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. The vertical distribution in the physical and chemical properties ofsubmicron aerosol has been characterised across northern India for the firsttime using airborne in situ measurements. This study focusses primarily onthe Indo-Gangetic Plain, a low-lying area in the north of India whichcommonly experiences high aerosol mass concentrations prior to the monsoonseason. Data presented are from the UK Facility for Airborne AtmosphericMeasurements BAe-146 research aircraft that performed flights in the regionduring the 2016 pre-monsoon (11 and 12 June) and monsoon(30 June to 11 July) seasons. Inside the Indo-Gangetic Plain boundary layer, organic matter dominated thesubmicron aerosol mass (43 %) followed by sulfate (29 %), ammonium(14 %), nitrate (7 %) and black carbon (7 %). However, outside theIndo-Gangetic Plain, sulfate was the dominant species, contributing 44 %to the total submicron aerosol mass in the boundary layer, followed byorganic matter (30 %), ammonium (14 %), nitrate (6 %) and black carbon(6 %). Chlorine mass concentrations were negligible throughout thecampaign. Black carbon mass concentrations were higher inside theIndo-Gangetic Plain (2  µ g m −3 ) compared to outside (1  µ g m −3 ).Nitrate appeared to be controlled by thermodynamic processes,with increased mass concentration in conditions of lower temperature andhigher relative humidity. Increased mass and number concentrations wereobserved inside the Indo-Gangetic Plain and the aerosol was more absorbingin this region, whereas outside the Indo-Gangetic Plain the aerosol waslarger in size and more scattered in nature, suggesting greater dustpresence, especially in north-western India. The aerosol composition remainedlargely similar as the monsoon season progressed, but the total aerosol massconcentrations decreased by ∼50  % as the rainfall arrived;the pre-monsoon average total mass concentration was 30  µ g m −3 compared to a monsoon average total mass concentration of 10–20  µ g m −3 .However, this mass concentration decrease was less noteworthy( ∼20  %–30 %) over the Indo-Gangetic Plain, likely due to thestrength of emission sources in this region. Decreases occurred in coarsemode aerosol, with the fine mode fraction increasing with monsoon arrival.In the aerosol vertical profile, inside the Indo-Gangetic Plain during thepre-monsoon, organic aerosol and absorbing aerosol species dominated in thelower atmosphere (  km), with sulfate, dust and other scatteringaerosol species enhanced in an elevated aerosol layer above 1.5 km withmaximum aerosol height ∼6  km. The elevated concentration ofdust at altitudes >1.5  km is a clear indication of dusttransport from the Great Indian Desert, also called the Thar Desert, innorth-western India. As the monsoon progressed into this region, theelevated aerosol layer diminished, the aerosol maximum height reduced to ∼2  km. The dust and sulfate-dominated aerosol layer aloftwas removed upon monsoon arrival, highlighted by an increase in fine modefraction throughout the profile.

atmospheric chemistry and physics

Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons