Climate warming induced wildfires are rapidly increasing at high latitudes, yet their climate impacts remain poorly understood. These deeply smoldering fires may release long-stored carbon and thus perturbate the global carbon cycle and further emit light-absorbing carbonaceous particles enhancing snow and ice melt after deposition. We newly investigate the carbon isotopic and light-absorbing characteristics of carbonaceous particles produced in laboratory combustion experiments on Arctic-boreal peats and compare these with biomass from boreal forest and savanna environments. We provide the first observational evidence that boreal and especially Arctic peat smoldering may release millennial-aged carbon into the atmosphere, which upsets radiocarbon-based source attribution, separating fossil-fuel-derived sources from modern biomass. Moreover, above- and below-ground material combust differently, and hence the fraction of modern carbon (F14C), i.e., the average age, of the original biomass and the produced carbonaceous particles may differ. Furthermore, we show that peat smoldering produces significant amounts of Brown Carbon, which absorbs light at a similar magnitude to Black Carbon in these samples. Our results indicate that the increasing number of Arctic-boreal peat fires may exacerbate Arctic warming more than previously estimated.

Whereas residual light-absorbing particles remaining after combustion are taken as proxy for mineral dust, lightabsorbing particles removed by combustion in the thermal-optical analysis are attributed to carbonaceous material. The latter is assumed to mainly be composed of black carbon. The temporal evolution of residual particles observed in a Svalbard ice core (Holtedahlfonna), covering the period 1700 to 1998, shows strong relations with the reanalyzed local temperature and wind data. This indicates possible local sources for the mineral dust proxy. The relative contribution to light attenuation caused by residual particles prior to 1875 is in the range 0.2 to 0.5. After 1875 the contribution is generally above 0.4 with a maximum of about 0.7 in the middle of the 20th century. The increasing fraction in the beginning of the 20th century is attributed to enhanced emission of local mineral dust due to increased local summertime wind speeds. The decrease in the relative contribution follows a lowering in the wind speed and a concurrent increase in the deposition of BC from about the 1970 ' s. Enhanced light attenuation in the red wavelength not explained by the MD and BC proxies remains a conundrum but show clear covariations with positive monthly mean summer temperatures above zero and non-sea salt K+ ions.

Central Asia is a region with high-altitude mountains, hosting numerous glaciers and widespread seasonal snow cover, both of which play an integral role in the regional hydrological cycle. Despite their importance, observations of cryospheric and atmospheric variables are scarce in this area but are essential to assess the temporal and spatial changes induced by climate change. To address this gap, we present a diverse data set of cryospheric and atmospheric variables from the Zarafshon River Basin and the Hydrographic Party Glacier (GGP) in Tajikistan, spanning 2018–2025. The dataset includes glacier terminus positions and snow conditions, glacier ablation, high resolution aerial photography, meteorological variables, surface reflectance combined with snow chemistry, and atmospheric aerosol concentrations. These observations provide a valuable basis for research on glacier dynamics, snow processes, and atmosphere-cryosphere interactions in a region where monitoring has been sparse. The data are available at the Finnish Meteorological Institute data repository METIS: https://doi.org/10.57707/fmi-b2share.t4vwg-gf542 (Svensson et al., 2026).

Street canyons are well-known hot spots due to the harmful exposure to high concentrations of atmospheric pollutants emitted mainly by motor vehicles. We report on measurements of air pollutants conducted in a street canyon in Stockholm (Sweden) in spring 2006. Particle number size distributions (PNSD) were measured in the 25–606 nm range, along with total particle number, light-absorbing carbon mass concentration (MLAC), PM10, NOx, CO, traffic rate (TR), vehicle speed and meteorological variables. We used PNSD as input to the positive matrix factorisation (PMF) analysis to identify and apportion the pollutant sources. All pollutants showed distinct diurnal patterns, with highest concentrations in weekday mornings (08:00–09:00). TR was always higher on weekdays, except for the early hours (00:00–06:00). The raise in the weekend early-hour TR was accompanied by the largest MLAC of the day, a higher NOx/CO ratio compared to weekdays and a modal shift of PNSD towards larger diameters (47–56 nm), indicates a change in the vehicle fleet share to being dominated by diesel-run taxis. The largest contribution to the submicron particles was observed for winds blowing along the canyon, transporting particles emitted by vehicles accelerating from the traffic lights at the intersection, uphill towards the measurement site, and from the nearby streets. Three PMF factors were identified: local emissions from a mixed fleet dominated by gasoline engines, local traffic emissions highly impacted by diesel vehicles, and urban background aerosol. On average, gasoline-fuelled vehicles largely contributed to NOx, and particle number concentrations (54–65

The absorption of shortwave irradiance in snow depends on the physical properties of snow (e.g., snow grain size and shape, liquid water content, etc.) and light-absorbing particles (LAP). Originating from natural and anthropogenic sources, LAP has been reported to accelerate snowmelt significantly in different regions globally. Yet, our process-level understanding of LAP after deposition onto snow remains rather limited. Here we investigate the impacts of artificial deposition of different LAP onto snow surfaces in an outdoor environment of northern Finland. Following LAP dry deposition into a custom-made tent standing on top of the snowpack, the albedo was followed along with the properties of snow in snow pit measurements throughout the spring season. The results showed that the albedo decay at the end of the season for the different spots were linked to the initial amount and type of LAP that were deposited onto the snowpack. Measured snow temperature profiles from LAP doped snow versus natural reference snow illustrated that the LAP affected snow had higher temperatures in the subsurface snow layers. Collected snow samples analyzed for size distribution of soot particles revealed no apparent agglomeration of soot particles during thaw-freezing events taking place during the experiment. Despite the relatively large perturbation of the experimentally deposited LAP, their impact on the season length was only up to 3 days. Additional experiments are, nevertheless, needed to better constrain the effects of LAP on snow albedo, melt rate, and other associated processes.

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.

Seasonal snow cover duration is the net result from many processes acting on snow fallen on the Earth's surface. Several of these processes feed back into the atmosphere-cryosphere system causing non-linear interactions. The timing of snow retreat is of essential importance, but the duration of snow cover has large spatiotemporal variabilities. However, from a large data set of observed snow depth changes in northern Finland, systematic similar evolutions are identified that allow for a considerable simplification and reduction of the complexity in snow depth changes. Here, a novel conceptual framework is designed based on dividing the season into two main periods (dark and bright period, based on solar irradiance), for which snow depth decrease is parameterized based on three variables, average temperature, incoming shortwave radiation, and light-absorbing particles (LAP) in the snow. The processes are simplified into two linear relations, and a new formulation for concentration enhancement of LAP, which is dependent on snow depth decrease, is given. The results show that the seasonal snow cover duration is shifted by about one day for every 10 mm snow water equivalent of precipitation. This effect is comparable in scale to that of doubling of the amount of LAP concentration in snow. We also found that the combined shift in snow cover duration from interannual variability in ambient temperature and shortwave radiation (warm and bright vs. cold and dark season) is large enough to explain the variability of a couple of weeks for a given precipitation amount in Northern Finland.

Black carbon (BC) particles produced by incomplete combustion of biomass and fossil fuels warm the atmosphere and decrease the reflectivity of snow and ice, hastening their melt. Although the significance of BC in Arctic climate change is widely acknowledged, observations on its deposition and sources are few. We present BC source types in a 300‐year (1700–2005) Svalbard ice core by analysis of particle‐bound organic compounds, radiocarbon, and trace elements. According to the radiocarbon results, 58% of the deposited elemental carbon (EC, thermal‐optical proxy of BC) is of non‐fossil origin throughout the record, while the organic compounds suggest a higher percentage (68%). The contribution of fossil fuels to EC is suggested to have been elevated between 1860 and 1920, particularly based on the organics and trace element data. A second increase in fossil fuel sources seems to have occurred near the end of the record: according to radiocarbon measurements between 1960 and 1990, while the organics and trace element data suggest that the contribution of fossil fuels has increased since the 1970s to the end of the record, along with observed increasing EC deposition. Modeled atmospheric transport between 1948 and 2004 shows that increasing EC deposition observed at the glacier during that period can be associated with increased atmospheric transport from Far East Asia. Further observational BC source data are essential to help target climate change mitigation efforts. The combination of robust radiocarbon with organic compound analyses requiring low sample amounts seems a promising approach for comprehensive Arctic BC source apportionment.

Abstract. Black carbon (BC), the most efficient atmospheric aerosol for absorbing light in the visible spectrum, exerts a warming effect on a region undergoing unprecedented climatic changes. Here, BC is studied indirectly using filter-based methods to ascertain aerosol light absorption parameters. We investigated long-term changes using a harmonised 21-year data set of light absorption measurements, in conjunction with air mass source analysis. The measurements were performed at Zeppelin Observatory (ZEP), Svalbard, from 2002 to 2022. We report a statistically significant (s.s.) decreasing long-term trend for the light absorption coefficient, measured at the site for the entirety of the data set. However, the last 7 years, 2016–2022, showed a slightly increasing s.s. trend for the haze season. In addition, we observed an increasing trend in the single scattering albedo from 2002 to 2022. Five distinct source regions were identified; the trends involving air masses from the five regions showed decreasing absorption coefficients, except for the air masses influenced by emissions 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 by the influence from high-emission regions. 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 rainfall, as a sink of aerosol, plays a role in the long-term trends in the absorption coefficient, explaining approximately a quarter of the overall trend. A decreasing trend in the scavenging ratio further suggests an increase in the aerosol removal processes. We note that there is an increasing potential influence from active forest fires, particularly in the last few summers (i.e. 2015–2022). Active fires have been shown to have a significant impact on the mean seasonal absorption coefficient especially during northern hemispheric summer. However, no noticeable alteration in annual long-term trends can be observed.

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.

Introduction The Arctic region is particularly sensitive to global climate change, experiencing warming at twice the rate of the global average. Changes within and outside of the Arctic (e.g., meteorology, atmospheric transport, and precipitation patterns) can have consequences for the sources and sinks of aerosols. Atmospheric aerosols can alter the planetary radiation balance directly through scattering and absorption and indirectly through modification of cloud properties. Understanding the direction and magnitude of recent changes in the Arctic aerosol population is key to elucidating the implications for the changing Arctic, although this remains a scientific challenge. Here we report a Pan-Arctic view of recent trends for aerosol particle number concentrations in different size fractions. Measurement Site & Methods Measurements were obtained from different stations around the Arctic including Villum Research Station (Villum, 81°36’ N, 16°39’ W, 24 m a.s.l) in northeastern Greenland, Alert (81°28’ N, 62°30’ W, 210 m a.s.l.) in the Canadian Archipelago, Zeppelin Observatory (78°56’ N, 11°53’ E, 474 m a.s.l.) on Svalbard, Pallas (67°58’N, 24°07’ E, 560 m a.s.l.) in northern Finland, and Tiksi (71°36’ N, 128°53’E, 1 m a.s.l.) in the Siberian Arctic. Particle number size distributions (PNSD) were measured using a Scanning Mobility Particle Sizer (SMPS) at Villum and Alert, and a Differential Mobility Particle Sizer (DMPS) at Zeppelin, Pallas, and Tiksi. Measurements were collected from 2010 to 2018 at all sites except for Zeppelin (2011 to 2019). Number concentrations were calculated by integrating the PNSD for three size fractions: Nucleation (10-35 nm), Aitken (35-80 nm), and Accumulation (80-300 nm). Nucleation number concentrations were unavailable for Zeppelin. The trends in the number concentration for these size fractions were identified and quantified using the Mann-Kendal test and Theil Sen slope on the 90th % confidence interval via the 3PW algorithm, using the daily median as temporal aggregation and meteorological seasons as temporal segmentation. Only statistically significant trends are discussed. Results Although the sites Villum, Alert, and Zeppelin are all located in the High Arctic (> 75° N) and relatively close to one another, there are differences between the direction and magnitude of trends for the size fractions. For example, at Villum, increasing trends are observed for the Nucleation fraction during spring and summer. Interestingly, at Alert, decreasing trends are observed for the Accumulation fraction during spring and autumn. At Zeppelin, no significant trends were observed for any fraction during any season. Similar to the High Arctic sites, for the continental sites, Tiksi and Pallas, no uniform picture for the direction and magnitude of trends in the size fractions is observed. At Tiksi, decreasing trends for both the Aitken and Accumulation fractions are detected during summer and autumn. While at Pallas, no significant trends were observed during any season. This work offers insight into the climatic implications (i.e., radiative balance and cloud properties) for a future Arctic climate by monitoring changes of aerosol concentrations in optically and cloud-relevant sizes. Future work will investigate the causes of these trends.

Snow darkening by deposited light-absorbing particles (LAP) accelerates snowmelt and shifts the snow melt-out date (MOD). Here, we present a simple approach to estimate the snow albedo variability due to LAP deposition and test this method with data for 2 seasons (February–May 2016 and December 2016–June 2017) at a high-altitude valley site in the Central Himalayas, India. We derive a parameterization for the snow albedo that only depends on the daily observations of average ambient temperature and change in snow depth, as well as an assumed average concentration of LAP in snow precipitation. Linear regression between observed and parameterized albedo for the base case assuming an equivalent elemental carbon concentration [ECeq] of 100 ng g–1 in snow precipitation yields a slope of 0.75 and a Pearson correlation coefficient r2 of 0.76. However, comparing the integrated amount of shortwave radiation absorbed during the winter season using observed albedo versus base case albedo resulted in rather small differences of 11% and 4% at the end of Seasons 1 and 2, respectively. The enhanced energy absorbed due to LAP at the end of the 2 seasons for the base case scenario (assuming an [ECeq] of 100 ng g–1 in snow precipitation) was 40% and 36% compared to pristine snow. A numerical evaluation with different assumed [ECeq] in snow precipitation suggests that the relative sensitivity of snow albedo to changes in [ECeq] remains rather constant for the 2 seasons. Doubling [ECeq] augments the absorption by less than 20%, highlighting that the impact on a MOD is small even for a doubling of average LAP in snow precipitation.

Aerosol optical thickness (AOT) has decreased substantially in Europe in the summer half year (April–September) since 1980, with almost a 50% reduction in Central and Eastern Europe, according to Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) reanalysis. At the same time, strong positive trends in ERA5 reanalysis surface solar radiation downward for all‐sky and clear‐sky conditions (SSRD and SSRDc, respectively) and temperature at 2 m are found for Europe in summer during the period 1979–2020. The GEBA observations show as well strong increases in SSRD during the latest four decades. Estimations of changes in SSRDc, using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model, show similarly strong increases when fed by MERRA‐2 AOT. The estimates of warming in this study, caused by increases in SSRD and SSRDc, are based on energy budget approximations and the Stefan Boltzmann law. The increases in near surface temperature, estimated both for clear‐sky and all‐sky conditions, are up to about 1°C for Central and Eastern Europe. The total warming over large parts of this region for clear‐sky conditions is however nearly double the global mean temperature increase of 1.1°C, while somewhat less for all‐sky conditions. The effects of aerosols on warming over the southerly Iberian Peninsula are weaker compared to countries further north. The rapid total warming over the Iberian Peninsula is probably caused by greenhouse warming, drier surface conditions, and to some degree decline in aerosols. Reduced cloud cover is found for large parts of Europe in summer during the latest four decades.

The Zeppelin Observatory (78.90∘ N, 11.88∘ E) is located on Zeppelin Mountain at 472 m a.s.l. on Spitsbergen, the largest island of the Svalbard archipelago. Established in 1989, the observatory is part of Ny-Ålesund Research Station and an important atmospheric measurement site, one of only a few in the high Arctic, and a part of several European and global monitoring programmes and research infrastructures, notably the European Monitoring and Evaluation Programme (EMEP); the Arctic Monitoring and Assessment Programme (AMAP); the Global Atmosphere Watch (GAW); the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS); the Advanced Global Atmospheric Gases Experiment (AGAGE) network; and the Integrated Carbon Observation System (ICOS). The observatory is jointly operated by the Norwegian Polar Institute (NPI), Stockholm University, and the Norwegian Institute for Air Research (NILU). Here we detail the establishment of the Zeppelin Observatory including historical measurements of atmospheric composition in the European Arctic leading to its construction. We present a history of the measurements at the observatory and review the current state of the European Arctic atmosphere, including results from trends in greenhouse gases, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), other traces gases, persistent organic pollutants (POPs) and heavy metals, aerosols and Arctic haze, and atmospheric transport phenomena, and provide an outline of future research directions.

Here we apply new analysis methods and approaches to existing long-term measurement series that provide additional insights into the atmospheric processes that control black carbon (BC) in the Arctic. Based on clustering size distribution data from Zeppelin Observatory for the years 2002–2010, observations classified as ‘Polluted’ were further investigated based on BC properties. The data were split into two subgroups, and while the microphysical and chemical fingerprints of the two subgroups are very similar, they show larger differences in BC concentration and correlation with the particle size distribution. Therefore, a source–receptor analysis was performed with HYSPLIT 10-days backward trajectories for both subsets. We demonstrate that within this ‘Polluted’ category, the airmasses that contributed to the largest BC signal at the Zeppelin station are not necessarily associated with traditional transport pathways from Eurasia. Instead, the strongest signal is from a region east of the Ural Mountains across the continent to the Kamchatka Peninsula.

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 µg 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 µg 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.

The aim of this study was to explore particle size dependent properties by combining long-term observations of equivalent black carbon (eBC) and number size distributions to investigate their correlation as function of particle size. The work was conducted in two main parts. The first part consisted of a short laboratory experiment to compare observed total particle light absorption (σabs) with that observed according to particle size by using a combination of a Differential Mobility Analyzer (DMA) and a Particle Soot Absorption Photometer (PSAP). The laboratory study confirmed strong similarities between the observed and derived σabs. In the second part the statistical approach using correlation between the σabs and the dN of each bin of the number size distribution was tested on long-term data ranging from 2002 to 2010 observed at Zeppelin station, Ny-Ålesund Svalbard. The data was clustered according to the number size distribution and grouped in four major categories: Washout, Nucleation, Intermediate and Polluted. Each category presented different features with respect to the derived eBC mass distributions, the Intermediate category showed similarities to the few available Single Particle Soot Photometer (SP2) observations in the Arctic. Overall, the statistical distribution of eBC, according to particle size, presented a larger dynamical range in the location of the mode(s). To check for consistency, the eBC mass distributions were transformed into number based eBC size distribution and compared to the observed total number size distribution. Whereas the Washout, Nucleation and Intermediate categories presented plausible number distributions, the Polluted category displayed a mode at small sizes (about 50 nm) that was significantly exaggerated.

Oceanic dimethyl sulfide (DMS) emissions have been recognized as a biological regulator of climate by contributing to cloud formation. Despite decades of research, the climatic role of DMS remains ambiguous largely because of limited observational evidence for DMS‐induced cloud condensation nuclei (CCN) enhancement. Here, we report concurrent measurement of DMS, physiochemical properties of aerosol particles, and CCN in the Arctic atmosphere during the phytoplankton bloom period of 2010. We encountered multiple episodes of new particle formation (NPF) and particle growth when DMS mixing ratios were both low and high. The growth of particles to sizes at which they can act as CCN accelerated in response to an increase in atmospheric DMS. Explicitly, the sequential increase in all relevant parameters (including the source rate of condensable vapor, the growth rate of particles, Aitken mode particles, hygroscopicity, and CCN) was pronounced at the DMS‐derived NPF and particle growth events. This field study unequivocally demonstrates the previously unconfirmed roles of DMS in the growth of particles into climate‐relevant size and eventual CCN activation.

Abstract. 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⁡(〖EC〗_eq )+1)PP±0.0033(log⁡(〖EC〗_eq )+1)PP, where PP is the seasonal precipitation in mm snow water equivalent. A change in ECeq by 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.