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Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We have developed the first model with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation. The model can reproduce the observed NPF, HOM gas-phase composition and SOA formation over the Boreal forest. During the spring, HOM SOA formation increases the CCN concentration by ~10 % and causes a direct aerosol radiative forcing of −0.10 W/m 2 . In contrast, NPF reduces the number of CCN at updraft velocities < 0.2 m/s, and causes a direct aerosol radiative forcing of +0.15 W/m 2 . Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest. Forests emit compounds into the atmosphere that are oxidized into highly oxygenated molecules that serve as precursors for cloud condensation nuclei–a process that impacts the climate, but is poorly represented in models. Here the authors create a new model that accurately depicts highly oxygenated molecule and climate dynamics over Boreal forests.

Wetlands cover only 3 % of the global land surface area, but boreal wetlands are experiencing an unprecedented warming of four times the global average. These wetlands emit isoprene and terpenes (including monoterpenes (MT), sesquiterpenes (SQT), and diterpenes (DT)), which are climate-relevant highly reactive biogenic volatile organic compounds (BVOCs) with an exponential dependence on temperature. In this study, we present ecosystem-scale eddy covariance (EC) fluxes of isoprene, MT, SQT, and DT (hereafter referred to together as terpenes) at Siikaneva, a boreal fen in southern Finland, from the start to the peak of the growing season of 2021 (19 May 2021 to 28 June 2021). These are the first EC fluxes reported using the novel state-of-the-art Vocus proton transfer reaction mass spectrometer (Vocus-PTR) and the first-ever fluxes reported for DTs from a wetland. Isoprene was the dominant compound emitted by the wetland, followed by MTs, SQTs, and DTs, and they all exhibited a strong exponential temperature dependence. The Q10 values, the factor by which terpene emissions increases for every 10 ∘C rise in temperature, were up to five times higher than those used in most BVOC models. During the campaign, the air temperature peaked above 31 ∘C on 21–22 June 2021, which is abnormally high for boreal environments, and the maximum flux for all terpenes coincided with this period. We observed that terpene emissions were elevated after this abnormally “high-temperature stress period”, indicating that past temperatures alter emissions significantly. The standardized emission factor (EF) of the fen for isoprene (EFiso) was 11.1 ±  0.3 nmol m−2 s−1, which is at least two times higher than in previous studies and as high as the emission factors typical for broadleaf and other forests in the lower latitudes. We observed EFMT of 2.4 ±  0.1 nmol m−2 s−1, EFSQT of 1.3 ± 0.03 nmol m−2 s−1, higher than typical for needle leaf and broadleaf tree functional types, and EFDT of 0.011 ± 0.001 nmol m−2 s−1. We also compared the landscape average emissions to the model of emissions of gases and aerosols from nature (MEGAN) v2.1 and found that the emissions were underestimated by over 9 times for isoprene, over 300 times for MTs, and 800 times for SQTs. Our results show that due to very high EFs and high sensitivity to increasing temperatures, these high-latitude ecosystems can be a large source of terpenes to the atmosphere, and anthropogenic global warming could induce much higher BVOC emissions from wetlands in the future.

Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O 2 . This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NO X , which typically rapidly terminates autoxidation in urban areas, the studied C 6 –C 10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality. Efficient autoxidation of organic compounds typically requires that they possess double bonds or oxygen-containing moieties, which is why alkanes were thought to contribute little to atmospheric organic aerosol formation. Here, mass spectrometry shows significant autoxidation of alkanes under both atmospheric and combustion conditions.

The metallurgical industry in the Kola Peninsula, north-west Russia, form, after Norilsk, Siberia, the second largest source of air pollution in the Arctic and subarctic domain. Sulfur dioxide (SO2) emissions from the ore smelters are transported to wide areas, including Finnish Lapland. We performed investigations on concentrations of SO2, aerosol precursor vapours, aerosol and ion cluster size distributions together with chemical composition measurements of freshly formed clusters at the SMEAR I station in Finnish Lapland relatively close (∼ 300 km) to the Kola Peninsula industrial sites during the winter 2019–2020. We show that highly concentrated SO2 from smelter emissions is converted to sulfuric acid (H2SO4) in sufficient concentrations to drive new particle formation hundreds of kilometres downwind from the emission sources, even at very low solar radiation intensities. Observed new particle formation is primarily initiated by H2SO4–ammonia (negative-)ion-induced nucleation. Particle growth to cloud condensation nuclei (CCN) sizes was concluded to result from sulfuric acid condensation. However, air mass advection had a large role in modifying aerosol size distributions, and other growth mechanisms and condensation of other compounds cannot be fully excluded. Our results demonstrate the dominance of SO2 emissions in controlling wintertime aerosol and CCN concentrations in the subarctic region with a heavily polluting industry.

A multilayer gas dry deposition model has been developed and implemented into a one-dimensional chemical transport model SOSAA (model to Simulate the concentrations of Organic vapours, Sulphuric Acid and Aerosols) to calculate the dry deposition velocities for all the gas species included in the chemistry scheme. The new model was used to analyse in-canopy sources and sinks, including gas emissions, chemical production and loss, dry deposition, and turbulent transport of 12 featured biogenic volatile organic compounds (BVOCs) or groups of BVOCs (e.g. monoterpenes, isoprene+2-methyl-3-buten-2-ol (MBO), sesquiterpenes, and oxidation products of mono- and sesquiterpenes) in July 2010 at the boreal forest site SMEAR II (Station for Measuring Ecosystem–Atmosphere Relations). According to the significance of modelled monthly-averaged individual source and sink terms inside the canopy, the selected BVOCs were classified into five categories: 1. Most of emitted gases are transported out of the canopy (monoterpenes, isoprene + MBO). 2. Chemical reactions remove a significant portion of emitted gases (sesquiterpenes). 3. Bidirectional fluxes occur since both emission and dry deposition are crucial for the in-canopy concentration tendency (acetaldehyde, methanol, acetone, formaldehyde). 4. Gases removed by deposition inside the canopy are compensated for by the gases transported from above the canopy (acetol, pinic acid, β-caryophyllene's oxidation product BCSOZOH). 5. The chemical production is comparable to the sink by deposition (isoprene's oxidation products ISOP34OOH and ISOP34NO3). Most of the simulated sources and sinks were located above about 0.2 hc (canopy height) for oxidation products and above about 0.4 hc for emitted species except formaldehyde. In addition, soil deposition (including deposition onto understorey vegetation) contributed 11–61 % to the overall in-canopy deposition. The emission sources peaked at about 0.8–0.9 hc, which was higher than 0.6 hc where the maximum of dry deposition onto overstorey vegetation was located. This study provided a method to enable the quantification of the exchange between atmosphere and biosphere for numerous BVOCs, which could be applied in large-scale models in future. With this more explicit canopy exchange modelling system, this study analysed both the temporal and spatial variations in individual in-canopy sources and sinks, as well as their combined effects on driving BVOC exchange. In this study 12 featured BVOCs or BVOC groups were analysed. Other compounds could also be investigated similarly by being classified into these five categories.

We measured volatile organic compounds (VOCs), carbon dioxide (CO2) and carbon monoxide (CO) at an urban background site near the city centre of Helsinki, Finland, northern Europe. The VOC and CO2 measurements were obtained between January 2013 and September 2014 whereas for CO a shorter measurement campaign in April–May 2014 was conducted. Both anthropogenic and biogenic sources were identified for VOCs in the study. Strong correlations between VOC fluxes and CO fluxes and traffic rates indicated anthropogenic source of many VOCs. The VOC with the highest emission rate to the atmosphere was methanol, which originated mostly from traffic and other anthropogenic sources. The traffic was also a major source for aromatic compounds in all seasons whereas isoprene was mostly emitted from biogenic sources during summer. Some amount of traffic-related isoprene emissions were detected during other seasons but this might have also been an instrumental contamination from cycloalkane products. Generally, the observed VOC fluxes were found to be small in comparison with previous urban VOC flux studies. However, the differences were probably caused by lower anthropogenic activities as the CO2 fluxes were also relatively small at the site.

With several cities worldwide pursuing carbon neutrality in the upcoming decades, there is an increasing interest in quantifying cities’ anthropogenic carbon emissions using atmospheric observations. The challenge with both in-situ and remote sensing methods is, however, that the observations include both anthropogenic and biogenic signals. To reduce uncertainties in anthropogenic emission estimations, it is critical to partition biogenic fluxes of carbon dioxide (CO 2 ) from the observed data. In this study, we, for the first time, examine the suitability of carbonyl sulfide (COS), a proxy for photosynthesis, on partitioning biogenic CO 2 uptake from the ecosystem exchange measured with the eddy covariance (EC) technique over an urban area in Helsinki, Finland. The urban vegetation acts as a clear sink for COS whereas anthropogenic processes show minimal COS emissions within the source area of the measured net carbon flux. We show that two different COS flux-based methods are able to produce the dynamics of photosynthesis by an independent light-response curve-based estimation. Together with commonly used soil and vegetation respiration proxy, we removed biogenic signals from the urban net CO 2 exchange and demonstrated that together with CO 2 fluxes, COS flux can successfully be used to get realistic estimations of anthropogenic carbon emissions using the EC method.

This paper reports the fluxes and mixing ratios of biogenically emitted volatile organic compounds (BVOCs) 4 m above a mixed oak and hornbeam forest in northern Italy. Fluxes of methanol, acetaldehyde, isoprene, methyl vinyl ketone + methacrolein, methyl ethyl ketone and monoterpenes were obtained using both a proton-transfer-reaction mass spectrometer (PTR-MS) and a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) together with the methods of virtual disjunct eddy covariance (using PTR-MS) and eddy covariance (using PTR-ToF-MS). Isoprene was the dominant emitted compound with a mean daytime flux of 1.9 mg m−2 h−1. Mixing ratios, recorded 4 m above the canopy, were dominated by methanol with a mean value of 6.2 ppbv over the 28-day measurement period. Comparison of isoprene fluxes calculated using the PTR-MS and PTR-ToF-MS showed very good agreement while comparison of the monoterpene fluxes suggested a slight over estimation of the flux by the PTR-MS. A basal isoprene emission rate for the forest of 1.7 mg m−2 h−1 was calculated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) isoprene emission algorithms (Guenther et al., 2006). A detailed tree-species distribution map for the site enabled the leaf-level emission of isoprene and monoterpenes recorded using gas-chromatography mass spectrometry (GC–MS) to be scaled up to produce a bottom-up canopy-scale flux. This was compared with the top-down canopy-scale flux obtained by measurements. For monoterpenes, the two estimates were closely correlated and this correlation improved when the plant-species composition in the individual flux footprint was taken into account. However, the bottom-up approach significantly underestimated the isoprene flux, compared with the top-down measurements, suggesting that the leaf-level measurements were not representative of actual emission rates.

Aerosols and their interaction with clouds constitute the largest uncertainty in estimating the radiative forcing affecting the climate system. Secondary aerosol formation is responsible for a large fraction of the cloud condensation nuclei in the global atmosphere. Wetlands are important to the budgets of methane and carbon dioxide, but the potential role of wetlands in aerosol formation has not been investigated. Here we use direct atmospheric sampling at the Siikaneva wetland in Finland to investigate the emission of methane and volatile organic compounds, and subsequently formed atmospheric clusters and aerosols. We find that terpenes initiate stronger atmospheric new particle formation than is typically observed over boreal forests and that, in addition to large emissions of methane which cause a warming effect, wetlands also have a cooling effect through emissions of these terpenes. We suggest that new wetlands produced by melting permafrost need to be taken into consideration as sources of secondary aerosol particles when estimating the role of increasing wetland extent in future climate change.

The subject of the study was the effect of monoterpene oxidation on the growth of particles during new-particle formation (NPF) events at the SMEAR II measurement station in Hyytiala, southern Finland, during 2006-2011. The nighttime oxidation capacity, i.e. how readily the atmosphere can oxidize monoterpenes, was found to be dominated by the nitrate radical, whereas the daytime oxidation capacity was mainly dominated by ozone. The mean lifetimes of monoterpenes ranged from about one hour to several hours, depending on the time of year and day. A strong link was found between the growth rate of particles of 7-20 nm in diameter during the NPF events and monoterpene oxidation by ozone during the preceding night. Our findings suggest that during nighttime a build-up of primarily oxidized monoterpenes in the atmosphere occurs, and that these compounds can be oxidized by the hydroxyl radical after sunrise, promoting the particle growth.