Measurement report: Long-term measurements of aerosol precursor concentrations in the Finnish sub-Arctic boreal forest

Abstract. Aerosol particles form in the atmosphere by clustering of certain atmospheric vapors. After growing to larger particles by condensation of low volatile gases, they can affect the Earth’s climate directly by scattering light and indirectly by acting as cloud condensation nuclei. Observations of low-volatility aerosol precursor gases have been reported around the world but longer-term measurement series and any Arctic data sets showing seasonal variation are close to non-existent. In here, we present ~7 months of aerosol precursor gas measurements performed with the nitrate based chemical ionization mass spectrometer (CI-APi-TOF). We deployed our measurements ~150 km North of the Arctic Circle at the continental Finnish sub-Arctic field station, SMEAR I, located in Värriö strict nature reserve. We report concentration measurements of the most common new particle formation related compounds; sulfuric acid (SA), methane sulfonic acid (MSA), iodic acid (IA) and a total concentration of highly oxygenated organic compounds (HOMs). At this remote measurement site, SA is originated both from anthropogenic and biological sources and has a clear diurnal cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle with concentrations peaking in the summer. Of the measured compounds, iodic acid concentrations are the most stable throughout the measurement period, except in April, when the concentration of IA is significantly higher than during the rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and autumn, and on new particle formation event or non-event days. HOMs are abundant during the summer months and low in winter months. Due to the low winter concentrations and their high correlation with ambient air temperature, we suggest that most of HOMs are products of biogenic emissions, most probably monoterpene oxidation products. New particle formation events at SMEAR I happen under relatively low temperatures with a fast temperature rise in the morning followed by decreasing relative humidity during the day. The ozone concentrations are on average ~10 ppbv higher on NPF days than non-event days. During NPF days, we have on average higher SA concentration peaking at noon, higher MSA concentrations in the afternoon and slightly higher IA concentration than during non-event days. All together, these are the first long term measurements of aerosol forming vapors from the SMEAR I in the sub-arctic region, and the results help us to understand atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.