Abstract. Alpha-dicarbonyl compounds are believed to form brown
carbon in the atmosphere via reactions with ammonium sulfate (AS) in cloud
droplets and aqueous aerosol particles. In this work, brown carbon formation
in AS and other aerosol particles was quantified as a function of relative
humidity (RH) during exposure to gas-phase glyoxal (GX) in chamber
experiments. Under dry conditions (RH < 5 %), solid AS,
AS–glycine, and methylammonium sulfate (MeAS) aerosol particles brown within
minutes upon exposure to GX, while sodium sulfate particles do not. When GX
concentrations decline, browning goes away, demonstrating that this dry
browning process is reversible. Declines in aerosol albedo are found to be a
function of [GX]2 and are consistent between AS and AS–glycine
aerosol. Dry methylammonium sulfate aerosol browns 4 times more than dry
AS aerosol, but deliquesced AS aerosol browns much less than dry AS aerosol.
Optical measurements at 405, 450, and 530 nm provide an estimated
Ångstrom absorbance coefficient of -16±4. This coefficient and
the empirical relationship between GX and albedo are used to estimate an
upper limit to global radiative forcing by brown carbon formed by 70 ppt GX
reacting with AS (+7.6×10-5 W m−2). This quantity is
< 1 % of the total radiative forcing by secondary brown carbon
but occurs almost entirely in the ultraviolet range.