Carbonaceous aerosol over semi-arid region of western India: Heterogeneity in sources and characteristics

Carbonaceous species (elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC)) and water-soluble inorganic species (Na+, NH4+, K+, Ca2+, Mg2+, Cl−, NO3−, SO42−) in PM10 and PM2.5 from Ahmedabad and Jodhpur (urban and semi-urban locations, respectively) in western India were measured during May–September, 2011. Stable isotope composition of carbonaceous aerosol (δ13C of TC) in PM10 samples was also determined. Average EC concentration in PM10 at Ahmedabad was 1μgm−3 (range: 0.34 to 3.4μgm−3), almost 80% of which remained in PM2.5. Similarly, 70% of EC in PM10 (average: 0.9μgm−3) resided in PM2.5 at Jodhpur. Average OC concentration at Ahmedabad was 6.4μgm−3 and ~52% of this was found in PM2.5. On the contrary, OC concentration at Jodhpur was 40μgm−3, 80% of which was found in coarse particles contributing substantially to aerosol mass. δ13C of TC (average: −27.5‰, range: −29.6 to −25.8‰) along with WSOC/EC ratio shows an increasing trend at Jodhpur suggesting the possibility of aging of aerosol, since aging results in enrichment of heavier isotope. OC and WSOC show significant correlations with K+ and not with EC, indicating biogenic origin of OC. Different size distributions are also exhibited by WSOC at the two stations. On the other hand, δ13C exhibits an inverse trend with sea-salt constituents at Ahmedabad, indicating the influence of air masses transported from the western/south-western region on carbonaceous aerosol. These results suggest that a strong heterogeneity exists in the sources of carbonaceous aerosol over this region and potential sources of non-combustion emissions such as bio-aerosol that need further investigation.