Metallic element composition and possible source of PM2.5 in the downtown and uptown ambient air of Chiang Mai province, Thailand

Abstract

PM[subscript 2.5] and its metallic element compositions were measured at two different sites in Chiang Mai, site no.1 and site no.2 as the representatives of downtown and uptown area, from December 2004 to February 2005. Twenty-four-hour PM[subscript 2.5] samples were collected six days a week by MicroVol1100 particulate samplers. The concentrations of PM[subscript 2.5] at sites nos.1 and 2 varied from 23-145 microgram/std m[superscript 3] and 0-93 microgram/std m[superscript 3].The average PM[subscript 2.5] concentrations at sites nos.1 and 2 were 93 microgram/std m[superscript 3] and 44 microgram/std m[superscript 3], respectively. The increasing trends of PM[subscript 2.5] were observed at both sites during the study period. The significant positive correlations between the PM[subscript 2.5] concentrations and the daily average temperature and between the PM[subscript 2.5] concentrations and daily sunshine hour were observed at both sites. The average of metal elements were ordered as Ca> Mg> K> Zn> Mn> Fe> Cr> Pb> Ni> Cu> Cd for site no.1 and ordered as Ca> Mg> K> Zn> Fe> Mn> Cr > Pb> Ni> Cu> Cd for site no.2. There were 27 days for site no.1 and 6 days for site no.2 that the PM[subscript 2.5] concentrations were higher than the USEPA 24-h standard (65 microgram/std m[superscript 3]). Based on the average concentrations of Cd, Pb, Cr and Ni, the excess cancer risk resulting from exposure to Chiang Mai ambient air at sites nos.1 and 2 were 1.6x10[superscript -3] and 1.1x10[superscript -3], respectively. The principal component analysis (PCA) yielded 4 sources of PM[subscript 2.5] with the percentage of variance explained by each source as followings: traffic activity (50.7%), agricultural/forest fires (16.7%), soil (10.8%) and open burning (9.1%). Thus, traffic activity is considered as the major source of PM[subscript 2.5] in Chiang Mai ambient air. It was noticed that the maximum and minimum values of PC scores for samples collected from downtown appeared in the same level of those collected from uptown. This can be interpreted that the influence of each source type to PM[subscript 2.5] at the both sites are similar. Moreover, the agricultural/forest fires can be considered as the regional pollution source of PM[subscript 2.5] in Chiang Mai due to the high correlation coefficient between the PC scores of the second component for site no.1 and site no.2