Fresh and aged PM2.5 and their ion composition in rural and urban atmospheres of Northern Thailand in relation to source identification

Abstract

This study aims to investigate ion composition of PM_2.5 in various sites and seasons and to identify the main sources on spatial and temporal basis. PM_2.5 compositions of two urban and two rural areas in Northern Thailand in 2019 were investigated to distinguish urban traffic and rural open burning sources. During the burning season, average PM_2.5 concentrations in rural areas (104 ± 45 μg m^-3) were slightly higher than those in urban areas (94 ± 39 μg m^-3). Source identification of PM_2.5 by cluster analysis during burning season in urban sites and one rural site revealed mixed sources of aged aerosols from biomass burning, traffic and transboundary pollution, characterized by (NH₄)₂SO₄ and KNO₃. Only PM_2.5 in one rural area (Chiang Dao), where intense open burning activities observed, contained significant KCl level in addition to other compounds. KCl is being used as a tracer for fresh aerosols from biomass burning as opposes to KNO₃ for aged aerosols. It was found that KNO₃ proportion in total ions increased with PM_2.5 concentrations both in urban and rural areas, indicating prominent open burning influences in regional scale. Source identification in other seasons was more distinguishable between urban and rural areas, and more varied depending on local emissions. Urban PM_2.5 sources were secondary inorganic aerosols from traffic gas conversion in contrast with rural PM_2.5 which were mainly from biomass burning.

Keywords: Air pollution; KCl; Open burning; PM(2.5); Smoke haze; Water-soluble ions.