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1. Distinct surface response to black carbon aerosols

   https://doi.org/10.5194/acp-21-13797-2021  

   Tang, Tao ; Shindell, Drew ; Zhang, Yuqiang ; Voulgarakis, Apostolos ; Lamarque, Jean-Francois ; Myhre, Gunnar ; Faluvegi, Gregory ; Samset, Bjørn H. ; Andrews, Timothy ; Olivié, Dirk ; et al ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. For the radiative impact of individual climate forcings,most previous studies focused on the global mean values at the top of theatmosphere (TOA), and less attention has been paid to surface processes,especially for black carbon (BC) aerosols. In this study, the surface radiativeresponses to five different forcing agents were analyzed by using idealizedmodel simulations. Our analyses reveal that for greenhouse gases, solarirradiance, and scattering aerosols, the surface temperature changes aremainly dictated by the changes of surface radiative heating, but for BC,surface energy redistribution between different components plays a morecrucial role. Globally, when a unit BC forcing is imposed at TOA, the netshortwave radiation at the surface decreases by -5.87±0.67 W m−2 (W m−2)−1 (averaged over global land without Antarctica), which ispartially offset by increased downward longwave radiation (2.32±0.38 W m−2 (W m−2)−1 from the warmer atmosphere, causing a netdecrease in the incoming downward surface radiation of -3.56±0.60 W m−2 (W m−2)−1. Despite a reduction in the downward radiationenergy, the surface air temperature still increases by 0.25±0.08 Kbecause of less efficient energy dissipation, manifested by reduced surfacesensible (-2.88±0.43 W m−2 (W m−2)−1) and latent heat flux(-1.54±0.27 W m−2 (W m−2)−1), as well as a decrease inBowen ratio (-0.20±0.07 (W m−2)−1). Such reductions of turbulentfluxes can be largely explained by enhanced air stability (0.07±0.02 K (W m−2)−1), measured as the difference of the potential temperaturebetween 925 hPa and surface,more »and reduced surface wind speed (-0.05±0.01 m s−1 (W m−2)−1). The enhanced stability is due to the fasteratmospheric warming relative to the surface, whereas the reduced wind speedcan be partially explained by enhanced stability and reduced Equator-to-poleatmospheric temperature gradient. These rapid adjustments under BC forcingoccur in the lower atmosphere and propagate downward to influence thesurface energy redistribution and thus surface temperature response, whichis not observed under greenhouse gases or scattering aerosols. Our studyprovides new insights into the impact of absorbing aerosols on surfaceenergy balance and surface temperature response.« less
2. Aerosol Direct Radiative and Cloud Adjustment Effects on Surface Climate over Eastern China: Analyses of WRF Model Simulations

   https://doi.org/10.1175/JCLI-D-18-0236.1  

   Song, Yangyang ; Chen, Guoxing ; Wang, Wei-Chyung ( February 2019 , Journal of Climate)

   The WRF-simulated changes in clouds and climate due to the increased anthropogenic aerosols for the summers of 2002–08 (vs the 1970s) over eastern China were used to offline calculate the radiative forcings associated with aerosol–radiation (AR) and aerosol–cloud–radiation (ACR) interactions, which subsequently facilitated the interpretation of surface temperature changes. During this period, the increases of aerosol optical depth (ΔAOD) averaged over eastern China range from 0.18 in 2004 to 0.26 in 2007 as compared to corresponding cases in the 1970s, and the multiyear means (standard deviations) of AR and ACR forcings at the surface are −6.7 (0.58) and −3.5 (0.63) W m⁻², respectively, indicating the importance of cloud changes in affecting both the aerosol climate forcing and its interannual variation. The simulated mean surface cooling is 0.35°C, dominated by AR and ACR with a positive (cooling) feedback associated with changes in meteorology (~10%), and two negative (warming) feedbacks associated with decreases in latent (~70%) and sensible (~20%) heat fluxes. More detailed spatial characteristics were analyzed using ensemble simulations for the year 2008. Three regions—Jing-Jin-Ji (ΔAOD ~ 0.63), Sichuan basin (ΔAOD ~ 0.31), and middle Yangtze River valley (ΔAOD ~ 0.26)—at different climate regimes were selected to investigate the relative rolesmore »of AR and ACR. While the AR forcing is closely related to ΔAOD values, the ACR forcing presents different regional characteristics owing to cloud changes. In addition, the surface heat flux feedbacks are also different between regions. The study thus illustrates that ACR forcing is useful as a diagnostic parameter to unravel the complexity of climate change to aerosol forcing over eastern China.

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3. Pan-Arctic seasonal cycles and long-term trends of aerosol properties from 10 observatories

   https://doi.org/10.5194/acp-22-3067-2022  

   Schmale, Julia ; Sharma, Sangeeta ; Decesari, Stefano ; Pernov, Jakob ; Massling, Andreas ; Hansson, Hans-Christen ; von Salzen, Knut ; Skov, Henrik ; Andrews, Elisabeth ; Quinn, Patricia K. ; et al ( January 2022 , Atmospheric Chemistry and Physics)

   Abstract. Even though the Arctic is remote, aerosol properties observed there arestrongly influenced by anthropogenic emissions from outside the Arctic. Thisis particularly true for the so-called Arctic haze season (January throughApril). In summer (June through September), when atmospheric transportpatterns change, and precipitation is more frequent, local Arctic sources,i.e., natural sources of aerosols and precursors, play an important role.Over the last few decades, significant reductions in anthropogenic emissionshave taken place. At the same time a large body of literature shows evidencethat the Arctic is undergoing fundamental environmental changes due toclimate forcing, leading to enhanced emissions by natural processes that mayimpact aerosol properties. In this study, we analyze 9 aerosol chemical species and 4 particleoptical properties from 10 Arctic observatories (Alert, Kevo, Pallas,Summit, Thule, Tiksi, Barrow/Utqiaġvik, Villum, and Gruvebadet and ZeppelinObservatory – both at Ny-Ålesund Research Station) to understand changesin anthropogenic and natural aerosol contributions. Variables includeequivalent black carbon, particulate sulfate, nitrate, ammonium,methanesulfonic acid, sodium, iron, calcium and potassium, as well asscattering and absorption coefficients, single scattering albedo andscattering Ångström exponent. First, annual cycles are investigated, which despite anthropogenic emissionreductions still show the Arctic haze phenomenon. Second, long-term trendsare studied using the Mann–Kendall Theil–Sen slope method. We find in total41 significant trendsmore »over full station records, i.e., spanning more than adecade, compared to 26 significant decadal trends. The majority ofsignificantly declining trends is from anthropogenic tracers and occurredduring the haze period, driven by emission changes between 1990 and 2000.For the summer period, no uniform picture of trends has emerged. Twenty-sixpercent of trends, i.e., 19 out of 73, are significant, and of those 5 arepositive and 14 are negative. Negative trends include not only anthropogenictracers such as equivalent black carbon at Kevo, but also natural indicatorssuch as methanesulfonic acid and non-sea-salt calcium at Alert. Positivetrends are observed for sulfate at Gruvebadet. No clear evidence of a significant change in the natural aerosolcontribution can be observed yet. However, testing the sensitivity of theMann–Kendall Theil–Sen method, we find that monotonic changes of around 5 % yr−1 in an aerosol property are needed to detect a significanttrend within one decade. This highlights that long-term efforts well beyonda decade are needed to capture smaller changes. It is particularly importantto understand the ongoing natural changes in the Arctic, where interannualvariability can be high, such as with forest fire emissions and theirinfluence on the aerosol population. To investigate the climate-change-induced influence on the aerosolpopulation and the resulting climate feedback, long-term observations oftracers more specific to natural sources are needed, as well as of particlemicrophysical properties such as size distributions, which can be used toidentify changes in particle populations which are not well captured bymass-oriented methods such as bulk chemical composition.« less
4. Influence of cloud microphysical processes on black carbon wet removal, global distributions, and radiative forcing

   https://doi.org/10.5194/acp-19-1587-2019  

   Xu, Jiayu ; Zhang, Jiachen ; Liu, Junfeng ; Yi, Kan ; Xiang, Songlin ; Hu, Xiurong ; Wang, Yuqing ; Tao, Shu ; Ban-Weiss, George ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Parameterizations that impact wet removal of black carbon (BC)remain uncertain in global climate models. In this study, we enhance thedefault wet deposition scheme for BC in the Community Earth System Model (CESM)to (a) add relevant physical processes that were not resolved in thedefault model and (b) facilitate understanding of the relative importanceof various cloud processes on BC distributions. We find that the enhancedscheme greatly improves model performance against HIPPO observationsrelative to the default scheme. We find that convection scavenging, aerosolactivation, ice nucleation, evaporation of rain or snow, and below-cloudscavenging dominate wet deposition of BC. BC conversion rates for processesrelated to in-cloud water–ice conversion (i.e., riming, the Bergeronprocess, and evaporation of cloud water sedimentation) are relativelysmaller, but have large seasonal variations. We also conduct sensitivitysimulations that turn off each cloud process one at a time to quantify theinfluence of cloud processes on BC distributions and radiative forcing.Convective scavenging is found to have the largest impact onBC concentrations at mid-altitudes over the tropics and even globally. Inaddition, BC is sensitive to all cloud processes over the NorthernHemisphere at high latitudes. As for BC vertical distributions, convectivescavenging greatly influences BC fractions at different altitudes.Suppressing BC droplet activation in clouds mainly decreases the fraction ofcolumn BC below 5 km, whereas suppressing BC ice nucleation increases thatabovemore »10 km. During wintertime, the Bergeron process also significantlyincreases BC concentrations at lower altitudes over the Arctic. Oursimulation yields a global BC burden of 85 Gg; corresponding directradiative forcing (DRF) of BC estimated using the Parallel Offline RadiativeTransfer (PORT) is 0.13 W m−2, much lower than previous studies. Therange of DRF derived from sensitivity simulations is large, 0.09–0.33 W m−2,corresponding to BC burdens varying from 73 to 151 Gg. Due todifferences in BC vertical distributions among each sensitivity simulation,fractional changes in DRF (relative to the baseline simulation) are alwayshigher than fractional changes in BC burdens; this occurs because relocating BCin the vertical influences the radiative forcing per BC mass. Our resultshighlight the influences of cloud microphysical processes on BC concentrationsand radiative forcing.« less
5. Contribution of biomass burning to black carbon deposition on Andean glaciers: consequences for radiative forcing

   https://doi.org/10.1088/1748-9326/acb371  

   Bonilla, E. X. ; Mickley, L. J. ; Beaudon, E. G. ; Thompson, L. G. ; Rodriguez, W. E. ; Encarnación, R. Cruz ; Whicker, C. A. ; Flanner, M. G. ; Schmitt, C. G. ; Ginot, P. ( February 2023 , Environmental Research Letters)

   Andean glaciers have melted rapidly since the 1960s. While some melting is likely due to anthropogenic climate change driven by increasing greenhouse gases, deposition of light-absorbing particles such as black carbon (BC) may also play a role. We hypothesize that BC from fires in the Amazon Basin and elsewhere may be deposited on Andean glaciers, reducing the surface albedo and inducing further melting. Here we investigate the role of BC deposition on albedo changes in the Andes for 2014–2019 by combining atmospheric chemistry modeling with observations of BC in snow or ice at four mountain sites in Peru (Quelccaya, Huascarán, Yanapaccha, and Shallap) and at one site in Bolivia (Illimani). We find that annual mean ice BC concentrations simulated by the chemical transport model GEOS-Chem for 2014–2019 are roughly consistent with those observed at the site with the longest record, Huascarán, with overestimates of 15%–40%. Smoke from fires account for 20%–70% of total wet and dry deposition fluxes, depending on the site. The rest of BC deposited comes from fossil fuel combustion. Using a snow albedo model, we find that the annual mean radiative forcing from the deposition of smoke BC alone on snow ranges from +0.1 to +3.2more »W m⁻²under clear-sky conditions, with corresponding average albedo reductions of 0.04%–1.1%. These ranges are dependent on site and snow grain size. This result implies a potentially significant climate impact of biomass burning in the Amazon on radiative forcing in the Andes.

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