Nighttime chemistry at a rural site in the Lower Fraser Valley

NO3 was measured at the Sumas Eagle Ridge Site during the Pacific 2001 Air Quality Study. The average maximum levels (30min average) observed on four nights was 34pptv. NO3 at these levels can play a significant role in oxidation of volatile oxidation compounds (VOC), particularly biogenic monoterpenes. N2O5 levels were calculated on two nights presuming that the equilibrium, NO2+NO3⇄N2O5, was maintained. Peak N2O5 levels were 200–300pptv. NO3 and N2O5 were found to contribute 7–9% of the total gaseous NOy on these two nights. Homogeneous and heterogeneous hydrolysis rates of N2O5 were calculated to estimate the nighttime production of gaseous and particulate nitric acid, respectively. Ignoring losses, the overnight total accumulated production of nitric acid was found to be 1.4–2.1μgm-3. In a comparison, it was found that the calculated heterogeneous production of fine particle nitrate can account for most of that which was observed experimentally on the two nights, indicating that this process is significant. Particulate pinonaldehyde measured throughout the study showed a significant night/day enhancement, in contrast to other carbonyls, indicating that NO3 also plays a role in formation of secondary organic aerosols at the site.