doi Ang Khang. The mass concentrations of most species and the amount of biomass smoke contribution to OC (%) increased during nighttime, indicating higher strength of biomass burning emission during nighttime. Compared the chemical composition data from 2013 to 2015 at Doi Ang Khang site, the maximum concentrations of biomass burning tracers, levoglucosan and nss-K+, was observed in 2015. For SOA formed from oxidation of isoprene, a strong correlation between two diastereoisomeric 2-methyltetrols (2-methylerythritol and 2-methylthreitol) was observed, indicating the similar photochemical origin and formation mechanism of these two SOA species. The 2-methyltetrols also exhibited a significant relationship with relative humidity and temperature, while the positive correlation between 2-methyltetrols and NO3- implied that high concentrations of NO3- can increase the formation rate of 2-methyltetrols from isoprene. The significant high concentrations of 2-methyltetrols and consistent trends with 2-methyltetrols, WSOC and OCsec during the sampling period, suggesting SOA formations might impose strong influence on ambient air quality besides the biomass burning emission at Doi Ang Khang site during the dry season.