Variability of Black Carbon mass concentration in surface snow at Svalbard 1

21 Black Carbon (BC) is a significant forcing agent in the Arctic, but substantial uncertainty remains 22 to quantify its climate effects due to the complexity of the different mechanisms involved, in particular 23 related to processes in the snow-pack after deposition. In this study, we provide detailed and unique 24 information on the evolution and variability of BC content in the upper surface snow layer during the 25 spring period in Svalbard (Ny-Ålesund). Two different snow-sampling strategies were adopted during 26 spring 2014 and 2015, providing the refractory BC (rBC) mass concentration variability on a 27 seasonal/daily and daily/hourly time scales. The present work aims to identify which atmospheric 28 variables could interact and modify the mass concentration of BC in the upper snowpack, the snow layer 29 which BC particles affects the snow albedo. Atmospheric, meteorological, and snow-related physico30 chemical parameters were considered in a multiple linear regression model to identify the factors that 31 could explain the variations of BC mass concentrations during the observation period. Precipitation 32 https://doi.org/10.5194/acp-2021-39 Preprint. Discussion started: 7 April 2021 c © Author(s) 2021. CC BY 4.0 License.

to quantify its climate effects due to the complexity of the different mechanisms involved, in particular 23 related to processes in the snow-pack after deposition. In this study, we provide detailed and unique 24 information on the evolution and variability of BC content in the upper surface snow layer during the 25 spring period in Svalbard (Ny-Ålesund). Two different snow-sampling strategies were adopted during

Introduction 37
In the last two decades, the Arctic region has been exposed to dramatic changes in terms of 38 atmospheric temperature rise, sea ice decrease, and increase of air mass transport from lower latitudes 39 bringing warmer and humid air masses containing pollutants and anthropogenic derived compounds (Law  The impact of BC particles absorbing the incoming solar radiation has indeed a non-negligible role in the 58 Arctic region, which is already threatened by a two-fold temperature increase compared to the mid-59 latitude areas, the so-called "Arctic Amplification" (Bond et  showed that the cryosphere is affected both by the BC-induced warming of the atmosphere and by direct 63 and indirect BC effects on the snow once deposited over it (Flanner, 2013), Although wet deposition is suggested to be the main driver of BC concentration in the snow, little is 90 known about other environmental processes potentially affecting the BC particles concentration once 91 deposited, i.e. physical post-depositional processes. 92 In this study we present two unique experiments performed in a clean area close to the town of 93 Ny-Ålesund (Svalbard)  In the 85-days experiment, the first 10 cm of surface snow were collected on a daily basis 136 (approximately at 11.00 am, GMT+2) in the same area, using a 5 cm diameter and 10 cm long Teflon 137 tube. The samples were collected following a straight line leaving about 15 cm between the sampling 138 points to minimize the spatial variability. The collected snow was homogenized in a pre-cleaned plastic 139 bag and then, without melting, 50 mL was transferred into vial (Falcon™ 50mL Conical Centrifuge 140 Tubes) for BC, coarse mode particles number concentration and electrical conductivity analyses. The 85-141 days experiment was designed with the aim to investigate the BC presence in the upper snow layer, where 142 most of the snow-radiation interaction takes place and where BC particles' presence can decrease the 143 snow albedo (Doherty et al., 2010). Moreover, this sampling strategy allowed to evaluate the variation of 144 BC on a seasonal basis and to capture the impacts of wind, precipitation or melting. 145 During the 3-days experiment, the first 3 cm of surface snow were collected on an hourly basis in 146 pre-cleaned vials in a delimited area of 2 x 2 m using the same sampling tools as above (Spolaor et al., 147 2019). In this case the samples were collected following a straight line leaving about 5 cm between the 148 sampling points. The aim of the 3-days experiment was to investigate the potential daily cycle of surface 149 BC concentration; therefore, we foresaw that small variations could derive from the impact of the daily 150 variation of solar zenith angle and subsequent induced snow metamorphism at the surface of the 151 snowpack, often at cm scale. To avoid dilution of the signal, we reduced the vertical sampling thickness 152 to 3 cm to enhance our chances of observing variation in the rBC mass concentration if such variation 153

exists. 154
The temperature at the surface of the snowpack (at 7 cm for 85-days and at 3 cm for 3-days 155 experiment) was always measured. The daily/hourly snow accumulation was determined by measuring 156 the emerging part of 4 poles placed around the sampling area. The average standard deviation calculated 157 from the four poles provides us a reasonable estimate of the variability in snow accumulation\depletion 158 within the sampling area. The standard deviation obtained ranges from 2 to 4 cm for the entire periods, 159 indicating a limited spatial variability. volumetric airflow before and after the field campaign. A 5 minutes temporal resolution was used for data 170 acquisition. However, due to the low background concentration in the Arctic, the signal/noise ratio is 171 high, so that data were hourly averaged. The data presented in this study were processed according to 172 g -1 ), which we used to estimate MAC at 520 nm (10.2 m 2 g -1 ). 178 179

Particle Soot Absorption Photometer (PSAP) 180
During the 85-days sampling period the aerosol absorption coefficient was also measured by 181 means of a 3-wavelengths PSAP (this instrument was not available during the 3-days experiment period). for the 85-days experiment was retrieved using the Aethalometer (first period) and the PSAP (second 188 period), with an overlapping period with simultaneous measurements of 5 days. For the retrieved eBC 189 mass concentration from the two instruments to be equal during the overlapping period, the PSAP eBC 190 was calculated dividing the absorption measurements (at 530 nm) with a MAC equal to 7.25 m 2 g -1 191 (keeping the AE31 data as reference). Daily averages were calculated from the 1-minute data to compare 192 with the rBC daily data obtained from the snow.

rBC Measurement -SP2 208
The rBC mass concentration and mass size distribution were measured following the methods 209 described in Lim et al. (2014). The snow samples were melted at room temperature prior to the analyses. 210 The vials with the melted snow were sonicated for ten minutes at room temperature. The samples were 211 nebulized before the injection in the Apex-Q desolvation system (APEX-Q, Elemental Scientific Inc., 212 Omaha, USA). The nebulization efficiency was evaluated daily by injecting Aquadag® solutions with 213 different mass concentrations, ranging from 0.1 to 100 ng g -1 , obtaining an average value of 61%, that The SP2 data were analyzed using the IGOR based toolkit from M. Gysel (Laboratory of 217 Atmospheric Chemistry, Paul Scherrer Institute, Switzerland). The large amount of signals derived from 218 every single particle are elaborated achieving rBC mass and number concentrations and size distributions. 219 220

Meteorological Parameters 221
Meteorological parameters, in addition to the atmospheric and snow ancillary measurements, 222 were used in the statistical exercise to study the variability of rBC mass concentration in surface snow 223 samples as a function of the atmospheric conditions. BC particles are deposited on the snowpack 224 following a combination of wet and dry deposition. However, once deposited on/in the snowpack other 225 processes can potentially induced a significant variability in the surface BC content. The wind direction 226 and its velocity can modify the BC distribution in the upper snowpack due to snow-mobilization. The 227 solar radiation and relative humidity may enhance snow sublimation and surface hoar formation thus 228 modifying the relative BC concentration in the upper snow layer by removing or adding "water" mass to 229 the snow surface. 230 Air temperature and relative humidity at 2 meter height have been retrieved from a meteorological station 231 located about 800 meters north of the sampling site, using a ventilated PT-100 thermo-couple by Thies 232 Clima and a HMT337 humicap sensor by Vaisala, respectively. Wind speed and direction at 10 meter 233 height were obtained from a Combined Wind Sensor Classic by Thies Clima (see Maturilli et al., 2013).

Parameters consider in the statistical analysis 243
The snow pack evolution is primarily driven by meteorological parameters, which are responsible for 244 adding/removing mass to the annual snow pack. Wind can affect the snow pack evolution in several 245 ways: 1) by snow redistribution, 2) favouring the ablation\sublimation, and 3) lifting particles from 246 nearby sources and areas. Surface snow and air temperatures are two fundamental parameters required to 247 fully understand the varying conditions of the snow pack. In our study, the temperature variables are 248 proxies for the melting episodes and for the presence of liquid water potentially affecting the 249 concentration of impurities. The incoming solar radiation is not expected to be directly linked to the 250 surface mass concentration of rBC, however the surface process could affect it indirectly by favouring 251 sublimation (water mass removal), as well as hoar formation (water mass addition) during the colder parts 252 of the day (night/early morning). The relative humidity gives an idea of the amount of water present in the 253 atmosphere and the high RH might favour the deposition of BC suspended by the formation of water 254 droplets through the cloud condensation nuclei, this is especially significant for the selected sampling 255 location, nearby to the shore. The last meteorological parameter considered is the precipitation amount. 256 This is important to understand the wet deposition processes able to transfer BC particles from the 257 atmosphere to the snow surface. 258 The additional selected parameters are 1) the atmospheric eBC mass concentration, an interesting 259 parameter to investigate the potential transfer function of BC particles from the atmosphere into the snow 260 surface, 2) the coarse mode particles (dust) that could have a similar transport pathways to the black 261 carbon and gives an idea of the amount of total impurities deposition and 3) the total water conductivity, 262 an indirect measurement of the salinity content of the snow. Considering the location of the sampling site 263 (<1km from the coast line), the contribution of the ocean emissions to the snow pack chemical 264 composition is significant. We considered the total conductivity as an indication of sea spray deposition, 265 and to investigate common deposition pattern and/or similarities to the behaviour of BC (although BC is 266 not emitted from ocean surface). The conductivity was also considered to determine if there was a large well on the BC concentration as discussed in Section 2.6. All the atmospheric parameters described in the 275 previous section (wind, snow and air temperature, incoming solar radiation, relative humidity and snow 276 precipitation amount) were initially considered as covariates to be included in the multiple linear 277 regression. However, wind speed and direction, as well as the atmospheric stability, expressed as vertical 278 wind speed, were removed because preliminary statistical analyses indicate that none of them is 279 associated with the observed variations in snow rBC mass concentrations. This does not mean that such 280 parameters do not play a role in controlling the BC concentration, but that no statistically significant 281 associations were found with the data collected in our study and thus these parameters no longer 282 considered in the statistical analyses discussed below.   Table 1 reporting the standardized estimated coefficients and the corresponding p-values). 349 In order to interpret the statistical results, the description of the 85-days campaign is split into two periods 353 identified as the transition from the "cold" to the "melting" state. The first period occurred before the end 354 concentration (Figure 2). A possible explanation is that the precipitation amounts were small so that the 366 precipitation events did not alter significantly the atmospheric BC reservoir. 367 In the second period, from the beginning of June, the atmospheric temperature increases, causing the In this study, the estimated statistical association between snow rBC mass concentration and the daily 396 snow temperature is negative and strongly significant (p < 0.001). During the 85-days experiment, we can distinguish two events where the temperature appeared to play a role in the BC concentration. Both of 398 them show an increase in rBC mass concentration during melting/refreezing episodes, in agreement with 399 other studies (Aamaas et al., 2011). The first event occurred between May 5 to 12 and the second after 400 May 20, when the proper snow melting began (Figure 2). The first event was characterized by a rapid rise 401 of daily air temperature (from -6°C to -1°C) in concomitance to a snow precipitation event, followed by a 402 rapid temperature decrease to -6 °C. The surface snow (10 cm) mirrored this behaviour, first rising from -  (Figure 3). The average value over the whole sampling period is 9.5 ± 5.2 ng g -1 (approximately 6 424 times higher than during the 85-days experiment). The rBC mass size distribution was characterized by a 425 median value of the geometric means of about 230 ± 32 nm, significantly lower than that which was 426 reported and described in Figure S4; the interpretation of the differences between the rBC and the EC 429 measurements in snow samples was beyond this manuscript's objectives. 3 . The water conductivity shows a similar behaviour, and it is characterized by an average of 39 ± 9 µS 433 (30% higher than during the 85-days experiment).

Statistical Results 449
The multiple linear regression model for the 3-days experiment explains the 83% of the total snow rBC 450 mass concentration variance, a percentage higher than the 85-days experiment, likely due to the more 451 stable atmospheric conditions and the greater interaction with the atmosphere of the upper 3 cm compared 452 to the 10 cm used for the seasonal experiment. The fitted multiple linear regression model indicates a 453 statistically significant association between the rBC mass concentration in the snow and the conductivity 454 (p < 0.001), the number concentration of coarse-mode particles (p = 0.003), the snow precipitation 455 amount (p < 0.001), the incoming solar radiation (p = 0.009) and the snow temperature (p = 0.01). The 456 standardized estimated coefficients are reported in Table 1, displayed along with 90% and 95% 457 confidence intervals in Figure 4. condition is essential for the hoar formation) able to dilute the surface snow BC concentration. 472 Specifically, the lowest rBC mass concentration value is found between 5 a.m. and 12 a.m. and in the 473 same time interval the solar radiation increases from 100 to 400 W m -2 , followed by a delay of the air and 474 the snow temperatures increase. In these time frames, the temperature offset between the air and the 475 surface snow is the highest, up to 4°C, with the surface snow being the coldest between the two. 476 Condensation of water vapour on the top of the snow crystals is likely adding "water" mass (without BC The snow precipitation amount is negatively associated with the rBC mass concentration in the 487 snow (p < 0.001). As previously remarked, the aerosol scavenging intensity is not measurable with snow 488 sampling strategies based on the sampling of a constant snow thickness from the surface (3 cm in this 489 case). We tentatively explain the negative relation observed in this study with the high frequency 490 sampling, being able to follow the evolution of the BC particles scavenged during a snow episode (from 3 491

Conclusions and Future Perspectives 513
The seasonal and daily experiments (85-and 3-days long, respectively) suggest that the rBC 514 concentration in the upper snow layer is not only due to a cumulative process such as when evaluating the 515 entire annual snow pack but, rather by a more complex process involving atmospheric, meteorological 516 and snowpack conditions. Our results based on a multiple linear regression models suggest that the 517 amount of BC in the surface snow is decoupled from the BC atmospheric load. This finding suggests that, 518 despite the potentially high atmospheric BC concentrations (as in the case of long-range transport of 519 biomass burning plumes), the surface snow BC mass concentration can potentially remain unaffected. In sampling experiment). On the other hand, the surface melting episodes enrich the BC content in the 532 surface layer not because of enhanced deposition but mainly because of water mass loss. In particular, the 533 snow mass loss is stronger during the snow-melting season, where an increase in the rBC concentration 534 could significantly alter the snow albedo and further enhance the radiative absorption, hence promoting a 535 positive feedback. We believe our results to be representative at least of the Arctic costal areas, 536 characterized by similar processes and seasonality. 537 The remarkable diurnal and daily variability, as well as the complex interdependent mechanisms    "snow" -amount of fresh snow from the precipitation episodes, "SWR" -solar radiation, "temp" -the 642 snow temperature. The plot is produced with the R package (R Core Team, 2020) jtools (Long, 2020).