Measurement report: Long-term measurements of aerosol precursor concentrations in the Finnish sub- 1 Arctic boreal forest 2

: 12 Aerosol particles form in the atmosphere by clustering of certain atmospheric vapors. After growing to larger 13 particles by condensation of low volatile gases, they can affect the Earth’s climate directly by scattering light 14 and indirectly by acting as cloud condensation nuclei. Observations of low-volatility aerosol precursor gases 15 have been reported around the world but longer-term measurement series and any Arctic data sets showing 16 seasonal variation are close to non-existent. In here, we present ~7 months of aerosol precursor gas 17 measurements performed with the nitrate based chemical ionization mass spectrometer (CI-APi-TOF). We 18 deployed our measurements ~150 km North of the Arctic Circle at the continental Finnish sub-Arctic field 19 station, SMEAR I, located in Värriö strict nature reserve. We report concentration measurements of the most 20 common new particle formation related compounds; sulfuric acid (SA), methane sulfonic acid (MSA), iodic 21 acid (IA) and a total concentration of highly oxygenated organic compounds (HOMs). At this remote 22 measurement site, SA is originated both from anthropogenic and biological sources and has a clear diurnal 23 cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle with concentrations 24 peaking in the summer. Of the measured compounds, iodic acid concentrations are the most stable throughout 25 the measurement period, except in April, when the concentration of IA is significantly higher than during the 26 rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and 27 autumn, and on new particle formation event or non-event days. HOMs are abundant during the summer 28 months and low in winter months. Due to the low winter concentrations and their high correlation with ambient 29 air temperature, we suggest that most of HOMs are products of biogenic emissions, most probably 30 monoterpene oxidation products. New particle formation events at SMEAR I happen under relatively low 31 temperatures with a fast temperature rise in the morning followed by decreasing relative humidity during the 32 day. The ozone concentrations are on average ~10 ppbv higher on NPF days than non-event days. During NPF 33 days, we have on average higher SA concentration peaking at noon, higher MSA concentrations in the 34 afternoon and slightly higher IA concentration than during non-event days. All together, these are the first long 35 term measurements of aerosol forming vapors from the SMEAR I in the sub-arctic region, and the results help 36 us to understand atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.

Aerosol particles form in the atmosphere by clustering of certain atmospheric vapors. After growing to larger 13 particles by condensation of low volatile gases, they can affect the Earth's climate directly by scattering light 14 and indirectly by acting as cloud condensation nuclei. Observations of low-volatility aerosol precursor gases 15 have been reported around the world but longer-term measurement series and any Arctic data sets showing 16 seasonal variation are close to non-existent. In here, we present ~7 months of aerosol precursor gas 17 measurements performed with the nitrate based chemical ionization mass spectrometer (CI-APi-TOF). We 18 deployed our measurements ~150 km North of the Arctic Circle at the continental Finnish sub-Arctic field 19 station, SMEAR I, located in Värriö strict nature reserve. We report concentration measurements of the most 20 common new particle formation related compounds; sulfuric acid (SA), methane sulfonic acid (MSA), iodic 21 acid (IA) and a total concentration of highly oxygenated organic compounds (HOMs). At this remote 22 measurement site, SA is originated both from anthropogenic and biological sources and has a clear diurnal 23 cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle with concentrations 24 peaking in the summer. Of the measured compounds, iodic acid concentrations are the most stable throughout 25 the measurement period, except in April, when the concentration of IA is significantly higher than during the 26 rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and 27 autumn, and on new particle formation event or non-event days. HOMs are abundant during the summer 28 months and low in winter months. Due to the low winter concentrations and their high correlation with ambient 29 air temperature, we suggest that most of HOMs are products of biogenic emissions, most probably 30 monoterpene oxidation products. New particle formation events at SMEAR I happen under relatively low 31 temperatures with a fast temperature rise in the morning followed by decreasing relative humidity during the 32 day. The ozone concentrations are on average ~10 ppbv higher on NPF days than non-event days. During NPF 33 days, we have on average higher SA concentration peaking at noon, higher MSA concentrations in the 34 afternoon and slightly higher IA concentration than during non-event days. All together, these are the first long 35 term measurements of aerosol forming vapors from the SMEAR I in the sub-arctic region, and the results help 36 us to understand atmospheric chemical processes and aerosol formation in the rapidly changing Arctic. 37

Introduction: 38
The climate of sub-Arctic region is characterized with some of the most extreme temperature variations on 39 Earth. We expect that during the course of the 21 st century, the boreal forest is to experience the largest increase 40 in temperatures of all forest biomes (IPCC, 2013), making it the most vulnerable to climate change. The boreal 41 forest (taiga) covers most of the sub-Arctic and encompasses more than 30% of all forests on Earth, being one 42 of the largest biome in the world (Brandt et al., 2013). The expected rate of changes, may overwhelm the 43 resilience of forest ecosystems and possibly lead to significant biome-level changes (Reyer et al., 2015). The 44 forest-atmosphere systems are closely interlinked to one another. The forest stores carbon and water in the 45 https://doi.org/10.5194/acp-2021-735 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License.
peat, soil and as biomass while at the same time vegetation emits volatile organic compounds (VOC) into the 46 atmosphere (Bradshaw and Warkentin, 2015). In the Arctic, summer is short, but solar radiation is abundant 47 and extends the daylight hours all the way to midnight and beyond. On the other hand, during the polar night 48 air pollutants accumulate in the atmosphere due to cold and stable atmosphere, while turbulent mixing is 49 inhibited, and the lack of removal processes lead to the formation of Arctic haze (Stohl, 2006). These features 50 make the Arctic an interesting study region for photochemistry of reduced atmospheric compounds. Oxidation 51 processes that dominantly occur in the summer time control the processes removing VOCs and other traces 52 gases, such as SO2 and NOx, from the atmosphere in the Arctic. Detailed understanding of atmospheric 53 processes leading to aerosol precursor formation and gas-to-particle conversion and their role in feedback 54 mechanisms help in assessing the future climate. 55 Aerosol and trace gas measurements in the sub-Arctic field station SMEAR I, go back to the 90s (Ahonen et 56 al., 1997;Kulmala et al., 1998;Mäkelä et al., 1997). Trace gas and aerosol measurements at SMEAR I started 57 in 1992 making them one of the longest continuous measurements of aerosol particle number and size 58 distributions in the Arctic (Ruuskanen et al., 2003). These long-term measurements show that aerosol particles 59 regularly form and grow from very small sizes (< 8 nm diameter) with the highest frequency in the spring, 60 between March and May (Dal Maso et al., 2007;Vehkamäki et al., 2004). It is suggested, that spring promotes 61 new particle formation (NPF) because of the awakening of biological processes after the winter. At SMEAR I 62 the snow only melts away in May-June and thus, many biological processes (photosynthesis) activate while 63 the snow is still deep. This makes the Arctic spring a very complex environment for atmospheric chemistry 64 with possible emission sources from melting snow, ice, melt water, vegetation and transport from other areas. 65 At SMEAR I, most of the observed NPF events are either connected to clean air arriving from the Northern 66 sector (originating from The Arctic Ocean and transported over boreal forest, Dal Maso et al., 2007) or the 67 polluted air masses from the Eastern sector (Kyrö et al., 2014;Sipilä et al., 2021). Annually, around 30-60 68 NPF events are recorded at SMEAR I, of which around half could be initiated by anthropogenic air pollutants 69 from the Kola Peninsula (Kyrö et al., 2014;Pirjola et al., 1998;Sipilä et al., 2021) leaving half of the events 70 occurring from natural sources. The trend of NPF occurrence in Värriö is decreasing, as the anthropogenic 71 sulfur dioxide emissions are decreasing in Russia (Kyrö et al., 2014). 72 Formation and growth of new particles at SMEAR I usually happen during daylight, highlighting the 73 importance of photochemical activities. However, unlike most other locations, NPF is also observed during 74 nighttime or polar night (Kyrö et al., 2014;Vehkamäki et al., 2004). Formation and growth processes of 75 aerosols seem not to be correlated with each other at SMEAR I (Vehkamäki et al., 2004). Earlier literature 76 reports, that the formation rate (J) has no clear seasonal trend, while the growth rates (GR) of small particles 77 clearly peak during summer (Ruuskanen et al., 2007). This indicates that different chemistry drives the initial 78 cluster formation and the subsequent growth processes. From the observed nucleation rates it has been 79 proposed that NPF at SMEAR I could be due to sulfuric acid -ammonia (-water) nucleation (Napari et al., 80 2002) likely dominated by ion-induced channel at least during winter months (Sipilä et al., 2021). Kyrö et al.,81 2014 concludes that 20-50% of the condensational growth can also be explained by sulfuric acid in Värriö.

82
Other studies speculate about the possibility of different organic compounds participating in NPF in the sub-83 Arctic. Tunved et al., 2006 studied the air masses arriving to SMEAR I station and concluded that the aerosol 84 mass increased linearly with time that the air masses travelled over land. The concentration of condensing 85 gases over the boreal forest was concluded to be high and most likely consisting mainly of oxidation products 86 of terpenes (VOCs) that are emitted by the forest. At SMEAR II station in Hyytiälä, approximately 700 km 87 South-West of Värriö, oxidized organics mostly explain the growth of newly formed particles (Bianchi et al., 88 2017;Ehn et al., 2014). However, direct measurements of the aerosol forming and growing vapor species are 89 still lacking from SMEAR I except during wintertime without biogenic activity when sulfuric acid has been 90 shown to be primarily responsible on formation and growth (Sipilä et al., 2021). In Värriö, the role of NPF is 91 critical in forming of cloud condensation nuclei (CCN), since measurements show that the number of CCN 92 can increase up to 800 % as a result of NPF (Kerminen et al., 2012). In other locations in the boreal forest and 93 Arctic, some measurements shed light into the possible chemical components that could be forming particles 94 in Värriö. Currently, the closest continuous measurements with the nitrate based CI-APi-TOF are conducted 95 in Hyytiälä at the SMEAR II-station (Jokinen et al., 2012(Jokinen et al., , 2017Kulmala et al., 2013). In Hyytiälä there is 96 direct evidence on the key role of the photochemical production of sulfuric acid and HOMs maintaining 97 atmospheric NPF (Bianchi et al., 2017;Ehn et al., 2014;Jokinen et al., 2017;Kulmala et al., 2013). 98 Other chemical composition measurements of aerosol precursors have been conducted only in a few locations 99 in the High-Arctic and over the Arctic Ocean (Baccarini et al., 2020;Beck et al., 2021;He et al., 2021;Sipilä 100 et al., 2016). These studies show that in the Arctic, the marginal ice zone and the coast of the Arctic Ocean is 101 a source of atmospheric iodic acid that is efficiently forming new particles. Sulfuric acid and MSA 102 concentrations were also reported (Beck et al., 2021), but they were much lower in concentration than iodic 103 acid (Baccarini et al., 2020). However, the chemistry behind NPF is not that simple, even the pristine Arctic 104 air. The clean air above the Arctic Ocean is abundant in dimethyl sulfide (DMS) emitted by phytoplankton, 105 that rapidly oxidizes into sulfuric acid and MSA on sunny days and consequently forms cloud condensation 106 nuclei (Charlson et al., 1987;Park et al., 2018). Beck et al., (2021) report, that in Svalbard in the Arctic Ocean, 107 sulfuric acid and methane sulfonic acid contribute to the formation of secondary aerosol. They also observed 108 that these compounds formed particles large enough to contribute to some extent to cloud condensation nuclei 109 (CCN). This is supported by measurements of aerosol chemical composition from the Arctic that commonly 110 report MSA in particulate matter (Dall Osto et al., 2018;Kerminen et al., 1997). According to Beck et al.

111
(2021) the initial aerosol formation in the high Arctic occurs via ion-induced nucleation of sulfuric acid and 112 ammonia and subsequent growth by mainly sulfuric acid and MSA condensation during springtime and highly 113 oxygenated organic molecules (HOM) during summertime. By contrast, in an ice-covered region around 114 Villum, Greenland, Beck et al. (2021) observed new particle formation driven by iodic acid, but the particles 115 remained small and did not grow to CCN sizes due to insufficient concentration of condensing vapors. Since 116 the Arctic CCN number concentrations are low in general, formation of new particles is a very sensitive process 117 affecting the composition of the aerosol population and CCN numbers in the area. Also in Värriö, the role of 118 NPF is critical in forming of cloud condensation nuclei (CCN), since measurements show that the number of 119 CCN can increase up to 800 % as a result of NPF (Kerminen et al., 2012). 120 In this article, we present the measurements of aerosol precursor molecules from the continental SMEAR I 121 station, ~150 km North of the Arctic Circle and ~150 km from the Arctic Ocean. We measured sulfuric acid, 122 methane sulfonic acid, iodic acid and highly oxygenated organic compound concentrations with a sulfuric acid 123 calibrated CI-APi-TOF (Jokinen et al., 2012;Kürten et al., 2012) to determine their levels in the sub-Arctic 124 boreal forest and to understand whether these species are connected with the aerosol formation process in the 125 area. 126

Methods, measurement site and instrumentation: 127
The core of this work is measurements of gas phase aerosol precursors. We use the nitrate chemical ionization 128 atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-ToF) that has been operational at the 129 SMEAR I-station (N67°46, E29°36) in Eastern-Lapland since the early spring of 2019. SMEAR stands for 130 Station for Measuring Ecosystem -Atmosphere Relations. Measurements were done on top of Kotovaara hill 131 (390 m a.s.l.), close to ground level in an air-conditioned small log wood cottage. The cottage is surrounded 132 by ~65-year-old Scotts pine forest. More details about the station can be found in earlier publications (Hari et 133 al., 1994;Kyrö et al., 2014). The mass spectrometric measurements are designed to start a long-term 134 measurement series of atmospheric aerosol forming trace gases in the Finnish Lapland and the measurements 135 are ongoing to this day. We measure e.g. sulfuric acid, iodic acid, highly oxygenated organic molecules and 136 methane sulfonic acid with high time resolution and precision. The measurements are running in Finnish winter 137 time (UTC+2) throughout the year. 138 We calibrated the CI-APi-TOF twice during the measurement period and run the instrument with the same 139 settings for the whole measurement period reported in this paper. We calibrated the instrument using a sulfuric 140 acid calibrator described in Kürten et al., 2012. The calibration factor from the two separate calibrations were 141 1) 7 · 10 9 and 2) 8 · 10 9 and we use the average 7.5 · 10 9 in our study calculate the concentrations of all reported 142 compounds. This factor includes the loss parameter due to the ~1 m long unheated inlet tube (3/4" stainless 143 https://doi.org/10.5194/acp-2021-735 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. steel). HOMs and iodic acid have been estimated to be charged similarly at the kinetic limit as sulfuric acid 144 (Ehn et al., 2014;Sipilä et al., 2016), so the calibration factor for them should be similar, but please note, that 145 the concentration of other compounds than SA can be highly uncertain due to different ionizing efficiencies, 146 sensitivities and other unknown uncertainties. The sulfuric acid, iodic acid and MSA data presented in this 147 study are all results of high-resolution peak fitting of the CI-APi-TOF, in order to avoid inaccurate 148 identification of compounds and to separate overlapping peaks. The HOM data is a sum of mass-to-charge 149 ratios from 300 to 400 Th, representing the monomer HOM range (C10 compound range), 401 to 500 Th for 150 the slightly larger HOMs (C15 compound range) and 501 to 600 Th for the dimer species (C20 compound range).

151
We also give the sum of these all (from 300 to 600 Th). The goal of this article is not to specify different HOM 152 compounds or to study NPF in mechanistic details but to give an overview of general seasonal trends and 153 variations of these selected species. Note that since this is a sum of all peaks in the selected mass range, we 154 cannot assure that all the compounds included are HOMs. However, the investigation in laboratory conditions 155 show that the nitrate-CI-APi-TOF is highly selective and sensitive towards HOMs with O > 5 ( Riva et al.,156 2019) and with hydroperoxide (-OOH) functionalities (Hyttinen et al., 2015). All data obtained from the CI-157 APi-TOF we analyzed using tofTools program described in (Junninen et al., 2010) and averaged over an hour.

158
The original data time resolution is 5 sec. The uncertainty range of the measured concentrations reported in 159 this study is estimated to be −50%/+100% and the limit of detection, LOD 4·10 4 molecules cm −3 (Jokinen et 160 al., 2012). 161 To classify NPF events recorded during the measurement period, we used the data measured by a Differential 162 Mobility Particle Sizer (DMPS). The DMPS instrument and earlier statistics of NPF events in Värriö has been 163 documented by (Dal Maso et al., 2007;Vana et al., 2016;Vehkamäki et al., 2004). The NPF events were 164 classified according to (Dal Maso et al., 2005). uninterruptedly. Only a few short power cuts stopped our measurements during this time. Iodic acid data is 177 missing from late July since its peak could not be separated well enough from overlapping peaks in the spectra 178 during this time. This was due to poor resolution (low signal of IO3close to another peak) that makes peak 179 integration to give negative, unreal values and we thus decided to flag them out. After late October, the 180 instrument malfunctioned and stopped our measurements. In this particular article, we present data from spring 181  concentrations at SMEAR I in April to October 2019. All data in panels A-C are resulting from high-resolution 187 peak fitting. HOM data are sums of certain mass ranges; from 300 to 400 Th in green, representing C10 or 188 HOM monomer compounds, from 401 to 500 Th in red, representing C15 compound and from 501 to 600 Th 189 on light blue, representing C20 or HOM dimer compounds. The sum of HOM (blue) is a sum of the 190 aforementioned mass ranges. The sum of HOMs is approximately one order of magnitude higher than SA, 191 MSA or IA concentrations during this measurement period. 192 In Figure 2 we show some of the most interesting environmental and meteorological parameters that influence 193 the atmospheric gas composition during the measurements period; temperature, global radiation and snow 194 depth ( Figure 2A). There are some special features in year 2019; the summer had two heat waves, when the 195 air temperature rose up to 29.2 °C in early June and to almost the same values in late July. These episodes are 196 getting more common in Lapland due to climate change. These warmer conditions will probably change the 197 emissions of trace gases including the composition and abundance of aerosol precursors in the future Arctic 198 environment (Schmale et al., 2021). The springtime diurnal solar cycle is clearly visible with all studied compounds. All measured aerosol 213 precursor compounds are abundant even during the period when snow covers the ground in the spring. The 214 HOM concentrations follow the increasing solar radiation and rising temperature. MSA has a stronger diurnal 215 cycle before the snow melt than after it. This may be due to rain and cloudy conditions that are more common 216 in the summer. Sulfuric acid and iodic acid do not have such strong seasonal variation than HOMs and MSA. 217 The aerosol precursor concentrations are discussed in more detail in the following sections. 218 Seasonal and monthly variation of SA, MSA, iodic acid and HOM concentrations 219 We present the diurnal variation of aerosol precursors; sulfuric acid, methane sulfonic acid, iodic acid and 220 highly oxygenated molecule, concentrations separately for different seasons in Figure 3. Strong seasonality is 221 most evident in sulfuric acid and HOM concentrations. SA is at its highest in the spring, decreasing toward 222 summer and autumn while HOMs reach their maximum in the summer. The increase in HOMs in the summer 223 at SMEAR I is linked to the increased emissions of VOCs from vegetation that oxidize into HOMs via 224 ozonolysis (Ehn et al., 2014) and OH-radical reactions (Berndt et al., 2016;Jokinen et al., 2014Jokinen et al., , 2017Wang 225 et al., 2018). The overall lowest aerosol precursor concentrations we detect during autumn (winter data was 226 missing from this study, see Sipilä et al. 2021, for winter time observations made promptly after the period 227 reported here). MSA shows very similar concentrations during spring and summer, and drops down to the limit 228 of detection level for autumn. Iodic acid acts very differently than the other compounds. We observe iodic acid 229 to have a similar level of concentration throughout the measurement period and the concentration almost seem 230 to "saturate" during daylight hours. This daytime maximum stays at the same level about 5 hours longer during 231 spring than in the autumn. The day length getting shorter towards the autumn explains this behavior. The 232 maximum hourly median concentrations for the measured compounds are ~2 · 10 6 cm -3 for SA (spring), ~5 · 233 10 5 cm -3 for MSA (summer), ~3· 10 5 cm -3 for iodic acid (all seasons) and ~5 · 10 7 cm -3 for the sum of HOMs 234 (summer, mass range from 300 to 600 Th). 235 We can compare these numbers to SMEAR II long-term (5-year median concentration) observations, were the 236 median peak SA concentrations are ~1.5 · 10 6 cm -3 , ~1 · 10 6 cm -3 and ~3· 10 5 cm -3 for spring, summer and 237 autumn, respectively (Sulo et al., 2021). These measured concentrations are very similar to SMEAR I 238 observations except a slightly higher summer and autumn SA concentration at SMEAR II. However, it should 239 be noted that the springtime measurements from SMEAR I do not include March data, which makes the 240 springtime comparison uncertain. There is also a difference in the timing of the peak SA concentration in the 241 summer. At SMEAR I the peak concentration is reached at noon and at SMEAR II it can be found some hours 242 https://doi.org/10.5194/acp-2021-735 Preprint. a maximum around noon, spanning to the afternoon (Figure 3). At SMEAR II, HOMs have two maxima, one 249 at noon and another one in the early evening. From these, the latter is connected to non-nitrate monomer and 250 dimer HOMs and nitrate dimer HOMs. At SMEAR I the lack of an evening maximum could indicate that 251 HOM dimer formation is less dominant at SMEAR I compared to SMEAR II due to lower air temperatures, 252 or due to the different diurnal cycle of oxidants due to longer hours of solar radiation North of the Arctic Circle. When analyzing the monthly aerosol precursor profiles in Figure 4, we observe that the springtime atmosphere 258 is abundant in SA and iodic acid that have the highest median concentrations in April. MSA and HOMs 259 concentrations peak in June. The MSA behavior is likely connected to the algae blooms in the Arctic Ocean 260 that peak around midsummer. The marine emissions of DMS oxidize in the atmosphere to sulfur dioxide, 261 sulfuric acid and to MSA (e.g. Park et al., 2018). However, sulfuric acid has more sources, since SO2, has also 262 anthropogenic sources. At SMEAR I we cannot distinguish these sources precisely (more discussion about this 263 in section 3.3.). It is notable that the peak concentration of MSA is earlier in the day in April, around 12 o'clock 264 noon, than it is later in the year when the peak concentration is reached in the late afternoon (from 13:00 to 265 18:00 o'clock). There are no previous MSA concentration reports from the SMEAR stations but some gas 266 phase MSA results from Antarctica show maximum of 1 · 10 5 cm -3 to 1 · 10 7 cm -3 concentrations (Mauldin et 267 al., 2004, Mauldin et al., 2010, Jokinen et al., 2018. In the Arctic, around half a year measurement series from 268 Villum in Greenland show MSA concentrations <10 6 cm -3 (Mar -Sep) and from10 5 cm -3 to 10 7 cm -3 with the  al., 2019) due to the fact that the ocean surface is a major source of iodine (Carpenter et al., 2013). While it is 279 not precisely known how iodic acid forms in the gas phase, its formation requires oxidation of the initial 280 precursors (IOx species) by ozone and the last steps of its formation is potentially driven by reaction with OH 281 (Chameides and Davis, 1980). 282 Compared to the other precursor compounds, iodic acid has the most stable concentration between seasons, 283 with a long increasing period in April during the snow-melting season. This is likely due to the simultaneously 284 increasing ozone concentrations ( Fig 2B) and solar radiation. In contrast to measurements from the Arctic 285 Ocean (Baccarini et al., 2020), we did not observe a clear increase in iodic acid concentration in the autumn 286 due to freezing. We find that September had only marginally higher concentrations compared to August or 287 July (Fig 4). Winter measurements would be necessary to estimate the effect of freezing in the concentration 288 of IA. 289 The source of iodic acid on a continental site like the SMEAR I is an interesting subject to speculate. The 290 observed HIO3 peak in April could indicate that there could be an influence from air masses exposed to Arctic 291 marine environment, due to ocean surface acting as a major source of atmospheric iodine (Carpenter et al., 292 2013). The increasing temperature in the spring induce a higher activity of phytoplankton in the nearby Barents 293 Sea and Norwegian Sea that remains ice free, even during the winter, and could result in the higher emission 294 of precursors for iodic acid (Lai et al., 2011). Higher temperature would also result in more efficient advection, 295 which would transport species faster from emission points to SMEAR I. The calculated back trajectories 296 https://doi.org/10.5194/acp-2021-735 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. support the idea that iodine-rich air masses arrive from the West or northwest to SMEAR I (discussed in details 297 in section 3.3. New Particle Formation evens and Figure 10). This would be the hypothesis of the long-range 298 transport for source of iodic acid in SMEAR I. On the contrary, the strong diurnal variation on iodic acid 299 concentration seen as one order of magnitude difference between noon and midnight, suggests fast on site 300 chemistry, which is not consistent with long-range transport. Also iodic acid life time against condensational 301 loss is expected to be short, in the range of an ~hour, this suggests that the source of HIO3 is close to or at the 302 site of measurements. Land vegetation is a source of methyl iodide (CH3I) that could be the source of iodic 303 acid at SMEAR I, at least during summer (Sive et al., 2007). 304 Most interestingly, we seem to have an emission source of iodine during all seasons. There are no reports on 305 iodine emissions from continental snow, but we hypothesize that one possible source of iodine in SMEAR I 306 during spring is the melting snowpack. This is possible due to the deposition of sea salts on snow particularly 307 during dark periods that activate during the spring and are re-emitted to the atmosphere through heterogeneous 308 photochemistry of iodide, and iodate ions (Raso et al., 2017;Spolaor et al., 2019). There are also possible 309 forest emissions of iodinated organics, similar to New England growing season (Raso et al., 2017), that might 310 be enhanced by higher temperature or high ozone concentrations. This type of emissions of iodinated gases, 311 or their implications, have not been studied before but these observations might direct research into emission 312 studies at SMEAR I, since our findings indicate that vegetation could be an emission source of iodine. 313 The sum of HOMs in SMEAR I reaches up to a median ~5 · 10 7 cm -3 concentration in the summer. This is 314 about one order of magnitude lower than the concentrations reported from the SMEAR II station in Hyytiälä 315 (Yan et al., 2016), about 700 km south, where HOMs are at a maximum of ~6 · 10 8 cm -3 during spring daytime. 316 It is striking how well the concentration of HOMs follow the air temperature ( Figure 5). From the temperature 317 dependency, we can speculate that most VOCs emitted by vegetation close to Värriö could be monoterpenes 318 due to their strong temperature dependency. This is supported by emission rate measurements of VOCs 319 showing that in northern Finland 60 to 85 % are accounted by α-and β-pinene emissions (Tarvainen et al., 320 2004). However, sesquiterpene emissions from nearby wetlands could contribute to HOMs since their 321 emissions are also temperature dependent and they are emitted by the boreal wetlands (Hellén et al., 2020;322 Seco et al., 2020). As HOMs are oxidation products of VOCs, it is evident that the HOM concentration will 323 increase in SMEAR I in the future with the increasing VOC emissions, including isoprene, monoterpenes and 324 sesquiterpenes, due to temperature rise (Ghirardo et al., 2020;Tiiva et al., 2008;Valolahti et al., 2015).

New particle formation events; 331
During the measurement period from 4 April 2019 to 27 October 2019, we observed 36 regional NPF events 332 in total and our CI-APi-TOF data covers 33 of these NPF days. During the same period, we observed 75 non-333 event days without clear signs of particle formation (Dal Maso et al., 2005). Rest of the days during our 334 measurement period were defined as undefined, bad data or partly bad data days and these were excluded from 335 our analysis. In this chapter, we focus on trace gases, meteorological parameters and aerosol precursor detected 336 gases during NPF days and compare them to non-event days. 337 We plot NPF and non-event days median average number size distribution of aerosol particles (from 3 to 800 338 nm) in Figure 6, and the total number concentration and the 2-7 nm air ion concentrations in Figure 7. In figure  339 6, in the case of NPF event days we see a distinct "banana" plot, where small < 10 nm, particles are forming 340 and growing with time. The DMPS data is plotted from 2.82 nm to 708 nm but note that the channels below 341 ~5 nm have much larger uncertainties than those above. The median event start time is located around noon 342 and the growth of particles continues steadily until midnight. However, when looking at individual days, there 343 is a large variation in the start-times of the particle formation, some events start early in the morning or even 344 in the night, while some start in late afternoon. Non-event days show very few particles in the < 10 nm size 345 bins.  SMEAR I during NPF event (red, n = 33) and non-event days (black, n = 75). The total particle number 354 concentration is recorded with a CPC and air ion concentrations with a NAIS. 355 The total number of particles measured at the site during NPF events rises up to ~2400 cm -3 reaching the 356 maximum concentration at ~17 o'clock in the evening. This shows that NPF is an important source of aerosol 357 particles in Värriö as previously reported (Vehkamäki et al., 2004). Non-event days have clearly lower particle 358 concentrations throughout the day, staying lower than 1000 cm -3 on average. The measured 2-7 nm anion 359 concentrations stay very low during non-event days. As intermediate ions form mainly during NPF, their 360 concentrations are used as indicator of NPF events in boreal environments (Leino et al., 2016). On NPF days, 361 we see a peak in the anion concentration at noon, the concentration being about six times higher than during 362 non-event days. This indicates that negative ions may play a role in SMEAR I particle formation events. 363 Figure 8 shows the differences in temperature, relative humidity, global radiation and ozone concentration 364 between NPF event days (in red) and non-event days (black). In Värriö, NPF events preferably happen in 365 relatively low temperatures with a fast temperature rise in the early morning hours, lower and decreasing RH 366 during the NPF days compared to non-event days. NPF days have clearly higher global irradiance values and 367 about 10 ppbv higher ozone concentrations than non-event days. The meteorological conditions favoring NPF 368 are thus similar than at the SMEAR II station in Hyytiälä, where sunny clear sky days with low RH and 369 condensation sink along with wind directions from the cleaner northerly sector are forecasting NPF events 370 (Nieminen et al., 2014). and ozone concentration (ppbv) in D), all measured at SMEAR I during NPF event (red, n = 33) and non-event 374 days (black, n = 75). 375 Next, we show the concentrations of aerosol precursor compounds during NPF and non-event days in figure  376 9. (Kulmala et al., 2013)The sulfuric acid concentrations closely follow the solar irradiation profile (Figure 8). Similarly to the results obtained from the high Arctic, Svalbard, also MSA is elevated during NPF events, 378 especially during summer, and could possibly contribute aerosol growth (Beck et al., 2021). We observe close 379 to an order of magnitude higher MSA concentration between the events and non-events days, highlighting the 380 dominant role of sulfur species to nucleation and growth in general at this site. In order to attribute the source 381 of sulfur species and IA during the event and non-event days we performed a cluster analysis using a 382 geographical information system (GIS) based software, Trajstat (Wang et al., 2009). The NCEP/NCAR 383 reanalysis data was used as meteorological input for the model (Kalnay et al., 1996). The simulations were 384 performed at an arrival height of 250 m. a.g.l. SMEAR I station is located approximately at similar height (390 385 m a.s.l), thus representing the air masses arriving at the station even during strong temperature inversions 386 (Sipilä et al., 2021). 387 388 Figure 9. Aerosol precursor gases in SMEAR I during NPF (red, n = 33) and non-event days (black, n = 75).

389
The data is hourly median average. 390 Higher concentrations of aerosol precursors SA, MSA and IA are connected to the air masses that arrive to 391 SMEAR I from the Arctic Ocean ( Figure 10). Cluster analysis of air mass back trajectories arriving to Värriö 392 during NPF days clearly shows, that most NPF events occur when the air mass was exposed to marine 393 environments within the last 72 hours. In our case, mainly the Norwegian Sea in the West (58 %) or the Barents 394 (21 %) and Kara Seas (21 %) in the Arctic Ocean. This seems relevant to our results since the marine 395 environment in the North is emitting large amounts of dimethyl sulfide (DMS), a precursor for SA and MSA 396 (Levasseur, 2013) and iodine species that further oxidize to IA (Baccarini et al., 2020;Sherwen et al., 2016).

397
A fraction of air masses that are connected to both NPF (21 %) and non-event days (37 %) are coming to 398 SMEAR I from the Kola peninsula that is connected to high SO2 emissions, higher particle number 399 concentrations and winter time NPF events (Sipilä et al., 2021). Most non-event air masses arrive to Värriö 400 from South-West (49 %) crossing northern Finland and Sweden. 401 In addition from Figure 9 we observe that we cannot rule out the contribution of iodic acid in NPF in SMEAR 402 I, but with the recorded concentration, it usually is not enough to initiate NPF (He et al., 2021). Although iodic 403 acid concentrations are slightly larger on NPF days than non-event days, the rise in concentration happens 404 already early in the morning, clearly before the average event start-time. The possible source of iodic acid was 405 https://doi.org/10.5194/acp-2021-735 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. discussed earlier in chapter 3.2 and we hypothesize that the source of iodine at SMEAR I could be both; i) the 406 long distance transport from the Arctic Ocean combined to ii) the local emissions from the snow pack and 407 vegetation. The hypothesis of vegetation emitted iodine species is supported by the minor difference between 408 NPF (mostly marine) and non-event day (mostly continental) concentrations. At SMEAR I, HOMs are the 409 only species that are at a (marginally) lower level during non-event than NPF days indicating that the total 410 HOM concentration does not determine when NPF events occur. However, this does not exclude the possible 411 participation of certain HOMs in NPF together with sulfur compounds (Lehtipalo et al., 2018) or at later stages 412 of the NPF process, especially during particle growth. However, pure biogenic nucleation involving ions and 413 HOMs (Kirkby et al., 2016) seems not to be a major NPF pathway in Värriö. 414 Our measurements do not unveil the detailed mechanism of nucleation or growth of particles. We lack 415 measurements of ambient bases that are needed to stabilize sulfuric acid clusters in ambient conditions (e.g. 416 Almeida Jen et al., 2014;Kirkby et al., 2011;Kürten et al., 2014;Myllys et al., 2018). With the 417 given observations comparing NPF days with non-event days it is likely that most regional NPF events require 418 sulfuric acid, but the NPF process can involve other compounds as well, especially IA and MSA, which show 419 higher concentrations on NPF days, very similarly that the results reported from Ny-Ålesund (Beck et al., and the NCEP/NCAR reanalysis data used as meteorological input. Red = NPF event, black = non-NPF 424

Conclusions: 425
We report ~7 months of nitrate based CI-APi-TOF measurements of sulfuric acid, methane sulfonic acid, iodic 426 acid and highly oxygenated organic compounds from a remote sub-Arctic field station SMEAR I in Finland.

427
The measurements aim to increase our understanding of the Arctic aerosol forming precursors and atmospheric 428 chemistry in more detail. The reason for measuring these compounds ~150 km north of the Arctic Circle is 429 simple; the Arctic is warming twice the speed as the planet on average. Lapland is already facing environmental 430 changes when e.g. woody plants disperse further north and influence the tundra ecosystem (Aakala et al., 2014;431 Kemppinen et al., 2021). These changes will in turn affect the emissions of aerosol precursor gases, which 432 may have feedback effects on to the climate (e.g. Kulmala et al., 2020;Paasonen et al., 2013). 433 https://doi.org/10.5194/acp-2021-735 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License.
The area surrounding SMEAR I station has snow cover for almost 8 months a year. Accumulating snow during 434 the autumn is a good reservoir to e.g. halogens, similarly than in the high Arctic (and Arctic Ocean) 435 environment. The snow pack also acts as a cover for biogenic emissions entering the atmosphere from the 436 ground. Any changes in the temperature and snow cover in the sub-Arctic regions will effect on atmospheric 437 chemistry and composition that are undeniably changing the way aerosol particles form and what their number 438 concentration is in the region. 439 In this study, we report seasonal and monthly variations of SA, MSA, IA and HOM concentrations and find 440 all these compounds abundant in springtime. SA has a peak concentration in the spring, decreasing for the rest 441 of the seasons. We detect high concentrations of MSA and IA that are usually connected to marine and coastal 442 environments, although Värriö is located ~130 km from the nearest coast of the White Sea. While MSA is 443 abundant in the spring, summer and decreases to limit of detection levels for autumn, IA continues at the same 444 concentration throughout the seasons. It seems likely that these two compounds are connected to emissions 445 from phytoplankton or the Arctic ice pack and arrive to SMEAR I by long transport routes. In the case of iodic 446 acid, we suggest that the source of iodine emissions is a combination of transport and local emission from the 447 continental snow pack and vegetation at the site. Further work is needed to confirm this hypothesis. 448 The most striking correlation we found in HOM concentrations and ambient air temperature. The vegetation 449 at SMEAR I is the source of VOCs even in the snow covered spring season and these volatile gases are oxidized 450 into HOMs with different reaction rates depending on the oxidant. In the case of such strong temperature 451 controlled HOM concentrations, we conclude that HOMs in the mass range of 300 -600 Th are most likely 452 products of monoterpene oxidation. 453 We also studied the abundance of these aerosol precursors separately during NPF and non-event days. We 454 observed that new particles at SMEAR I preferably form in relatively low temperatures (< 10°C), low relative 455 humidity that decreases with rising temperature during the day, ~10 ppbv higher ozone concentration than 456 during non-event days, high SA concentration in the morning and high MSA concentrations in the afternoon.

457
Cluster analysis of air masses show that NPF usually happens in marine air masses travelling to the site from 458 North west -West. All together, these are the first long term measurements of aerosol forming precursor from 459 the sub-arctic region helping us to understand atmospheric chemical processes and aerosol formation in the 460 rapidly changing Arctic. 461