Long term characterization of aerosol optical properties: Implications for radiative forcing over the desert region of Jodhpur, India

- Long term continuous data on aerosol optical properties at a desert location in India.
- Dominance of coarse absorbing dust particles in AOT, especially during pre-monsoon.
- Pronounced diurnal variation in AOT and α during all seasons.
- ARF indicated cooling at SUF and TOA and warming of ATM.
- ARF at ATM twice more in pre monsoon than rest of the year.

AOT data for eight years period (2004-2012) using the MICROTOPS II Sun photometer has been used to study the wavelength dependent optical characteristics of aerosols over Jodhpur, situated in the desert region in NW India. The daily mean AOT at 500 nm for the present study period was 0.66 ± 0.14 with an average Angstrom exponent as 0.71 ± 0.20. Linear regression analysis of monthly AOT and Angstrom Exponent indicated an increasing trend of both. Seasonal variations of daily AOT and α as well as spectral dependence of seasonal mean AOT are presented. Diurnal variation of AOT and α in different season is studied. Impact of dust storm events on the aerosol characteristics over Jodhpur during the study period is studied. AOT values derived from MICROTOPS II were cross checked with Sun Sky Radiometer (Model POM-01, Prede Inc.) data for the period from May 2011 to April 2012 and were found to be in good agreement. Short wave aerosol radiative forcing (ARF) was computed for one year period of May 2011 to April 2012. Spectral variation of AOT, SSA and ASP showed more AOT and ASP during pre monsoon period when SSA was comparatively low; indicating towards more prevalence of coarse size absorbing dust in this period. The ARF at SUF and TOA was negative during all the seasons indicating dominance of scattering type aerosols mainly dust particles whereas that at ATM was positive in all the seasons indicating heating of the atmosphere, especially more during pre monsoon (+40.5 W/m2) than during rest of the year (+19.5 W/m2). A high degree of correlation between ARF at the SUF with AOT (R2 = 0.94) indicated that ARF is a strong function of AOT. The radiative forcing efficiency inferred to scattering nature of aerosols at SUF (−4.2 W/m2/AOD) and TOA (−63.2 W/m2/AOD) indicating cooling at surface and top of the atmosphere whereas, there was warming of the atmosphere in between (+59 W/m2/AOD). The atmospheric heating rates varied from 0.49 K/day in post monsoon to 1.13 K/day in pre monsoon. This study has enabled us to understand the long term nature and physical characteristics of atmospheric aerosols over Jodhpur.