Optical and physical properties of aerosols during active fire events occurring in the Indo-Gangetic Plains: Implications for aerosol radiative forcing

The climatological characteristics of biomass burning over the western Indo-Gangetic Plain (IGP) are examined during the post-monsoon (October-November) season from 2001-2018 using the Moderate resolution Imaging Spectrometer (MODIS) fire products. The intensity of the biomass burning are estimated from the fire pixels detected by the MODIS satellites in the form of fire counts, surface brightness temperature, and fire radiative power. Such biomass burning are mostly affected over Lahore and Patiala region of the western IGP. These fire products are examined along with optical and physical properties of aerosols and its radiative forcing estimated from AErosol RObotic NETwork (AERONET) data. Due to non-availability of long-term aerosol data, the study was performed only at Lahore (AERONET) and Patiala (MODIS) of the western IGP region. The current study shows increasing trends of average brightness temperature by 16% year⁻¹ over Patiala and 14% year⁻¹ over Lahore, which enhances the aerosol optical depth (AOD) by 3%–7% year⁻¹ over the study region. The sphericity of the fine-mode aerosols are rising from 23% to 61% during non-fire to fire events and the dominance of fine-mode aerosols are observed during the active fire events. Further, the absorptivity of AOD (at 440 nm) has enhanced from 0.07 to 0.14 during non-fire to fire events. Asymmetric parameter (AS) of fine-mode aerosol increases from 0.65 to 0.68 during non-fire to active fire events. The characteristics of elevated aerosol layers due to the active fire events are clearly distinguished from the non-fire events within 0-2 km above the surface as noticed from high aerosol extinction and attenuated back scatter coefficients, obtained from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Absorbing aerosols associated from biomass-burning contribute to the surface radiative forcing by -173.96 ± 47 W m⁻² and atmospheric forcing by 123.57 ± 41 W m⁻² over Lahore. The impact of the biomass-burning over Lahore has making the atmosphere more warming by 3.50 ± 1.14 K Day⁻¹. The estimated heating rates (HR) in the atmosphere are rising during both fire and non-fire events by 3.79% and 3.44% per year, respectively.