Competition of Partitioning and Reaction Controls Brown Carbon Formation from Butenedial in Particles

Organic reactions in atmospheric particles impact human health and climate, such as by the production of brown carbon. Previous work suggests that reactions are faster in particles than in bulk solutions because of higher reactant concentrations and pronounced surface-mediated processes. Additionally, dialdehydes may have accelerated reactions in particles, as has been shown for the glyoxal reaction with ammonium sulfate (AS). Here, we examine the competition between evaporation and reaction of butenedial, a semivolatile dialdehyde, and reduced nitrogen (NH_X) in bulk solutions and levitated particles with mass spectrometry (MS). Pyrrolinone is the major product of butenedial/AS bulk solutions, indicating brown carbon formation via accretion reactions. By contrast, pyrrolinone is completely absent in all MS measurements of comparable levitated particles suspended in a pure N₂ stream. Pyrrolinone is only produced in levitated butenedial particles exposed to gas-phase ammonia, without enhanced reaction kinetics previously observed for glyoxal and other systems. Despite butenedial's large Henry's law constant and fast reaction with NH_X compared to glyoxal, the brown carbon pathway competes with evaporation only in polluted regions with extreme NH_X. Therefore, accurate knowledge of effective volatilities or Henry's law constants for complex aerosol matrices is required when chemistry studied in bulk solutions is extrapolated to atmospheric particles.

Investigation of the role of bicyclic peroxy radicals in the oxidation mechanism of toluene

The products of the primary OH-initiated oxidation of toluene were investigated using the turbulent flow chemical ionization mass spectrometry technique under different oxygen, NO, and initial OH radical concentrations as well as a range of total pressures. The bicyclic peroxy radical intermediate, a key proposed intermediate species in the Master Chemical Mechanism (MCM) for the atmospheric oxidation of toluene, was detected for the first time. The toluene oxidation mechanism was shown to have a strong oxygen concentration dependence, presumably due to the central role of the bicyclic peroxy radical in determining the stable product distribution at atmospheric oxygen concentrations. The results also suggest a potential role for bicyclic peroxy radical + HO(2) reactions at high HO(2)/NO ratios. These reactions are postulated to be a source of the inconsistencies between environmental chamber results and predictions from the MCM.