Vertical and horizontal distribution of aerosol chemical composition across Northern India

James Brooks1, H. Coe1, J. D. Allan1,7, W. Morgan1, D. Liu1, J. M. Haywood2,3, E. J. Highwood4, S. S. Babu5, S. K. Satheesh6, A. G. Turner4

(1) Centre for Atmospheric Science, Manchester, UK. (2) Met Office, UK. (3) College of Engineering, Mathematics & Physical Sciences, Exeter, UK. (4) Department of Meteorology, University of Reading, UK. (5) Space Physics Laboratory, Vikram Sarabhai Space Centre, India. (6) Centre for Atmospheric & Oceanic Sciences, Indian Institute of Science, India. (7) National Centre for Atmospheric Science, UK.

Summary

For the first time, a synthesis of India based airborne in-situ measurements of inorganic and organic submicron aerosol properties using a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS) is presented. The vertical distribution of sub-micron aerosol modulates its impact on meteorology and thus the type, magnitude and sign of associated climate forcing. However, aerosol vertical and horizontal distributions are poorly characterised over Northern India. We address these uncertainties via analysis of in-situ aerosol chemical composition data collected by the South-West Asian Aerosol Monsoon Interactions (SWAAMI) field experiment of 2016. The major focus of the study is the vertical and horizontal distribution of sub-micron aerosol chemical composition. Vertical profiles and straight-and-level-runs (SLRs) are derived from a cToF-AMS. Sub-micron aerosol varies dependant on location. Data analysis shows that inside the IGP, organic aerosol (OA) dominates NR-PM1, whereas outside the IGP, sulphate dominates. BC mass concentrations indicate greater values inside the IGP. Nitrate appears to be determined by thermodynamic processes, with increased nitrate mass concentration in conditions of lower temperature and higher relative humidity. The total aerosol mass concentrations decrease as monsoon rainfall arrives over the study regions. NR-PM1 also displays variations in the vertical profiles. Inside the IGP, profiles undergo changes with monsoon progression. During the pre-monsoon, OA dominates in the lower atmosphere (<1.5km) with sulphate dominating in a clear elevated aerosol layer (EAL) above (>1.5km). As the monsoon progresses into the IGP region, this EAL structure diminishes, with the profile decreases in vertical extent, mass concentration and a switch to being dominated by OA. Outside the IGP, the profiles display sulphate aerosol dominating throughout.