AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Effect of Relative Humidity on Secondary Brown Carbon Formation in Evaporating Droplets
NETHMI KASTHURIARACHCHI, Laura-Helena Rivellini, Xi Chen, Yongjie Li, Alex Lee, National University of Singapore
Abstract Number: 227 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosols (SOA) formed via aqueous-phase reactions have been identified as a prominent pathway for brown carbon (BrC) formation. Water-soluble volatile organic compounds such as glyoxal and methylglyoxal produce BrC from reactions with inorganic and organic reduced-nitrogen species in bulk solutions on the order of days. Such reactions can be substantially accelerated upon droplet evaporation, where BrC is formed on the order of seconds. However, despite aerosols being exposed to a wide range of relative humidity (RH) throughout their atmospheric lifetime, BrC formation in response to surrounding RH remains poorly understood.
This is the first study to investigate BrC formation in evaporating droplets and to quantify their mass absorption coefficient (MAC) as a function of RH through reactions of glyoxal and methylglyoxal with ammonium sulfate (AS) or glycine. The general variations of MAC at respective peak wavelengths were similar for all systems; low MAC was observed in the bulk solutions, but MAC steadily increased and peaked at ~55-65% RH and declined towards lower RH (~30%). The initial increase of MAC is attributed to the acceleration of reactions promoted by the highly concentrated solute environment, while further drying may have led to evaporative loss of reactants and reduced mobility of molecules undergoing aqueous reactions. Hygroscopic studies revealed that phase-transition may be related to the observed variation. Furthermore, organo-nitrogen compounds can have substantial contribution to light absorption in these chemical systems. Aerosol mass spectrometric measurements showed vastly different variations in N:C ratio as a function of RH for the AS and glycine systems, indicating the potential difference of their underlying chemistry and/or product volatilities. Overall, our results highlight the significance of investigating BrC formation under various RH conditions. The effects of RH on BrC formation in complex SOA mixtures will be further discussed.