10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Physical, Chemical, and Optical Properties of Aged Smoke Aerosols and Impacts on Boundary Layer Clouds over the South Atlantic during CLARIFY
JONATHAN TAYLOR, HuiHui Wu, Ian Crawford, Keith Bower, James Allan, Dantong Liu, Michael Flynn, Paul Williams, James Dorsey, Tom Choularton, Justin Langridge, Cathryn Fox, Michael I. Cotterell, Nicholas Davies, Kate Szpek, Hugh Coe, Jim Haywood, University of Manchester
Abstract Number: 161 Working Group: Carbonaceous Aerosol
Abstract We present an overview of aerosol and cloud microphysical measurements taken during the CLARIFY campaign during August – September 2017 on the UK FAAM Bae-146 Airborne Research Aircraft. CLARIFY flights targeted the semi-permanent stratocumulus deck that caps the marine boundary layer over the South Atlantic. Biomass burning in central/southern Africa lofts smoke layers up to altitudes of several kilometres during the dry season (June-September). These smoke layers descend as they travel West over the south Atlantic, eventually mixing into the boundary layer and often entraining into the cloud deck. Our observations constitute the most detailed and comprehensive set of measurements of highly aged smoke in this region, and may be used to improve constraint on aerosol and cloud radiative forcing.
28 research sorties were flown out of Ascension Island in the remote South Atlantic. Ascension is uniquely placed to receive a variety of pollution conditions; we will discuss case studies showing aged pollution in lofted layers before and after descent and mixing into the boundary layer, and contrast aerosol and cloud properties with cleaner cases. We will also discuss average and variation in properties such as organic/inorganic aerosol composition, size distribution, and black carbon mixing state in these heavily aged plumes, and the corresponding effects on aerosol optical properties.
Submicron aerosol composition was dominated by organics, which were heavily photochemically aged after several days transport time in the tropics. Inorganic composition showed temporal and vertical variation; boundary layer aerosol was acidic in mid-August and was lacking in nitrate. Later in the campaign nitrate was more prominent and at times increased with altitude. Black carbon was thickly coated throughout the campaign, though calculated coating thicknesses sometimes showed similar vertical variation with nitrate. Absorption enhancements were observed by quantifying the effect of passing particles through a thermodenuder on aerosol absorption.
Cloud properties were found to be very sensitive to the boundary layer aerosol concentrations. In clean conditions, cloud drop number concentrations (CDNCs) as low as 20/cm3 were measured. In polluted conditions these reached as high as several hundred per cubic centimetre. This difference was also accompanied by differences in the median volume diameter and drizzle concentrations, which together show a clear effect of boundary layer aerosol on cloud properties.