Journal Article
Atmospheric Chemistry and Physics, vol. 18, iss. 3, pp. 1643-1652, 2018
Authors
Mallory L. Hinks, Julia Montoya-Aguilera, Lucas Ellison, Peng Lin, Alexander Laskin, Julia Laskin, Manabu Shiraiwa, Donald Dabdub, Sergey A. Nizkorodov
Abstract
Abstract. The effect of relative humidity (RH) on the chemical composition of secondary
organic aerosol (SOA) formed from low-NOx toluene oxidation in the
absence of seed particles was investigated. SOA samples were prepared in an
aerosol smog chamber at < 2 % RH and 75 % RH, collected on Teflon
filters, and analyzed with nanospray desorption electrospray ionization
high-resolution mass spectrometry (nano-DESI–HRMS). Measurements revealed a
significant reduction in the fraction of oligomers present in the SOA
generated at 75 % RH compared to SOA generated under dry conditions. In a
separate set of experiments, the particle mass concentrations were measured
with a scanning mobility particle sizer (SMPS) at RHs ranging from < 2 to
90 %. It was found that the particle mass loading decreased by
nearly an order of magnitude when RH increased from < 2 to
75–90 % for low-NOx toluene SOA. The volatility distributions of the
SOA compounds, estimated from the distribution of molecular formulas using
the “molecular corridor” approach, confirmed that low-NOx toluene SOA
became more volatile on average under high-RH conditions. In contrast, the
effect of RH on SOA mass loading was found to be much smaller for
high-NOx toluene SOA. The observed increase in the oligomer fraction and
particle mass loading under dry conditions were attributed to the enhancement of
condensation reactions, which produce water and oligomers from smaller
compounds in low-NOx toluene SOA. The reduction in the fraction of
oligomeric compounds under humid conditions is predicted to partly counteract
the previously observed enhancement in the toluene SOA yield driven by the
aerosol liquid water chemistry in deliquesced inorganic seed particles.
