Surface temperature changes due to aerosols using Satellite Observations over India

Abstract

Atmospheric aerosols are an essential component of Earth’s climate system that interact with the radiation falling on Earth through direct, indirect and semi-direct effects. Multiple studies have explored the impact of aerosols on surface-reaching solar radiation. These studies have shown an overall negative radiative forcing globally. It is also well known that aerosol-induced radiation changes have a corresponding effect on surface air temperature, an essential parameter for meteorology and human life on the surface. However, not many studies have explored this issue on local or regional scales, especially using observational data due to the complex nature of this relationship which is influenced by land surface parameters in addition to short and longwave radiation. This study attempts to quantify the effect of aerosols on surface air temperature using long term satellite observations (2002-2019) over the Indian region on annual and seasonal time scales. It is found that aerosols reduce the daytime surface temperature by as much as -1.0 to - 1.5 ° C/unit aerosol optical depth (AOD) during the winter season (Dec-Jan-Feb, DJF) and ~ -0.5 to -1 ° C/unit AOD during the post-monsoon season (Sep-Oct-Nov, SON) over most of North India. A surprising observation was observed during March to May (MAM), where aerosols induced warming of ~ 0.03 ° C/unit AOD - 0.97 ° C/unit AOD over Northern India.This warming signature’s timing and spatial distribution point to the possibility of absorption by predominantly dust aerosols and the highly reflective nature (surface albedo) of land surface during this period to be the plausible causes. A similar analysis for nighttime reveals an all seasons warming of ~ 1 - 2 ℃/unit AOD during DJF and SON while being in the range of 2-3 ℃/unit AOD over most of India during MAM. The net daily aerosol effect is smaller during DJF and SON than MAM (significant positive aerosol effect) due to the similarity in signs of aerosol forcing during daytime and nighttime (overall addition). Overall, net daily aerosol effect is observed to be in the range of -0.2 - 0.4 ℃ annually over different regions. Aerosols are also observed to reduce the diurnal temperature range (DTR) during all seasons. Annually, 8the DTR varies around -0.7 to -0.4 ℃. Northern India shows higher aerosol effect compared to Southern India. Similar results were obtained using MERRA reanalysis AOD dataset with the temperature having a slightly higher sensitivity to reanalysis AOD than that observed using satellite aerosol information (MODIS). Several sensitivity analyses by potentially varying aerosol loading, temperature and size are also discussed.