Assessment of PM2.5 chemical compositions in Delhi: primary vs secondary emissions and contribution to light extinction coefficient and visibility degradation

Haze-fog conditions over northern India are associated with visibility degradation and severe attenuation of solar radiation by airborne particles with various chemical compositions. PM 2.5 samples have been collected in Delhi, India from December 2011 to November 2012 and analyzed for carbonaceous and inorganic species. PM 10 measurements were made simultaneously such that PM 10–2.5 could be estimated by difference. This study analyzes the temporal variation of PM 2.5 and carbonaceous particles (CP), focusing on identification of the primary and secondary aerosol emissions, estimations of light extinction coefficient (b ext ) and the contributions by the major PM 2.5 chemical components. The annual mean concentrations of PM 2.5 , organic carbon (OC), elemental carbon (EC) and PM 10–2.5 were found to be 153.6 ± 59.8, 33.5 ± 15.9, 6.9 ± 3.9 and 91.1 ± 99.9 μg m −3 , respectively. Total CP, secondary organic aerosols and major anions (e.g., SO 4 2− and NO 3 − ) maximize during the post-monsoon and winter due to fossil fuel combustion and biomass burning. PM 10–2.5 is more abundant during the pre-monsoon and post-monsoon. The OC/EC varies from 2.45 to 9.26 (mean of 5.18 ± 1.47), indicating the influence of multiple combustion sources. The b ext exhibits highest values (910 ± 280 and 1221 ± 371 Mm −1 ) in post-monsoon and winter and lowest in monsoon (363 ± 110 and 457 ± 133 Mm −1 ) as estimated via the original and revised IMPROVE algorithms, respectively. Organic matter (OM =1.6 × OC) accounts for ~39 % and ~48 % of the b ext , followed by (NH 4 ) 2 SO 4 (~21 % and ~24 %) and EC (~13 % and ~10 %), according to the original and revised algorithms, respectively. The b ext estimates via the two IMPROVE versions are highly correlated (R 2 = 0.95, root mean square error = 38 % and mean bias error = 28 %) and are strongly related to visibility impairment ( r = −0.72), mostly associated with anthropogenic rather than natural PM contributions. Therefore, reduction of CP and precursor gas emissions represents an urgent opportunity for air quality improvement across Delhi.