10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effect of Relative Humidity on Light Absorbing Secondary Organic Aerosol Formation
NETHMI KASTHURIARACHCHI, Alex Lee, National University of Singapore
Abstract Number: 258 Working Group: Aerosol Chemistry
Abstract The uptake of water-soluble volatile organic compounds into aqueous aerosol, fog and cloud droplets can result in subsequent aqueous-phase reactions producing secondary organic aerosols (SOA). Glyoxal (Gly), an α-dicarbonyl compound formed by the oxidation of isoprene and aromatic compounds, is extensively studied as a precursor for aqueous SOA. Its reactions with nitrogen containing compounds, both inorganic and organic, have been found to result in the formation of light absorbing aerosols.
Throughout its lifetime, atmospheric aerosols can undergo exposure to different levels of relative humidity (RH). Droplet evaporation at low RH, resulting in a highly concentrated solute environment within the droplet, has been found to accelerate the formation light absorbing compounds. However, studies have shown that the existence of liquid water is highly essential to the production of light absorbing aerosol species, prompting the need for investigating a critical RH level which can effectively facilitate this chemistry.
This study aims to quantitatively describe the effect of RH on the formation of light absorbing aerosols through the reactions of Gly with ammonium sulphate (AS) and Glycine (GL). An initial concentration of 10mM of Gly was mixed with 20mM of AS or 40mM of GL. Each solution was atomized and subsequently dried between 20% and 85% RH. The dried particles were collected on a PTFE filter and extracted with deionised water. Light absorption of aerosol extracts was measured using a UV-vis spectrometer with a liquid waveguide capillary cell. The absorption signals were normalised by the total organic carbon content to obtain the mass absorption coefficient (MAC, m2/g C) of reaction products observed at different RH levels.
Consistent with previous studies, the Gly-AS system gave a single peak at 287nm while the Gly-GL system gave multiple peaks, with the peak at 380nm being the most prominent. The MAC at the peak wavelengths were plotted against the RH for each system. The MAC of reaction products from the Gly-AS system increased from 100% RH (i.e. bulk solution), reaching a maximum at ∼65% RH. A steady decline of MAC was then observed towards the lower RH. A similar trend was observed for the Gly-GL system with a steady increase of MAC up to ∼55% RH followed by a gradual decline towards the lower RH. Formation of light absorbing aerosol species was insignificant at very low RH condition for both systems. In particular, at ~20% RH, the MAC of reaction products were only comparable to those observed in the bulk solution. Results from both chemical systems highlight the significance of the effect of RH on the production of light absorbing species upon droplet evaporation. The potential influence of phase transition on the formation of light absorbing materials will be discussed.