Atmosphere

Studies of the Secondary Organic Aerosol Component of PM2.5 Arising from the NEPM Air Toxic Precursors, Toluene and m-Xylene

Final Report
CSIRO, November 2008

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Executive summary

This Clean Air Research Program (CARP) presents an account of 39 smog chamber experiments performed using toluene and m-xylene vapour. These experiments included a reactive organic carbon (ROC) matrix study comprising ROC/NOx regime/threshold experiments at three NOx concentrations of ~69, ~50, and 34 ppbv and four hydrocarbon concentrations of 89, 63, 34 and 13 ppbv. Five toluene/propene/NOx perturbation experiments and six secondary organic aerosol (SOA) collection experiments were also performed.

A field collection of PM2.5 aerosol was carried out over 3 days at Carlingford, a suburb of Sydney. An additional field sample collected at the NSW DEC Liverpool site by CSIRO Marine and Atmospheric Research (CMAR) was also acquired.

SOA mass concentration yields were observed to vary with time. The observed mass concentrations and yields for SOA formed from toluene ranged from 3.0 to 19.6 µg/m3 and 2.0 to 8.8 %, respectively at a UVA intensity equal to a JNO2 of 0.80 min-1. At a JNO2 of 0.41 min-1, the observed toluene mass concentrations and yields ranged from 8.2 to 30.8 µg/m3 and 5.5 to 11.0 %, respectively. At a JNO2 of 0.41 min-1, m-xylene observed SOA mass concentrations and yields ranged from 0.36 to 34.9 µg/m3 and 0.022 to 10.4%, respectively.

The toluene-SOA and m-xylene-SOA collected on borosilicate glass fibre filters were pale yellow, the colour-saturation of which was dependent upon the initial NOx concentration. It is not known whether the colour is caused by conjugation and/or the presence of organic nitrates. The colour disappears immediately with addition of formic acid to sample extracts.

Toluene-SOA and m-xylene-SOA are readily soluble in water. The NMR results indicate that both forms of SOA resist hydrolysis and are consistent with the view that they are polymeric. The FTIR results indicate that SOA is significantly oxygenated and is likely to incorporate organic bound nitrogen. GC-MS and APCI-MS confirm the presence of oxygenated species such as glyoxal, methylglyoxal, oxalic acid and pyruvic acid.

SOA is likely to form from toluene or m-xylene when they are in such concentration that promotes the oxidisation of NO to NO2 to a concentration below which NO is no longer able to control O3 by titration to form NO2 and O2. Generally this condition occurs under high ROC/NOx ratios. If the absolute hydrocarbon and NOx concentrations increase whilst maintaining a fixed ROC/NOx ratio, then the SOA mass concentration is expected to increase with increasing absolute concentration of ROC and NOx.

There were obvious similarities between the oxygenated-group characteristics of the FTIR spectra of the field sample collected in summer with all smog chamber samples. This was not the case for the winter sample. The FTIR spectra of the winter sample suggested that organic incorporated oxygen was very low. The solubility of the summer sample in water was high whereas the winter sample was low. These comparisons support the view that the summer sample was produced by strong oxidising conditions and that the oxygenated characteristics exhibited by the summer aerosol were produced by secondary reactions.

Cover of Studies of the Secondary Organic Aerosol Component

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