Abstract. Traditional yield curve analysis shows that semi-volatile organic compounds are a major component of secondary organic aerosols (SOAs). We investigated the volatility distribution of SOAs from α-pinene ozonolysis using positive electrospray ionization mass analysis and dilution- and heat-induced evaporation measurements. Laboratory chamber experiments were conducted on α-pinene ozonolysis, in the presence and absence of OH scavengers. Among these, we identified not only semi-volatile products, but also less volatile highly oxygenated molecules (HOMs) and dimers. Ozonolysis products were further exposed to OH radicals to check the effects of photochemical aging. HOMs were also formed during OH-initiated photochemical aging. Most HOMs that formed from ozonolysis and photochemical aging had 10 or fewer carbons. SOA particle evaporation after instantaneous dilution was measured at < 1 and ∼ 40 % relative humidity. The volume fraction remaining of SOAs decreased with time and the equilibration timescale was determined to be 24–46 min for SOA evaporation. The experimental results of the equilibration timescale can be explained when the mass accommodation coefficient is assumed to be 0.1, suggesting that the existence of low-volatility materials in SOAs, kinetic inhibition, or some combined effect may affect the equilibration timescale measured in this study.
Abstract. Studies of the volatility distribution of secondary
organic aerosol (SOA) from aromatic compounds are limited compared with SOA
from biogenic monoterpenes. In this study, the volatility distribution was
investigated by composition, heating, and dilution measurements for SOA
formed from the photooxidation of 1,3,5-trimethylbenzene in the presence of
NOx. Composition studies revealed that highly oxygenated monomers
(C9H14Ox, x = 4–7) and dimers (C18H26Ox, x = 8–12) are the major products in SOA particles. Highly oxygenated
molecules (HOMs) with five or more oxygens were formed during photochemical
aging, whereas dimers degraded during photochemical aging. HOMs with five or
more oxygens may be produced from the photooxidation of phenol-type gaseous
products, whereas dimers in the particle phase may be photolyzed to smaller
molecules during photochemical aging. The results of composition, heating,
and dilution measurements showed that fresh SOA that formed from
1,3,5-trimethylbenzene (TMB) photooxidation includes low-volatility compounds with
<1 µg m−3 saturation concentrations, which are
attributed to dimers. Similar results were reported for α-pinene SOA
in previous studies. Low-volatility compounds with <1 µg m−3 saturation concentrations are not included in the
volatility distributions employed in the standard volatility basis-set (VBS) approach. Improvements
in the organic aerosol model will be necessary for the study of anthropogenic
SOA as well as biogenic SOA.
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