Vertical and horizontal distribution of sub-micron aerosol chemical composition and physical characteristics across Northern India, during the pre-monsoon and monsoon seasons

Abstract

<p><strong>Abstract.</strong> The vertical distribution in the physical and chemical properties of submicron aerosol has been characterised across northern India for the first time using airborne in-situ measurements. This study focusses primarily on the Indo-Gangetic Plain, a low-lying area in the north of India which commonly experiences high aerosol mass concentrations prior to the monsoon season. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights in the region during the 2016 pre-monsoon (11<sup>th</sup> and 12<sup>th</sup> June) and monsoon (30<sup>th</sup> June to 11<sup>th</sup> July) seasons.</p> <p> Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the submicron aerosol mass (43 %) followed by sulphate (29 %), ammonium (14 %), nitrate (7 %) and black carbon (7 %). However, outside the Indo-Gangetic Plain, sulphate was the dominant species contributing 44 % to the total submicron aerosol mass in the boundary layer, followed by organic matter (30 %), ammonium (14 %), nitrate (6 %) and black carbon (6 %). Chlorine mass concentrations were negligible throughout the campaign. Black carbon mass concentrations were higher inside the Indo-Gangetic Plain (2 µg/m<sup>3</sup> std) compared to outside (1 µg/m<sup>3</sup> std). Nitrate appeared to be controlled by thermodynamic processes, with increased mass concentration in conditions of lower temperature and higher relative humidity. Increased mass and number concentrations were observed inside the Indo-Gangetic Plain and the aerosol was more absorbing in this region, whereas outside the Indo-Gangetic Plain the aerosol was larger in size and more scattering in nature, suggesting greater dust presence especially in northwest India. The aerosol composition remained largely similar as the monsoon season progressed, but the total aerosol mass concentrations decreased by ~ 50 % as the rainfall arrived; the pre-monsoon average total mass concentration was 30 µg/m<sup>3</sup> std compared to a monsoon average total mass concentration of 10–20 µg/m<sup>3</sup> std. However, this mass concentration decrease was less noteworthy (~ 20–30 %) over the Indo-Gangetic Plain, likely due to the strength of emission sources in this region. Decreases occurred in coarse mode aerosol, with the fine mode fraction increasing with monsoon arrival. In the aerosol vertical profile, inside the Indo-Gangetic Plain during the pre-monsoon, organic aerosol and absorbing aerosol species dominated in the lower atmosphere (< 1.5 km) with sulphate, dust and other scattering aerosol species enhanced in an elevated aerosol layer above 1.5 km with maximum aerosol height ~ 6 km. As the monsoon progressed into this region, the elevated aerosol layer diminished, the aerosol maximum height reduced to ~ 2 km and the total mass concentrations decreased by ~ 50 %. The dust and sulphate-dominated aerosol layer aloft was removed upon monsoon arrival, highlighted by an increase in fine mode fraction throughout the profile.</p>