Aerosol radiative forcing during clear, hazy, and foggy conditions over a continental polluted location in north India

Abstract

From the simultaneous measurements of aerosol optical, physical, and chemical characteristics over Hisar, a semiurban location in northern India, aerosol radiative (shortwave (SW), longwave (LW), and net) forcings are estimated using a radiative transfer model. The submicron aerosol mass concentrations are found to be similar, while the supermicron mass concentrations on hazy and foggy days are found to be higher than those found during clear days. Aerosol optical depths are found to be high on foggy days, and they decrease on hazy and clear days. Black carbon (BC) aerosol mass concentration is found to be low during clear and hazy periods and increases by about a factor of 5 during foggy days. Single-scattering albedo (ω) values at 0.5 µm are found to be 0.88, 0.86, and 0.76 for clear, hazy, and foggy conditions, quite in agreement with varying BC amounts. The ω values over Hisar are found to exhibit close correspondence with ω derived from other locations in India in winter. SW atmospheric (ATM) forcing is found to increase from 16 W m^-2 during clear periods to 49 W m^-2 for foggy days. LW cooling of the ATM increases from about -2 W m^-2 for clear conditions to about -3 W m^-2 during foggy periods. LW ATM forcings are found to contribute 11-14% to the net ATM forcing. As the LW ATM forcings are negative, they partially cancel the large SW ATM warmings. Sensitivity study shows that LW ATM cooling becomes more prominent with an increase in the amount of absorbing aerosols and decrease in water vapor, while LW forcings are found to vary only by 1% for differing ozone amounts.