Photooxidation of alpha-pinene at high relative humidity in the presence of increasing concentrations of NOx

The photooxidation of similar to 1 ppm a-pinene in the presence of increasing concentrations of NO2 was studied in a Teflone chamber at 72-88% relative humidity and 296-304K. The loss of alpha-pinene and formation of gas-phase products were followed using proton-transfer reaction mass spectrometry (PTR-MS). Gas-phase reaction products (and their yields) include formaldehyde (5 +/- 1 %), formic acid (2.5 +/- 1.4%), methanol (0.6 +/- 0.3 %), acetaldehyde (3.9 +/- 1.7%), acetic acid (8.6 +/- 1.9%), acetone (12 +/- 3%), pinonaldchyde (22 +/- 6 %), and pinene oxide (0.9 +/- 0.1 %), There was evidence of organic nitrates;, and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition and density probed using single particle mass spectrometry (SPLAT II). SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and oxygenates having a density of 1.21 +/- 0.02 g cm(-3), 20% larger than often assumed in calculating SOA yields. Three-wavelength light scattering measurements were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon. (c) 2008 Elsevier Ltd. All rights reserved.