Quantitative long-term measurements of VOC concentrations by PTR-MS: annual cycle at a boreal forest site

Abstract. Atmospheric concentrations of volatile organic compounds (VOCs) vary diurnally, seasonally and annually. Therefore continuous long-term measurements are needed to properly describe the atmospheric levels of these chemically reactive compounds. We conducted on-line measurements of atmospheric volume mixing ratios of several VOCs in the European boreal region with a proton transfer reaction – mass spectrometer (PTR-MS) over a 15 month period. The measurements lasted from spring 2006 to fall 2007 and were conducted at SMEAR II station in south-western Finland. The measured compounds included for example methanol, acetone, sum of isoprene and 2-methyl-3-buten-2-ol (MBO), monoterpenes and benzene. The VOC mixing ratios were measured inside and above canopy, during every second or third hour. The PTR-MS was calibrated regularly and background signals of VOCs were measured every second or third hour from the zero air and subtracted from the measured volume mixing ratio. In the continuous measurements at moderate VOC mixing ratios we observed that the PTR-MS has to be calibrated and SEM operation voltage checked regularly, at least twice a month to ensure reliable measurements. Of the measured VOCs, methanol and acetone were the most abundant ones, their volume mixing ratio medians were in the order of 1 ppbv. Volume mixing ratios of methanol, acetone, isoprene-MBO and monoterpenes were high during summer and low in winter indicating mostly biogenic or photochemical local or regional origin. Benzene behaved in the opposite way, while seasonal variation of acetaldehyde and methacrolein-methylvinylketone (MACR-MVK) was less profound. Methanol, acetone, isoprene and monoterpene volume mixing ratios had clear diurnal patterns during summers, while the mixing ratios of other VOCs did not exhibit this behavior. During winter we did not observe systematic diurnal cycles in the VOC volume mixing ratios.