Soot Aerosols from Wheat Stubble Burning Lead to Ice Nucleation and Heavy Rainfall Over Arid Rajasthan, India

In the Indo-Gangetic Plains, extensive crop residue burning (CRB) of wheat stubble is done during the month of May. Soot emitted following CRB, a serious environmental pollutant, affects cloud properties. Though important, it is poorly understood as an ice nucleating particle. During the summer season, on 13 May 2020, an unusually heavy rainfall event occurred over Rajasthan, India, which was studied using ground- and satellite-data. The sun photometer observations of the AERONET station on 12 May 2020 yielded an absorption Ångstorm exponent value of 1.01. This value corresponds to black carbon (BC) and indicates its dominance as a fine aerosol associated with CRB. The HYSPLIT model indicated a downwind trajectory towards Rajasthan from Haryana and Punjab (source of CRB). Atmospheric ageing and oxygenation of BC probably increased the number of hydrophilic surface sites. Thus, soot comprising BC particles got activated into cloud droplets through the heterogeneous nucleation of water vapour. As a result, the prevailing sub-saturation condition with 80% RH led to the wetting of BC particles through the nucleation of water vapour and increased ice concentration in the cloud anvils in the late night of 12 May 2020 and precipitated on 13 May 2020 (up to 58.2 mm). AERONET data highlights that the region had an abundance of BC produced from CRB. Apart from its water affinity, other particle characteristics such as porosity and polarity also augmented ice nucleation.