Measurement report: Long-term measurements of aerosol precursor concentrations in the Finnish subarctic boreal forest
Authors: Jokinen Tuija , Lehtipalo Katrianne , Thakur Roseline Cutting , Ylivinkka Ilona , Neitola Kimmo , Sarnela Nina , Laitinen Totti , Kulmala Markku , Petäjä Tuukka , Sipilä Mikko 
Published: February 2022

Abstract. Aerosol particles form in the atmosphere via the clustering of certain
atmospheric vapors. After growing into larger particles by the condensation of low-volatility gases, they can affect the Earth's climate by scattering light and acting as cloud condensation nuclei (CCN). 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 nonexistent. Here, we present ∼7 months of
aerosol precursor gas measurements performed with a nitrate-based chemical
ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer. We deployed our measurements
∼150 km north of the Arctic Circle at the SMEAR I (Station for Measuring Ecosystem–Atmosphere Relations) continental Finnish
subarctic field station, located in the Värriö strict nature reserve. We
report concentration measurements of the most common compounds related to new particle formation
(NPF): sulfuric acid (SA), methane sulfonic acid (MSA),
iodic acid (IA) and the total concentration of highly oxygenated organic
molecules (HOMs). At this remote measurement site, SA originates from both
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, IA
concentrations are the most stable throughout the measurement period, except
in April during which time 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 NPF event or non-event
days. HOMs are abundant during the summer months and low in the autumn
months. Due to their low autumn concentrations and high correlation with
ambient air temperature, we suggest that most HOMs are products of
biogenic emissions, most probably monoterpene oxidation products. NPF events
at SMEAR I happen under relatively low-temperature (1–8 ∘C) conditions,
with a fast temperature rise in the early morning hours as well as lower and
decreasing relative humidity (RH, 55 % vs. 80 %) during NPF days
compared with non-event days. NPF days have clearly higher global irradiance
values (∼450 m−2 vs. ∼200 m−2) and
about 10 ppbv higher ozone concentrations than non-event days. During NPF
days, we have, on average, higher SA concentrations, peaking at noon; higher MSA
concentrations in the afternoon; and slightly higher IA concentration than
during non-event days. In summary, these are the first long-term
measurements of aerosol-forming vapors from SMEAR I in the subarctic
region, and the results of this work will help develop an understanding of atmospheric chemical processes
and aerosol formation in the rapidly changing Arctic.