On the seasonal change of fluxes and the climatology of acetone and methanol at a rural mountain site in California

Methanol and acetone are oxygenated volatile organic compounds (VOCs) with long atmospheric lifetimes. They are transported higher into the troposphere, where they can influence the odd hydrogen budget and thereby ozone chemistry. At our research site near Blodgett Forest Research Station, elevation 1300 m, on the western slope of the Sierra Nevada mountains, we have measured the seasonal cycle of acetone and methanol mixing ratios and fluxes since spring 2000. Owing to their long atmospheric lifetimes and local emissions, methanol and acetone are the most abundant VOCs throughout the seasons, with highest levels in summer (methanol = 60 ppb, acetone = 9 ppb) and lowest levels in winter (methanol = 0.4 ppb, acetone = 0.1 ppb). Both methanol and acetone fluxes were close to zero in the spring, increasing towards the summer and dropping during the fall. Both showed a typical, exponential temperature-dependence as demonstrated for this site before (F_{30°C = 0.35 and 0.8 mg C m^-2 h^-1 for acetone and methanol, respectively). Temperature-normalized fluxes did not show a strong seasonality for either acetone or methanol, however, both were slightly elevated during active leaf growth, acetone fluxes appeared to increase again in the fall, and methanol fluxes increased during a routine thinning procedure in the plantation. Rain events did not seem to increase OVOC deposition, and low-temperature data indicate that the dry deposition velocity was likely less than 0.1 cm s^-1 for acetone and 0.3 cm s^-1 for methanol. Emissions of both OVOCs were correlated with their mixing ratios and the local biogenic VOC MBO. As the mixing ratio correlations continued throughout the winter, it is likely that both OVOCs show net emissions all year round.