Determination of ambient PM2.5-bound metals in traffic congestion area of Chiang Mai city

Abstract

The upper part of Northern Thailand including Chiang Mai, has been confronted with severe air pollution during the dry season almost every year. This study aims to monitor ambient concentrations from traffic-congested areas in Chiang Mai city to identify significant tracers for traffic emissions. Daily PM2.5 samples were collected at Rin Kham Intersection, one of the most condense traffic congestion in the city, during pre-smoke haze period (pre-SH) (November 2020 to December 2020), Smoke Haze period (SH); High-PM2.5 (H-PM2.5) (January 2021 to February 2021) and Extreme- PM2.5 (Ex-PM2.5) (March 2021 to April 2021), and Post-Smoke Haze period (Post-SH) (April 2021) using a high-volume air sampler. Mean PM2.5 concentrations for each sampling periods were 41.55±5.59 µg/m3 (n = 30), 61.71±14.92 µg/m3 (n = 31), 96.89±25.24 µg/m3 (n=27), and 32.03±8.14 µg/m3 (n=15), respectively. Analysis of elemental compositions of ambient PM2.5 revealed significant associations and potential sources across different periods. K (biomass burning tracer) had the highest concentration in the Ex-PM2.5 period, followed by H-PM2.5, indicating the dominant biomass burning during the high PM2.5 period. Ca and Fe have higher concentrations during pre-SH than post-SH but less than H-PM2.5 and Ex-PM2.5. This study revealed that Zn, Pb, Cr, and Cu are important tracers for traffic emission in this area. Moreover, Cr, Fe, and Zn showed significant correlations with PM2.5 levels during pre-SH, while K, Mg, and Mn exhibited stronger associations during SH, especially Ex-PM2.5. During post-SH, a strong correlation with K remained, but an increased correlation with Zn was also observed. This indicates that PM2.5 during SH (H-PM2.5 and Ex-PM2.5) were influenced by biomass burning, while traffic emission was contributed more during pre-SH and post-SH due to less biomass burning.