Abstract:
The present work examines the influence of relative humidity (RH), physical and optical
aerosol properties on the light-scattering enhancement factor [f(RH=85%)] over central Indian Himalayas during the Ganges Valley Aerosol Experiment (GVAX). The aerosol hygroscopic properties were measured by means of DoE/ARM (US Department of Energy, Atmospheric Radiation Measurement) mobile facility focusing on periods with the regular instrumental operation (November – December 2011). The measured optical properties include
aerosol light-scattering (sp) and absorption (ap) coefficients and the intensive parameters
i.e., single scattering albedo (SSA), scattering Ångström exponent (SAE), absorption
Ångström exponent (AAE) and light scattering enhancement factor (f(RH) = sp(RH,)/sp(RHdry, )). The measurements were separated for sub-micron (<1 µm, D1µm) and particles with diameter <10 µm (D10µm) in order to examine the influence of particle size on
f(RH) and enhancement rate (γ). The particle size affects the aerosol hygroscopicity since
mean f(RH = 85%) of 1.27 ± 0.12 and 1.32 ± 0.14 are found for D10µm and D1µm, respectively.
These f(RH) values are relatively low suggesting the enhanced presence of soot and carbonaceous particles from biomass burning activities, which is verified via backward air-mass trajectories. Similarly, the light-scattering enhancement rates are ~0.20 and 0.17 for the D1µm
and D10µm particles, respectively. However, a general tendency for increasing f(RH) and γ is
shown for higher sp and ap values indicating the presence of rather aged smoke plumes,
coated with industrial aerosols over northern India, with mean SSA, SAE and AAE values of
0.92, 1.00 and 1.15 respectively. On the other hand, a moderate-to-small dependence of
f(RH) and γ on SAE, AAE, and SSA was observed for both particle sizes. Furthermore,
f(RH) exhibits an increasing tendency with the number of cloud condensation nuclei (NCCN)
indicating larger particle hygroscopicity but without significant dependence on the activation
ratio.
