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Aerosol hygroscopic growth and cloud droplet formation influence the radiation transfer budget of the atmosphere and thereby the climate. In the Arctic, these aerosol properties may have a more pronounced effect on the climate compared to the mid-latitudes. Hygroscopic growth and cloud condensation nuclei (CCN) concentrations of High Arctic aerosols were measured during two field studies in the spring and summer of 2016. The study site was the Villum Research Station (Villum) at Station Nord in the northeastern region of Greenland. Aerosol hygroscopic growth was measured with a hygroscopic tandem differential mobility analyzer (HTDMA) over a total of 23 days, and CCN concentrations were measured over a period of 95 days. Continuous particle number size distributions were recorded, facilitating calculations of aerosol CCN activation diameters and aerosol kappa (κ)-values. In spring, average CCN concentrations, at supersaturations (SS) of 0.1 to 0.3 %, ranged from 53.7 to 85.3 cm-3, with critical activation diameters ranging from 130.2 to 80.2 nm, and κCCN ranging from 0.28–0.35. In summer, average CCN concentrations were 20.8 to 47.6 cm-3, while critical activation diameters and κCCN were from 137.1 to 76.7 nm and 0.23–0.35, respectively. Mean particle hygroscopic growth factors ranged from 1.60 to 1.75 at 90 % relative humidity in spring, while values between 1.47 and 1.67 were observed in summer depending on initial dry size. Although the summer aerosol number size distributions were characterized by frequent new particle formation events, the CCN population at cloud-relevant supersaturations was determined by accumulation mode aerosols. This emphasizes the importance of accumulation mode aerosol sources to provide available CCN during summer. The influence of particle hygroscopic growth on the radiative transfer through aerosol-radiation interactions could be of major importance. The results of this study are directly applicable in the modeling of direct and indirect climate effects of Arctic aerosols. Targeted chemical and morphological analysis, based on filter samples or on-line techniques, could further clarify the role of primary organic marine influence on Arctic aerosol CCN concentrations and therewith climate effects.

The present report povides technical background information to the study of Egerer et al. (2023) on the role of a low-level jet (LLJ) for stirring the stable atmospheric surface layer in the Arctic. Based on micrometeorological tower measurements and tethered balloon-borne hot-wire anemometer measurements conducted during a 15 h observation period during the PAMARCMiP field experiment at the Villum Research Station (VRS, Station Nord) in North-East Greenland (March 28-30, 2018). Egerer et al. (2023) analyzed the transition of a stable atmospheric boundary layer (ABL) with a LLJ into a traditional stable ABL in the late-winter central Arctic.

The depth of the aerosol layer at the Villum Research Station at Station Nord in the high Arctic is analysed based on 8 years of observations from a ceilometer and one full year from a wind lidar. The layer is of particular interest for aerosol process modelling and atmospheric chemistry studies. The depth of the aerosol layer is assigned to the inflection point in the attenuated backscatter profile by two methods; one is based on polynomial approximation of the profile and the other is direct numerical differentiation. The analysis is based on two types of hourly profiles; one consists of averaging the attenuated backscatter observations and the other by computing the median. Due to sporadic occurrence of outliers in the ranges around 50 m in the ceilometer observations, this part of the profile is not used in this study. Restricting the observations to heights above 100 m, the depths of the aerosol layer are found to be typically ≈230 m. It varies little between winter and summer, but the spread in the depth is larger during the winter as compared to summer. To extend the study of the aerosol-layer depth below 100 m, a method is applied that combines the ceilometer measurements with the carrier-to-noise ratio from the wind lidar. The results are available for 2018 only, and they show aerosol-layer depths below ≈80 m as well as depths around 230 m and they show few observations between ≈80 and ≈230 m. Near the ground, the observed backscatter exhibits a pronounced seasonal variation, having low values during the summer and high values during the winter. The strength of the seasonal variability decreases with height, especially above the aerosol-layer depth, and is virtually absent at 1 km.

The depth of the aerosol layer at the Villum Research Station at Station Nord in the high Arctic is analyzed based on eight years of observations from a ceilometer. The depth of the aerosol layer is assigned to the inflection point in the attenuated backscatter profile by two methods; one is based on polynomial approximation of the profile and the other is direct numeric differentiation. The analysis is based on two types of hourly profiles; one consists of averaging the attenuated backscatter observations and the other by computing the median. It is noted that the depth of the aerosol layer in the Arctic might be different from the depth of the turbulent layer near the ground.Near the ground, the observed backscatter exhibits a pronounced seasonal variation, having low values during the summer and high values during the winter. The strength of the seasonal variability decreases with height and is virtually absent at 1 km.Due to sporadic occurrence of outliers in the range between 50 m and 80 m in the ceilometer observations, this part of the profile is not used in this study. Restricting the observations to heights above100 m, the depths of the aerosol layer are found to be typically 230-250 m. It varies little between winter and summer, but the spread in the depth was larger during the winter as compared to summer.To extend the profile of the attenuated backscatter below 100 m, the ceilometer measurements were combined with Carrier-To-Noise-Ratio observations from a Doppler wind Lidar. Thus, hourly profiles of attenuated backscatter starting at 40 m, are obtained. The results are available for 2018 only and they show aerosol-layer depths below 100 m as well as depths around 230-250 m and they show few observations of aerosol-layer depths between 100 m and 230 m.Acknowledgement: This article is based upon work from COST Action CA18235 PROBE, supported by COST (European Cooperation in Science and Technology). www.cost.eu.

Although coastal regions only amount to 7 % of the global oceans, their contribution to the global oceanic air–sea CO2 exchange is proportionally larger, with fluxes in some estuaries being similar in magnitude to terrestrial surface fluxes of CO2. Across a heterogeneous surface consisting of a coastal marginal sea with estuarine properties and varied land mosaics, the surface fluxes of CO2 from both marine areas and terrestrial surfaces were investigated in this study together with their impact in atmospheric CO2 concentrations by the usage of a high-resolution modelling framework. The simulated terrestrial fluxes across the study region of Denmark experienced an east–west gradient corresponding to the distribution of the land cover classification, their biological activity and the urbanised areas. Annually, the Danish terrestrial surface had an uptake of approximately −7000 GgC yr−1. While the marine fluxes from the North Sea and the Danish inner waters were smaller annually, with about −1800 and 1300 GgC yr−1, their sizes are comparable to annual terrestrial fluxes from individual land cover classifications in the study region and hence are not negligible. The contribution of terrestrial surfaces fluxes was easily detectable in both simulated and measured concentrations of atmospheric CO2 at the only tall tower site in the study region. Although, the tower is positioned next to Roskilde Fjord, the local marine impact was not distinguishable in the simulated concentrations. But the regional impact from the Danish inner waters and the Baltic Sea increased the atmospheric concentration by up to 0.5 ppm during the winter months.

Flux measurements with the aerodynamic gradient method (AGM) performed with a single analyzer measuring non-simultaneously at two heights have routinely been conducted. This study investigates the effect of this practice with calculations of single analyzer derived fluxes compared to fluxes derived from simultaneous concentration measurements at two heights for NH3. The results show a mean relative difference of less than 7% for the half-hour averaging intervals, whereas the relative difference in the cumulative loss of total ammoniacal nitrogen (TAN) is less than 4%. Scatter plots and linear regression show linear behavior with slope and intercept close to one and zero, respectively. The regression coefficients were between 0.913 and 0.966 for the simulations, but with large deviations for the single half-hour measurement interval. Changes in the starting height and averaging duration at each height for the single analyzer calculations yield small differences, but the effect is minimal compared to the general uncertainty of flux determination with AGM.

The transfer of CO2 between air and water can be quantified by the use of a transfer or exchange coefficient. Here we have studied the effect on the exchange coefficient of CO2 from inclusion of the carbonate buffer system in water. The methodology has been to solve the flux budget for the chemical reaction equations in the water column, combining the vertical flux in the air and the flux across the interface by the introduction of a so called resistance model for the whole system where the system have been simplified with respect to variation of certain parameters. The thermal effects on the flow and chemistry modelling have in the present study not been included, although this also plays a significant role on the air‐sea exchange. The model shows the effects on the air‐sea exchange coefficient from the carbonate chemistry in the water column are negligible if the measurements are taken at sufficient depth in the water. Hence, it is found that the skin‐layer or interface layer contains the largest resistance and therefore controls the flux.It is also shown that measurement of the near‐ surface CO2 gradients in ocean water is very difficult, as the gradients of CO2 occur within the first meter from the water surface. In reality the equilibrium is reached within the first meter from the water surface. The results justify the use of standard methods to estimate the flux of CO2 by measuring a concentration at typical larger than 3 meters depth and 10 meters above the surface.

There is the need for further study of whether global estimates of air-sea CO 2 exchange in estuarine systems capture the relevant temporal variability and, as such, the temporal variability of bulk parameterized and directly measured CO 2 fluxes was investigated in the Danish estuary, Roskilde Fjord. The air-sea CO 2 fluxes showed large temporal variability across seasons and between days and that more than 30% of the net CO 2 emission in 2013 was a result of two large fall and winter storms. The diurnal variability of ΔpCO 2 was up to 400 during summer changing the estuary from a source to a sink of CO 2 within the day. Across seasons the system was suggested to change from a sink of atmospheric CO 2 during spring to near neutral during summer and later to a source of atmospheric CO 2 during fall. Results indicated that Roskilde Fjord was an annual low-emission estuary, with an estimated bulk parameterized release of 3.9 ± 8.7 mol CO 2 m −2 y −1 during 2012–2013. It was suggested that the production-respiration balance leading to the low annual emission in Roskilde Fjord, was caused by the shallow depth, long residence time and high water quality in the estuary. In the data analysis the eddy covariance CO 2 flux samples were filtered according to the H 2 O CO 2 cross-sensitivity assessment suggested by Landwehr et al. (2014). This filtering reduced episodes of contradicting directions between measured and bulk parameterized air-sea CO 2 exchanges and changed the net air-sea CO 2 exchange from an uptake to a release. The CO 2 gas transfer velocity was calculated from directly measured CO 2 fluxes and ΔpCO 2 and agreed to previous observations and parameterizations.

Eddy covariance observations of CO2 fluxes were conducted during March–April 2012 in a temporally sequential order for 8, 4 and 30 days, respectively, at three locations on fast sea ice and on newly formed polynya ice in a coastal fjord environment in northeast Greenland. CO2 fluxes at the sites characterized by fast sea ice (ICEI and DNB) were found to increasingly reflect periods of strong outgassing in accordance with the progression of springtime warming and the occurrence of strong wind events: FCO2ICE1 = 1.73 ± 5 mmol m−2 day−1 and FCO2DNB = 8.64 ± 39.64 mmol m−2 day−1, while CO2 fluxes at the polynya site (POLYI) were found to generally reflect uptake FCO2POLY1 = −9.97 ± 19.8 mmol m−2 day−1. Values given are the mean and standard deviation, and negative/positive values indicate uptake/outgassing, respectively. A diurnal correlation analysis supports a significant connection between site energetics and CO2 fluxes linked to a number of possible thermally driven processes, which are thought to change the pCO2 gradient at the snow–ice interface. The relative influence of these processes on atmospheric exchanges likely depends on the thickness of the ice. Specifically, the study indicates a predominant influence of brine volume expansion/contraction, brine dissolution/concentration and calcium carbonate formation/dissolution at sites characterized by a thick sea-ice cover, such that surface warming leads to an uptake of CO2 and vice versa, while convective overturning within the sea-ice brines dominate at sites characterized by comparatively thin sea-ice cover, such that nighttime surface cooling leads to an uptake of CO2 to the extent permitted by simultaneous formation of superimposed ice in the lower snow column.

Measurements of equivalent black carbon (EBC) in aerosols at the high Arctic field site Villum Research Station (VRS) at Station Nord in North Greenland showed a seasonal variation in EBC concentrations with a maximum in winter and spring at ground level. Average measured concentrations were about 0.067 ± 0.071 for the winter and 0.011 ± 0.009 for the summer period. These data were obtained using a multi-angle absorption photometer (MAAP). A similar seasonal pattern was found for sulfate concentrations with a maximum level during winter and spring analyzed by ion chromatography. Here, measured average concentrations were about 0.485 ± 0.397 for the winter and 0.112 ± 0.072 for the summer period. A correlation between EBC and sulfate concentrations was observed over the years 2011 to 2013 stating a correlation coefficient of R2 Combining double low line 0.72. This finding gives the hint that most likely transport of primary emitted BC particles to the Arctic was accompanied by aging of the aerosols through condensational processes. BC and sulfate are known to have only partly similar sources with respect to their transport pathways when reaching the high Arctic. Aging processes may have led to the formation of secondary inorganic matter and further transport of BC particles as cloud processing and further washout of particles is less likely based on the typically observed transport patterns of air masses arriving at VRS. Additionally, concentrations of EC (elemental carbon) based on a thermo-optical method were determined and compared to EBC measurements. EBC measurements were generally higher, but a correlation between EC and EBC resulted in a correlation coefficient of R2 Combining double low line 0.64. Model estimates of the climate forcing due to BC in the Arctic are based on contributions of long-range transported BC during spring and summer. The measured concentrations were here compared with model results obtained by the Danish Eulerian Hemispheric Model, DEHM. Good agreement between measured and modeled concentrations of both EBC/BC and sulfate was observed. Also, the correlation between BC and sulfate concentrations was confirmed based on the model results observed over the years 2011 to 2013 stating a correlation coefficient of R2 Combining double low line 0.74. The dominant source is found to be combustion of fossil fuel with biomass burning as a minor, albeit significant source.

Minimising the uncertainties in estimates of air–sea CO2 exchange is an important step toward increasing the confidence in assessments of the CO2 cycle. Using an atmospheric transport model makes it possible to investigate the direct impact of atmospheric parameters on the air–sea CO2 flux along with its sensitivity to, for example, short-term temporal variability in wind speed, atmospheric mixing height and atmospheric CO2 concentration. With this study, the importance of high spatiotemporal resolution of atmospheric parameters for the air–sea CO2 flux is assessed for six sub-basins within the Baltic Sea and Danish inner waters. A new climatology of surface water partial pressure of CO2 (pCO2w) has been developed for this coastal area based on available data from monitoring stations and on-board pCO2w measuring systems. Parameterisations depending on wind speed were applied for the transfer velocity to calculate the air–sea CO2 flux. Two model simulations were conducted – one including short-term variability in atmospheric CO2 (VAT), and one where it was not included (CAT). A seasonal cycle in the air–sea CO2 flux was found for both simulations for all sub-basins with uptake of CO2 in summer and release of CO2 to the atmosphere in winter. During the simulated period 2005–2010, the average annual net uptake of atmospheric CO2 for the Baltic Sea, Danish straits and Kattegat was 287 and 471 Gg C yr−1 for the VAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr−1 was found to be significant, and thus ignoring short-term variability in atmospheric CO2 does have a sizeable effect on the air–sea CO2 exchange. The combination of the atmospheric model and the new pCO2w fields has also made it possible to make an estimate of the marine part of the Danish CO2 budget for the first time. A net annual uptake of 2613 Gg C yr−1 was found for the Danish waters. A large uncertainty is connected to the air–sea CO2 flux in particular caused by the transfer velocity parameterisation and the applied pCO2w climatology. However, as a significant difference of 184 Gg C yr−1 is obtained between the VAT and CAT simulations, the present study underlines the importance of including short-term variability in atmospheric CO2 concentration in future model studies of the air–sea exchange in order to minimise the uncertainty.

Ultrafine particles (UFP, particles with diameters (Dp)

We compare 4 planetary boundary layer (PBL) schemes of Weather Research and Forecasting model for high Artic conditions, documented during summer and winter campaigns at Station Nord, Greenland. During March 2012, 22 radiosonds were launched at 00 and 12 UTC. During July-August 2011, 25 radiosondes were launched at 00, 06, 12, and 18 UTC. The chosen PBL schemes are 3 TKE schemes: MYJ, MYNN and QNSE and non-local YSU. Comparison is performed between data from radiosoundings and corresponding in time model results up to different height from 100 m to 8000 m. Sensitivity of model to vertical and spatial resolution is examined with MYJ through 4 configurations combining 26 or 42 vertical levels and 4 km or 1.33 km horizontal grid step. Sensitivity to the resolution tests showed that increasing horizontal resolution from 4 km to 1.33 km did not improve model performance. Increasing the number of vertical layers lead to closer to observed profiles and slightly improved statistics by layers. Sensitivity to the lead time (24 h or 48 h) is examined with MYJ at 1.33 km grid step and 42 vertical layers. Quality of forecast for day 1 and day 2 is similar for the summer. Temperature and wind speed (WS) biases for the winter are with 1 K and 1 ms-1 larger for 48 h compared to 24 h lead time. The lack of diurnal variability during both campaigns is correctly simulated by all PBL schemes. The performed tests show that TKE schemes outperform YSU and as a whole MYNN gives the highest scores.

Understanding and predicting the ammonia (NH3) exchange between the biosphere and the atmosphere is important due to the environmental consequences of the presence of reactive nitrogen (N-r) in the environment. The dynamics of the natural sources are, however, not well understood, especially not for forest ecosystems due to the complex nature of this soil-vegetation-atmosphere system. Furthermore, the high reactivity of NH3 makes it technically complex and expensive to measure and understand the forest atmospheric NH3 exchange. The aim of this study is to investigate the NH3 flux partitioning between the ground layer, cuticle and stomata compartments for two temperate deciduous forest ecosystems located in Midwestern, USA (MMSF) and in Denmark (DK-Sor). This study is based on measurements and simulations of the surface energy balance, fluxes of CO2 and NH3 during two contrasted periods of the forest ecosystems, a period with full developed canopy (MMSF) and a senescent period for the DK-Sor site, with leaf fall and leaf litter build-up. Both datasets indicate emissions of NH3 from the forest to the atmosphere. The two-layer NH3 compensation point model SURFATM-NH3 was used in combination with a coupled photosynthesis-stomatal conductance model to represent seasonal variation in canopy physiological activity for simulating both net ecosystem CO2 exchange rates (R-2=0.77 for MMSF and R-2 = 0.84 for DK-Sor) and atmospheric NH3 fluxes (R-2 = 0.43 for MMSF and R-2 = 0.60 for DK-Sor). A scaling of the ground layer NH3 emission potential (Gamma(g)) was successfully applied using the plant area index (PM) to represent the build-up of a litter layer in the leaf fall period. For a closed green forest canopy (MMSF), unaffected by agricultural NH3 sources, NH3 was emitted with daytime fluxes up to 50 ng NH3-N m(-2) s(-1) and nighttime fluxes up to 30 ng NH3-N m(-2) s(-1). For a senescing forest (DK-Sor), located in an agricultural region, deposition rates of 250 ng NH3-N m(-2) s(-1) were measured prior to leaf fall, and emission rates up to 670 ng NH3-N m(-2) s(-1) were measured following leaf fall. For MMSF, simulated stomatal NH3 emissions explain the daytime flux observations well, and it is hypothesized that cuticular desorption is responsible for the observed NH3 emissions at night. During leaf fall in DK-Sor, ground fluxes dominate the NH3 flux with a mean emission rate of 150 ng NH3-N m(-2) s(-1). This study shows that forests potentially comprise a natural source of NH3 to the atmosphere, and that it is crucial to take into account the bi-directional exchange processes related to both the stomatal, cuticular and ground layer pathways in order to realistically simulate forest-atmosphere fluxes of NH3. (C) 2017 Elsevier B.V. All rights reserved.

The air–sea transfer velocity of \(\mathrm{CO}_{2}\, (k_{\mathrm{CO}_{2}})\) was investigated in a shallow estuary in March to July 2012, using eddy-covariance measurements of \(\mathrm{CO}_{2}\) fluxes and measured air–sea \(\mathrm{CO}_{2}\) partial-pressure differences. A data evaluation method that eliminates data by nine rejection criteria in order to heighten parametrization certainty is proposed. We tested the data evaluation method by comparing two datasets: one derived using quality criteria related solely to the eddy-covariance method, and the other derived using quality criteria based on both eddy-covariance and cospectral peak methods. The best parametrization of transfer velocity normalized to a Schmidt number of 600 \((k_{600})\) was determined to be: \(k_{600} = 0.3\,{U_{10}}^{2.5}\) where \(U_{10}\) is the wind speed in m \(\mathrm{s}^{-1}\) at 10 m; \(k_{600}\) is based on \(\mathrm{CO}_{2}\) fluxes calculated by the eddy-covariance method and including the cospectral peak method criteria. At low wind speeds, the transfer velocity in the shallow water estuary was lower than in other coastal waters, possibly a symptom of low tidal amplitude leading to low intensity water turbulence. High transfer velocities were recorded above wind speeds of 5 m \(\mathrm{s}^{-1}\), believed to be caused by early-breaking waves and the large fetch (6.5 km) of the estuary. These findings indicate that turbulence in both air and water influences the transfer velocity.

The understanding of biochemical feedback mechanisms in the climate system is lacking knowledge in relation to bi-directional ammonia (NH3) exchange between natural ecosystems and the atmosphere. We therefore study the atmospheric NH3 fluxes during a 25-day period during autumn 2010 (21 October to 15 November) for the Danish beech forest Lille Bøgeskov to address the hypothesis that NH3 emissions occur from deciduous forests in relation to leaf fall. This is accomplished by using observations of vegetation status, NH3 fluxes and model calculations. Vegetation status was observed using plant area index (PAI) and leaf area index (LAI). NH3 fluxes were measured using the relaxed eddy accumulation (REA) method. The REA-based NH3 concentrations were compared to NH3 denuder measurements. Model calculations of the atmospheric NH3 concentration were obtained with the Danish Ammonia MOdelling System (DAMOS). The relative contribution from the forest components to the atmospheric NH3 flux was assessed using a simple two-layer bi-directional canopy compensation point model. A total of 57.7% of the fluxes measured showed emission and 19.5% showed deposition. A clear tendency of the flux going from deposition of −0.25 ± 0.30 μg NH3-N m−2 s−1 to emission of up to 0.67 ± 0.28 μg NH3-N m−2 s−1 throughout the measurement period was found. In the leaf fall period (23 October to 8 November), an increase in the atmospheric NH3 concentrations was related to the increasing forest NH3 flux. Following leaf fall, the magnitude and temporal structure of the measured NH3 emission fluxes could be adequately reproduced with the bi-directional resistance model; it suggested the forest ground layer (soil and litter) to be the main contributing component to the NH3 emissions. The modelled concentration from DAMOS fits well the measured concentrations before leaf fall, but during and after leaf fall, the modelled concentrations are too low. The results indicate that the missing contribution to atmospheric NH3 concentration from vegetative surfaces related to leaf fall are of a relatively large magnitude. We therefore conclude that emissions from deciduous forests are important to include in model calculations of atmospheric NH3 for forest ecosystems. Finally, diurnal variations in the measured NH3 concentrations were related to meteorological conditions, forest phenology and the spatial distribution of local anthropogenic NH3 sources. This suggests that an accurate description of ammonia fluxes over forest ecosystems requires a dynamic description of atmospheric and vegetation processes.

The knowledge of climate effects of atmospheric aerosols is associated with large uncertainty, and a better understanding of their physical and chemical properties is needed, especially in the Arctic environment. The objective of the present study is to improve our understanding of the processes affecting the composition of aerosols in the high Arctic. Therefore size-segregated aerosols were sampled at a high Arctic site, Station Nord (Northeast Greenland), in March 2009 using a Micro Orifice Uniform Deposit Impactor. The aerosol samples were extracted in order to analyse three water-soluble anions: chloride, nitrate and sulphate. The results are discussed based on possible chemical and physical transformations as well as transport patterns. The total concentrations of the ions at Station Nord were 53–507 ng m−3, 2–298 ng m−3 and 535–1087 ng m−3 for chloride (Cl−), nitrate (NO3−) and sulphate (SO42−), respectively. The aerosols in late winter/early spring, after polar sunrise, are found to be a mixture of long-range transported and regional to local originating aerosols. Fine particles, smaller than 1 μm, containing SO42−, Cl− and NO3−, are hypothesized to originate from long-range transport, where SO42− is by far the dominating anion accounting for 50–85% of the analyzed mass. The analysis suggests that Cl− and NO3− in coarser particles (> 1.5 μm) originate from local/regional sources. Under conditions where the air mass is transported over sea ice at high wind speeds, very coarse particles (> 18 μm) are observed, and it is hypothesized that frost flowers on the sea ice are a source of the very coarse nitrate particles.

Reactive nitrogen (Nr) compounds have different fates in the atmosphere due to differences in governing processes of physical transport, deposition and chemical transformation. Nr compounds addressed here include reduced nitrogen (NHx: ammonia (NH3) and its reaction product ammonium (NH4+)), oxidized nitrogen (NOy: nitrogen monoxide (NO) + nitrogen dioxide (NO2) and their reaction products) as well as organic nitrogen compounds (organic N). Pollution abatement strategies need to take into account these differences in the governing processes of these compounds when assessing their impact on ecosystem services, biodiversity, human health and climate. NOx (NO + NO2) emitted from traffic affects human health in urban areas where the presence of buildings increases the residence time in streets. In urban areas this leads to enhanced exposure of the population to NOx concentrations. NOx emissions have little impact on nearby ecosystems because of the small dry deposition rates of NOx. These compounds need to be converted into nitric acid (HNO3) before removal through deposition is efficient. HNO3 sticks quickly to any surface and is thereby either dry deposited or incorporated into aerosols as nitrate (NO3−). In contrast to NOx compounds, NH3 has potentially high impacts on ecosystems near the main agricultural sources of NH3 because of its large ground-level concentrations along with large dry deposition rates. Aerosol phase NH4+ and NO3− contribute significantly to background PM2.5 and PM10 (mass of aerosols with a diameter of less than 2.5 and 10 μm, respectively) with an impact on radiation balance as well as potentially on human health. Little is known quantitatively and qualitatively about organic N in the atmosphere, other than that it contributes a significant fraction of wet-deposited N, and is present in both gaseous and particulate forms in the atmosphere. Further studies are needed to characterize the sources, air chemistry and removal rates of organic N emissions.

Data collected at open water stations (depth>400m) in all major ocean basins in 2006-2008 are used to examine the relationship between the size structure of the phytoplankton community (determined by size fractionated chlorophyll filtration), temperature and inorganic nutrient availability. A significant relationship (p2 from the atmosphere. The reported results suggest that ocean warming may directly alter plankton community structure. This, in turn, may alter the structure of marine food webs and impact the performance of the open ocean as a natural carbon sink.

Measurements of aerosol particle physical and chemical properties, gas phase concentrations and meteorological parameters were made along a transect in southern Indiana during the Nucleation In ForesTs (NIFTy) experiment conducted in May 2008. These measurements indicate nucleation was observed at all three measurement sites on almost half of all sampling days. The intensity of the nucleation events, as measured by the increase in ≥10 nm aerosol particle number concentrations of approximately 2×104 cm−3 over a layer of at least 300 m depth, is in good agreement with recent model results for the Midwestern USA derived using PMCAMx-UF. During the hour after termination of nucleation approximately half of the number concentration reduction is due to coagulation, while the remainder is due in equal parts to dry deposition and entrainment of relatively ultra-fine aerosol particle free troposphere air. Clear nucleation with continuous subsequent growth is only observed on days when the morning fractional cloud cover was less than 30%. It is associated with a clear transition from a strongly stratified atmosphere with low turbulence intensity and weak vertical velocities, to much a weaker vertical gradient of wind speed, increased turbulence intensity and stronger downwards vertical velocities, consistent with growth of the mixed layer and entrainment of air from the residual layer. Nucleation intensity is not very strongly determined by the prevailing condensational sink. However, there is a strong correlation between both a modified version of the Nucleation Parameter from Boy and Kulmala (2002) and ultrafine aerosol particle number concentrations, and mean morning H2SO4 concentrations and ultrafine aerosol particle number concentrations. Five A-class event days during NIFTy were characterized by values of the dimensionless nucleation parameter of Kuang et al. (2010) that are below 0.3, further indicating the applicability of their postulate that nucleation is favored by LΓ values below 0.7. Based on aerosol particle composition measurements it appears that aerosol particle formation and initial growth to approximately 30 nm diameter is dominated by ammonium and sulfate. Conservative estimates of the percent contribution of H2SO4 to aerosol particle growth (for sub-30 nm aerosol particles) on five A-class event days ranged from 23 to 85%.

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the ocean's external (nonrecycled) nitrogen supply and up to ∼3% of the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide (N 2 O) per year. Although ∼10% of the ocean's drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N 2 O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.

This paper reports a study designed to test, evaluate and compare micro-meteorological methods for determining the particle number flux above forest canopies. Half-hour average particle number fluxes above a representative broad-leaved forest in Denmark derived using eddy covariance range from -7x10(7) m(-2) s(-1) (1st percentile) to 5x10(7) m(-2) s(-1) (99th percentile), and have a median value of -1.6x10(6) m(-2) s(-1). The statistical uncertainties associated with the particle number flux estimates are larger than those for momentum fluxes and imply that in this data set approximately half of the particle number fluxes are not statistically different to zero. Particle number fluxes from relaxed eddy accumulation (REA) and eddy covariance are highly correlated and of almost identical magnitude. Flux estimates from the co-spectral and dissipation methods are also correlated with those from eddy covariance but exhibit higher absolute magnitude of fluxes.

Over the past two decades, with recognition that the ocean's sea-ice cover is neither insensitive to climate change nor a barrier to light and matter, research in sea-ice biogeochemistry has accelerated significantly, bringing together a multi-disciplinary community from a variety of fields. This disciplinary diversity has contributed a wide range of methodological techniques and approaches to sea-ice studies, complicating comparisons of the results and the development of conceptual and numerical models to describe the important biogeochemical processes occurring in sea ice. Almost all chemical elements, compounds, and biogeochemical processes relevant to Earth system science are measured in sea ice, with published methods available for determiningbiomass, pigments, net community production, primary production, bacterial activity, macronutrients, numerous natural and anthropogenic organic compounds, trace elements, reactive and inert gases, sulfur species, the carbon dioxide system parameters, stable isotopes, and water-ice-Atmosphere fluxes of gases, liquids, and solids. For most of these measurements, multiple sampling and processing techniques are available, but to date there has been little intercomparison or intercalibration between methods. In addition, researchers collect different types of ancillary data and document their samples differently, further confounding comparisons between studies. These problems are compounded by the heterogeneity of sea ice, in which even adjacent cores can have dramatically different biogeochemical compositions. We recommend that, in future investigations, researchers design their programs based on nested sampling patterns, collect a core suite of ancillary measurements, and employ a standard approach for sample identification and documentation. In addition, intercalibration exercises are most critically needed for measurements of biomass, primary production, nutrients, dissolved and particulate organic matter (including exopolymers), the CO2 system, air-ice gas fluxes, and aerosol production. We also encourage the development of in situ probes robust enough for long-Term deployment in sea ice, particularly for biological parameters, the CO2 system, and other gases., SCOPUS: ar.j, info:eu-repo/semantics/published

The uptake rates of atmospheric CO2 in the Nordic Seas, and particularly the shelf waters around Greenland, are among the highest in the world’s oceans. The driving factors behind the air-sea exchange of CO2 in open waters are the difference between the partial pressure of CO2 (pCO2) in the atmosphere and the surface waters, leading to an uptake in areas where the pCO2 of surface waters is lower. Because the coastal area of Greenland is very sensitive to climate change, and because it takes up more CO2 relative to other marine areas, a realistic estimate of the exchange rates is crucial in order to obtain reliable assessments of the CO2 uptake by the Greenlandic coastal area. The results from present study reveal the importance of a diminishing sea ice cover; and it is clear that the wind climate is essential to the surface uptake of CO2.

Summertime nitric acid concentrations over a deciduous forest in the midwestern United States are reported, which range between 0.36 and 3.3 μg m−3. Fluxes to the forest are computed using the relaxed eddy accumulation technique and gradient methods. In accord with previous studies, the results indicate substantial uncertainties in the gradient-based calculations. The relaxed eddy accumulation (REA) derived fluxes are physically reasonable and are shown to be of similar magnitude to dry deposition estimates from gradient sampling. The REA derived mean deposition velocity is approximately 3 cm s−1, which is also comparable to growing season estimates derived by Meyers et al. for a similar deciduous forest. Occasional inverted concentration gradients and fluxes are observed but most are not statistically significant. Data are also presented that indicate substantial through canopy penetration of nitric acid.

Ikaite (CaCO3 · 6H2O) is a metastable phase of calcium carbonate that normally forms in a cold environment and/or under high pressure. Recently, ikaite crystals have been found in sea ice, and it has been suggested that their precipitation may play an important role in air–sea CO2 exchange in ice-covered seas. Little is known, however, of the spatial and temporal dynamics of ikaite in sea ice. Here we present evidence for highly dynamic ikaite precipitation and dissolution in sea ice grown at an outdoor pool of the Sea-ice Environmental Research Facility (SERF) in Manitoba, Canada. During the experiment, ikaite precipitated in sea ice when temperatures were below −4 °C, creating three distinct zones of ikaite concentrations: (1) a millimeter-to-centimeter-thin surface layer containing frost flowers and brine skim with bulk ikaite concentrations of >2000 μmol kg−1, (2) an internal layer with ikaite concentrations of 200–400 μmol kg−1, and (3) a bottom layer with ikaite concentrations of −1. Snowfall events caused the sea ice to warm and ikaite crystals to dissolve. Manual removal of the snow cover allowed the sea ice to cool and brine salinities to increase, resulting in rapid ikaite precipitation. The observed ikaite concentrations were on the same order of magnitude as modeled by FREZCHEM, which further supports the notion that ikaite concentration in sea ice increases with decreasing temperature. Thus, varying snow conditions may play a key role in ikaite precipitation and dissolution in sea ice. This could have a major implication for CO2 exchange with the atmosphere and ocean that has not been accounted for previously.

We suggest the application of a flux parameterization commonly used over terrestrial areas for calculation of CO2 fluxes over sea ice surfaces. The parameterization is based on resistance analogy. We present a concept for parameterization of the CO2 fluxes over sea ice suggesting to use properties of the atmosphere and sea ice surface that can be measured or calculated on a routine basis. Parameters, which can be used in the conceptual model, are analysed based on data sampled from a seasonal fast-ice area, and the different variables influencing the exchange of CO2 between the atmosphere and ice are discussed. We found the flux to be small during the late winter with fluxes in both directions. Not surprisingly we find that the resistance across the surface controls the fluxes and detailed knowledge of the brine volume and carbon chemistry within the brines as well as knowledge of snow cover and carbon chemistry in the ice are essential to estimate the partial pressure of pCO2 and CO2 flux. Further investigations of surface structure and snow cover and driving parameters such as heat flux, radiation, ice temperature and brine processes are required to adequately parameterize the surface resistance.

Knowledge on the forcing mechanisms (meteorological and chemical) that come into play in the Arctic environment is highly uncertain. We analyse data from measurements and mesoscale meteorological modelling for periods in summer 2011 and winter 2011/2012 to elucidate the boundary-layer features at Station Nord situated in Northern Greenland (81.6 N, 16.7 W). A major challenge for modelling is to connect local-scale observations with larger scales modelling of the atmosphere. In particular, in summer, bare soil in the vicinity of the station may not be present in the global boundary and surface conditions. Thus, in summer the deviations of modeled from measured values of temperature and humidity near the surface are larger compared to winter. We found that the underestimation of temperature near the ground is larger at clear sky compared to cloudy conditions; and the underestimation reached up to height 1-1, 5 km at clear sky and up to the first 100 m for the cloudy days. The measured wind speed profiles showed high variability, while the modeled were smoothed. During summer the modeled wind speed was close to or larger than the measured without clear indication for the role of clouds. In winter, the over-estimation of wind speed was more pronounced.

Regardless of study type, site topography, homogeneity and large-scale meteorological flows, estimating ecosystem-scale surface-fluxes using the micrometeorological eddy covariance method inevitably leads to questions concerning inclusion or exclusion of advective flux contributions, in providing representative results. For process-oriented studies in which fluxes are linked to local physical parameters and up-scaled through numerical modeling efforts, advection represents a site-specific component which interferes with our ability to isolate local biochemical processes of interest, as represented by turbulent fluxes. Yet, outside of discarding data reflecting excessive advective interference, no method currently exists to disentangle these contributions on flux estimates. Here, we present a novel comprehensive numerical scheme to identify and separate out advective contributions to exchanges in the surface layer. Comparison between the presented method and conventional methodology on observations of sensible heat, latent heat and CO2-fluxes from a number of sites suggests the presence of absolute flux thresholds at |QSENS|=30 Wm−2, |QLAT|=16 Wm−2 and |FCO2|=2.0 μmol m−2 s−1 marking clear shifts in the influence of advection. Above the thresholds, the relative difference of flux estimates δ remained fixed at δ =5–25% suggesting arguably negligible advection influence. Below the thresholds, however, relative difference rises to δSENS=⟨ 51%|88%|225%⟩, δLAT=⟨14%|28%|99%⟩ and δCO2=⟨ 41%|83%|521%⟩, where bracketed values are the 13.6th percentile, 50th percentile (the median) and the 86.4th percentile respectively, suggesting non-negligible relative influence of advection on low flux estimates. The thresholds thus serve as lower limits to local-scale flux resolvability by conventional methodology. The presented method is shown to allow for flux estimation during severe signal disruption and to yield fewer estimates for an enclosed gas analyzer during low-flux conditions suggesting the presence of a lower detection limit with this particular instrument setup, as well as a superiority of open path gas analyzers, in low-flux environments. Overall the notion of a dynamic and generally non-negligible overlap of advective and turbulent frequency-wise flux contributions is confirmed, suggesting the inevitable indiscriminate inclusion/exclusion of both when setting a fixed averaging time according to conventional methodology, leading ultimately to misrepresentation of actual local fluxes.

We present measurements of ammonia (NH3) over a deciduous forest in southern Indiana collected during four field campaigns; two in the spring during the transition to leaf-out and two during the winter. Above canopy NH3 concentrations measured continuously using two Wet Effluent Diffusion Denuders indicate mean concentrations of 0.6-1.2μgm-3 during the spring and 0.3μgm-3 during the winter. Measurements suggest that on average the forest act as a sink of NH3, with a representative daily deposition flux of 1.8mg-NH3m-2 during the spring. However, on some days during the spring inverted concentration gradients of NH3 were observed resulting in an apparent upward flux of nearly 0.2mg-NH3m-2h-1. Analyses suggest that this apparent emission flux may be due to canopy emission but evaporation of ammonium nitrate particles may also be partly responsible for the observed inverted concentration gradients.

Ikaite is a hydrous calcium carbonate mineral (CaCO3 · 6H2O). It is only found in a metastable state, and decomposes rapidly once removed from near-freezing water. Recently, ikaite crystals have been found in sea ice and it has been suggested that their precipitation may play an important role in air-sea CO2 exchange in ice-covered seas. Little is known, however, of the spatial and temporal dynamics of ikaite in sea ice. Here we present evidence for highly dynamic ikaite precipitation and dissolution in sea ice grown at an out-door pool of the Sea-ice Environmental Research Facility (SERF). During the experiment, ikaite precipitated in sea ice with temperatures below −3 °C, creating three distinct zones of ikaite concentrations: (1) a mm to cm thin surface layer containing frost flowers and brine skim with bulk concentrations of > 2000 μmol kg−1, (2) an internal layer with concentrations of 200–400 μmol kg−1 and (3) a~bottom layer with concentrations of < 100 μmol kg−1. Snowfall events caused the sea ice to warm, dissolving ikaite crystals under acidic conditions. Manual removal of the snow cover allowed the sea ice to cool and brine salinities to increase, resulting in rapid ikaite precipitation. The modeled (FREZCHEM) ikaite concentrations were in the same order of magnitude as observations and suggest that ikaite concentration in sea ice increase with decreasing temperatures. Thus, varying snow conditions may play a key role in ikaite precipitation and dissolution in sea ice. This will have implications for CO2 exchange with the atmosphere and ocean.

Many previous studies have indicated the importance of nitric acid (HNO3) reactions on sea salt particles for flux divergence of HNO3 in the marine surface layer. The potential importance of this reaction in determining the spatial and temporal patterns of nitrogen dry deposition to marine ecosystems is investigated using models of sea spray generation and particle- and gas-phase dry deposition. Under horizontally homogeneous conditions with near-neutral stability and for wind speeds between 3.5 and 10 m s−1, transfer of HNO3 to the particle phase to form sodium nitrate may decrease the deposition velocity of nitrogen by over 50%, leading to greater horizontal transport prior to deposition to the sea surface. Conversely, for wind speeds above 10 m s−1, transfer of nitrogen to the particle phase would increase the deposition rate and hence decrease horizontal transport prior to surface removal.

Fluxes of HNO 3 and NH 3 to the sea-surface have been obtained from measurements of vertical concentration profiles. The obtained fluxes have been compared to fluxes calculated by the use of the resistance method, and the fluxes calculated from measurements based on the extrapolation of a log-linear profile were found to be an order of a magnitude higher than the fluxes obtained from the resistance method. The difference between these two calculated fluxes is explained by scavenging of the gases by sea-spray and chemical reactions. A simple model is constructed to calculate the vertical profiles for HNO 3 in the case of high chemical reactions. The high fluxes and the measured profiles are explained by the calculated profiles of HNO 3 where chemical reactions are taken into account. Since both sink/sources and horizontal inhomogeneity are influencing the NH 3 flux, it has not been possible to calculate profiles for this component by taking chemical reactions into account. DOI: 10.1034/j.1600-0889.1998.t01-1-00001.x

Nordic Ministers of Environment adopted in March 2012 the “Svalbard Declaration” with decisions to reduce the negative impacts of the climate changes and air pollution caused by the emission of the ...

An increasingly ice-free Arctic Ocean could have significant implications for greenhouse-gas sources and sinks in the Northern high latitudes. In this Review, the impact of diminishing sea-ice extent on greenhouse-gas exchange in both marine and terrestrial Arctic environments is synthesised and discussed.

In order to develop strategies for controlling and reducing Arctic air pollution, there is a need to understand the basic mechanisms for determining the fate of air pollution in the Arctic. Sources of atmospheric particles at Station Nord (81° 36' N, 16° 40' W) in North East Greenland were evaluated for a two-year period from March 2008 to February 2010. Source apportionment using Positive Matrix Factorization (PMF) and COnstrained Physical REceptor Model (COPREM) was based on measurements of black carbon, elements (Al, Si, S, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Zr, Pb) and inorganic ions (SO2, SO42−, Na+, NH4+, NO3−, Cl2−. In general, source apportionment results by PMF and COPREM showed good agreement. Five sources adequately explained the measurements, which included a Marine and a Soil source of natural origin and three additional anthropogenic sources, which were all influenced by metal industries. One anthropogenic source was dominated by Zn of which air mass back trajectories using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model suggested a Canadian Arctic origin, despite certain influences from Southern and Eastern origins. Another anthropogenic source was characterised by high concentrations of Pb and As, which has been historically referred to as a Combustion source at Station Nord. The impacts of large-scale industry in Siberia, Russia were evident through high Cu concentrations in both the Combustion source and an additional Cu/Ni source. Br correlated well with the anthropogenic species S and Pb though the elements are unlikely to have a common origin. More likely, sulphuric acid aerosols serve as transport containers for Br species of marine origin. Of particular relevance to climate, sources of black carbon were identified to be mainly anthropogenic and most probably of Siberian origin (80–98%).

The understanding of biochemical feed-back mechanisms in the climate system is lacking knowledge in relation to bi-directional ammonia (NH3) exchange between natural ecosystems and the atmosphere. We therefore study the atmospheric NH3 fluxes during a 25 days period during autumn 2010 (21 October–15 November) for the Danish beech forest, Lille Bøgeskov, to address the hypothesis that NH3 emissions occur from deciduous forests in relation to leaf fall. This is accomplished by using observations of vegetation status, NH3 fluxes and model calculations. Vegetation status was observed using plant area index (PAI) and leaf area index (LAI). NH3 fluxes were measured using the relaxed eddy accumulation (REA) method. The REA based NH3 concentrations were compared to NH3 denuder measurements. Model calculations were obtained with the Danish Ammonia MOdelling System (DAMOS). 57.7% of the fluxes measured showed emission and 19.5% showed deposition. The mean NH3 flux was 0.087 ± 0.19 μg NH3-N m−2 s−1. A clear tendency of the flux going from negative (deposition) to positive (emission) fluxes of up to 0.96 ± 0.40 μg NH3-N m−2 s−1 throughout the measurement period was found. In the leaf fall period (23 October–8 November), an increase in the atmospheric NH3 concentrations was related to the increasing forest NH3 flux. The modelled concentration from DAMOS fits well the measured concentrations before leaf fall. During and after leaf fall, the modelled concentrations are too low. The results indicate that the missing contribution to atmospheric NH3 concentration from vegetative surfaces related to leaf fall are of a relatively large magnitude. We therefore conclude that emissions from deciduous forests are important to include in model calculations of atmospheric NH3 for forest ecosystems. Finally, diurnal variations in the measured NH3 concentrations were related to meteorological conditions, forest phenology and the spatial distribution of local anthropogenic NH3 sources. This suggests that an accurate description of ammonia fluxes over forest ecosystems requires a dynamic description of atmospheric and vegetation processes.

Reactive nitrogen (Nr) compounds have different fates in the atmosphere due to differences in the governing processes of physical transport, deposition and chemical transformation. Nr compounds addressed here include reduced nitrogen (NHx: ammonia (NH3) and its reaction product ammonium (NH4&plus;)), oxidized nitrogen (NOy: nitrogen monoxide (NO) &plus; nitrogen dioxide (NO2) and their reaction products) as well as organic nitrogen compounds (organic N). Pollution abatement strategies need to take into account the differences in the governing processes of these compounds when assessing their impact on ecosystem services, biodiversity, human health and climate. NOx (NO &plus; NO2) emitted from traffic affects human health in urban areas where the presence of buildings increases the residence time in streets. In urban areas this leads to enhanced exposure of the population to NOx concentrations. NOx emissions generally have little impact on nearby ecosystems because of the small dry deposition rates of NOx. These compounds need to be converted into nitric acid (HNO3) before removal through deposition is efficient. HNO3 sticks quickly to any surface and is thereby either dry deposited or incorporated into aerosols as nitrate (NO3−). In contrast to NOx compounds, NH3 has potentially high impacts on ecosystems near the main agricultural sources of NH3 because of its large ground-level concentrations along with large dry deposition rates. Aerosol phase NH4&plus; and NO3− contribute significantly to background PM2.5 and PM10 (mass of aerosols with an aerodynamic diameter of less than 2.5 and 10 μm, respectively) with an impact on radiation balance as well as potentially on human health. Little is known quantitatively and qualitatively about organic N in the atmosphere, other than that it contributes a significant fraction of wet-deposited N, and is present in both gaseous and particulate forms. Further studies are needed to characterise the sources, air chemistry and removal rates of organic N emissions.