Evidence for pyrazine-based chromophores in cloudwater mimics containing methylglyoxal and ammonium sulfate

Abstract

Abstract. Simulating aqueous brown carbon (aqBrC) formation from small molecule amines and aldehydes in cloud water mimics provides insight into potential humic-like substance (HULIS) contributors and their effect on local and global aerosol radiative forcing. Previous work has shown that these (Maillard type) reactions generate products that are chemically, physically, and optically similar to atmospheric HULIS in many significant ways, including in their complexity. Despite numerous characterization studies, attribution of the intense brown color of many aqBrC systems to specific compounds remains in-5 complete. In this work, we present evidence of novel pyrazine-based chromophores (PBC) in the product mixture of aqueous solutions containing methylglyoxal and ammonium sulfate. PBC observed here include 2,5-dimethyl pyrazine (DMP) and products of methylglyoxal addition to the pyrazine ring. This finding is significant as the literature of Maillard reactions in food chemistry tightly links the formation of pyrazine (and related compounds) to browning in foods. We investigated both the roles of cloud processing (by bulk evaporation) and pH on absorptivity and product distribution in microliter samples to 10 understand the contribution of these PBC to aqBrC properties. In agreement with previous work, we observed elevated absorptivity across the entire UV/visible spectrum following simulated cloud processing as well as higher absorptivity in more basic samples. Absorptivity of the pH 2 sample, following evaporation, exceeded that of the unevaporated pH 9 sample, indicating that cloud processing can overcome the previously observed kinetic barrier imposed on aqBrC formation in acidic conditions. Further, the fraction of pyrazine compounds in the product mixture increased by up to a factor of four in response to drying 15 with a maximum observed contribution of 16% at pH 5. Therefore, cloud processing under more acidic conditions may produce PBC at the expense of imine and imidazole-derived compounds. This finding has implications for further BrC reactivity and degradation pathways.1

In Situ Surface Tension Measurements of Hanging Droplet Methylglyoxal/Ammonium Sulfate Aerosol Mimics under Photooxidative Conditions

“…Upon exposure to UV radiation, however, masses at m / z 83 and 97 are observed to increase dramatically in magnitude when irradiated (150 and 230%, respectively); these peaks are attributable to methylimidazole and dimethylimidazole, both of which have been observed in MG/AS dark-chemistry reaction systems (Table ). ,, While not particularly light-absorbing in the 280 nm region, these aqSOA products notably share the common quality of containing a relatively small, polar overall structure (imidazole) with nonpolar methyl groups. Because this trait is similar to MG itself, which acts as a surfactant even in the absence of other chemistry, this may be sufficient to enable these molecules to partition to gas–droplet surfaces or persist at the interface upon formation.…”

“…4,17−19 These reactions can range from selfoligomerization processes to reactions with inorganic components, such as ammonium ions that are also present within the aqueous phase. 17,20−24 Resulting nitrogen-containing products, such as pyrazine 25 or imidazole 26,27 derivatives, also retain some oligomeric behavior in their own right, continuing to take on additional dicarbonyl units as they continue to age. MG and its dark-chemistry products have been demonstrated to depress surface tension in aerosol mimic solutions, 20,28 while G and its dark-chemistry products were observed to not be surface-active under similar reaction conditions.…”

“…Carbonyl-containing VOCs (CVOCs) are a particularly important class of aqueous SOA (aqSOA) precursors that arise through gas-phase oxidation of biogenic and anthropogenic organic emissions. − These compounds readily partition into and reversibly hydrate in the aqueous phase, leading to high contributions to overall SOA mass in both aqueous aerosols and cloudwater droplets. Furthermore, α-dicarbonyl compounds, such as glyoxal (G) and methylglyoxal (MG), can undergo additional particle-phase processing to form higher weight SOA products, leading to enhanced uptake into aqueous aerosols. ,− These reactions can range from self-oligomerization processes to reactions with inorganic components, such as ammonium ions that are also present within the aqueous phase. ,− Resulting nitrogen-containing products, such as pyrazine or imidazole , derivatives, also retain some oligomeric behavior in their own right, continuing to take on additional dicarbonyl units as they continue to age. MG and its dark-chemistry products have been demonstrated to depress surface tension in aerosol mimic solutions, , while G and its dark-chemistry products were observed to not be surface-active under similar reaction conditions .…”

Formation and photochemical properties of aqueous brown carbon through glyoxal reactions with glycine

“…They found that the MG–AS brown carbon system may affect the atmospheric photochemistry in aerosol particles, but with limited impact on the direct radiative forcing of climate. Hawkins et al 8 provided evidence for the pyrazine-based chromophores in cloud-water mimics containing MG and AS, which had implications in the further clarification of the reactivity and degradation pathways of brown carbon. According to the optical properties and chemical composition of individual brown carbon components produced through the reactions of MG and AS, Lin et al 21 found that the reduced-nitrogen organic compounds formed in the reactions between atmospheric carbonyls and ammonia/amines are important brown carbon chromophores.…”

Nitrogen-containing Secondary Organic Aerosols Formation by Acrolein Reaction with Ammonia/Ammonium

“…Reactions of carbonyl compounds with ammonia or ammonium to form hemiaminals and imines contribute to the formation of light-absorbing SOA in the atmosphere (De Haan et al, 2011;Hawkins et al, 2018;Lee et al, 2013;Powelson et al, 2014). In this work, we found that acrolein, the smallest α,β-unsaturated aldehyde, has the potential to form light-absorbing heterocyclic NOC, as shown in Figure 9.…”