Aqueous Aerosol Processing of Glyoxal and Methylglyoxal: Recent Measurements of Uptake Coefficients, SOA Production, and Brown Carbon Formation

Methylglyoxal uptake coefficients (gamma) are needed in order to predict the amount of secondary organic aerosol (SOA) and brown carbon formed by heterogeneous processing of methylglyoxal. We compare recent laboratory and theoretical estimates of uptake coefficients for glyoxal and methylglyoxal on various aqueous aerosol surfaces. Measured methylglyoxal uptake coefficients on pre-reacted glycine aerosol particles increased from gamma = 0.0004 to 0.0057 between 72 and 99% RH. At high (>= 95%) RH, the measured uptake coefficients for methylglyoxal are greater than those measured for glyoxal, and four orders of magnitude higher than theoretical estimates for methylglyoxal, likely due to irreversible reactions between methylglyoxal and ammonia or glycine. Both laboratory and theoretical studies of methylglyoxal uptake found similar dependence on RH, however, where gamma increases with RH due to a "salting out" effect. Methylglyoxal uptake measured on cloud droplets was too rapid to allow uptake coefficients to be extracted from the data, but was largely reversible. Recent results have also demonstrated that in evaporating aqueous aerosol particles, brown carbon is formed much more rapidly and is much more resistant to photolytic bleaching than in bulk-phase simulations. Together, these results suggest that SOA and brown carbon formation by methylglyoxal in aqueous aerosol is larger than models currently predict, but SOA formation by methylglyoxal in cloud droplets may be smaller than current predictions.