Comparative assessment of aerosol optical properties over a mega city and an adjacent urban area in India

The present work revolves around the comparative analysis of aerosol optical properties in a mega city, Delhi and in a nearby urban area, Rohtak. It is pertinent to note that despite of the close proximity and similar meteorological conditions, the two study locations show significant differences in aerosol characteristics. The study is conducted using ground based Sky-radiometer measurements for a period of one year. The mean annual Aerosol Optical Depth (AOD) at 500 nm over Delhi and Rohtak is observed to be 1.01 and 0.73 respectively, with correlation coefficient of 0.67 and mean absolute difference of 0.51. The magnitude of AOD in Delhi is higher than in Rohtak throughout the year, except in post-monsoon season. The difference in Angstrom exponent (alpha) between the stations is minimal. However, lower magnitude of alpha is observed in Rohtak, indicating presence of more concentration of coarse-mode particles. Single Scattering Albedo (SSA) also shows seasonal variation with significantly lower values in Delhi throughout the year, indicating contribution of absorbing type of aerosols (like black carbon). The volume concentration in fine-size is found to be higher in Delhi than in Rohtak, indicating combined effect of dust, vehicular, biomass burning and industrial emissions. The aerosol classification via relationship between AOD and alpha shows that urban/biomass burning (U/B) aerosols are dominant in Delhi and mixed type (MT) aerosols in Rohtak during winter and pre-monsoon.