Glyoxal secondary organic aerosol chemistry: effects of dilute nitrate and ammonium and support for organic radical–radical oligomer formation

Abstract

Environmental context. Atmospheric waters (clouds, fogs and wet aerosols) are media in which gases can be converted into particulate matter. This work explores aqueous transformations of glyoxal, a water-soluble gas with anthropogenic and biogenic sources. Results provide new evidence in support of previously proposed chemical mechanisms. These mechanisms are beginning to be incorporated into transport models that link emissions to air pollution concentrations and behaviour.Abstract. Glyoxal (GLY) is ubiquitous in the atmosphere and an important aqueous secondary organic aerosol (SOA) precursor. At dilute (cloud-relevant) organic concentrations, OH radical oxidation of GLY has been shown to produce oxalate. GLY has also been used as a surrogate species to gain insight into radical and non-radical reactions in wet aerosols, where organic and inorganic concentrations are very high (in the molar region). The work herein demonstrates, for the first time, that tartarate forms from GLY þ OH . Tartarate is a key product in a previously proposed organic radical-radical reaction mechanism for oligomer formation from GLY oxidation. Previously published model predictions that include this GLY oxidation pathway suggest that oligomers are major products of OH radical oxidation at the high organic concentrations found in wet aerosols. The tartarate measurements herein provide support for this proposed oligomer formation mechanism. This paper also demonstrates, for the first time, that dilute (cloud or fog-relevant) concentrations of inorganic nitrogen (i.e. ammonium and nitrate) have little effect on the GLY þ OH chemistry leading to oxalate formation in clouds. This, and results from previous experiments conducted with acidic sulfate, increase confidence that the currently understood dilute GLY þ OH chemistry can be used to predict GLY SOA formation in clouds and fogs. It should be recognised that organic-inorganic interactions can play an important role in droplet evaporation chemistry and in wet aerosols. The chemistry leading to SOA formation in these environments is complex and remains poorly understood.

Photochemical organonitrate formation in wet aerosols

“…In fact, several key questions remain to be answered. For example, the major sink of nitrate radicals in the aqueous phase is the formation of HNO 3 (Kirkland et al, 2013). However, it should be noted that organonitrates are commonly observed in rainwater (Altieri et al, 2009), clouds (Boone et al, 2015), and wet aerosols (Hodas et al, 2014).…”

R1

“…On the other hand, m / z − 226 is likely to be a Cl − adduct organonitrate. Cl − adducts for organonitrates have been observed (Bouma and Jennings, 1981;Lawrence et al, 2001;Rajapakse et al, 2016;Zhu and Cole, 2000), and MIDAS does not propose realistic organonitrates without Cl − . The uncertainty for the mass of Cl adducts is reasonably low (∼ 50 ppm).…”

Fundamental Heterogeneous Reaction Chemistry Related to Secondary Organic Aerosols (SOA) in the Atmosphere

“…11 (Tan et al, 2009). Acidity and the presence of ammonium, nitrate and sulfate ions have little effect on the formation of these products, which implies the pathways are dominated by radical-radical reactions Kirkland et al, 2013). Thus, TTRD can be formed by the recombination reaction of hydrated glyoxal radicals after H-abstraction from glyoxal dihydrated gem-diol as proposed by Lim et al (2010):…”

“…It has been shown that a cloud-relevant concentration of sulfuric acid (Tan et al, 2009), ammonium sulfate and nitric acid (Kirkland et al, 2013) have little effect on oxalic acid formation. The aqueous medium enables the formation of glyoxal gem-diols whose functional groups are oxidized during reactions with OH, while the C-C bond structure is preserved to allow the formation of oxalic acid in cloud water being different from in the gas phase.…”