Search

We investigated long-term changes using a harmonised 22-year data set of aerosol light absorption measurements, in conjunction with air mass history and aerosol source analysis. The measurements were performed at Zeppelin Observatory, Svalbard, from 2002 to 2023. We report a statistically significant decreasing long-term trend for the light absorption coefficient. However, the last 8 years of 2016–2023 showed a slight increase in the magnitude of the light absorption coefficient for the Arctic haze season. In addition, we observed an increasing trend in the single-scattering albedo from 2002 to 2023. Five distinct source regions, representing different transport pathways, were identified. The trends involving air masses from the five regions showed decreasing absorption coefficients, except for the air masses from Eurasia. We show that the changes in the occurrences of each transport pathway cannot explain the reductions in the absorption coefficient observed at the Zeppelin station. An increase in contributions of air masses from more marine regions, with lower absorption coefficients, is compensated for by an influence from high-emission regions. The proportion of air masses en route to Zeppelin, which have been influenced by active fires, has undergone a noticeable increase starting in 2015. However, this increase has not impacted the long-term trends in the concentration of light-absorbing aerosol. Along with aerosol optical properties, we also show an increasing trend in accumulated surface precipitation experienced by air masses en route to the Zeppelin Observatory. We argue that the increase in precipitation, as experienced by air masses arriving at the station, can explain a quarter of the long-term reduction in the light absorption coefficient. We emphasise that meteorological conditions en route to the Zeppelin Observatory are critical for understanding the observed trends. [ABSTRACT FROM AUTHOR]

The Arctic is a rapidly changing ecosystem, with complex ice–ocean–atmosphere feedbacks. An important process is new particle formation (NPF), from gas-phase precursors, which provides a climate forcing effect. NPF has been studied comprehensively at different sites in the Arctic, ranging from those in the High Arctic and those at Svalbard to those in the continental Arctic, but no harmonised analysis has been performed on all sites simultaneously, with no calculations of key NPF parameters available for some sites. Here, we analyse the formation and growth of new particles from six long-term ground-based stations in the Arctic (Alert, Villum, Tiksi, Zeppelin Mountain, Gruvebadet, and Utqiaġvik). Our analysis of particle formation and growth rates in addition to back-trajectory analysis shows a summertime maxima in the frequency of NPF and particle formation rate at all sites, although the mean frequency and particle formation rates themselves vary greatly between sites, with the highest at Svalbard and lowest in the High Arctic. The summertime growth rate, condensational sinks, and vapour source rates show a slight bias towards the southernmost sites, with vapour source rates varying by around an order of magnitude between the northernmost and southernmost sites. Air masses back-trajectories during NPF at these northernmost sites are associated with large areas of sea ice and snow, whereas events at Svalbard are associated with more sea ice and ocean regions. Events at the southernmost sites are associated with large areas of land and sea ice. These results emphasise how understanding the geographical variation in surface type across the Arctic is key to understanding secondary aerosol sources and providing a harmonised analysis of NPF across the Arctic. [ABSTRACT FROM AUTHOR]

The Arctic is a rapidly changing ecosystem, with complex ice-ocean-atmosphere feedbacks. An important process new particle formation (NPF) from gas phase precursors, which provide a climate forcing effect. NPF has been studied comprehensively at different sites in the Arctic ranging from those in the high Arctic, those at Svalbard, and those in the continental Arctic, but no harmonized analysis has been performed on all sites simultaneously, with no calculations of key NPF parameters available for some sites. Here, we analyse the formation and growth of new particles from six long-term ground-based stations in the Arctic (Alert, Villum, Tiksi, Mt. Zeppelin, Gruvebadet. & Utqiagvik). Our analysis of particle formation and growth rates, as well as back trajectory analysis shows summertime maxima in frequency of NPF and particle formation rate at all sites, although the mean frequency and particle formation rates themselves vary greatly between sites, highest at Svalbard, and lowest in the high Arctic. Growth rate, condensational sinks and vapour source rates show a slight bias towards the southernmost sites, with vapour source rates varying by around an order of magnitude between the northernmost and southernmost sites. Air masse back trajectories during NPF at these northernmost sites are associated with large areas of sea ice and snow, whereas events at Svalbard are associated with more sea ice and ocean regions. Events at the southernmost sites are associated with large areas of land, and sea ice. These results emphasize how understanding the geographical variation in surface type across the Arctic is key to understanding secondary aerosol sources, and provide harmonised analysis of NPF across the Arctic. [ABSTRACT FROM AUTHOR]

Snow darkening by deposited light-absorbing particles (LAP) has the potential to accelerate snowmelt and shift the snow melt-out date. Here we investigate the sensitivity of the seasonal snow cover duration to changes in LAP at a high altitude valley site in the Central Himalayas, India. First, the variation of the albedo of the seasonal snow was emulated using two seasons of automatic weather station (AWS) data and applying a constant, but realistic deposition of LAP to the snow. Then, the number of days with snowmelt were evaluated based on the estimated net energy budget of the seasonal snow cover and the derived surface temperature. The impact on the energy budget by LAP combined with the melt-day analysis resulted in very simple relations to determine the contribution of LAP to the number of days with snowmelt of the seasonal snow in Himalaya. Above a concentration of 1 ng g-1(Elemental Carbon equivalent, ECeq, which in this study includes EC and the absorption equivalent EC contribution by other light absorbing particles, such as mineral dust) in new snow, the number of days with snowmelt can be estimated by; days = 0.0109(log(ECeq) + 1) PP ± 0.0033 (log(ECeq) + 1)PP, where PP is the seasonal precipitation in mm snow water equivalent. A change in ECeqby a factor of two corresponds to about 1/3 of a day per 100 mm precipitation. Although the change in the number of days with melt caused by the changes in ECeq is small, the estimated total change in the snow melt-out date by LAP can be significant. For our realistic base case scenario for the Sunderdhunga Valley, Central Himalayas, India, of ECeq=100 ng g-1 and PP=400 mm, this yields in an advancement of the melt-out date of about 13 days. [ABSTRACT FROM AUTHOR]

Light-absorbing carbonaceous aerosols emitted by biomass or fossil fuel combustion can contribute to amplifying Arctic climate warming by lowering the albedo of snow. The Svalbard archipelago, being near to Europe and Russia, is particularly affected by these pollutants, and improved knowledge of their distribution in snow is needed to assess their impact. Here we present and synthesize new data obtained on Svalbard between 2007 and 2018, comprising measurements of elemental (EC) and water-insoluble organic carbon (WIOC) in snow from 37 separate sites. We used these data, combined with meteorological data and snowpack modeling, to investigate the variability of EC and WIOC deposition in Svalbard snow across latitude, longitude, elevation and time. Overall, EC concentrations (CsnowEC) ranged from <1.0 to 266.6 ng g -1 , while WIOC concentrations (CsnowWIOC) ranged from <1 to 9426 ng g -1 , with the highest values observed near Ny-Ålesund. Calculated snowpack loadings (LsnowEC , LsnowWIOC) on glaciers surveyed in spring 2016 were 0.1 to 2.6 mg m -2 and 2 to 173 mg m -2 , respectively. The median CsnowEC and the LsnowEC on those glaciers were close to or lower than those found in earlier (2007–2009), comparable surveys. Both LsnowEC and LsnowWIOC increased with elevation and snow accumulation, with dry deposition likely playing a minor role. Estimated area-averaged snowpack loads across Svalbard were 1.1 mg EC m -2 and 38.3 mg WIOC m -2 for the 2015–2016 winter. An ∼11 -year long dataset of spring surface snow measurements from the central Brøgger Peninsula was used to quantify the interannual variability of EC and WIOC deposition in snow. In most years, CsnowEC and CsnowWIOC at Ny-Ålesund (50 m a.s.l.) were 2–5 times higher than on the nearby Austre Brøggerbreen glacier (456 m a.s.l.), and the median EC/WIOC in Ny-Ålesund was 6 times higher, suggesting a possible influence of local EC emission from Ny-Ålesund. While no long-term trends between 2011 and 2018 were found, CsnowEC and CsnowWIOC showed synchronous variations at Ny-Ålesund and Austre Brøggerbreen. When compared with data from other circum-Arctic sites obtained by comparable methods, the median CsnowEC on Svalbard falls between that found in central Greenland (lowest) and those in continental sectors of European Arctic (northern Scandinavia, Russia and Siberia; highest), which is consistent with large-scale patterns of BC in snow reported by surveys based on other methods. [ABSTRACT FROM AUTHOR]

Anthropogenic activities on the Indo-Gangetic Plain emit vast amounts of light-absorbing particles (LAPs) into the atmosphere, modifying the atmospheric radiation state. With transport to the nearby Himalayas and deposition to its surfaces the particles contribute to glacier melt and snowmelt via darkening of the highly reflective snow. The central Himalayas have been identified as a region where LAPs are especially pronounced in glacier snow but still remain a region where measurements of LAPs in the snow are scarce. Here we study the deposition of LAPs in five snow pits sampled in 2016 (and one from 2015) within 1 km from each other from two glaciers in the Sunderdhunga Valley, in the state of Uttarakhand, India, in the central Himalayas. The snow pits display a distinct enriched LAP layer interleaved by younger snow above and older snow below. The LAPs exhibit a distinct vertical distribution in these different snow layers. For the analyzed elemental carbon (EC), the younger snow layers in the different pits show similarities, which can be characterized by a deposition constant of about 50 µ g m -2 mm -1 snow water equivalent (SWE), while the old-snow layers also indicate similar values, described by a deposition constant of roughly 150 µ g m -2 mm -1 SWE. The enriched LAP layer, contrarily, displays no similar trends between the pits. Instead, it is characterized by very high amounts of LAPs and differ in orders of magnitude for concentration between the pits. The enriched LAP layer is likely a result of strong melting that took place during the summers of 2015 and 2016, as well as possible lateral transport of LAPs. The mineral dust fractional absorption is slightly below 50 % for the young- and old-snow layers, whereas it is the dominating light-absorbing constituent in the enriched LAP layer, thus, highlighting the importance of dust in the region. Our results indicate the problems with complex topography in the Himalayas but, nonetheless, can be useful in large-scale assessments of LAPs in Himalayan snow. [ABSTRACT FROM AUTHOR]

The study of long-term trends in aerosol optical properties is an important task to understand the underlying aerosol processes influencing the change of climate. The Arctic, as the place where climate change manifests most, is an especially sensitive region of the world. Within this work, we use a unique long-term data record of key aerosol optical properties from the Zeppelin Observatory, Svalbard, to ask the question of whether the environmental changes of the last 2 decades in the Arctic are reflected in the observations. We perform a trend analysis of the measured particle light scattering and backscattering coefficients and the derived scattering Ångström exponent and hemispheric backscattering fraction. In contrast to previous studies, the effect of in-cloud scavenging and of potential sampling losses at the site are taken explicitly into account in the trend analysis. The analysis is combined with a back trajectory analysis and satellite-derived sea ice data to support the interpretation of the observed trends. We find that the optical properties of aerosol particles have undergone clear and significant changes in the past 2 decades. The scattering Ångström exponent exhibits statistically significant decreasing of between - 4.9 % yr -1 and - 6.5 % yr -1 (using wavelengths of λ=450 and 550 nm), while the particle light scattering coefficient exhibits statistically significant increasing trends of between 2.6 % yr -1 and 2.9 % yr -1 (at a wavelength of λ=550 nm). The magnitudes of the trends vary depending on the season. These trends indicate a shift to an aerosol dominated more by coarse-mode particles, most likely the result of increases in the relative amount of sea spray aerosol. We show that changes in air mass circulation patterns, specifically an increase in air masses from the south-west, are responsible for the shift in aerosol optical properties, while the decrease of Arctic sea ice in the last 2 decades only had a marginal influence on the observed trends. [ABSTRACT FROM AUTHOR]

Anthropogenic activities on the Indo-Gangetic Plain emit vast amounts of light-absorbing particles (LAP) into the atmosphere, modifying the atmospheric radiation scheme. With transport to the nearby Himalayan mountains and deposition to its surfaces the particles contribute to glacier and snowmelt via darkening of the highly reflective snow. The Central Himalayas have been identified as a region where LAP are especially pronounced in glacier snow, but still remain a region where measurements of LAP in the snow are scarce. Here we study the deposition of LAP in five snow pits sampled in 2016 (and one from 2015) from two glaciers in the Sunderdhunga valley, state of Uttarakhand, India, Central Himalaya. The snow pits display a distinct melt layer interleaved by younger snow above, and older snow below. The LAP exhibit a large vertical distribution in these different snow layers. For the analyzed elemental carbon (EC), the younger snow layers in the different pits show similarities, and can be characterized by a deposition constant of about 50 µgm-2 mm-1 while the old snow layers also indicate similar values, and can be described with deposition constant of roughly 150 µgm-2 mm-1. The melt layer, contrarily, display no similar trends between the pits. Instead, it is characterized by very high amounts of LAP, and differ in orders of magnitude for concentration between the pits. The melt layer is likely a result of strong melting that took place during the summers of 2015 and 2016. The mineral dust fractional absorption is slightly below 50 % for the young and old snow layer, whereas in the melt layer is the dominating light absorbing constituent, thus, highlighting the importance of dust in the region. Our results indicate the problems with complex topography in the Himalaya, but nonetheless, can be useful in large-scale assessments of LAP in Himalayan snow. [ABSTRACT FROM AUTHOR]

The study of long-term trends in aerosol optical properties is an important task to understand the underlying aerosol processes influencing the change of climate. The Arctic, as the place where climate change manifests most, is an especially sensitive region of the world. Within this work, we use a unique long-term data record of key aerosol optical properties from Zeppelin observatory, Svalbard, to ask the question of whether the environmental changes of the last two decades in the Arctic are reflected in the observations. We perform a trend analysis of the measured particle light scattering and backscattering coefficients and the derived scattering Ångström exponent and hemispheric backscattering fraction. In contrast to previous studies, the effect of in-cloud scavenging and potential sampling losses at the site is taken explicitly into account in the trend analysis. The analysis is combined with a back trajectory analysis and satellite-derived sea ice data, to support the interpretation of the observed trends. We find that the optical properties of aerosol particles have undergone clear and significant changes in the past two decades. The scattering Angström exponent and the particle light scattering coefficient exhibit statistically significant decreasing of between -4.9 and -6.3 % per year (using wavelengths of λ = 450 and 550 nm) and increasing trends of between 2.3 and 2.9 % per year (at a wavelength of λ = 550 nm), respectively. The magnitudes of the trends vary depending on the season. These trends indicate a shift to an aerosol dominated more by coarse-mode particles, most likely the result of increases in the relative amount of sea spray aerosol. We show that changes in air mass circulation patterns, specifically an increase in air masses from the south-west, are responsible for the shift in aerosol optical properties, while the decrease of Arctic sea ice in the last two decades had only a marginal influence on the observed trends. [ABSTRACT FROM AUTHOR]

Light-absorbing carbonaceous aerosols emitted by biomass or fossil fuel combustion can contribute to amplify Arctic climate warming by lowering the albedo of snow. The Svalbard archipelago, being near to Europe and Russia, is particularly affected by these pollutants, and improved knowledge of their distribution in snow is needed to assess their impact. Here we present and synthesize new data obtained on Svalbard between 2007 and 2018, comprising 324 measurements of elemental (EC) and organic carbon (OC) in snow from 49 sites. We used these data, combined with meteorological and aerosol data and snowpack modelling, to investigate the variability of EC and OC deposition in Svalbard snow across latitude, longitude, elevation and time. Overall, EC concentrations (…) ranged from <1.0 to 266.6 ng g-1, while OC concentrations (…) ranged from <1.0 to 9449.1 ng g-1, with the highest values observed near Ny-Ålesund. Calculated snowpack loadings (…) m April 2016 were 0.1 to 16.2 mg m-2and 1.7 to 320.1 mg m-2, respectively. The median … and … in the late 2015-16 winter snowpack on glaciers were close to or lower than those found in earlier (2007-09), comparable surveys. Both … and … increased exponentially with elevation and snow accumulation, with dry deposition likely playing a minor role. Estimated area-averaged snowpack loads across Svalbard were 1.8 mg EC m-2 and 71.5 mg OC m-2 in April 2016. An ~11-year long dataset of spring surface snow measurements from central Brøgger Peninsula was used to quantify the interannual variability of EC and OC deposition in snow. On average, … and … at Ny-Ålesund (50 m a.s.l.) were 3 and 7 times higher, respectively, than on the nearby Austre Brøggerbreen glacier (456 m a.s.l.), and the median EC/OC in Ny-Ålesund was 6 times higher, pointing to some local EC emission from Ny-Ålesund. While no long-term trends between 2011 and 2018 were found, … and … showed synchronous variations at Ny-Ålesund and Austre Brøggerbreen. Comparing … at Austre Brøggerbreen with aerosol data from Zeppelin Observatory, we found that snowfall washout ratios between 10 and 300 predict a range of … in agreement with that measured in surface snow. Together, results from this study and comparable surveys confirm the existence of a longitudinal gradient in EC deposition across the Arctic and sub-Arctic, with the lowest … found in the western Arctic (Alaska, Yukon) and central Greenland, and the highest in northwestern Russia and Siberia. [ABSTRACT FROM AUTHOR]

Size-resolved aerosol trends were investigated based on a 17-year data set (2000–2017) from the rural background site Aspvreten located in southern Sweden (58.8 ∘ N, 17.4 ∘ E). Cluster analysis of the size distributions was performed to aid in the interpretation of the data. The results confirm previous findings of decreasing aerosol mass and number during the last decades as a result of reduced anthropogenic emissions in Europe. We show that both particle modal number concentration and size have substantially been reduced during the last 17 years. Negative trends in particle number concentration of about 10 cm -3 yr -1 are present for nuclei, Aitken, and accumulation modes. In total, integral particle number concentration has decreased by 30 %, from 1860 to ca. 1300 cm -3. The reduction in modal number concentration is accompanied by a decrease in modal size, and this decrease is largest for the accumulation mode (2 nm yr -1 or about 17 % for the whole period). These reductions have resulted in a decrease in submicron particle mass (< 390 nm) by more than 50 % over the period 2000–2017. These decreases are similar to observations found at other stations in northern Europe. Although all size classes show a downward trend as annual averages, we also show that observed trends are not evenly distributed over the year and that a rather complex picture emerges where both sign and magnitude of trends vary with season and size. The strongest negative trends are present during spring (accumulation mode) and autumn (Aitken mode). The strongest positive trends are present during summer months (Aitken mode). The combined trajectory and data analyses do not present evidence for an increase in new particle formation formed locally, although some evidence of increased new particle formation some distance away from the receptor is present. Observed aerosol size distribution data, together with an adiabatic cloud parcel model, were further used to estimate the change in cloud droplet concentration for various assumptions of updraught velocities and aerosol chemical composition. The results indicate a substantial increase in the atmospheric brightening effect due to a reduction in cloud reflectivity corresponding to 10 %–12 % reduction in cloud albedo over the period 2000–2017. [ABSTRACT FROM AUTHOR]

The deposition of light-absorbing aerosol (LAA) onto snow initiates processes that lead to increased snowmelt. Measurements of LAA, such as black carbon (BC) and mineral dust, have been observed globally to darken snow. Several measurement techniques of LAA in snow collect the particulates on filters for analysis. Here we investigate micro-quartz filters' optical response to BC experiments in which the particles are initially suspended in air or in a liquid. With particle soot absorption photometers (PSAPs) we observed a 20 % scattering enhancement for quartz filters compared to the standard PSAP Pallflex filters. The multiple-scattering correction factor (Cref) of the quartz filters for airborne soot aerosol is estimated to be ∼3.4. In the next stage correction factors were determined for BC particles mixed in water and also for BC particles both mixed in water and further treated in an ultrasonic bath. Comparison of BC collected from airborne particles with BC mixed in water filters indicated a higher mass absorption cross section by approximately a factor of 2 for the liquid-based filters, which is probably due to the BC particles penetrating deeper in the filter matrix. The ultrasonic bath increased absorption still further, roughly by a factor of 1.5, compared to only mixing in water. Application of the correction functions to earlier published field data from the Himalaya and Finnish Lapland yielded mass absorption coefficient (MAC) values of ∼7 –10 m 2 g -1 at λ=550 nm, which is in the range of the published MAC of airborne BC aerosol. [ABSTRACT FROM AUTHOR]

Size resolved aerosol trends were investigate based on a 17-year data set (2000–2017) from the rural background site Aspvreten located in southern Sweden (58.8° N, 17.4° E). Cluster analysis of the size distributions was performed to aid in the interpretation of the data. The results confirm previous findings of decreasing aerosol mass during last decades as a result of reduced anthropogenic emissions in Europe. We show that both particle modal number concentration and size substantially has been reduced during last decades. Negative trends in particle number concentration of about 10 cm−3 y−1 is present for nuclei, Aitken and accumulation modes. In total, integral particle number concentration has decreased by 30 %, from 1860 cm−3 to ca 1300 cm−3. This decrease is similar to observations found at other stations in Northern Europe. The reduction in modal number concentration is accompanied by a decrease in modal size, and this decrease is largest for the accumulation mode (2 nm y−1 or about 17 % for the whole period). These reductions have resulted in a decrease in submicron particle mass (< 390 nm) by more than 50 % over the period 2000–2017. Although all size classes show a downward trend as annual averages, we also show that observed trends are not evenly distributed over the year, and that a rather complex picture emerges where both sign and magnitude of trends vary with season and size. The strongest negative trends are present during spring (accumulation mode) and autumn (Aitken mode). The strongest positive trends are present during summer months (Aitken mode). The data analyses do not present evidence for an increase in new particle formation formed locally, although some evidence of increased new particle formation some distance away from the receptor is present. Observed aerosol size distribution data, together with an adiabatic cloud parcel model, was further used to estimate the change in cloud droplet concentration for various assumptions of updraft velocities and aerosol chemical composition. The results indicate a substantial increase in the atmospheric brightening effect due to a reduction in cloud reflectivity corresponding to 10–12 % reduction in cloud albedo over the period 2000–2017. [ABSTRACT FROM AUTHOR]

The deposition of light-absorbing aerosols (LAA) onto snow initiates processes that lead to increased snowmelt. Measurements of LAA, such as black carbon (BC) and mineral dust, have been observed globally to darken snow. Several measurement techniques of LAA in snow collects the particulates on filters for analysis. Here we investigate micro-quartz filters optical response to BC experiments where the particles initially are suspended in air or in a liquid. With particle soot absorption photometers (PSAP) we observed a 20 % scattering enhancement for quartz filters compared to the standard PSAP Pallflex filters. The multiple-scattering correction factor (Cref) of the quartz filters for airborne soot aerosol is estimated to ~3.4. In the next stage correction factors were determined for BC particles mixed in water and also for BC particles both mixed in water and further treated in an ultrasonic bath. Comparison of BC collected from airborne particles with BC mixed in water filters indicated approximately a factor of two higher mass absorption cross section for the liquid based filters, probably due to the BC particles penetrating deeper in the filter matrix. The ultrasonic bath increased absorption still further, roughly by a factor of 1.5 compared to only mixing in water. Application of the correction functions to earlier published field data from the Himalaya and Finnish Lapland yielded MAC values of ~7-10 m² g-1 at λ= 550 nm which is in the range of published MAC of airborne BC aerosol. [ABSTRACT FROM AUTHOR]

Atmospheric black carbon (BC) particles are a concern due to their impact on air quality and climate. Their net climate effect is, however, still uncertain. This uncertainty is partly related to the contribution of coated BC particles to the global cloud condensation nuclei (CCN) budgets. In this study, laboratory measurements were performed to investigate CCN activity of BC (REGAL 400R pigment black) particles, in pure state or coated through evaporating and subsequent condensation of glutaric acid, levoglucosan (both water-soluble organics) or oleic acid (an organic compound with low solubility). A combination of soot particle aerosol mass spectrometer (SP-AMS) measurements and size distribution measurements with a scanning mobility particle sizer (SMPS) showed that the studied BC particles were nearly spherical agglomerates with a fractal dimension of 2.79 and that they were coated evenly by the organic species. The CCN activity of BC particles increased after coating with all the studied compounds and was governed by the fraction of organic material. The CCN activation of the BC particles coated by glutaric acid and levoglucosan were in good agreement with the theoretical calculations using the shell-andcore model, which is based on a combination of the CCN activities of the pure compounds. The oleic acid coating enhanced the CCN activity of the BC particles, even though the pure oleic acid particles were CCN inactive. The surprising effect of oleic acid might be related to the arrangement of the oleic acid molecules on the surface of the BC cores or other surface phenomena facilitating water condensation onto the coated particles. Our results show that present theories have potential for accurately predicting the CCN activity of atmospheric BC coated with organic species, given that the identities and amounts of the coating species are known. Furthermore, our results suggest that even relatively thin soluble coatings (around 2 nm for the compounds studied here) are enough to make the insoluble BC particles CCN active at typical atmospheric supersaturations and thus be efficiently taken up by cloud droplets. This highlights the need for an accurate description of the composition of atmospheric particles containing BC to unravel their net impact on climate. [ABSTRACT FROM AUTHOR]

Light-absorbing impurities (LAIs) deposited in snow have the potential to substantially affect the snow radiation budget, with subsequent implications for snow melt. To more accurately quantify the snow albedo, the contribution from different LAIs needs to be assessed. Here we estimate the main LAI components, elemental carbon (EC) (as a proxy for black carbon) and mineral dust in snow from the Indian Himalayas and paired the results with snow samples from Arctic Finland. The impurities are collected onto quartz filters and are analyzed thermal-optically for EC, as well as with an additional optical measurement to estimate the light-absorption of dust separately on the filters. Laboratory tests were conducted using substrates containing soot and mineral particles, especially prepared to test the experimental setup. Analyzed ambient snow samples show EC concentrations that are in the same range as presented by previous research, for each respective region. In terms of the mass absorption cross section (MAC) our ambient EC surprisingly had about half of the MAC value compared to our laboratory standard EC (chimney soot), suggesting a less light absorptive EC in the snow, which has consequences for the snow albedo reduction caused by EC. In the Himalayan samples, larger contributions by dust (in the range of 50% or greater for the light absorption caused by the LAI) highlighted the importance of dust acting as a light absorber in the snow. Moreover, EC concentrations in the Indian samples, acquired from a 120 cm deep snow pit (possibly covering the last five years of snow fall), suggest an increase in both EC and dust deposition. This work emphasizes the complexity in determining the snow albedo, showing that LAI concentrations alone might not be sufficient, but additional transient effects on the light-absorbing properties of the EC need to be considered and studied in the snow. Equally as imperative is the confirmation of the spatial and temporal representativeness of these data by comparing data from several and deeper pits explored at the same time. [ABSTRACT FROM AUTHOR]

Atmospheric black carbon (BC) particles are a concern due to their impact on air quality and climate. Their net climate effect is, however, still uncertain. This uncertainty is partly related to the contribution of coated BC-particles to the global CCN budgets. In this study, laboratory measurements were performed to investigate cloud condensation nuclei (CCN) activity of BC (Regal black) particles, in pure state or coated through evaporating and subsequent condensation of glutaric acid, levoglucosan (both water-soluble organics) or oleic acid (an organic compound with low solubility). A combination of Soot Particle Aerosol Mass Spectrometer (SP-AMS) measurements and size distribution measurements with Scanning Mobility Particle Sizer (SMPS) showed that the studied BC particles were nearly spherical agglomerates with a fractal dimension of 2.79 and that they were coated evenly by the organic species. The CCN activity of BC particles increased after coating with all the studied compounds and was governed by the fraction of organic material. The CCN activation of the BC particles coated by glutaric acid and levoglucosan were in good agreement with the theoretical calculations using shell-and-core model, which is based on a combination of the CCN activities of the pure compounds. The oleic acid coating enhanced the CCN activity of the BC particles, even though the pure oleic acid particles were CCN inactive. The surprising effect of oleic acid might be related to the arrangement of the oleic acid molecules on the surface of the BC cores or other surface phenomena facilitating water condensation onto the coated particles. Our results show potential in accurately predicting the CCN activity of atmospheric BC coated with organic species by present theories, given that the identities and amount of the coating species are known. Furthermore, our results suggest that even relatively thin soluble coatings (around 2 nm for the compounds studied here) are enough to make the insoluble BC particles CCN active at typical atmospheric supersaturations and thus be efficiently taken up by cloud droplets. This highlights the need of an accurate description of the composition of atmospheric particles containing BC to unravel their net impact on climate. [ABSTRACT FROM AUTHOR]

The climate impact of black carbon (BC) is notably amplified in the Arctic by its deposition, which causes albedo decrease and subsequent earlier snow and ice spring melt. To comprehensively assess the climate impact of BC in the Arctic, information on both atmospheric BC concentrations and deposition is essential. Currently, Arctic BC deposition data are very scarce, while atmospheric BC concentrations have been shown to generally decrease since the 1990s. However, a 300-year Svalbard ice core showed a distinct increase in EC (elemental carbon, proxy for BC) deposition from 1970 to 2004 contradicting atmospheric measurements and modelling studies. Here, our objective was to decipher whether this increase has continued in the 21st century and to investigate the drivers of the observed EC deposition trends. For this, a shallow firn core was collected from the same Svalbard glacier, and a regional-to-meso-scale chemical transport model (SILAM) was run from 1980 to 2015. The ice and firn core data indicate peaking EC deposition values at the end of the 1990s and lower values thereafter. The modelled BC deposition results generally support the observed glacier EC variations. However, the ice and firn core results clearly deviate from both measured and modelled atmospheric BC concentration trends, and the modelled BC deposition trend shows variations seemingly independent from BC emission or atmospheric BC concentration trends. Furthermore, according to the model ca. 99 % BC mass is wet-deposited at this Svalbard glacier, indicating that meteorological processes such as precipitation and scavenging efficiency have most likely a stronger influence on the BC deposition trend than BC emission or atmospheric concentration trends. BC emission source sectors contribute differently to the modelled atmospheric BC concentrations and BC deposition, which further supports our conclusion that different processes affect atmospheric BC concentration and deposition trends. Consequently, Arctic BC deposition trends should not directly be inferred based on atmospheric BC measurements, and more observational BC deposition data are required to assess the climate impact of BC in Arctic snow. [ABSTRACT FROM AUTHOR]

Light-absorbing impurities (LAI) have the potential to substantially affect snow albedo, with subsequent changes on snow melt and impact on climate. To more accurately quantify the snow albedo, the contribution from different LAI needs to be assessed. Here we estimate the main LAI components, elemental carbon (EC) (as a proxy for black carbon) and mineral dust in snow from Indian Himalaya and compared it to snow samples from Arctic Finland. The impurities are collected onto quartz filters and are analyzed thermal-optically for EC, as well as with an additional optical measurement to estimate the light-absorption of dust separately on the filters. Laboratory tests were conducted using substrates containing soot and mineral particles specially prepared to test the experimental setup. Analyzed ambient snow samples show EC concentrations that are in the same range as presented by previous research, for each respective region. In terms of the mass absorption cross section (MAC) our ambient EC had surprisingly about half of the MAC value compared to our laboratory standard EC (chimney soot), suggesting a less light absorptive EC in the snow, which has consequences for the snow albedo reduction caused by EC. In the Himalayan samples, larger contributions by dust (in the range of 50% or greater for the light absorption caused by the LAI) highlighted the importance of dust acting as a light absorber in the snow. Moreover, EC concentrations in the Indian samples, acquired from a 120 cm deep snow pit (covering possibly the last five years of snow fall), suggest an increase in both EC and dust, while at the same time there is a tendency for a reduction in the MAC value with snow depth. This work emphasizes the complexity in determining the snow albedo, showing that LAI concentrations alone might not be sufficient, but additional transient effects on the light-absorbing properties of the EC need to be considered and studied in the snow. Equally imperative is to confirm the spatial and temporal representativeness of these data by comparing data from several and longer pits explored at the same time. [ABSTRACT FROM AUTHOR]

A new, more physically based wet removal scheme for aerosols has been implemented in the Lagrangian particle dispersion model FLEXPART. It uses three-dimensional cloud water fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) to determine cloud extent and distinguishes between in-cloud and below-cloud scavenging. The new in-cloud nucleation scavenging depends on cloud water phase (liquid, ice or mixed-phase), based on the aerosol's prescribed efficiency to serve as ice crystal nuclei and liquid water nuclei, respectively. The impaction scavenging scheme now parameterizes below-cloud removal as a function of aerosol particle size and precipitation type (snow or rain) and intensity. Sensitivity tests with the new scavenging scheme and comparisons with observational data were conducted for three distinct types of primary aerosols, which pose different challenges for modeling wet scavenging due to their differences in solubility, volatility and size distribution: (1) 137Cs released during the Fukushima nuclear accident attached mainly to highly soluble sulphate aerosol particles, (2) black carbon (BC) aerosol particles, and (3) mineral dust. Calculated e-folding lifetimes of accumulation mode aerosols for these three aerosol types were 11.7, 16.0, and 31.6 days respectively, when well mixed in the atmosphere. These are longer lifetimes than those obtained by the previous removal schem, and, for mineral dust in particular, primarily result from very slow in-cloud removal, which globally is the primary removal mechanism for these accumulation mode particles. Calculated e-folding lifetimes in FLEXPART also have a strong size dependence, with the longest lifetimes found for the accumulation-mode aerosols. For example, for dust particles emitted at the surface the lifetimes were 13.8 days for particles with 1 µm diameter and a few hours for 10 µm particles. A strong size dependence in below-cloud scavenging, combined with increased dry removal, is the primary reason for the shorter lifetimes of the larger particles. The most frequent removal is in-cloud scavenging (85% of all scavenging events) but it occurs primarily in the free troposphere, while below-cloud removal is more frequent below 1000m (52%of all events) and can be important for the initial fate of species emitted at the surface, such as those examined here. For assumed realistic in-cloud removal efficiencies, both BC and sulphate have a slight overestimation of observed atmospheric concentrations (a factor of 1.6 and 1.2 respectively). However, this overestimation is largest close to the sources and thus appears more related to overestimated emissions rather than underestimated removal. The new aerosol wet removal scheme of FLEXPART incorporates more realistic information about clouds and aerosol properties and it compares better with both observed lifetimes and concentration than the old scheme. [ABSTRACT FROM AUTHOR]

A new, more physically based wet removal scheme for aerosols has been implemented in the Lagrangian particle dispersion model FLEXPART. It uses three-dimensional cloud water fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) to determine cloud extent and distinguishes between in-cloud and below-cloud scavenging. The in-cloud nucleation scavenging differentiates between cloud water phases (liquid, ice or mixed-phase) to allow for aerosol and cloud type specific removal. The impaction scavenging scheme parameterizes below-cloud removal as a function of aerosol size and precipitation type (snow or rain) and intensity. Sensitivity tests with the new scavenging scheme and comparisons with observational data were conducted for three distinct types of primary aerosols, which pose different challenges for modelling wet scavenging due to their differences in solubility, volatility and size distribution: (1) 137Cs released during the Fukushima nuclear accident attached mainly to highly soluble sulphate aerosol, (2) black carbon (BC) aerosol, and (3) mineral dust. Calculated e-folding lifetimes of accumulation mode aerosols for these three aerosol types were, 11.7, 16.0, and 31.6 days respectively, when well mixed in the atmosphere. The long lifetimes of mineral dust in particular are primarily a result of slow in-cloud removal which is the primary removal mechanism. Calculated e-folding lifetimes in FLEXPART also have a strong size dependence, with the longest lifetimes found for the accumulation-mode aerosols. For example, for dust particles emitted at the surface the lifetimes were 13.8 days for particles with 1 &956;m diameter and a few hours for 10 &956;m particles. A strong size dependence in below cloud scavenging, combined with increased dry deposition and gravitational settling, is the primary reason for the shorter lifetimes of the larger particles. The most frequent removal is in-cloud scavenging (85 % of all scavenging events) but it occurs primarily in the free troposphere, while below-cloud removal is more frequent below 1000 m (52 % of all events) and can be important for the initial fate of species emitted at the surface, such as those examined here. For assumed realistic in-cloud removal efficiencies, both BC and sulphate have a slight overestimation of observed atmospheric concentrations (a factor of 1.6 and 1.2 respectively). However, this overestimation is largest close to the sources and thus appears more related to overestimated emissions rather than underestimated removal. [ABSTRACT FROM AUTHOR]