A global perspective on aerosol from low-volatility organic compounds

Abstract

Abstract. Organic aerosol from primary semivolatile and intermediate volatility compounds is estimated using a global chemical transport model. Semivolatile organic compound (SVOC, saturation concentrations between about 0.1 and 104 μg/m3) oxidation is predicted to be a much larger global source of net aerosol production than oxidation of traditional parent hydrocarbons (terpenes, isoprene, and aromatics). Using a prescribed rate constant and reduction in volatility, the yield of aerosol (defined as the net mass of aerosol formed divided by the total mass of the parent hydrocarbon emitted) from SVOCs is predicted to be about 75% on a global, annually averaged basis. Intermediate volatility compound (IVOC, saturation concentrations between about 104 and 106 μg/m3) emissions and oxidation are highly uncertain since they are not typically measured. The use of a naphthalene-like surrogate with different high-NOx and low-NOx parameterizations produces an aerosol yield of about 30% or roughly 5 Tg/yr of aerosol from IVOC oxidation on a global basis. Estimates of the total global organic aerosol source presented here range between 60 and 100 Tg/yr. This range reflects uncertainty in the parameters for SVOC volatility, SVOC oxidation, SVOC emissions, and IVOC emissions, as well as wet deposition. The highest estimates result if SVOC emissions are significantly underestimated (by more than a factor of 2) or if wet deposition of the gas-phase semivolatile species is less effective than previous estimates. Compared to a traditional non-volatile primary organic aerosol model without IVOCs, the global estimate of organic aerosol production is at most roughly 10% higher than previous studies. Additional information is needed to constrain the emissions and treatment of SVOCs and IVOCs, which have traditionally not been included in models. Comparisons to winter organic carbon observations over the US indicate that SVOC emissions are significantly underestimated by the traditional POA inventories. The degree to which IVOC emissions or other parameters are uncertain is unknown.