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

Abstract

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^-, SO₄^2-) in PM₁₀ and PM_2.5 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 1 μg m^-3 (range: 0.34 to 3.4 μg m^-3), almost 80% of which remained in PM_2.5. Similarly, 70% of EC in PM10 (average: 0.9 μg m^-3) resided in PM_2.5 at Jodhpur. Average OC concentration at Ahmedabad was 6.4 μg m^-3 and ∼52% of this was found in PM_2.5. On the contrary, OC concentration at Jodhpur was 40 μg m^-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.