Assessment of the effects of acid-coated ice nuclei on the Arctic cloud microstructure, atmospheric dehydration, radiation and temperature during winter

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. (2012)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/joc.3454

Assessment of the effects of acid-coated ice nuclei on theArctic cloud microstructure, atmospheric dehydration,

radiation and temperature during winter

Eric Girard,a* Guillaume Dueymes,a Ping Du,a and Allan K. Bertramb

a Department of Earth and Atmospheric Sciences, University of Quebec at Montreal, Montreal, Quebec, Canadab Chemistry Department, University of British Columbia, Vancouver, BC, Canada

ABSTRACT: Owing to the large-scale transport of pollution-derived aerosols from the mid-latitudes to the Arctic, mostof the aerosols are coated with acidic sulfate during winter in the Arctic. Recent laboratory experiments have shownthat acid coating on dust particles substantially reduces the ability of these particles to nucleate ice crystals. Simulationsperformed using the Limited Area version of the Global Multiscale Environmental Model (GEM-LAM) are used to assessthe potential effect of acid-coated ice nuclei on the Arctic cloud and radiation processes during January and February 2007.Ice nucleation is treated using a new parameterization based on laboratory experiments of ice nucleation on sulphuricacid-coated and uncoated kaolinite particles. Results show that acid coating on dust particles has an important effect oncloud microstructure, atmospheric dehydration, radiation and temperature over the Central Arctic, which is the coldest partof the Arctic. Mid and upper ice clouds are optically thinner while low-level mixed-phase clouds are more frequent andpersistent. These changes in the cloud microstructures affect the radiation at the top of the atmosphere with longwavenegative cloud forcing values ranging between 0 and −6 W m−2 over the region covered by the Arctic air mass. Copyright 2012 Royal Meteorological Society

KEY WORDS Arctic climate; arctic haze; aerosols; clouds; ice nuclei; aerosol-cloud-radiation interactions

Received 6 May 2011; Revised 30 January 2012; Accepted 5 February 2012

1. Introduction

Wintertime cloud cover in the troposphere over the Arcticranges between 40 to 50% (Wyser et al., 2008), andlow-level mixed phase clouds and optically thin iceclouds dominate. Thin ice clouds are difficult to detectby passive imagery and this results in a substantialunderestimation of cloud cover (Curry et al., 1996; Wyserand Jones, 2005; Karlsson and Dybbroe, 2010). Grenieret al. (2009) have shown that there are two types ofoptically thin ice clouds. The first one is formed by alarge concentration of small ice crystals and the secondone is formed by larger precipitating ice crystals insmaller concentration. The latter cloud type is correlatedwith aerosol concentration (Grenier et al., 2009). Theseresults suggest that there might be a relationship betweenthe second type of thin ice clouds and large aerosolconcentrations possibly of anthropogenic sources.

The Arctic is highly polluted during the cold seasonwith high concentrations of aerosols often observed(Schnell, 1984; Yli-Tuomi et al., 2003; Law and Stohl,2007). These aerosols, which are mainly emitted overnorthern European cities, China and Siberia (Shaw,

∗ Correspondence to: E. Girard, Department of Earth and AtmosphericSciences, University of Quebec at Montreal, P.O. Box 8888, Station“Downtown”, Montreal, Quebec H3C 3P8, Canada.E-mail: [email protected]

1995), are transported from the mid-latitudes to theArctic by large-scale atmospheric circulations (Barrieet al., 1989; Shaw, 1995; Bourgeois and Bey, 2011;Fisher et al., 2011). This transport is favoured by thesouthward progression of the polar front during winterin the Northern Hemisphere. Anthropogenic aerosols areemitted north of the front in an environment with littleprecipitation, resulting in limited loss of aerosols bywet deposition. The aerosol component consists of asignificant fraction of highly acidic sulphate, and previouswork has shown that most of the submicron aerosolparticles are coated with this highly acidic sulphate (Bigg,1980; Cantrell et al., 1997).

Laboratory experiments and field observations suggestthat acid coatings on ice nuclei (IN) can have animportant effect on heterogeneous ice nucleation, whichcan occur either in the deposition, condensation-freezing,immersion and contact modes (Pruppacher and Klett,1997). In the immersion mode, ice nucleation occurs ona solid particle immersed in either an aqueous solutionin subsaturated air with respect to liquid water or inan activated cloud water droplet. In the condensation-freezing mode, ice nucleation occurs on a solid particleimmersed in an aqueous solution and above liquid watersaturation. In the deposition mode, ice nucleation occurson a solid particle or a solid particle only partiallyimmersed in an aqueous solution. Finally, in the contact

Copyright 2012 Royal Meteorological Society

E. GIRARD et al.

mode, ice nucleation occurs on a solid particle in contactwith a water droplet.

Archuleta et al. (2005) have shown that the decrease ofice nucleation at temperatures below −40 °C by immer-sion and condensation modes due to sulphuric acid coat-ing is variable and depends on the IN chemical compo-sition. Other laboratory experiments performed at tem-peratures ranging between −10 and −40 °C also showthat the heterogeneous freezing temperature initiated byimmersion of various mineral dust particles decreases asthe percentage by weight of sulphuric acid in the particleincreases (Ettner et al., 2004). More recently, Eastwoodet al. (2009) have shown that deposition ice nucleation onkaolinite particles is considerably altered at temperaturesbelow 243 K, requiring an additional 30% ice supersat-uration for ice nucleation to occur when compared touncoated particles. According to Sullivan et al. (2010a),the de-activation effect of sulphuric acid on dust parti-cle is irreversible and is still active once the acid hasbeen neutralized with ammonia. Chernoff and Bertram(2010) have shown that other coatings such as ammo-nium bisulphate also increases the onset relative humidityfor ice nucleation when compared to uncoated dust parti-cles but to a lesser extent than sulphuric acid coating.Other laboratory experiments on coated and uncoatedmineral dust particles have been performed by Knopf andKoop (2006), Salam et al. (2007), Cziczo et al. (2009),Niedermeier et al. (2010) and Sullivan et al. (2010b).The coating effect on ice nucleation was also indirectlyobserved during the Arctic Gas and Aerosol SamplingProgram (AGASP) by Borys (1989). In this study, theauthors showed that the IN concentration is decreased by3 to 4 orders of magnitude during highly polluted (Arctichaze) events (Borys, 1989). While deposition ice nucle-ation seems strongly affected by acid coating, immersionfreezing of activated cloud water droplets does not seemto be affected (Sullivan et al., 2010b).

Blanchet and Girard (1994, 1995) were the first tohypothesize that the ice nucleation inhibition effect ofacid coatings on aerosols can have an important effecton cloud microstructure and on the surface energy budgetduring winter in the Arctic. According to their hypothesis,the decrease of the IN concentration leads to the forma-tion of fewer but larger ice crystals. This process leads tothe formation of optically thin ice clouds recently identi-fied by Grenier et al. (2009). Acid coating on IN and theresulting larger ice crystals dehydrate the troposphere byincreasing the precipitation over large areas. This resultsin the decrease of the greenhouse effect due to the strongeffect of water vapour, primarily in the broad rotationalband in the far infrared. The reduced greenhouse effectfurther promotes dehydration and cooling (Blanchet andGirard, 1995; Curry et al., 1995). This hypothesis isreferred to as the dehydration-greenhouse feedback.

This process was first evaluated by Girard et al. (2005)using a single-column model. They simulated four coldseasons over Alert (Canada) using observed aerosol com-position and concentration. The acid aerosol scenarioshows a tropospheric cooling ranging between 0 and

2 K when compared to an uncoated aerosol scenario.Girard and Stefanof (2007) have used a regional climatemodel with prognostic aerosols to evaluate the effect ofacid aerosols on the Arctic surface radiative budget forFebruary 1990. They have assumed two aerosol scenar-ios: (1) an acid scenario in which the reduction of theIN concentration depends on the sulphate concentrationand (2) a natural aerosol scenario in which the IN con-centration is unaltered. Their results show an increase ofprecipitation and dehydration of the troposphere and upto 3 K in cooling in the coldest part of the Arctic.

In these previous investigations, the acid aerosol sce-nario was treated rather subjectively and crudely becausefew laboratory studies on the effects of coating on icenucleation were available. The decrease of IN concen-tration due to acid coating was a function of the sul-phate concentration using an exponential function andwas based on Borys (1989) observations of IN decreasein polluted events. The choice of an exponential func-tion was made to produce substantial IN reduction forcases where the sulphate concentration was large. Fur-thermore, it was assumed that all ice nucleation modeswere altered equally by the presence of acid coatings.Finally, the one-moment microphysics scheme used inthe Girard and Stefanof (2007) investigation was rela-tively simple with no detailed representation of the icecrystal sedimentation process.

This research aims to refine the representation of theeffects of acid coating on IN to get a more realistic evalu-ation of the potential effect of anthropogenic aerosols onarctic wintertime clouds and radiation budgets. Labora-tory data from Eastwood et al. (2008, 2009) on ice nucle-ation properties of uncoated and sulphuric acid-coatedkaolinite particles are used to develop a more physi-cally based and refined parameterization of ice nucle-ation in subsaturated air with respect to liquid water.This new treatment of heterogeneous ice nucleation isimplemented into an elaborate two-moment cloud micro-physics scheme. In the second and third sections of thismanuscript, the model is described with an emphasis onthe new parameterization. In the fourth section, resultsof the simulations showing the effect of acid coatings onclouds and surface radiation budget for the months ofJanuary and February 2007 are described and analyzed.Section 5 presents a brief summary and a discussion ofthe results.

2. Model description

The limited-area version of the Global Multiscale Envi-ronmental Model (GEM-LAM) is used in this study. Thenumerical formulation of the model is described by Coteet al. (1998). The radiation scheme is from Li and Barker(2005) and is based on the correlation-k method withnine bands in the longwave frequencies and three bandsin the shortwave frequencies. Emission and absorptionof the following gaseous species are accounted for: H2O,CO2, O3, N2O, CH4, CFC11-14. The land-surface scheme

Copyright 2012 Royal Meteorological Society Int. J. Climatol. (2012)

EFFECTS OF ACID-COATED ICE NUCLEI ON ARCTIC CLOUD AND RADIATION DURING WINTER

ISBA (Interactions Soil-Biosphere-Atmosphere) devel-oped by Noilhan and Planton (1989) is used to determinethe lower boundary conditions for the vertical diffusion oftemperature, moisture, and momentum, as well as evalu-ating the evolution of 10 prognostic variables: the surfacetemperature, the mean soil temperature, the near-surfacesoil moisture, the liquid and frozen bulk soil water con-tent, the liquid water retained on the foliage of the veg-etation canopy, the equivalent water content of the snowreservoir, the liquid water retained in the snow pack, thesnow albedo, and the relative snow density.

The microphysics scheme used in this study is fromMilbrandt and Yau (2005). Two versions of this schemeare available in GEM-LAM (single and double moments).In this study, we use the two-moment version. Thescheme includes the following prognostic variables: themixing ratio and the number concentration of cloud liquidwater, cloud ice water, rain, snow, hail and graupel.The description of the various microphysical processesis available in Milbrandt and Yau (2005). In this section,the emphasis is put on the parameterization of ice crystalnucleation due to the importance of this microphysicalprocess in this investigation.

Homogeneous freezing of cloud liquid droplets isbased on the parameterization of DeMott et al. (1994)in which the ice nucleation rate is a polynomial functionof temperature. The fraction of cloud droplets that freezehomogeneously gradually increases from 0 at −30 to 1at −50 °C. Therefore, both homogeneous and heteroge-neous freezing processes can occur simultaneously in thistemperature range. Contact freezing is parameterized fol-lowing Young (1974), in which the number concentrationof contact IN is parameterized as a function of tempera-ture. Immersion freezing of activated rain and cloud waterdroplets follows the parameterization of Bigg (1953).The representation of heterogeneous ice nucleation bywater vapour deposition and condensation freezing is par-ticularly important in this investigation. In the originalversion of the Milbrandt and Yau (2005) microphysicsscheme, deposition and condensation freezing dependson the ice supersaturation following the empirical rela-tionship of Meyers et al. (1992). This parameterizationfor deposition ice nucleation has been modified to dis-tinguish ice nucleation on sulphuric acid-coated IN fromice nucleation on uncoated IN.

The new parameterization for ice nucleation in sub-saturated air with respect to liquid water is based onthe classical theory of heterogeneous ice nucleation ofFletcher (1962). The new parameterization can representboth deposition nucleation on uncoated IN and immer-sion freezing of haze droplets in subsaturated air withrespect to liquid water, which will be referred to asthe deposition-immersion nucleation mode in this study.It is assumed that the surface of the IN is energeti-cally uniform for ice nucleation. The only additionalunknown parameter is the contact angle (�) between theice embryo and the IN. Following the single contact angleapproach (Hung et al., 2003; Chen et al., 2008; East-wood et al., 2008, 2009; Fornea et al., 2009; Chernoff

and Bertram, 2010), the contact angle has been derivedusing the results of the laboratory experiments of East-wood et al. (2008, 2009) for uncoated kaolinite particles(�uncoated = 12°) and for kaolinite particles coated withsulphuric acid (�coated = 27°). The following equation isthen used to determine the number of ice crystals (Nice)

nucleated in a given time step (�t):

Nice(�t) = Nkaolinite[1 − exp(−JAkaolinite�t)] (1)

where Akaolinite is the surface area of the kaoliniteparticles, Nkaolinite is the total concentration of kaoliniteparticles and J is the nucleation rate of ice embryo perunit area of the particle and is defined as:

J (cm−2 s−1) = B exp(−�G∗

kT

)

where �G∗ = 16πσ 3ivf (cos �)

3(ρiRvT ln Si)2

where B is the pre-exponential factor (Pruppacher andKlett, 1998), �G∗ is the critical Gibbs free energy for theformation of an ice embryo, k is the Boltzman constant,σiv is the surface tension between ice and water vapour,ρi is the bulk ice density, Rv is the gas constant for watervapour, T is the temperature and Si is the saturation ratiowith respect to ice. f (cos �) is a function that dependson the contact angle as defined by Pruppacher and Klett(1998) for an infinite plane surface.

Recent studies have shown that more refined modelsthan the single contact angle model used in this studybetter reproduce the laboratory experiments for dust parti-cles of different sizes (Marcolli et al., 2007; Niedermeieret al., 2010; Vali, 2010; Wheeler and Bertram, 2011).Contact angle probability density function and activesites models (both stochastic and deterministic) have beenproposed (Marcolli et al., 2007; Connolly et al., 2009;Luond et al., 2010). In our investigation, the simplersingle contact angle model is used. This model wasrecently evaluated against in situ measurements takenduring the Surface Heat and Energy Budget of the Arctic(SHEBA) field experiment. Du et al. (2011) have com-pared the deposition-immersion ice nucleation parameter-ization used in this study with the empirical approach ofMeyers et al. (1992). They have shown that, as opposedto the empirical approach of Meyers et al. (1992), thenew parameterization of deposition-immersion ice nucle-ation described above reproduces quite well the annualcycle of the cloud thermodynamic phase, the down-welling longwave radiation and the cloud radiative forc-ing (CRF) at the surface during the cold part of the year.These results suggest that the model is able to simulatereasonably well wintertime cloud and radiation processesin an Arctic environment, which is of prime importance inthis investigation. Further work will be needed to test thesensitivity of the results to other models to parameterizelaboratory results.


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3. Design of the experiment

The months of January and February 2007 are simulated.This choice was motivated by the fact that the trans-port of aerosols was particularly effective during this timeperiod with the presence of a series of uncommon strongextra tropical storms over the North Atlantic and North-ern Europe (Fink et al., 2009). These storms have con-tributed to enhance the transport of pollution emitted overNorthern Europe and Siberia to the Arctic. Two aerosolscenarios are considered. In the first scenario (hereafteraerosol scenario A), it is assumed that dust particlesare uncoated. In the second aerosol scenario (hereafteraerosol scenario B), dust particles are coated with sul-phuric acid. The appropriate contact angle for deposition-immersion ice nucleation (see previous section) is usedin each aerosol scenario. Note that the same parameter-izations for contact freezing and immersion freezing ofactivated cloud water droplets are used for both aerosolscenarios. Since the model does not simulate explicitlythe aerosol composition and concentration, it is assumedthat the concentration of dust particles is constant in timeand space with a value of 0.38 cm−3. This value is basedon observations taken during field experiments in theArctic, which shows variable dust mass concentrationsranging between 50 and 3000 ng m−3 (Winchester et al.,1984; Franzen et al., 1994) depending on the air massorigin. The assumed number concentration of dust par-ticles (Nkaolinite) and surface area of kaolinite particles(Akaolinite) in our simulations corresponds to a mass con-centration of 500 ng m−3 and a diameter of 0.5 µm. Thechosen concentration is representative of the dust concen-tration over the Arctic during winter (Quinn et al. 1996).The same experiment is repeated using a dust concentra-tion reduced by a factor 2 to test the sensitivity of theresults to the prescribed dust concentration. Aerosol sce-narios A2 and B2 will refer to aerosol scenarios A andB, respectively, with a reduced dust concentration (resultspresented in Section 4.4.).

Two main assumptions in this investigation can maxi-mize the effects that sulfuric acid coating on dust particlescan have on the Arctic clouds and radiation. Our parame-terization of deposition-immersion ice nucleation is basedon kaolinite particles, which is one of the most effec-tive IN in the atmosphere. Although similar results wereobtained for other mineral dust particles coated with sul-phuric acid (Cziczo et al., 2009; Eastwood et al., 2009;Chernoff and Bertram, 2010), there are still several IN ofdifferent chemical compositions that have not been testedin laboratory yet. These IN of different chemical compo-sitions could very well behave differently than kaoliniteparticles when they are coated with sulphuric acid. Itshould also be noted that the assumed constant dust con-centration in time and space is a simplification that wasmade to avoid unjustified spatial and temporal distribu-tion of dust in our simulation. Dust and other aerosolsare often observed in thin discrete layers in the low, midand upper troposphere and can vary in the horizontalaccording to the large-scale atmospheric circulation. It

is therefore hazardous to assume a given spatial distribu-tion of dust particles without either coupling GEM withan aerosol transport model or perform a comprehensivesensitivity study on different aerosol spatial distributions.Therefore, the effect of acid coating on dust particleson the Arctic clouds and radiation budget obtained inthis study should be viewed as an upper limit given theabove-mentioned assumptions.

The model internal variability for the Arctic climate isrelatively high during winter (Rinke et al., 2004; Girardand Bekcic, 2005). Hence, a large number of simulationsare required to distinguish the investigated climate signalfrom the model variability. Each simulation within anensemble is initialized with different conditions followingRinke and Dethloff (2000). In our experiments, 10simulations of each aerosol scenario were necessary toget reasonable statistics over a large area of the Arctic.Unless specified otherwise, results presented in this paperalways represent the January and February (JF) ensemblemean of either aerosol scenario A or B and shadowedareas indicate that results are statistically significant witha confidence level of 95%. The statistical test is describedin Girard and Bekcic (2005).

The integration domain is centred over the Arcticand covers all areas north of 50°N, which include mostof Europe, North Asia, Northern Canada, Siberia, andthe North Atlantic and Pacific Oceans. The simulationdomain has 364 by 304 grid points with a horizontalresolution of 0.25°. There are 53 vertical levels with thehighest resolution in the lower troposphere. Initial andboundary conditions for atmospheric fields are providedby the European Centre for Medium-range WeatherForecast (ECMWF) ERA-Interim reanalysis on a 2.5° by2.5° longitude/latitude grid. Monthly mean sea surfacetemperature and sea ice concentration are from theAtmospheric Model Intercomparison Project (AMIP II)(Hurrell et al., 2008) reanalysis on a 1° by 1° grid.These values are interpolated on the grid used for oursimulations.

4. Results

Figure 1 shows the averaged mean sea level pressurefor January and February 2007 from the ERA-Interimreanalysis. The typical atmospheric circulation associatedwith a positive North Atlantic Oscillation index with anintense low-pressure system located in the North Atlanticregion and an anticyclone over Siberia characterizes thisperiod. In January and February 2007, several strongstorms have passed over Europe causing strong windsand a lot of precipitation. The large-scale circulationpattern was therefore very favourable to the transportof anthropogenic sulphate emitted over Eurasia and dustoriginating from East Asian deserts to the Central Arctic.

Figure 2 shows the JF mean temperature at 850 hPaand the geopotential height at 500 hPa from ERA-Interim reanalysis and model simulations. Simulationsused for this comparison are the ensemble averaged ofthe aerosol scenario B (acid-coated aerosol scenario). The



Figure 1. JF mean MSLP (hPa) from ERA-Interim reanalysis.

(a) (b)

(c) (d)

Figure 2. JF mean geopotential height (dam) at 500 hPa (a, b) and mean temperature (K) at 850 hPa (c, d) from ERA-Interim (a, c) and modelsimulation (aerosol scenario B) (b, d).


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model reproduces reasonably well the geopotential heightat 500 hPa with differences between ERA-Interim andthe simulation varying between −4 and +4 decameters(dam). A larger difference of +6 to +8 dam occurs overSiberia. Temperature at 850 hPa is also generally wellreproduced by the model with errors of less than 2 Kexcept over Siberia with an overestimation between 2and 4 K, which is consistent with the geopotential heightbias over the same area. These errors are acceptablein the context of this sensitivity study and compareadvantageously with other limited-area model simulationsof the Arctic climate (Rinke et al., 2006; Wyser et al.,2008).

4.1. Cloud microstructure

Figure 3 shows the JF mean liquid and ice water pathanomaly (an anomaly is defined as the difference betweenaerosol scenarios B and A). Over the Arctic, the ice waterpath is generally slightly smaller in aerosol scenario Bcompared to aerosol scenario A except for small isolated

areas where it is larger. Over the mid-latitudes and sub-Arctic regions, the ice water path anomalies are alsonegative and larger (absolute values). The liquid waterpath difference between aerosol scenarios B and A ispositive over the entire domain. The vertical profile ofthe ice water content and liquid water content spatiallyaveraged over a sub-domain delimited by sea ice andopen water boundary and temporally averaged over JFis shown in Figure 4. The increase of the liquid watercontent is the largest in the lower troposphere at about850 hPa. This is the height corresponding to the highestoccurrence of mixed-phase clouds (discussed at the endof this section). However, the increase of the liquid waterpath is also positive higher in the troposphere up to600 hPa. The decrease of the ice water content in aerosolscenario B spreads over the whole troposphere from thesurface to 300 hPa with a maximum at 500 hPa.

Figure 5 shows the spatial and temporal average of thevertical profile of the ice crystal number concentrationand mean diameter. Below 500 hPa, the ice crystalnumber concentration is smaller and the ice crystal mean

(a) (b)

Figure 3. JF mean cloud (a) liquid and (b) ice water path anomaly (∗0.1 kg m−2). Shadowed areas indicate that anomalies are statisticallysignificant with a confidence level of 95%.

(a) (b)

Figure 4. Vertical profiles of cloud (a) liquid and (b) ice water content (g kg−1) for aerosol scenarios A and B averaged over time and spatiallyaveraged over a mask delimited by sea ice boundaries.



(a) (b)

Figure 5. Vertical profiles of cloud ice crystal (a) number concentration (kg−1) and (b) mean diameter (µm) for aerosol scenarios A and Baveraged over time and spatially averaged over a mask delimited by sea ice boundaries.

diameter is larger in aerosol scenario B. This result isconsistent with the hypothesis formulated in the previoussection. However, above 500 hPa, the ice crystal numberconcentration increases and the mean diameter of icecrystals decreases in aerosol scenario B.

The different effects of acid coating on cloudmicrostructure above and below 500 hPa is related totemperature. At the upper levels, above 500 hPa, temper-atures are generally below −40 °C. At these temperatures,the homogeneous freezing rate of cloud water droplets isrelatively large. In these conditions, the decrease of icenucleation in the deposition-immersion mode in aerosolscenario B leads to the formation of more water droplets.A large fraction of them freeze homogeneously as soonas they are nucleated. This results in an increase of theice crystal concentration and a smaller mean ice crystaldiameter in aerosol scenario B. In aerosol scenario A, thenucleation rate of ice crystals in the deposition mode pre-vents the saturation ratio to reach the saturation point withrespect to liquid water as often as in aerosol scenario B.The concentration of ice crystals remains smaller whencompared to aerosol scenario B as the contribution ofhomogeneous freezing of water droplets is much smallerin this scenario.

Below 500 hPa, the JF mean air temperature remainsmostly above −40 °C over much of the Central Arctic.The homogeneous freezing of water droplets is notdominant at these temperatures. The Central Arctic ischaracterized by the presence of either anticyclonesor cold decaying low pressure systems in which veryweak ascents or subsidence of air prevail. In such anenvironment, deposition ice nucleation can be significantas the air mass slowly cools by infrared radiation and thuscan stay long periods of time oversaturated with respectto ice but subsaturated with respect to liquid water. Inaerosol scenario A, the nucleation rate of ice crystals bywater vapour deposition is larger than in aerosol scenarioB. Ice crystals are therefore smaller compared to aerosolscenario B as more ice crystals can absorb the availablewater vapour.

In aerosol scenario B, the heterogeneous ice nucleationrate is much smaller when the atmosphere is subsaturated

with respect to liquid water. This results in a smallerconcentration of larger ice crystals as shown in Figure 5.The reduced concentration of ice crystals in this scenariohas also a consequence on the mean relative humiditywith respect to ice (RHi). The total flux of water vapouronto the existing ice crystals is smaller, thus allowingthe relative humidity to increase. Figure 6 shows the JFmean relative humidity with respect to ice (RHi) anomalyat 850 hPa and the spatially and temporally averagedRHi vertical profile within the sea ice boundary mask.The RHi increase in aerosol scenario B is the largestover the Arctic, Siberia and Northern Canada wherethe temperatures are the coldest. The JF RHi anomalyat 850 hPa reaches values up to 14% in the Canadianand Central Arctic. It is noteworthy to mention that thepositive RHi anomaly covers more or less the Arctic airmass, characterized by the persistence of cold decayinglows and anticyclones. This suggests that weak coolingrate associated to either weak air ascent and/or infraredradiative cooling is a necessary condition for acid coatingon IN to have an effect on cloud microstructure throughRHi increase. At lower latitudes, synoptic systems aremore active and air ascent velocity is larger. In theseconditions, the air cooling rate is much larger whencompared to the Arctic air mass. Consequently, icenucleation in subsaturated air with respect to liquid wateris not as important since the ice crystal concentrations aretypically not large enough to deplete the available watervapour. Therefore, in both aerosol scenarios, large RHivalues are reached and the cloud microstructure is similar.This explains why the RHi anomalies are close to 0 southof the Arctic front.

Figure 7 shows the JF mean frequency of mixed-phaseclouds for both aerosol scenarios at 850 hPa and thedifference between scenarios B and A (the anomaly).In both aerosol scenarios, the frequency of mixed-phase clouds is very large over Northern Europe andNortheastern Asia, where the Icelandic and Aleutian lowsrespectively are predominant. These baroclinic zonesare characterized by the development of low-pressuresystems with strong vertical ascents. Therefore, the effectof deposition ice nucleation is negligible. This is why


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(a) (b)

Figure 6. (a) JF mean relative humidity with respect to ice (%) at 850 hPa anomaly. Shadowed areas indicate that anomalies are statisticallysignificant with a confidence level of 95%. (b) Vertical profiles of the mean relative humidity with respect to ice (%) for aerosol scenarios A

and B averaged over time and spatially averaged over a mask delimited by sea ice boundaries.

(a) (b)

(c)

Figure 7. JF mean mixed-phase cloud frequency at 850 hPa in aerosol scenarios (a) A, (b) B and (c) the anomaly. In (c), shadowed areas indicatethat anomalies are statistically significant with a confidence level of 95%.



Figure 8. JF mean precipitation rate relative anomaly (%). Shadowed areas indicate that anomalies are statistically significant with a confidencelevel of 95%.

the differences between both aerosol scenarios over thesetwo areas are relatively small. However, differences aremuch larger in the Arctic air mass, which correspondsto the region where the RHi increase in aerosol scenarioB is the largest. In aerosol scenario A, the frequency ofmixed-phase clouds in the Arctic air mass varies between0 and 20% compared to a frequency ranging between30 and 50% for aerosol scenario B. The mixed-phasecloud frequency anomaly is positive at all levels between600 hPa and the surface (not shown).

Figure 8 shows the January–February mean precip-itation rate relative anomaly. The relative anomaly isdefined as the ratio of the difference of precipitationin aerosol scenarios B and A on the precipitation rateof aerosol scenario A. Precipitation is increased overmuch of the Central Arctic in aerosol scenario B withrelative anomalies ranging between 20 and 60%. Neg-ative relative anomalies within the Arctic air mass aremostly not statistically significant. Bigger ice crystalshave larger terminal velocity and precipitate more effi-ciently. The increase of precipitation can be explained bythe increased frequency of mixed-phase clouds in aerosolscenario B. Relatively few ice crystals are nucleated inthe thin liquid layer and rapidly grow by the Wegener-Bergeron-Findeisen effect. Precipitating ice crystals alsocome from upper layers. In aerosol scenario B, largerice crystals precipitate more efficiently and can seed themixed-phase clouds near the surface, thus favouring pre-cipitation to the surface.

4.2. Radiation and temperature

The energy budget at the surface and at the top ofthe atmosphere (TOA) is mostly driven by terrestrial

radiation owing to the quasi-absence of solar radiationduring January and February. The net infrared radiationat the surface or at the TOA strongly depends on thepresence of clouds and their microphysical properties.According to Shupe and Intrieri (2004), the downwardlongwave radiation at the surface can increase by as muchas 40 W m−2 if liquid water is present in low-level arcticclouds compared to an ice cloud. Modelling studies havealso shown a large sensitivity of infrared radiation fluxesto cloud microstructure (Du et al., 2011, Simjanovskiet al., 2011). It is therefore expected that changes inducedby acid coating on the cloud microphysical properties inaerosol scenario B will have an impact on the energybudget both at the surface and at the TOA.

To estimate the effect of clouds on the radiationbudget, CRF was proposed by Ramanathan et al. (1989)to characterize the cloud effect on the net radiation eitherat the surface or at the TOA. CRF is defined as thedifference between the net radiative flux in the presenceof clouds and the net radiative flux without the presenceof clouds. It can also be separated into its longwaveand shortwave components. Figure 9 shows the JF meanCRF anomaly at the TOA. In our analysis, only thelongwave component of the CRF is discussed since theshortwave radiation is quasi-absent during winter. In bothaerosol scenarios, the CRF is positive with smaller valuesvarying between 0 and 20 W m−2 over the Arctic andhigher values of up to 60 W m−2 south of the Arcticair mass (not shown). The CRF anomaly at the TOAis negative over most of the Arctic with values rangingbetween 0 and −6 W m−2. Relative to the CRF absolutevalues of about 20 W m−2, the CRF anomalies representa substantial reduction of the CRF at the TOA.


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Figure 9. JF mean cloud radiative forcing (W m−2) at the TOA anomaly. Shadowed areas indicate that anomalies are statistically significantwith a confidence level of 95%.

Mixed-phase clouds are mostly located in the lowertroposphere. Their temperatures are warmer than thesurface skin temperature. Therefore, the presence of theseclouds leads to an increase of upward longwave radiationemission when compared to the surface. Assuming thesame amount of mid and upper level ice clouds, theincrease of low-level liquid (or mixed) phase clouds willenhance the CRF at the TOA. Indeed, increasing themixed-phase cloud emissivity changes the thickness overwhich the mid and upper ice clouds absorb this energy.Therefore, the amount of longwave radiation going outto space increases because of the fact that the upperice cloud layer acting as blackbody thickens and thus iswarmer. Figure 10(a) shows the variation of the JF meanCRF at the TOA and the cloud liquid water path foraerosol scenarios A and B over a sub-domain delimitedby sea ice. As expected, the CRF at the TOA increases asthe cloud liquid water path increases in aerosol scenarioA. In aerosol scenario B, this is not the case sinceadding more cloud liquid water does not affect muchthe low-level mixed-phase cloud emissivity. Clouds withliquid water path values above 10 to 15 g m−2 emit asa blackbody (Shupe and Intrieri, 2004; Du et al., 2011).Increasing the cloud liquid water path above these valuesdoes not increase significantly cloud emissivity. As aresult, the CRF at the TOA does not depend anymoreof the cloud liquid water path. This can be seen inFigure 10(a) for aerosol scenario B in which cloud liquidpath values are above 12 g m−2.

Figure 10(b) shows that the CRF at the TOA increasesas the cloud ice water path increases in both aerosolscenarios. This result was expected since as the opticalthickness of mid and upper ice clouds increases, less

infrared radiation can escape to space. Indeed, increasingcloud ice water path reduces the thickness of blackbodylayers and therefore decreases the amount of energy thatescapes out to space.

In aerosol scenario B, the increased frequency andoptical thickness of mixed-phase clouds combined to thereduced cloud optical thickness in the mid and high tro-posphere contribute to increase the upward infrared radi-ation fluxes at the TOA. Figure 10(c) shows that the CRFanomaly at the TOA decreases when the cloud ice waterpath anomaly decreases. Such a relationship between theCRF at the TOA anomaly and the cloud liquid waterpath anomaly is not seen because the saturation point inmixed-phase cloud emissivity is reached in aerosol sce-nario B (Figure 10(d)). Therefore, at these cloud liquidwater path values, whatever the increase of the low-levelcloud liquid water path, the CRF at the TOA will essen-tially depends on the mid and upper cloud ice water path.

Figure 11 shows the JF mean air temperature anomalyat 1000, 850 and 500 hPa. The Central Arctic is colderin aerosol scenario B with a decrease of the temperatureby up to −3 K near the surface. This cooling, whichspreads over much of the Arctic air mass, is also obtainedhigher in the troposphere with values ranging from −2 to−4 K at 850 and 500 hPa. Enhanced atmospheric coolingin aerosol scenario B also leads to a decrease of thevertically integrated water vapour (not shown) owing tothe decrease of the saturated water vapour pressure withtemperature. This also contributes to the cooling of thesurface by a decrease of the water vapour greenhouseeffect.

Figure 12 shows the downward longwave radiationanomaly at the surface. Not surprisingly, the anomaly



(a) (b)

(c) (d)

Figure 10. Variation of the JF mean cloud radiative forcing at the TOA (W m−2) with (a) the JF mean cloud liquid water path (kg m−2) and (b)the JF mean cloud ice water path (kg m−2). Variation of the JF mean cloud radiative forcing at the TOA anomaly (W m−2) with (c) the cloud

ice water path anomaly (kg m−2) and (d) the JF mean cloud liquid water path anomaly (kg m−2).

is negative over much of the cooling area in aerosol sce-nario B with values ranging between 0 and −4 W m−2.However, it is important to note that the anomaly of thedownwelling infrared radiation at the surface is nega-tive despite the increase of mixed-phase cloud frequency,which produces a positive anomaly of the CRF at thesurface of up to 10 W m−2 (not shown). Therefore, thecooling near the surface is not directly related to changesin cloud microstructure in aerosol scenario B but ratherto the atmospheric dehydration and cooling of the midand upper atmosphere, which leads to a decrease of thewater vapour greenhouse effect.

4.3. Atmospheric circulation

Figure 13 shows the mean-sea level pressure (MSLP)anomaly. The Icelandic low is deeper in aerosol scenarioB and the Siberian anticyclone is strengthened. Thepressure gradient is thus significantly increased by 4 to5 hPa over the Northern part of Eurasia and Canada. Thestrengthening of the atmospheric circulation is due tothe enhanced meridional temperature gradient in aerosolscenario B. This favours the formation of more intense

synoptic storms, which is reflected in the JF MSLPanomaly. The strengthening of the pressure gradient overEurasia has also important consequences for aerosoltransport from the mid-latitudes to the Arctic. Indeed,Eurasia is the main pathway by which aerosols aretransported to the Arctic. In our simulations, aerosolsare not explicitly represented. Therefore, the increase inthe aerosol concentration over the Arctic and possiblefeedbacks with clouds cannot be taken into account.

4.4. Sensitivity of the results to the concentration ofdust particles

Dust concentrations are highly variable and depend on thelarge-scale atmospheric circulation. In this section, resultsobtained with a prescribed dust concentration reduced bya factor 2 are presented (hereafter aerosol scenarios A2and B2).

Table I shows a comparison between the reference andreduced dust concentration for the JF averaged absolutevalues (aerosol scenarios B and B2) and anomalies oftemperature and relative humidity with respect to iceat 1000, 850 and 500 hPa, liquid and ice water path,


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(a) (b)

(c)

Figure 11. JF mean temperature anomaly (K) at (a) 500 hPa, (b) 850 hPa and (c) 1000 hPa. Shadowed areas indicate that anomalies arestatistically significant with a confidence level of 95%.

and CRF. These variables are spatially averaged over amask delimited by sea ice. In general, temperatures, rel-ative humidity with respect to ice, CRF and cloud waterpath are very similar in both acid-coated aerosol sce-narios (B and B2). Reducing the dust concentration hasimplications on the available IN concentration at a giventemperature and ice supersaturation. Aerosol scenarios Aand A2 (the reference scenarios from which anomaliesare calculated) give different results (not shown) sinceaccording to Equation (1), the nucleation rate of ice crys-tals is proportional to the dust concentration. At the sametime, the acid coating effect on ice crystal nucleation rateis also proportional to the dust concentration. From Equa-tion (1), we have:

�NIC = �κNdust (2)

where � refers to the difference between the coatedaerosol scenario and uncoated aerosol scenario, NIC isthe number of ice crystals nucleated per unit time, Ndust

is the total aerosol concentration and κ is the fractionof dust particles nucleating ice crystals per unit time.The absolute values of the examined variables in aerosol

scenarios B and B2 do not differ much as κ is relativelysmall in both aerosol scenarios. In aerosol scenarios Aand A2, κ is larger and therefore, differences in theexamined variables are much larger between these aerosolscenarios. Consequently, the anomalies of these variablesare different for both aerosol scenarios, aerosol scenarioB having the largest anomalies. The magnitude of cloudand radiative changes caused by acid coating on dustparticles largely depends on the dust concentration asshown in Table I. The radiative effects of acid coatingson dust increases as the dust concentration increases.

One can quantify the CRF at the TOA as a functionof the dust vertical path using an acid forcing factor (Af)

normalized by the dust vertical path as follows:

F = AfD+b

where F is the cloud forcing at the TOA and D

the dust vertical path. Af and b depend on the dustvertical path. With the dust vertical path used in oursimulations (1.75 µg m−2 in aerosol scenarios A2 andB2 and 3.50 µg m−2 in aerosol scenarios A and B), it is



Figure 12. JF mean downward longwave radiation at the surface anomaly (W m−2). Shadowed areas indicate that anomalies are statisticallysignificant with a confidence level of 95%.

Figure 13. JF mean MSLP anomaly (hPa). Shadowed areas indicate that anomalies are statistically significant with a confidence level of 95%.


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Table I. Spatial (sea ice mask) and temporal averages of some variables and the respective anomalies in aerosol scenarios B andB2.

Scenario B2 Scenario B Anomalies Scenario B2 Anomalies Scenario B

Liquid water path (kg m−2) 0.0189 0.0189 0.0155 0.0153Ice water path (kg m−2) 0.0459 0.0466 −0.0048 −0.0126Relative humidity w.r.t. ice at 1000 hPa (%) 99.74 100.63 5.37 7.97Relative humidity w.r.t. ice at 850 hPa (%) 71.88 74.05 4.44 10.10Relative humidity w.r.t. ice at 500 hPa (%) 71.71 72.99 2.43 4.51Cloud radiative forcing at TOA (W m−2) 8.98 9.35 −0.41 −2.37Temperature at 1000 hPa (K) 251.50 251.13 −1.25 −2.10Temperature at 850 hPa (K) 254.45 253.95 −1.08 −2.13Temperature at 500 hPa (K) 233.81 233.26 0.18 −0.80

then possible to derive its variation with D assuming alinear relationship as follows:

Af = −1.12 and b = −1.55 for 1.75 µg m−2 <

D ≤ 3.50 µg m−2

Af = −0.234 and b = 0 for D ≤ 1.75 µg m−2

By using these values, one can get an approximatedvalue of F for any dust vertical path between 0 and3.50 µg m−2.

5. Summary and discussion

In this study, the effects of acid-coated IN on the Arcticcloud microstructure and radiation is investigated forJanuary and February 2007. A new parameterizationfor heterogeneous ice nucleation is implemented intothe two-moment microphysics scheme of the CanadianGlobal Environmental Multiscale (GEM) model. Themain objective of this study is to assess the impact ofthe de-activation effect of IN on wintertime Arctic cloudsand energy budget.

Results show that acid coatings on IN have an impor-tant effect on ice nucleation with the coatings modifyingboth ice and mixed-phase cloud microstructures. The pri-mary effect of acid coating on IN is to significantlydecrease the nucleation rate at a given ice supersatura-tion. The consequences of this change are a function oftemperature as illustrated in Figure 14.

In the upper part of the troposphere the temperature isoften below −40 °C and homogeneous nucleation is thedominant freezing mechanism. In this case, the inhibitioneffect of acid coatings on deposition ice nucleationleads to an increased concentration of water droplets,which freeze homogenously. In the uncoated aerosolscenario, larger heterogeneous ice crystal nucleation ratesin subsaturated air with respect to liquid water preventsliquid water saturation more often than in the coatedaerosol scenario. As a result, the ice crystal concentrationis lower and their size is larger when compared tothe coated aerosol scenario. Since a very high icesupersaturation is needed to nucleate ice crystals in theacid-coated aerosol scenario, the JF mean ice water

Acid coatingon IN

T > -30 T < -50

Mixed-phasecloudoccurrence

+

UpwellingatmosphericLW radiation

+

Acid coatingon IN

Mid and upperice cloudoptical thickness

Cloud forcingat the top ofthe atmosphere

_

_

_

Atmosphericcooling

+

Figure 14. Flowchart showing the linkage between acid coating, cloudmicrostructure and radiation at the TOA.

content is lower than the uncoated aerosol scenario atthese levels.

In the mid and lower part of the atmosphere, het-erogeneous ice nucleation dominates over homogeneousfreezing. In the uncoated aerosol scenario, larger icecrystal nucleation rates by deposition nucleation moreoften prevents the atmosphere from becoming saturatedwith respect to liquid water compared to the coatedaerosol scenario. This leads to an increased frequency ofmixed-phase clouds in the coated-aerosol scenario withan increase of the liquid water content and a decreaseof the ice water content when compared to the uncoatedaerosol scenario. This effect, associated with warmer tem-peratures, peaks at 850 hPa.

These two different effects have a common impact onthe infrared radiative budget at the top of the atmosphere.Optically thinner mid to upper ice clouds in the coatedaerosol scenario increase the atmospheric transmissivityof terrestrial radiation. At the same time, the upwardinfrared radiation flux is increased in the coated aerosolscenario due to optically thicker and more frequentmixed-phase clouds when compared to the uncoatedaerosol scenario. The end result is a decrease of the



CRF at the top of the atmosphere ranging between 0 and−6 W m−2. This leads to an atmospheric cooling thatvaries between 0 and −4 K. The atmospheric coolingfurther promotes the formation of clouds in the coatedaerosol scenario leading to a decrease in the watervapour greenhouse effect and precipitation for Januaryand February. Results show that this Arctic cooling islarge enough to strengthen the large-scale troposphericcirculation associated with the polar front through theintensification of the baroclinic zone.

The results obtained in this study show that icenucleation plays an important role for both mid andupper ice clouds and low-level mixed-phase clouds inthe Arctic, which in turn has an effect on radiation andatmospheric circulation. Quite interestingly, acid coatingshave little effect on cloud and radiation south of the arcticair mass. This suggests that deposition ice nucleation isimportant mainly in stable air masses that cool slowly,thus preventing the relative humidity from reaching liquidwater saturation rapidly.

Some assumptions made in this study could eitheramplify or reduce the tropospheric cooling resultingfrom acid coatings on IN. Firstly, the dust concentrationis assumed to be constant both in time and space.Observations show that the aerosol concentration ishighly variable in the Arctic during winter (Shaw, 1995).Results are sensitive to the dust concentration. Therefore,this assumption is likely to give an upper-limit value ofthe tropospheric cooling for a given dust concentration.It has also been assumed that the de-activation effectapplies to all atmospheric IN. Although many other dustparticles are equally affected by acidic coating, otherIN of biological origin are not affected (Chernoff andBertram, 2010). This assumption may lead to a slightoverestimation of the de-activation effect. Finally, icenucleation in the contact mode is assumed to be un-altered by acid coating. This assumption could contributeto underestimate the cooling obtained in this studyto some extent. Hoose et al. (2008) and Storelvmoet al. (2008) in their modeling studies have assumedthat acid coating on dust particles inhibits contact icenucleation. Such an alteration of contact nucleation byacid coating can only enhance the cloud microstructurechanges obtained in our study, that is an increaseof the cloud liquid water and a decrease of the icecrystal number concentration. The extent to which analteration of contact ice nucleation could contribute tocloud microstructure changes depends on its relativecontribution to the total ice nucleation rate. This aspectremains to be investigated.

Using a refined treatment of the IN de-activationeffect based on laboratory experiment, this research hasconfirmed the results obtained in previous modelinginvestigations (Girard et al., 2005; Girard and Stefanof,2007) on the effect of acid coating on the Arctic cloudsand radiative budget during winter. This indirect effectof acid aerosols on arctic clouds and the resultingatmospheric cooling could explain, at least in part,the unexpected observed cooling trend of surface air

temperature over the Arctic Ocean during winter in theperiod 1979–1999 as observed by boys (Rigor et al.,2000) and by satellite remote sensing (Wang and Key,2003).

Acknowledgements

The authors would like to thank the Canadian Foundationfor Climate and Atmospheric Sciences (CFCAS), theNatural Sciences and Engineering Research Councilof Canada (NSERC) and the Fonds Quebecois de laRecherche sur la Nature et la Technologie (FQRNT) forsupport funding.

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Assessment of the effects of acid-coated ice nuclei on the Arctic cloud microstructure, atmospheric dehydration, radiation and temperature during winter