Kinetics\nand Oligomer Products of the Multiphase Reactions\nof Hydroxyacetone with Atmospheric Amines, Ammonium Sulfate, and Cloud\nProcessing

Hydroxyacetone (HA) is an atmospheric oxidation product\nof isoprene\nand other organic precursors that can form brown carbon (BrC). Measured\nbulk aqueous-phase reaction rates of HA with ammonium sulfate, methylamine,\nand glycine suggest that these reactions cannot compete with aqueous-phase\nhydroxyl radical oxidation. In cloud chamber photooxidation experiments\nwith either gaseous or particulate HA in the presence of the same\nN-containing species, BrC formation was minor, with similar mass absorption\ncoefficients at 365 nm (<0.05 m<sup>2</sup> g<sup>–1</sup>). However, rapid changes observed in aerosol volume and gas-phase\nspecies concentrations suggest that the lack of BrC was not due to\nslow reactivity. Filter-based UHPLC/(+)ESI-HR-QTOFMS analysis revealed\nthat the SOA became heavily oligomerized, with average molecular masses\nof ∼400 amu in all cases. Oligomers contained, on average,\n3.9 HA, 1.5 ammonia, and 1.6 other small aldehydes, including, in\ndescending order of abundance, acetaldehyde, glycolaldehyde, glyoxal,\nand methylglyoxal. PTR-ToF-MS confirmed the production of these aldehydes.\nWe identify C<sub>17</sub>H<sub>26</sub>O<sub>5</sub>, C<sub>10</sub>H<sub>22</sub>O<sub>9</sub>, C<sub>15</sub>H<sub>27</sub>NO<sub>7</sub>, C<sub>17</sub>H<sub>23</sub>NO<sub>5</sub>, and C<sub>18</sub>H<sub>32</sub>N<sub>2</sub>O<sub>9</sub> as potential tracer ions for HA\noligomers. We hypothesize that efficient oligomerization without substantial\nBrC production is due to negligible N-heterocycle (e.g., imidazoles/pyrazines)\nformation. While HA photooxidation is unlikely a significant atmospheric\nBrC source, it may contribute significantly to aqueous SOA formation.