In-situ measurements of aerosol properties and estimates of radiative forcing efficiency over Gangetic-Himalayan region during the GVAX field campaign

Regular near-surface measurements of aerosol scattering and absorbing coefficients, along with other aerosol and meteorological parameters, were performed during June 2011 to March 2012 at Nainital, central Himalayas in the frameworks of Ganges Valley Aerosol Experiment (GVAX). The spectral scattering (0.45, 0.55 and 0.70 μm) and absorption (0.467, 0.53 and 0.66 μm) coefficients exhibit increased values in November, mostly affected by the biomass-burning aerosols in Indo-Gangetic Plains. Both parameters are considered for fine (D < 1 μm) and sub-10 μm particles (D < 10 μm) revealing the influence of particle size in aerosol properties. Furthermore, estimations of spectral (0.467, 0.55 and 0.66 μm) single scattering albedo (SSA) and aerosol radiative forcing efficiency (ARFE) at 0.55 μm were performed focussing on determination of the role of particle size in spectral SSA and climate implications. The results show relatively high SSA values ranging from 0.90 (±0.09) to 0.95 (±0.01) for D < 10 μm, and from 0.87 (±0.10) to 0.93 (±0.02) for D < 1 μm particles, on monthly basis, suggesting large heterogeneity in the aerosol sources. The SSA for the sub-micron aerosols decreases with wavelength in the majority of the cases, in contrast to the increase for the super-micron particles suggesting different source apportionment for the particle groups. The ARFE at the top of the atmosphere is found to range from −3 to −20 W m−2 with a mean of ∼−17 W m−2 for both particle-size groups; however, during the June–October period, the ARFE for the sub-10 μm particles is found to be more negative than that for the fine aerosols.