Impact of Long-Range Transport Biomass Burning (BB) Emissions on Cloud Condensation Nuclei (CCN) Activation in Continental Polluted Air of Delhi, India

Cloud Condensation Nuclei (CCN) and other aerosol properties were investigated in Delhi, India, from Feb. to Mar. 2018. The high anthropogenic influence on aerosol was studied with size-resolved CCN measurements (supersaturation (S) between 0.13 to 0.66% and selected diameters from 10 to 300 nm). Furthermore the chemical composition (Aerosol Chemical Speciation Monitor and Aethalometer AE33) of the particles was measured. The aerosol number size distribution was derived by size data inversion of Differential Mobility Particle Sizer (DMPS) from size-resolved CCN measurements. Based on multi-year back trajectory (BT) data, a spatial clustering analysis was done for the actual campaign period and two distinct clusters were identified: northwest- west northwest-long range transport (NW-LRT) and south-southeast-east southeast (SE).

There was preponderant organic mass fraction (f_org) in the aerosols throughout the campaign, with prominent diurnal variation except during the SE period. Pronounced diurnal variation was observed also in black carbon (BC) with an average concentration of 16 µg/m³ during NW-LRT, in contrast to a weak diurnal cycle with lower average concentration of 8 µg/m³ during SE. During the NW-LRT cluster the air masses traversed over agriculture fields with biomass burning (BB) activities identified using the fire radiative power (FRP) observations of Copernicus Atmosphere Monitoring Service (CAMS) Global Fire Assimilation System (GFAS). So it can be speculated that the BB emissions from the fields have contributed to enhanced BC concentrations during this period over Delhi. The remaining period, showing a mixture of local and long-range transported emissions also had a BC concentration higher than SE period when only local/regional emissions were observed. This is an important insight into the air pollution apocalypse in Delhi.

The overall average values of critical dry diameter (D_c) for CCN activation varied from 54 ± 8 nm at S = 0.66% to 139 ± 12 nm at S = 0.13%.The hygroscopicity parameter derived from CCN data (к_CCN) was in the range from 0.1 to 0.9 with an arithmetic mean of 0.27 ± 0.10, which is close to that of Beijing, another polluted continental region (0.31 ± 0.08, Gunthe et al., 2011). к_CCN also shows good agreement with the hygroscopicity parameter derived from the chemical composition measurements. A linear fit (Gunthe et al., 2009) applied to the relationship between refractory/non-refractory organic mass fraction and к_CCN at S = 0.13%, gives an effective hygroscopicity parameter к_org = 0.17 ± 0.09 and к_inorg = 0.80 ± 0.09, when extrapolated to f_org = 1 and f_org = 0, respectively. The presence of externally mixed inactive CCN particles is indicated by an average maximum activated fraction (MAF) of 0.82 ± 0.17 at S = 0.13%. The overall average D_c, к_CCN, and MAF did not vary much between NW-LRT and SE periods, although the particle number concentration was higher during NW-LRT. Moreover, high CCN efficiency was observed during NW-LRT, in spite of its enhanced BC concentration, indicating the presence of aged internally mixed aerosols. Further details will be presented.