Influence of aerosol acidity on the formation of secondary organic aerosol from biogenic precursor hydrocarbons

Abstract

Secondary organic carbon (SOC) concentrations in steadystate aerosol were measured in a series of R-pinene/NOx and one series of β-caryophyllene/NOx irradiation experiments. The acidity of the inorganic seed aerosol was varied while the hydrocarbon and NOx concentrations were held constant in each series of experiments. Measurements were made for acidity levels and SOC concentrations much closer to ambient levels than had been previously achieved for α-pinene, while there are no previous measurements for SOC increases due to acidity for β-caryophyllene. The observed enhancement in SOC concentration linearly increases with the measured hydrogen ion concentration in air for each system. For the conditions of these studies, SOC increased by 0.04% per nmol H+ m-3 for α-pinene under two conditions where the organic carbon concentration differed by a factor of 5. For α-pinene, this level of response to acidic aerosol was a factor of 8 lower than was reported by Surratt et al. for similar series of experiments for SOC from the photooxidation of isoprene/NOx mixtures. By contrast, SOC from β-caryophyllene showed an increase of 0.22% per nmol H+ m-3, roughly two-thirds of the response in the isoprene system. Mass fractions for SOC particlephase tracers for α-pinene decreased slightly with increasing aerosol acidity, although remaining within previously stated uncertainties. Below 200 nmol H+ m-3, the mass fraction of β-caryophyllenic acid, the only identified tracer for β-caryophyllene SOC, was constant, although β-caryophyllenic acid showed a substantial decrease for acidities greater than 400 nmol H+m-3. © 2009 American Chemical Society.