Contrasting compositions of PM_(2.5)in Northern Thailand during La Nina(2017)and El Nino(2019)years

There have been a very limited number of systematic studies on PM_(2.5)compositions and their source contribution in Southeast Asia.This study aims to explore the characteristics of PM_(2.5)composition collected in Chiang Mai(Thailand)during La Nina and El Nino years and to apportion their sources during smoke haze and non-haze periods.The average PM_(2.5)concentration of smoke haze episode in 2019(El Nino)was much higher than in 2017(La Nina).The ratios of organic carbon(OC)to elemental carbon(EC),as well as K(biomass burning(BB)tracer)to PM_(2.5),were higher during smoke haze episodes in 2019 than in 2017 indicating a significant influence from BB.The ratios of secondary organic carbon(SOC)levels to primary organic carbon(POC)levels during smoke haze episodes were higher than those in non-haze period,which indicated greater SOC contributions or more photo-oxidation of precursors in haze episodes with high ambient temperatures.However,the ratios of soil markers(Ca and Mg)during non-haze period were high implying that soil source contributed more to PM_(2.5)concentrations when there less BB occurred.The positive Matrix Factorization(PMF)model revealed that the source of BB,characterized by high K fractions,was the largest contributor during smoke haze episodes accounting for 50%(2017)and 79%(2019).Climate conditions influence meteorological patterns,particularly during incidences of extreme weather such as droughts,which affect the scale and frequency of open burning and thus air pollution levels.