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Dimethyl sulfide (DMS) influences climate via cloud condensation nuclei (CCN) formation resulting from its oxidation products (mainly methanesulfonic acid, MSA, and sulfuric acid, H2SO4). Despite their importance, accurate prediction of MSA and H2SO4from DMS oxidation remains challenging. With comprehensive experiments carried out in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN, we show that decreasing the temperature from +25 to -10 °C enhances the gas-phase MSA production by an order of magnitude from OH-initiated DMS oxidation, while H2SO4production is modestly affected. This leads to a gas-phase H2SO4-to-MSA ratio (H2SO4/MSA) smaller than one at low temperatures, consistent with field observations in polar regions. With an updated DMS oxidation mechanism, we find that methanesulfinic acid, CH3S(O)OH, MSIA, forms large amounts of MSA. Overall, our results reveal that MSA yields are a factor of 2-10 higher than those predicted by the widely used Master Chemical Mechanism (MCMv3.3.1), and the NOxeffect is less significant than that of temperature. Our updated mechanism explains the high MSA production rates observed in field observations, especially at low temperatures, thus, substantiating the greater importance of MSA in the natural sulfur cycle and natural CCN formation. Our mechanism will improve the interpretation of present-day and historical gas-phase H2SO4/MSA measurements. publishedVersion

Long-term exposure to ambient ozone is linked with respiratory-related mortality, while the emerging climate change is projected to pose double-edged challenges for ozone air quality. Here, we calculate the impact of emissions- and climate-change under SSP3-7.0 scenario on ozone-related mortality on a global scale, using historical (experiment histSST) and future simulations (experiments ssp370SST and ssp370pdSST) from three CMIP6 Earth System Models (ESMs) (GFDL-ESM4, EC-Earth3-AerChem, and UKESM1-0-LL). The ssp370SST experiment follows time-varying SSTs, while the SSP370pdSST follows a present-day climatology for SSTs. The chronic obstructive pulmonary disease mortality attributable to ozone pollution is estimated following the Global Burden Disease (GBD) 2019 approach, by using the ozone season daily maximum 8-hour mixing ratio (OSDMA8), the baseline mortality rate (from the GBD), and the SSP3-7.0 present and future gridded population. An increase in ozone-related mortality of approximately 2.5 million people per year globally is projected at the end of the century (2090) with respect to 2000 due to emissions and population changes. The climate-change footprint on ozone-related mortality exhibits large variability among the ESMs; yet, over India and China all ESMs project an increase of ozone-related mortality in the future, highlighting the importance of the ozone penalty due to global warming in regions with strong anthropogenic sources.

We introduce the Atmospherically Relevant Chemistry and Aerosol box model ARCA box (v.1.2.2). It is a zero-dimensional process model with a focus on atmospheric chemistry and submicron aerosol processes, including cluster formation. A novel feature in the model is its comprehensive graphical user interface, allowing for detailed configuration and documentation of the simulation settings, flexible model input, and output visualization. Additionally, the graphical interface contains tools for module customization and input data acquisition. These properties – customizability, ease of implementation and repeatability – make ARCA an invaluable tool for any atmospheric scientist who needs a view on the complex atmospheric aerosol processes. ARCA is based on previous models (MALTE-BOX, ADiC and ADCHEM), but the code has been fully rewritten and reviewed. The gas-phase chemistry module incorporates the Master Chemical Mechanism (MCMv3.3.1) and Peroxy Radical Autoxidation Mechanism (PRAM) but can use any compatible chemistry scheme. ARCA's aerosol module couples the ACDC (Atmospheric Cluster Dynamics Code) in its particle formation module, and the discrete particle size representation includes the fully stationary and fixed-grid moving average methods. ARCA calculates the gas-particle partitioning of low-volatility organic vapours for any number of compounds included in the chemistry, as well as the Brownian coagulation of the particles. The model has parametrizations for vapour and particle wall losses but accepts user-supplied time- and size-resolved input. ARCA is written in Fortran and Python (user interface and supplementary tools), can be installed on any of the three major operating systems and is licensed under GPLv3.

The Erasmus+ ClimEd (2021-2023; http://climed.network; “Multilevel Local, Nation- and Regionwide Education and Training in Climate Services, Climate Change Adaptation and Mitigation”) project is aimed at the development of competency-based curricula for continuous comprehensive training of specialists in the field of climate services and additional education in climate change for decision-makers, experts in climate-dependent economic sectors, and public.Some of the goals and objectives of the project are closely related to the Pan-Eurasian EXperiment (PEEX; www.atm.helsinki.fi/peex), and especially with multi-disciplinary, -scale and -component study climate change at resolving major uncertainties in the Earth system science and global sustainability issues.The ClimEd Trainings (http://climed.network/events/climed-trainings), in total 7, will be carried out during the project and will be focused on training the faculty/ teaching/ research staff and postgraduates at the ClimEd partner institutions and collaborating organizations in advanced educational and information-and-communication technologies for building a flexible multi-level integrated practice-based education system in the field of Climate Services, Climate Change Adaptation and Mitigation.Due to COVID pandemic situation, the originally planned face-to-face first trainings (in Estonia, Ukraine, and Finland) were converted into online training. Such online trainings were divided into 3 consecutive blocks: (i) online lecturing, (ii) home-work-assignments (HWAs) as group projects with established internal communication between the member of the groups and with an option of zoom-consulting during remote work, and (iii) final oral presentations (projects’ defenses) of HWAs with evaluation and feedback, discussions, and awarding certificates (corresponding to ECTS credits) with achieved learning outcomes. The majority of HWAs are based on the ClimEd main themes linking climate change vs. agriculture, energy, technical design and construction, urban economy, water management, health care; although other themes of interest can be selected by groups. Trainings also include questionnaires distributed among participants: evaluation of the training, and evaluation of own learning outcomes. Technically, the Moodle system, Zoom-hosting, e-evaluations, etc. are actively utilized in such trainings. All materials of the trainings are always publicly accessible online at the ClimEd project website as well as long-term stored at the Moodle system for each training.The outcomes/ summaries – including the lecture topics and learning outcomes, information resources, themes of group projects, feedbacks and training results, established network-community of the training participants (trainees and lecturers and teachers of HWAs) – of the online training approach will be presented for the ClimEd Trainings. Summaries are available for: 1st training “Competence-Based Approach to Curriculum Development for Climate Education”; 19 Apr – 12 May 2021; http://climed.network/events/climed-trainings/climed-training-1-online); 2nd – “Adaptation of the Competency Framework for Climate Services to conditions of Ukraine” (29 Jun – 26 Aug 2021; http://climed.network/events/climed-trainings/climed-training-2-online); 3rd – “Digital tools and datasets for climate change education” (26 Oct – 12 Nov 2021; http://climed.network/events/climed-trainings/climed-training-3-online); and 4th “Developing learning courses in climate services considering needs of different users” (7–11 February 2022; http://climed.network/events/climed-trainings/climed-training-4).

Corrigendum: The legend in Fig. 6e has been mislabeled. The gray colorcorresponds to “Missing” and the other colors should havecorresponded to the same species as in Fig. 6f. The figure,which is also the key figure of the article, can be found belowwith the correct legend. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiala, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with a gas chromatographer coupled to a mass spectrometer (GC-MS) and offline liquid chromatography. Up to 104 compounds, mostly volatile organic compounds (VOCs) and oxidized VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The monthly averaged experimental total OH reactivity was found to be higher in April and May (ca. 20 s(-1)) than in June and July (7.6 and 15.4 s(-1), respectively). The measured values varied much more in spring with high reactivity peaks in late afternoon, with values higher than in the summer, in particular when the soil was thawing. Total OH reactivity values generally followed the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the difference between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, and (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (

Strong, ongoing high latitude-warming is causing changes to vegetation composition and plant productivity, modifying plant emissions of Biogenic Volatile Organic Compounds (BVOCs). In the sparsely populated high latitudes, climatic feedbacks resulting from BVOCs as precursors of atmospheric aerosols could be more important than elsewhere on the globe. Here, we quantitatively assess the linkages between vegetation changes, BVOC emissions and secondary organic aerosol (SOA) under different climate scenarios and show that warming-induced vegetation changes determine the spatial patterns of BVOC impacts on SOA. The northward advances of boreal needle-leaved trees and shrubs result in an increase of up to 45% in regional SOA optical depth, causing a cooling feedback. In contrast, areas dominated by temperate broad-leaved trees show a large decline in monoterpene emissions and SOA formation, causing a warming feedback. We highlight the necessity of considering vegetation shifts when assessing radiative feedbacks on climate following the BVOC-SOA pathway.

Major atmospheric oxidants (OH, O3 and NO3) dominate the atmospheric oxidation capacity, while H2SO4 is considered as a main driver for new particle formation. Although numerous studies have investigated the long-term trend of ozone in Europe, the trends of OH, NO3 and H2SO4 at specific sites are to a large extent unknown. The one-dimensional model SOSAA has been applied in several studies at the SMEAR II station and has been validated by measurements in several projects. Here, we applied the SOSAA model for the years 2007–2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants OH and NO3, as well as H2SO4 at SMEAR II. The simulations were evaluated with observations from several shorter and longer campaigns at SMEAR II. Our results show that daily OH increased by 2.39% per year and NO3 decreased by 3.41% per year, with different trends of these oxidants during day and night. On the contrary, daytime sulfuric acid concentrations decreased by 2.78% per year, which correlated with the observed decreasing concentration of newly formed particles in the size range of 3–25 nm with 1.4% per year at SMEAR II during the years 1997–2012. Additionally, we compared our simulated OH, NO3 and H2SO4 concentrations with proxies, which are commonly applied in case a limited number of parameters are measured and no detailed model simulations are available., We applied the 1D model SOSAA to simulate the atmospheric concentrations of OH, NO3, and H2SO4 from 2007 to 2018 at SMEAR II. The results showed that daily OH increased by 2.39% per year and NO3 and H2SO4 decreased by 3.41% per year and 2.78% per year, respectively.

The pressure to increase forest and land carbon stocks simultaneously with increasing forest based biomass harvest for energy and materials emphasizes the need for dedicated analyses of impacts and possible trade-offs between these different mitigation options including also forest related biophysical factors, surface albedo and the formation of biogenic Secondary Organic Aerosols (SOA). We analyzed the change in global radiative forcing (RF) due to changes in these climatic agents as affected by the change in state of Finnish forests under increased or decreased harvest scenarios from a baseline. We also included avoided emissions due to wood material and energy substitution. Increasing harvests from baseline (65% of Current Annual Increment) decreased the total carbon sink (carbon in trees, soil and harvested wood products) at least for 50 years. When we coupled this change in carbon with other biosphere responses, surface albedo and aerosols, decreasing harvests from the baseline produced the largest cooling effect during 50 years. Accounting also for the avoided emissions due to increased wood use, the RF responses of the two lowest harvest scenarios were within uncertainty range. Our results show that the effects of forest management on SOA formation should be included in the analyses trying to deduce the net climate impact of forest use. The inclusion of the rarely considered SOA effects enforces the view that the lower the harvest, the more climatic cooling boreal forests provide. These results should act as a caution mark for policy makers who are emphasizing the increased utilization of forest biomass for short-living products and bioenergy as an efficient measure to mitigate climate change.

Major atmospheric oxidants (OH, O3 and NO3) dominate the atmospheric oxidation capacity, while H2SO4 is considered as a main driver for new particle formation events. Although numerous studies have investigated the long-term trend of ozone in Europe, the trend of OH, NO3 and H2SO4 at specific sites are to a large extent unknown. In this study, we investigated how the trends in major atmospheric oxidants (OH, O3 and NO3) and H2SO4 changed in southern Finland during the past 12 years and discuss how these trends relate to decreasing emissions of regulated air pollutants in Europe. The one-dimensional model SOSAA has been applied in several studies at the SMEAR II station, and has been validated by measurements in several projects. Here, we ran the SOSAA model for the years 2007–2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants and H2SO4 at SMEAR II. The simulations were evaluated with observations at SMEAR II for several shorter and longer campaigns. Our results show that OH increased by +1.56 (−0.8; +3.17) % yr−1 during daytime and NO3 decreased by −3.92 (−6.49; −1.79) % yr−1 during nighttime, indicating different trends of the oxidants during day and night. Sulphuric acid decreased during daytime by −5.12 (−11.39; −0.52) % yr−1, which correlated with the observed decreasing concentration of newly formed particles in the size range 3–25 nm by 1.4 % yr−1 at SMEAR II during the years 1997–2012 (Nieminen et al., 2014). Additionally we compared our simulated OH, NO3 and H2SO4 concentrations with proxies, which are commonly applied in case limited amount of parameters are measured and no detailed model simulations are available.

In this study we modeled secondary organic aerosol (SOA) mass loadings from the oxidation (by O3, OH and NO3) of five representative biogenic volatile organic compounds (BVOCs): isoprene, endocyclic bond-containing monoterpenes (α-pinene and limonene), exocyclic double-bond compound (β-pinene) and a sesquiterpene (β-caryophyllene). The simulations were designed to replicate an idealized smog chamber and oxidative flow reactors (OFRs). The Master Chemical Mechanism (MCM) together with the peroxy radical autoxidation mechanism (PRAM) were used to simulate the gas-phase chemistry. The aim of this study was to compare the potency of MCM and MCM + PRAM in predicting SOA formation. SOA yields were in good agreement with experimental values for chamber simulations when MCM + PRAM was applied, while a stand-alone MCM underpredicted the SOA yields. Compared to experimental yields, the OFR simulations using MCM + PRAM yields were in good agreement for BVOCs oxidized by both O3 and OH. On the other hand, a stand-alone MCM underpredicted the SOA mass yields. SOA yields increased with decreasing temperatures and NO concentrations and vice versa. This highlights the limitations posed when using fixed SOA yields in a majority of global and regional models. Few compounds that play a crucial role (>95 % of mass load) in contributing to SOA mass increase (using MCM + PRAM) are identified. The results further emphasized that incorporating PRAM in conjunction with MCM does improve SOA mass yield estimation.

Aerosol effects on low warm clouds over the Yangtze River Delta (YRD, eastern China) are examined using co-located MODIS, CALIOP and CloudSat observations. By taking the vertical locations of aerosol and cloud layers into account, we use simultaneously observed aerosol and cloud data to investigate relationships between cloud properties and the amount of aerosol particles (using aerosol optical depth, AOD, as a proxy). Also, we investigate the impact of aerosol types on the variation of cloud properties with AOD. Finally, we explore how meteorological conditions affect these relationships using ERA-Interim reanalysis data. This study shows that the relation between cloud properties and AOD depends on the aerosol abundance, with a different behaviour for low and high AOD (i.e. AOD 0.35). This applies to cloud droplet effective radius (CDR) and cloud fraction (CF), but not to cloud optical thickness (COT) and cloud top pressure (CTP). COT is found to decrease when AOD increases, which may be due to radiative effects and retrieval artefacts caused by absorbing aerosol. Conversely, CTP tends to increase with elevated AOD, indicating that the aerosol is not always prone to expand the vertical extension. It also shows that the COT–CDR and CWP (cloud liquid water path)–CDR relationships are not unique, but affected by atmospheric aerosol loading. Furthermore, separation of cases with either polluted dust or smoke aerosol shows that aerosol–cloud interaction (ACI) is stronger for clouds mixed with smoke aerosol than for clouds mixed with dust, which is ascribed to the higher absorption efficiency of smoke than dust. The variation of cloud properties with AOD is analysed for various relative humidity and boundary layer thermodynamic and dynamic conditions, showing that high relative humidity favours larger cloud droplet particles and increases cloud formation, irrespective of vertical or horizontal level. Stable atmospheric conditions enhance cloud cover horizontally. However, unstable atmospheric conditions favour thicker and higher clouds. Dynamically, upward motion of air parcels can also facilitate the formation of thicker and higher clouds. Overall, the present study provides an understanding of the impact of aerosols on cloud properties over the YRD. In addition to the amount of aerosol particles (or AOD), evidence is provided that aerosol types and ambient environmental conditions need to be considered to understand the observed relationships between cloud properties and AOD.

A multi-layer ozone (O3) dry deposition model has been implemented into SOSAA (a model to Simulate the concentrations of Organic vapours, Sulphuric Acid and Aerosols) to improve the representation of O3 within and above the forest canopy in the planetary boundary layer where O3 is a key oxidant agent of biogenic volatile organic compounds (BVOCs) and thus affecting organic aerosol processes. We aim to predict the O3 uptake by a boreal forest canopy under varying environmental conditions and analyse the influence of different factors on total O3 uptake by the canopy as well as the vertical distribution of deposition sinks inside the canopy. We evaluated the newly implemented canopy deposition model by an extensive comparison of simulated and observed O3 fluxes and concentration profiles within and above the boreal forest canopy at SMEAR II (the Station to Measure Ecosystem-Atmosphere Relation II) in Hyytiälä, Finland, in August, 2010. The first half of August showed extremely warm and dry conditions which were probably representative for summer conditions prevailing at this site in future. The simulated O3 turbulent fluxes at the canopy top and the O3 concentration profiles inside the canopy agreed well with the measurement, which indicated that the turbulent transport and the O3 dry deposition onto the canopy and soil surface appeared to be properly represented in the model. In this model, the fraction of wet surface on vegetation leaves was parameterised according to the ambient relative humidity (RH). Model results showed that when RH was larger than 70 % the O3 uptake onto wet skin contributed 48.6 % to the total deposition during nighttime and 22.0 % during daytime. In addition, most of the O3 deposition occurred below 0.8 hc (canopy height) at this site. The contribution of sub-canopy deposition below 4.2 m was modelled to be about 40 % of the total O3 deposition during daytime which was similar to previous studies. Whereas for nighttime, the simulated sub-canopy deposition contributed 40–65 % to the total O3 deposition which was about two times as that in previous studies (25–30 %). The overall contribution of soil uptake was estimated as 36.5 %. These results indicated the importance of non-stomatal O3 uptake processes, especially the uptake on wet skin and soil surface. Furthermore, a qualitative evaluation of the chemical removal time scales indicated that the chemical removal rate within canopy was about 5 % of the total deposition flux at daytime and 16 % at nighttime under current knowledge of air chemistry. The evaluation of the O3 deposition processes provides improved understanding about the mechanisms involved in the removal of O3 for this boreal forest site which are also relevant to the removal of other reactive compounds such as the BVOCs and their oxidation products, which will be focus of a follow-up study.

Aniline has been found to have frequent environmental occurrence and high toxicity. However, little study has been performed on its environmental fate. Here, we employed Grand Canonical Monte Carlo simulations (GCMC) to investigate the adsorption behavior of aniline on hexagonal ice surface at 200 K using our modified force field of aniline and TIP5P force field of water. The results indicate that the adsorption isotherm of aniline exhibits a “monolayer saturation plateau”, starting with a rapid increase, then a plateau, and finally a condensed phase. Under very low surface coverage, the adsorption isotherm apparently follows Langmuir type adsorption isotherm although anilines can be adsorbed to various sites. Within the range of the apparent Langmuir-type adsorption isotherm, adsorbed anilines are independent from each other and most anilines are almost parallel to the ice surface and form two N−H•••O hydrogen bonds. With the increase of coverage, the adsorbed anilines can interact with each other, resulting in the deviation from the apparent Langmuir-type adsorption isotherm. In addition, the adsorption energy from GCMC simulation (−65.91 kJ mol−1) is well consistent that from our validating quantum chemistry calculation (−69.34 kJ mol−1), further confirming the reliability of our GCMC simulation results.

Non

Although much efforts have been put on studying air pollution, our knowledge on the mechanisms of frequently occurred intense haze episodes in China is still limited. In this study, using three years of measurements of air pollutants at three different height levels on a 325-meter Beijing meteorology tower, we found that a positive particulate matter-boundary layer feedback mechanism existed at three vertical observation heights during intense haze polluted periods within the mixing layer. This feedback was characterized by a higher loading of PM2.5 with a shallower mixing layer. Measurements showed that the feedback was related to the decrease of solar radiation, turbulent kinetic energy and thereby suppression of the mixing layer. The feedback mechanism can explain the rapid formation of intense haze episodes to some extent, and we suggest that the feedback mechanism should be considered in air quality models for better predictions.

Highly oxygenated multifunctional compounds (HOM) play a key role in the growth of new particles during new particle formation (NPF) events, but their quantitative roles in different environments of the globe haven’t been well studied yet. In this study, the HOM concentrations and NPF events at a remote boreal forest site in Finland (SMEAR II) and a sub-urban site in the polluted eastern China (SORPES) were simulated with the MALTE-BOX model, and their roles in NPF at the two distinctly different environments are discussed. We found that sulfuric acid and HOM organonitrate concentrations in the gas phase are significantly higher but other HOM monomers and dimers from monoterpene oxidation are lower at SORPES compared to SMEAR II. The model can simulate the NPF events at SMEAR II with a good agreement but underestimate the growth of new particles at SORPES, indicating a dominant role of sulfuric acid and HOM originated from oxidation of aromatics in the polluted environment. This study highlight the needs of molecular-scale measurements in improving the understanding of NPF mechanisms in the polluted areas like eastern China.

Aerosol-cloud interaction is examined using four years of data from the MODIS/Terra (morning orbit) and MODIS/Aqua (afternoon orbit) satellites. Aerosol optical depth (AOD) and cloud properties retrieved from both sensors are used to explore in a statistical sense the morning-to-afternoon variation of cloud properties in conditions with low and high AOD, over both land and ocean. The results show that the morning-to-afternoon variation of cloud properties during the 3 hours between the Terra and Aqua overpasses have similar patterns (increase or decrease) over land under both low and high AOD conditions. The variation in d(Cloud_X), defined as the mean change in cloud property Cloud_X between the morning and afternoon overpasses in high AOD conditions minus that in low AOD conditions, is different over land and ocean. This applies to cloud droplet effective radius (CDR), cloud fraction (CF) and cloud top pressure (CTP), but not to cloud optical thickness (COT) and cloud liquid water path (CWP). The effects of initial cloud fraction and meteorological conditions on the change in CF are also explored, showing that upward motion of air parcels can enhance the cloud cover much more when AOD is high than when it is low. In contrast, the increase of cloud cover with increasing relative humidity is much stronger in a relatively clean atmosphere with low AOD than in a more polluted atmosphere. Meanwhile, stable atmospheric conditions favour the development of a low cloud cover, especially when AOD is high. Overall, the analysis of the diurnal variation of cloud properties provides a better understanding of aerosol-cloud interaction over land and ocean.

In this study, the processes behind observed new particle formation (NPF) events and subsequent organicdominated particle growth at the Pallas AtmosphereEcosystem Supersite in Northern Finland are explored with the one-dimensional column trajectory model ADCHEM. The modeled sub-micron particle mass is up to similar to 75% composed of SOA formed from highly oxidized multifunctional organic molecules (HOMs) with low or extremely low volatility. In the model the newly formed particles with an initial diameter of 1.5 nm reach a diameter of 7 nm about 2 h earlier than what is typically observed at the station. This is an indication that the model tends to overestimate the initial particle growth. In contrast, the modeled particle growth to CCN size ranges (> 50 nm in diameter) seems to be underestimated because the increase in the concentration of particles above 50 nm in diameter typically occurs several hours later compared to the observations. Due to the high fraction of HOMs in the modeled particles, the oxygen-to-carbon (O V C) atomic ratio of the SOA is nearly 1. This unusually high O V C and the discrepancy between the modeled and observed particle growth might be explained by the fact that the model does not consider any particle-phase reactions involving semi-volatile organic compounds with relatively low O V C. In the model simulations where condensation of low-volatility and extremely low-volatility HOMs explain most of the SOA formation, the phase state of the SOA (assumed either liquid or amorphous solid) has an insignificant impact on the evolution of the particle number size distributions. However, the modeled particle growth rates are sensitive to the method used to estimate the vapor pressures of the HOMs. Future studies should evaluate how heterogeneous reactions involving semi-volatility HOMs and other less-oxidized organic compounds can influence the SOA composition-and size-dependent particle growth.

A multi-layer gas dry deposition model has been developed and implemented into a 1-dimensional chemical transport model SOSAA (a 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 to Measure Ecosystem-Atmosphere Relations II). According to the significance of modeled 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 by the gases transported from above the canopy (acetol, pinic acid, β-caryophyllene's oxidation product BCSOZOH), and finally (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 4 m for oxidation products and above about 8 m for emitted species except formaldehyde. In addition, soil deposition (including deposition onto understory vegetation) contributed 11–61 % to the overall in-canopy deposition. The emission sources peaked at about 14–16 m which was higher than 10 m 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 modeling system this study analysed both the temporal and spatial variations of individual in-caonpy sources and sinks, as well as their combined effects on driving BVOCs exchange. Twelve featured BVOCs or BVOC groups were analyzed in this study, more compounds could also be investigated similarly by being classified into the five categories.

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.

Aerosol effects on summertime low warm clouds over the Yangtze River Delta (YRD) are examined using co-located MODIS, CALIOP and CloudSat observations. By taking the vertical locations of aerosol and cloud layers into account, we use simultaneously observed aerosol and cloud data to investigate relationships between cloud properties and the amount of aerosol particles (using aerosol optical depth, AOD, as a proxy). Also, we investigate the impact of aerosol types on the variation of cloud properties with AOD. Finally, we explore how meteorological conditions affect these relationships using ERA Interim Reanalysis data. This study shows that the relation between cloud droplet effective radius (CDR) and AOD depends on the aerosol abundance, with a different behaviour for low and high AOD (i.e. AOD 0.3). Cloud fraction (CF) is found to be little dependent on the AOD when aerosol and cloud physically interact, but has a positive relation in case of well-separated clouds. Cloud optical Thickness (COT) is found to decrease when AOD increases, which may be due to radiative effects and retrieval artefacts caused by absorbing aerosol. Conversely, cloud top pressure (CTP) tends to increase with elevated AOD, indicating that the aerosol is not always prone to expand the vertical extension. Furthermore, separation of cases with either polluted dust or smoke aerosol shows that COT and CF are smaller for clouds mixed with smoke aerosol which is ascribed to the higher absorption efficiency of smoke than dust. The variation of cloud properties with AOD is analysed for different values of relative humidity (RH) and boundary layer thermodynamic and dynamic conditions, showing that high relative humidity and upward motion of air parcels can enhance the strength of aerosol-cloud interaction, especially pronounced in heavily polluted conditions than in moderately polluted conditions. Meteorological conditions play a weak role in the COT-AOD and CTP-AOD relationships throughout the range of AOD. Overall, the interpretation of the observed relationships between cloud properties and AOD requires that ambient environmental conditions are considered in addition to the aerosol and cloud parameters.

One dimensional atmospheric boundary layer (ABL) model coupled with detailed atmospheric chemistry and aerosol dynamical model, the model SOSAA, was used to predict the ABL and detailed aerosol population (characterized by the number size distribution) time evolution. The model was applied over a period of ten days in May 2013 for a pine forest site in southern Finland. The period was characterized by frequent new particle formation events and simultaneous intensive aerosol transformation. Throughout this study we refer to nucleation, condensational growth and coagulation as aerosol dynamical processes, i.e. the processes that govern the particle size distribution evolution. The aim of the study was to analyze and quantify the role of aerosol and ABL dynamics in vertical transport of aerosols. It was of particular interest to what extent the fluxes above canopy deviate due to above mentioned processes from the particle dry deposition on the canopy foliage. The model simulations revealed that the particle concentration change due to aerosol dynamics can frequently exceed the effect of particle deposition even an order of magnitude or more. The impact is however strongly dependent on particle size and time. In spite of the fact that the time scale of turbulent transfer inside canopy is much smaller than the time scales of aerosol dynamics and dry deposition, letting to assume well mixed properties of air, the fluxes at the canopy top frequently deviate from deposition inside forest. This is due to transformation of aerosol concentration throughout the ABL and resulting complicated pattern of vertical transport. Therefore we argue that the comparison of time scales of aerosol dynamics and deposition defined for the processes below the flux measurement level do not unambiguously describe the importance of aerosol dynamics for vertical transport within canopy. We conclude that under dynamical conditions the micrometeorological particle flux measurements such as performed by the eddy covariance technique do not generally represent the dry deposition. The deviation can be systematic for certain size ranges so that the conclusion applies also to time averaged particle fluxes.

Eddy covariance fluxes of formic acid, HCOOH, were measured over a boreal forest canopy in spring/summer 2014. The HCOOH fluxes were bidirectional, but mostly upward during daytime, in contrast to studies elsewhere that reported mostly downward fluxes. Downward flux episodes were explained well by modeled dry deposition rates. The sum of net observed flux and modeled dry deposition yields an upward “gross flux” of HCOOH, which could not be quantitatively explained by literature estimates of direct vegetative/soil emissions nor by efficient chemical production from other volatile organic compounds (VOC), suggesting missing or greatly underestimated HCOOH sources in the boreal ecosystem. We implemented a vegetative HCOOH source into the GEOS-Chem chemical transport model to match our derived gross flux, and evaluated the updated model against air- and space-borne observations. Model biases in the boundary layer were substantially reduced based on this revised treatment, but biases in the free troposphere remain unexplained.

The East Pacific wavetrain (EPW) refers to here the intense stationary wave activity detected in the troposphere over the East Pacific and North America in 45 northern winters from 1958 to 2002. The EPW is generated in the lower troposphere over the East Pacific, propagating predominantly eastward into North America and slightly upward then eventually into the stratosphere. The intensity of the EPW varies from year to year and exhibits apparent decadal variability. For the period 1958–1964, the EPW was in its second maximum, and it was weakest for the period 1965–1975, then it was strongest for the period 1976–1987. After 1987, the EPW weakened again.