Low Molecular Weight Monocarboxylic Acids in PM2.5 and PM10: Quantification, Seasonal Variation and Source Apportionment

PM2.5 and PM10 aerosols from a semi-urban site of Agra (27 degrees 10'N, 78 degrees 05'E) in North-Central India were analyzed for carbonaceous aerosols (Organic and Elemental carbon), low molecular weight monocarboxylic acids (Acetic and Formic acid) along with inorganic ions (Cl, NO3 , SO42, K+ and Ca2+) during April 2014 to August 2015. The average PM2.5 and PM10 mass concentrations were 86.3 +/- 71.3 and 169.7 +/- 100.5 mu g m(-3), respectively; about 45% of PM2.5 and 67% of PM10 samples were above NAAQ (National Ambient Air Quality) standards. The average organic carbon (OC) and elemental carbon (EC) concentrations were 18.2 +/- 12.3 and 6.7 +/- 4.5 mu g m(-3), respectively in PM2.5 and 25.2 +/- 14.1 and 8.1 +/- 5.9 mu g m(-3) respectively in PM10. The average concentration of acetic acid (AA) in PM2.5 and PM10 were 330 +/- 211 and 392 +/- 224 ng m(-3) respectively. The average concentration of formic acid (FA) in PM2.5 and PM10 were 348 +/- 193 and 336 +/- 175 ng m(-3) respectively. Formic acid concentration was higher in PM2.5 than PM10 but the difference is not statistically significant. Both AA and FA showed similar seasonal variation: winter > post-monsoon > summer > monsoon. Low temperature and high relative humidity in winter season favours gas to particle conversion resulting in high concentrations. The average FA to AA (F/A) ratio was 0.69 indicating dominance of primary sources at the study site. Correlation analysis of AA and FA with major ions (Cl-, NO3(-), SO4(2-), K+ and Ca2+), EC, secondary organic carbon and trace gases (O-3 and CO) was performed to identify their primary or secondary origin. The results of correlation analysis suggest that AA is mainly contributed by primary sources while FA originates from secondary sources.