Effects of mixing state on optical and radiative properties of black carbon in the European Arctic

Atmospheric aging promotes internal mixing of black carbon (BC), leading to an enhancement of light absorption and radiative forcing. The relationship between BC mixing state and consequent absorption enhancement was never estimated for BC found in the Arctic region. In the present work, we aim to quantify the absorption enhancement and its impact on radiative forcing as a function of microphysical properties and mixing state of BC observed in situ at the Zeppelin Arctic station (78 degrees N) in the spring of 2012 during the CLIMSLIP (Climate impacts of short-lived pollutants in the polar region) project. Single-particle soot photometer (SP2) measurements showed a mean mass concentration of refractory black carbon (rBC) of 39 ngm(-3), while the rBC mass size distribution was of lognormal shape, peaking at an rBC mass-equivalent diameter (D-rBC) of around 240 nm. On average, the number fraction of particles containing a BC core with D-rBC > 80 nm was less than 5% in the size range (overall optical particle diameter) from 150 to 500 nm. The BC cores were internally mixed with other particulate matter. The median coating thickness of BC cores with 220 nm <D-rBC <260 nm was 52 nm, resulting in a core-shell diameter ratio of 1.4, assuming a coated sphere morphology. Combining the aerosol absorption coefficient observed with an Aethalometer and the rBC mass concentration from the SP2, a mass absorption cross section (MAC) of 9.8 m(2) g(-1) was inferred at a wavelength of 550 nm. Consistent with direct observation, a similar MAC value (8.4m(2) g(-1) at 550 nm) was obtained indirectly by using Mie theory and assuming a coated-sphere morphology with the BC mixing state constrained from the SP2 measurements. According to these calculations, the lensing effect is estimated to cause a 54% enhancement of the MAC compared to that of bare BC particles with equal BC core size distribution. Finally, the ARTDECO radiative transfer model was used to estimate the sensitivity of the radiative balance to changes in light absorption by BC as a result of a varying degree of internal mixing at constant total BC mass. The clear-sky noontime aerosol radiative forcing over a surface with an assumed wavelength-dependent albedo of 0.76-0.89 decreased, when ignoring the absorption enhancement, by -0.12 Wm(-2) compared to the base case scenario, which was constrained with mean observed aerosol properties for the Zeppelin site in Arctic spring. The exact magnitude of this forcing difference scales with environmental conditions such as the aerosol optical depth, solar zenith angle and surface albedo. Nevertheless, our investigation suggests that the absorption enhancement due to internal mixing of BC, which is a systematic effect, should be considered for quantifying the aerosol radiative forcing in the Arctic region.Peer reviewe

Deposition of light-absorbing particles in glacier snow of the Sunderdhunga Valley, the southern forefront of the central Himalayas

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 mu 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 mu 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.Peer reviewe

Snow albedo and its sensitivity to changes in deposited light-absorbing particles estimated from ambient temperature and snow depth observations at a high-altitude site in the Himalaya

Snow darkening by deposited light-absorbing particles (LAP) accelerates snowmelt and shifts the snow meltout 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 r(2) 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.Peer reviewe

Year-round trace gas measurements in the central Arctic during the MOSAiC expedition

Despite the key role of the Arctic in the global Earth system, year-round in-situ atmospheric composition observations within the Arctic are sparse and mostly rely on measurements at ground-based coastal stations. Measurements of a suite of in-situ trace gases were performed in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. These observations give a comprehensive picture of year-round near-surface atmospheric abundances of key greenhouse and trace gases, i.e., carbon dioxide, methane, nitrous oxide, ozone, carbon monoxide, dimethylsulfide, sulfur dioxide, elemental mercury, and selected volatile organic compounds (VOCs). Redundancy in certain measurements supported continuity and permitted cross-evaluation and validation of the data. This paper gives an overview of the trace gas measurements conducted during MOSAiC and highlights the high quality of the monitoring activities. In addition, in the case of redundant measurements, merged datasets are provided and recommended for further use by the scientific community.Peer reviewe

Substantial contribution of iodine to Arctic ozone destruction

Unlike bromine, the effect of iodine chemistry on the Arctic surface ozone budget is poorly constrained. We present ship-based measurements of halogen oxides in the high Arctic boundary layer from the sunlit period of March to October 2020 and show that iodine enhances springtime tropospheric ozone depletion. We find that chemical reactions between iodine and ozone are the second highest contributor to ozone loss over the study period, after ozone photolysis-initiated loss and ahead of bromine.Iodine chemistry plays a more important role than bromine chemistry in tropospheric ozone losses in the Arctic, according to ship-based observations of halogen oxides from March to October 2020.Peer reviewe

CMS physics technical design report : Addendum on high density QCD with heavy ions

Peer reviewe

Overview of the MOSAiC expedition - Atmosphere

With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic

A mechanism for the photochemical transformation of nitrate in snow

Photochemical reactions of trace compounds in snow have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. One of the prominent reaction is the photolysis of nitrate, which leads to the formation of OH radicals in the snow and to the release of reactive nitrogen compounds like nitrogen oxides (NO and NO2) and nitrous acid (HONO) to the atmosphere. We performed photolysis experiments using artificial snow containing variable initial concentrations of nitrate and nitrite to investigate the reaction mechanisms responsible for the formation of the reactive nitrogen compounds. Increasing the initial nitrite concentrations resulted in the formation of significant amounts of nitrate in the snow. A possible precursor of nitrate is NO2, which can be transformed into nitrate either by the attack of a hydroxy radical or the hydrolysis of the dimer (N2O4). A mechanism for the transformation of the nitrogen-containing compounds in snow was developed assuming that all reactions took place in a quasi-liquid layer (QLL) at the surface of the ice crystals. The unknown photolysis rates of nitrate and nitrite and the rates of NO and NO2 transfer from the snow to the gas phase, respectively, where adjusted to give an optimum fit of the calculated time series of nitrate, nitrite, and gas phase NOx with respect to the experimental data. Best agreement was obtained with a ~25 times faster photolysis rate of nitrite compared to nitrate. The formation of NO2 is probably the dominant channel for the nitrate photolysis. We used the reaction mechanism further to investigate the release of NOx and HONO under natural conditions. We found that the NOx emissions are by far dominated by the release of NO2. The release of HONO to the gas phase depends on the pH of the snow and the HONO transfer to the gas phase. However, due to the small amounts of nitrite produced under natural conditions, the formation of HONO in the QLL is probably negligible. We suggest that observed emissions of HONO from the surface snow are caused by the heterogeneous formation of HONO in the firn air. The reaction of NO2 on the surfaces of the ice crystals is the most likely HONO source

Peroxyactyl nitrate (PAN) distribution over the South Atlantic Ocean

The atmospheric concentration of peroxyacetyl nitrate (PAN) was measured over the South Atlantic Ocean in the range between 37° and 70° S inMarch 1999 in order to investigate its latitudinal distribution and its role in the NOy chemistry of the remote marine boundary layer (MBL) in the high latitudes of thesouthern hemisphere. The measurements were performed using an automatic gas chromatographic system aboard RV Polarstern. Mean mixing ratios of 62 (+/- 21)pptv were obtained between 37° and 49° S. South of 55° S, PAN concentrations were very low with values of the order of 10-30 pptv.Pronounced diurnal cycles could not be observed. The concentrations over the South Atlantic were considerably lower compared with the North Atlantic where theMBL is influenced by continental air masses