Abstract.A state-of-the-art regional model, the Weather Research and Forecasting (WRF) model (Skamarock et al., 2008) coupled with a chemistry component (Chem) (Grell et al., 2005), is coupled with the snow, ice, and aerosol radiative (SNICAR) model that includes the most sophisticated representation of snow metamorphism processes available for climate study. The coupled model is used to simulate black carbon (BC) and dust concentrations and their radiative forcing in seasonal snow over North China in January-February of 2010, with extensive field measurements used to evaluate the model performance. In general, the model simulated spatial variability of BC and dust mass concentrations in the top snow layer (hereafter BCS and DSTS, respectively) are consistent with observations. The model generally moderately underestimates BCS in the clean regions but significantly overestimates BCS in some polluted regions. Most model results fall within the uncertainty ranges of observations. The simulated BCS and DSTS are highest with > 5000 ng g −1 and up to 5 mg g −1 , respectively, over the source regions and reduce to < 50 ng g −1 and < 1 µg g −1 , respectively, in the remote regions. BCS and DSTS introduce a similar magnitude of radiative warming (∼ 10 W m −2 ) in the snowpack, which is comparable to the magnitude of surface radiative cooling due to BC and dust in the atmosphere. This study represents an effort in using a regional modeling framework to simulate BC and dust and their direct radiative forcing in snowpack.Although a variety of observational data sets have been used to attribute model biases, some uncertainties in the results remain, which highlights the need for more observations, particularly concurrent measurements of atmospheric and snow aerosols and the deposition fluxes of aerosols, in future campaigns.

Seasonal snow samples obtained at 46 sites in 6 provinces of China in January and February 2010 were analyzed for a suite of chemical species and these data are combined with previously determined concentrations of insoluble light-absorbing particles (ILAP), including all particles that absorb light in the 650–700 nm wavelength interval. The ILAP, together with 14 other analytes, are used as input to a positive matrix factorization (PMF) receptor model to explore the sources of ILAP in the snow. The PMF analysis for ILAP sources is augmented with backward trajectory cluster analysis and the geographic locations of major source areas for the three source types. The two analyses are consistent and indicate that three factors/sources were responsible for the measured light absorption of snow: a soil dust source, an industrial pollution source, and a biomass and / or biofuel burning source. Soil dust was the main source of the ILAP, accounting for ~53% of ILAP on average

Abstract. Black carbon (BC) particles over the Himalayas and Tibetan Plateau (HTP), both airborne and those deposited on snow, have been shown to affect snowmelt and glacier retreat. Since BC over the HTP may originate from a variety of geographical regions and emission sectors, it is essential to quantify the source–receptor relationships of BC in order to understand the contributions of natural and anthropogenic emissions and provide guidance for potential mitigation actions. In this study, we use the Community Atmosphere Model version 5 (CAM5) with a newly developed source tagging technique, nudged towards the MERRA meteorological reanalysis, to characterize the fate of BC particles emitted from various geographical regions and sectors. Evaluated against observations over the HTP and surrounding regions, the model simulation shows a good agreement in the seasonal variation of the near-surface airborne BC concentrations, providing confidence to use this modeling framework for characterizing BC source–receptor relationships. Our analysis shows that the relative contributions from different geographical regions and source sectors depend on seasons and the locations in the HTP. The largest contribution to annual mean BC burden and surface deposition in the entire HTP region is from biofuel and biomass (BB) emissions in South Asia, followed by fossil fuel (FF) emissions from South Asia, then FF from East Asia. The same roles hold for all the seasonal means except for the summer when East Asia FF becomes more important. For finer receptor regions of interest, South Asia BB and FF have the largest impact on BC in Himalayas and Central Tibetan Plateau, while East Asia FF and BB contribute the most to Northeast Plateau in all seasons and Southeast Plateau in the summer. Central Asia and Middle East FF emissions have relatively more important contributions to BC reaching Northwest Plateau, especially in the summer. Although local emissions only contribute about 10% to BC in the HTP, this contribution is extremely sensitive to local emission changes. Lastly, we show that the annual mean radiative forcing (0.42 W m−2) due to BC in snow outweighs the BC dimming effect (−0.3 W m−2) at the surface over the HTP. We also find strong seasonal and spatial variation with a peak value of 5 W m−2 in the spring over Northwest Plateau. Such a large forcing of BC in snow is sufficient to cause earlier snow melting and potentially contribute to the acceleration of glacier retreat.

Nitrate is one of the most important atmospheric aerosols that greatly impacts global nitrogen cycles, climate, and human health (Myhre, Shindell, et al., 2013, IPCC AR5 Chapter 8;Pagalan et al., 2018). Nitrate aerosols can affect climate through scattering shortwave (SW) radiation and acting as cloud condensation nuclei (CCN) affecting cloud microphysical properties. Anthropogenic nitrate aerosols are usually formed from gas precursors such as nitrogen oxides (NO x ), which is largely related to heavy air pollution caused by economic activities. Aerosol mass spectrometer measurements over the globe show that nitrate aerosols can contribute a significant fraction to total aerosol mass, especially over metropolitan areas during winter according to the measurements (Zhang et al., 2007).Despite the importance of nitrate aerosols, only a handful climate models simulate their formation and lifecycle in the atmosphere (