Brown carbon formation from linked aqueous-phase photooxidation of glyoxal and SO2

Aqueous reaction mixtures containing glyoxal, SO₂, and ambient oxidants from the air are known to form C₁ – C₃ sulfonates and brown carbon (BrC) in the dark at pH as low as 3. Here we explore the effects of sunlight, hydrogen peroxide (a hydroxy radical source), and aerosol phase on this reaction system. In experiments with bulk liquid phase mixtures of glyoxal and sulfite (dissolved SO₂), brown carbon formation is slowed, but not stopped or reversed, by exposure to sunlight. In aerosol experiments at the CESAM chamber, brown carbon was formed from gas-phase glyoxal and sodium sulfite aerosol in the dark, but only after aerosol deliquescence, and not when gas-phase SO₂(g) instead of aerosol-phase sulfite ions was used as the S(IV) source, presumably due to lower aerosol pH in SO₂(g) experiments. Starting with glyoxal(g), SO₂(g), and HOOH(g), aerosol nucleation events were observed, but only if the chamber RH was greater than 45%. The newly-formed particles contained SOA but not BrC. However, if glyoxal(g), SO₂(g), OH radicals (light + HOOH), and aqueous-phase aerosol particles were all present in the chamber, BrC formation was observed. These results provide support for an aqueous-phase brown carbon formation mechanism involving radical-initiated oligomerization of glyoxal – SO₂ sulfonate intermediates. Positive-mode ESI-HR-MS analysis of aerosol filter extracts finds a wide variety of CHO product species with a peak area weighted average of 15 carbon atoms and 4.5 oxygen atoms per molecule.