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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 (NHX) 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 NHX compared to glyoxal, the brown carbon pathway competes with evaporation only in polluted regions with extreme NHX. 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. 

Reactions in aqueous solutions containing dicarbonyls (especially the α-dicarbonyls methylglyoxal, glyoxal, and biacetyl) and reduced nitrogen (NHx) have been studied extensively. It has been proposed that accretion reactions from dicarbonyls and NHx could be a source of particulate matter and brown carbon in the atmosphere and therefore have direct implications for human health and climate. Other dicarbonyls, such as the 1,4-unsaturated dialdehyde butenedial, are also produced from the atmospheric oxidation of volatile organic compounds, especially aromatics and furans, but their aqueous-phase reactions with NHx have not been characterized. In this work, we determine a pH-dependent mechanism of butenedial reactions in aqueous solutions with NHx that is compared to α-dicarbonyls, in particular the dialdehyde glyoxal. Similar to glyoxal, butenedial is strongly hydrated in aqueous solutions. Butenedial reaction with NHx also produces nitrogen-containing rings and leads to accretion reactions that form brown carbon. Despite glyoxal and butenedial both being dialdehydes, butenedial is observed to have three significant differences in its chemical behavior: (1) as previously shown, butenedial does not substantially form acetal oligomers, (2) the butenedial/OH− reaction leads to light-absorbing compounds, and (3) the butenedial/NHx reaction is fast and first order in the dialdehyde. Building off of a complementary study on butenedial gas-particle partitioning, we suggest that the behavior of other reactive dialdehydes and dicarbonyls may not always be adequately predicted by α-dicarbonyls, even though their dominant functionalities are closely related. The carbon skeleton (e.g., its hydrophobicity, length, and bond structure) also governs the fate and climate-relevant properties of dicarbonyls in the atmosphere. If other dicarbonyls behave like butenedial, their reaction with NHx could constitute a regional source of brown carbon to the atmosphere. 

Abstract. An improved understanding of the fate and properties of atmospheric aerosolparticles requires a detailed process-level understanding of fundamentalfactors influencing the aerosol, including partitioning of aerosolcomponents between the gas and particle phases. Laboratory experiments withlevitated particles provide a way to study fundamental aerosol processesover timescales relevant to the multiday lifetime of atmospheric aerosolparticles, in a controlled environment in which various characteristicsrelevant to atmospheric aerosol can be prepared (e.g., highsurface-to-volume ratio, highly concentrated or supersaturated solutions,changes to relative humidity). In this study, the four-carbon unsaturatedcompound butenedial, a dialdehyde produced by oxidation of aromaticcompounds that undergoes hydration in the presence of water, was used as amodel organic aerosol component to investigate different factors affectinggas–particle partitioning, including the role of lower-volatility“reservoir” species such as hydrates, timescales involved inequilibration between higher- and lower-volatility forms, and the effect ofinorganic salts. The experimental approach was to use a laboratory systemcoupling particle levitation in an electrodynamic balance (EDB) withparticle composition measurement via mass spectrometry (MS). In particular,by fitting measured evaporation rates to a kinetic model, the effectivevapor pressure was determined for butenedial and compared under differentexperimental conditions, including as a function of ambient relativehumidity and the presence of high concentrations of inorganic salts. Even underdry (RH<5 %) conditions, the evaporation rate of butenedial isorders of magnitude lower than what would be expected if butenedial existedpurely as a dialdehyde in the particle, implying an equilibrium stronglyfavoring hydrated forms and the strong preference of certain dialdehydecompounds to remain in a hydrated form even under lower water contentconditions. Butenedial exhibits a salting-out effect in the presence ofsodium chloride and sodium sulfate, in contrast to glyoxal. The outcomes ofthese experiments are also helpful in guiding the design of future EDB-MSexperiments.