Secondary organic aerosol formation from xylenes and mixtures of toluene and xylenes in an atmospheric urban hydrocarbon mixture: Water and particle seed effects (II)

Secondary organic aerosol (SOA) formation from the photooxidation of o-, p-xylene, and toluene with xylene mixtures was investigated in the UNC dual outdoor smog chambers. Experiments were performed with different initial background aerosol concentrations and levels of relative humidity (RH) in the environment of an eleven component mixture of non-SOA-forming dilute urban hydrocarbon mixture, oxides of nitrogen and sunlight. Post-nucleation was observed in most of the experiments in the 14–20 nm range except under the conditions with high background aerosol (>5 μg m−3) and with low o-xylene concentrations (<0.092 ppmv). The SOA yields of o-xylene varied from 0.8% to 6.5% depending on the RH and initial seed concentrations. p-Xylene had a lower SOA yield compared with o-xylene and the yields in experiments with toluene and xylene mixtures ranged from 1.1% to 10.3%. SOA yield was found to be positively correlated with the particle water (H2Op) content. A new condensed aromatic kinetic mechanism employing uptake of organics in H2Op as a key parameter was applied to all the experiments and the simulations showed reasonable fits to the observed data.

► First paper to show that SOA from xylenes is strongly influenced by relative humidity and corresponding particle phase water.
► The SOA from xylenes can be two to three times higher under very humid vs. dry atmospheres.
► Simulations with an new condensed kinetics mechanism of outdoor chamber experiments with xylenes in a dilute non-forming SOA HC mixture showed reasonable predictions of O3, NOx, xylenes, and SOA behavior.