Aerosol optical properties and their relationship with meteorological parameters during wintertime in Delhi, India

•Aerosol and their relationship with meteorological parameters over Delhi.•Average AOD, σscat, σabs & SSA were found to be 0.95, 1027Mm-1, 86 Mm-1, 0.93.•During foggy days AOPs and BC are 2.3 times higher compared to clean days.•A factor of two aerosol concentrations were observed during lower MLDs.

In situ and columnar measurements of aerosol optical properties (AOPs) [Aerosol optical depth (AOD), Angstrom Exponent (AE), Aerosol scattering (σscat) and absorption (σabs) coefficients and single scattering albedo (SSA)] along with soot particles (Black carbon: BC) and fine particles (PM2.5: d ≤ 2.5) were continuously measured at an urban site in Delhi, India during winter period (December 2011 to March 2012). Average values of AOD, σscat, σabs, and SSA at 500 nm; and AE for the observation period were found to be 0.95 ± 0.32, 1027.36 ± 797.1 Mm− 1, 85.95 ± 73.2 Mm− 1 and 0.93 ± 0.03; and 0.94 ± 0.19, respectively. Higher values of σscat and σabs were occurred in the month of December (1857 and 148 Mm− 1) while relatively lower values of σscat (585 Mm− 1) and σabs (44 Mm− 1) were occurred in March and February respectively. SSA, however, was higher during January (0.94) and lower in March (0.89). The mass concentration of PM2.5 and BC were 195.34 ± 157.99 and 10.11 ± 8.83 μg m− 3 respectively during study period. Bimodal distributions were observed in σscat and σabs coefficients during 0800 and 0900 h LT (traffic rush hours) and at 2200 and 2300 h LT (low boundary layer conditions) with lower values during daytime between 1500 and 1700 h LT, respectively. The σscat peak in morning may be attributed to large emissions of aerosol in the traffic rush hours and production of secondary aerosols with increasing solar radiation and temperature. During study period, the σscat (mean) coefficient was 13% lower during daytime as compared to nighttime. An interesting feature was seen in monthly analysis of σscat in between day and nighttime which was 18% and 22% higher in December and January in nighttime however ~ 4% lower during February and March; it is due to effect of local meteorology. The impact of meteorological parameters such as wind speed (WS), wind direction (WD), visibility (VIS) and mixed layer depths (MLDs) on AOPs along with fine and soot particles were studied. A clear negative significant correlation between atmospheric visibility with σscat (− 0.64); σabs (− 0.57) and PM2.5 (− 0.56) were observed. During foggy days (VIS ≤ 1000 m), the AOPs, fine and soot particles were substantially (~ 1.8 times) higher as compared to clean days, however, it was ~ 2.3 times higher during dense foggy days (VIS ≤ 500 m). Similarly higher (~ 2 times) AOPs and aerosol concentrations were also seen below 200 m MLDs. In addition to this, ~ 4 times higher AOPs and aerosol mass concentrations were observed when WS was below 1 m/s. In view of the above results and regression analysis, we can say that the meteorological parameters play a crucial role in enhancement of aerosols at ground level during winter period over Delhi.