Explicit Simulation of Aerosol Physics in a Cloud-Resolving Model: Aerosol Transport and Processing in the Free Troposphere

Explicit Simulation of Aerosol Physics in a Cloud-Resolving Model: Aerosol Transportand Processing in the Free Troposphere

ANNICA M. L. EKMAN

Department of Meteorology, Stockholm University, Stockholm, Sweden

CHIEN WANG

Massachusetts Institute of Technology, Cambridge, Massachusetts

JOHAN STRÖM

Institute of Applied Environmental Research, Stockholm University, Stockholm, Sweden

RADOVAN KREJCI

Department of Meteorology, Stockholm University, Stockholm, Sweden

(Manuscript received 11 February 2004, in final form 12 July 2005)

ABSTRACT

Large concentrations of small aerosols have been previously observed in the vicinity of anvils of con-vective clouds. A 3D cloud-resolving model (CRM) including an explicit size-resolving aerosol module hasbeen used to examine the origin of these aerosols. Five different types of aerosols are considered: nucleationmode sulfate aerosols (here defined by 0 � d �5.84 nm), Aitken mode sulfate aerosols (here defined by 5.84nm � d � 31.0 nm), accumulation mode sulfate aerosols (here defined by d � 31.0 nm), mixed aerosols, andblack carbon aerosols.

The model results suggest that approximately 10% of the initial boundary layer number concentration ofAitken mode aerosols and black carbon aerosols are present at the top of the convective cloud as the cloudreaches its decaying state. The simulated average number concentration of Aitken mode aerosols in thecloud anvil (�1.6 � 104 cm�3) is in the same order of magnitude as observations. Thus, the model resultsstrongly suggest that vertical convective transport, particularly during the active period of the convection,is responsible for a major part of the appearance of high concentrations of small aerosols (corresponding tothe Aitken mode in the model) observed in the vicinity of cloud anvils.

There is some formation of new aerosols within the cloud, but the formation is small. Nucleation modeaerosols are also efficiently scavenged through impaction scavenging by precipitation. Accumulation modeand mixed mode aerosols are efficiently scavenged through nucleation scavenging and their concentrationsin the cloud anvil are either very low (mixed mode) or practically zero (accumulation mode).

In addition to the 3D CRM, a box model, including important features of the aerosol module of the 3Dmodel, has been used to study the formation of new aerosols after the cloud has evaporated. The possibilityof these aerosols to grow to suitable cloud condensation or ice nuclei size is also examined. Concentrationsof nucleation mode aerosols up to 3 � 104 cm�3 are obtained. The box model simulations thus suggest thatnew particle formation is a substantial source of small aerosols in the upper troposphere during and afterthe dissipation of the convective cloud. Nucleation mode and Aitken mode aerosols grow due to coagulationand condensation of H2SO4 on the aerosols, but the growth rate is low. Provided that there is enough OHavailable to oxidize SO2, parts of the aerosol population (�400 cm�3) can reach the accumulation mode sizebin of the box model after 46 h of simulation.

Corresponding author address: Annica Ekman, Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: [email protected]

682 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 63

© 2006 American Meteorological Society

JAS3645

1. Introduction

Anthropogenic aerosols are one of the major con-tributors to human-induced climate change (e.g.,Twomey 1974; Charlson et al. 1992). A substantial ef-fort has been made to quantify the magnitude of theaerosol effect both through direct scattering and ab-sorption of shortwave radiation as well as indirectly oncloud formation, cloud characteristics, and precipita-tion. Clouds themselves also play an important role intransporting, scavenging, and processing aerosols. Con-vective clouds have been recognized as an importantmechanism for transferring chemical compounds fromthe surface to the free troposphere (e.g., Ridley et al.2004; Lawrence et al. 2003; Pickering et al. 2001; Wangand Prinn 2000). Several observations (e.g., Clarke1992, 1993; Nyeki et al. 1999; Ström et al. 1999; Clarkeand Kapustin 2002; Petzold et al. 2002; Schröder et al.2002; Twohy et al. 2002; Hermann et al. 2003; Minikinet al. 2003; Lee et al. 2004) have indicated high numberconcentrations of small aerosols in the vicinity of theanvils of convective clouds. One theory has been thatthe environment in this area is favorable for the nucle-ation of particles as both relative humidity and concen-tration of aerosol precursors are relatively high. An-other explanation for the small aerosols found near thetop of convective clouds could be direct transport fromthe surface to the free troposphere by strong verticalupdrafts. However, a great number of aerosols are wa-ter soluble and/or effective as cloud condensation nu-clei (CCN) and are hence likely to be scavenged byheavy precipitation. In addition, aerosols in the freetroposphere can serve as ice nuclei and are thus likelyto be scavenged through the formation of ice clouds.An important fact is that the lifetime of aerosols ismuch longer in the free troposphere than in the plan-etary boundary layer. The free tropospheric aerosolsmay thereby have a major influence on the earth’s ra-diative budget.

A few adiabatic parcel model studies have been per-formed to examine the origin of free tropospheric aero-sols (deReus et al. 1998; Kulmala et al. 1998; Clement etal. 2002). Zhang et al. (1998) incorporated a two-moment aerosol model into a two-dimensional cloudand sulfate chemistry model to simulate the effects ofclouds on aerosol redistribution and production in cu-mulonimbus clouds. They found that the nucleationrate after cloud dissipation in the upper troposphereincreased by one order of magnitude compared to thenucleation rate before cloud formation. Using a 2Dcloud-resolving model including an explicit aerosolmodule, Ekman et al. (2004) found that the direct trans-port of small-/medium-sized particles (5.84 nm � d �

31.0 nm) within a convective cloud is substantial. Up to10% of the surface concentration may reach the freetroposphere.

In the present study, a 3D version of the aerosolcloud-resolving model (CRM) utilized by Ekman et al.(2004) is employed. We evaluate the performance ofthe aerosol CRM by examining if the model is able toreproduce important features of observed variableswithin a convective cloud. We also further examine themagnitude of direct transport of aerosols within a con-vective cloud and estimate the number of new aerosolsformed in the vicinity of the anvil. The model and thesimulated case are first described in section 2. Thereaf-ter we compare the 3D model with available observa-tions in section 3. An estimate of the aerosol transportwithin the convective cloud is presented in section 4. Insection 5 we present results of a box model calculationof new particle formation and aerosol growth per-formed at the top of the cloud and driven by the outputof the 3D model data fields. Summary and conclusionsare given in the last section.

2. Model and simulated case

a. Cloud-resolving model

The present version of the CRM is a 3D version ofthe model reported in Ekman et al. (2004). There aremany benefits of using a 3D model instead of a 2Dmodel. Using a 3D model, it is possible to representhorizontal variations in the initial tracer compounddata. These variations could be important for simulat-ing the correct average and maximum amount of tracercompounds that are transported from the surface to thecloud anvil. Deep convective clouds are also knownthemselves to have complicated spatial structures thatare impossible to be fully captured using a 2D model.Spatial variations in, for example, cloud updraft anddowndraft may be important when average statistics oftransported aerosol concentrations from the surface tothe cloud anvil are calculated.

The dynamics–physics module in the CRM consistsof the nonhydrostatic momentum equations, the conti-nuity equations for water vapor and airmass density,the thermodynamic equation, and the equation of state(Wang and Chang 1993a). Also included are prognosticequations for the mixing ratios as well as number con-centrations of cloud droplets, raindrops, ice crystals,and graupel particles. The microphysical transforma-tions are formulated based on a “two moment” schemeincorporating the size spectra of particles (Wang andChang 1993a; Wang et al. 1995). A �-four-stream radia-

FEBRUARY 2006 E K M A N E T A L . 683

tion module based on Fu and Liou (1993) is incorpo-rated in the model using predicted concentrations ofgases (including H2O and O3) and hydrometeors to cal-culate radiative fluxes and heating rates.

In the CRM, the number of cloud condensation nu-clei (CCN) and ice nuclei (IN) available for cloud drop-let nucleation is predicted using the aerosol module byincluding the transport, mixing, and various physicaland chemical conversions of aerosols in the model (cf.next section). The chemistry submodule predicts atmo-spheric concentrations of 25 gaseous and 16 aqueous (inboth cloud droplets and raindrops) chemical com-pounds including important aerosol precursors, such assulfate and nitrate, undergoing more than 100 reactionsas well as transport and microphysical conversions. Amodule of heterogeneous chemistry on ice particles hasbeen developed and is included in the present versionof the model (Wang 2005). This module calculates sur-face uptake of several key chemical species includingHNO3, SO2, H2O2, and CH3OOH by ice particles.

The CRM has been used in studies of dynamics, mi-crophysics, chemistry, and aerosol transport in conti-nental deep convection (e.g., Wang and Chang 1993a,b;Wang and Crutzen 1995; Ekman et al. 2004) and deepconvection over the Pacific (Wang et al. 1995; Wangand Prinn 1998, 2000). Results of the chemical and dy-

namical parts of the model have also been comparedwith available observations including aircraft, radar,and satellite data. The spatial resolution of the modelcan be flexibly set; a horizontal grid interval of 2 kmand a vertical grid interval of 400 m are used in thisstudy.

b. Aerosol module

The evolution in time and space of aerosols consist-ing of sulfate, organic carbon, black carbon, and mix-tures thereof is described using a multimodal aerosolmodel originally developed by Wilson et al. (2001) andmodified by Ekman et al. (2004). A schematic pictureof the module is shown in Fig. 1. Five different modesare used to represent the aerosol population. These fivemodes are 1) nucleation mode sulfate aerosols (heredefined by 0 � d � 5.84 nm), 2) Aitken mode sulfateaerosols (here defined by 5.84 nm � d � 31.0 nm), 3)accumulation mode sulfate aerosols (here defined byd � 31.0 nm), 4) mixed aerosols, 5) and black carbon(BC) aerosols. In the present version of the model,mixed mode aerosols are assumed to have basically thesame properties as sulfate aerosols; that is, they havethe same density as sulfate aerosols, are water soluble,and may serve as both CCN and IN. One difference

FIG. 1. Schematic picture of processes included in the aerosol model (following work byWilson et al. 2001).


Fig 1 live 4/C

though is that the mixed mode aerosols have a sourcefrom the BC mode, as BC aerosols “age” when H2SO4

condense on the aerosols.The size distribution within each aerosol mode is as-

sumed to be lognormal and is described by three pa-rameters: number, mass, and standard deviation. To re-duce the computational burden, the standard devia-tions are prescribed (1.59 for all sulfate modes and 2.0for the mixed and BC mode). To close these aerosolspectra, both number concentrations and mass mixingratios of the five aerosol modes, that is, all together 10variables, are incorporated in the cloud-resolvingmodel as prognostic variables undergoing transport,mixing, dry deposition, and nucleation as well as impactscavenging besides aerosol microphysical processes(Ekman et al. 2004). The advection scheme used tocalculate the transport of these aerosol variables is thesame revised Bott scheme as developed in Wang andChang (1993a).

In the default version of the model, due to the shortintegration time, emissions of both carbonaceous andsulfuric aerosols are set to zero. Therefore, carbon-aceous aerosols have no additional sources other thanthe given initial loadings during the model integration.A continuous source in addition to the given initialloading of the whole sulfate aerosol population (threemodes) is the nucleation of new aerosols from H2SO4

(supplied by SO2 oxidation calculated in the chemistrymodule of the model) and H2O (Vehkamäki et al.2002). The condensation coefficient as well as the intra-and intermodal coagulation coefficients for each aero-sol mode is determined from the theory of Fuchs(1964), using the geometric mean radius of each mode.

The activation of a drop at a certain supersaturationdepends on the composition of the solute. The numberof aerosols available to form cloud droplets is deter-mined by calculating the critical radius correspondingto the critical saturation ratio for drop activation usingthe Köhler equation (cf. Ekman et al. 2004). For anyaerosol size bin that has the critical radius R* within itsboundaries (Rmin � R* � Rmax), the bin is split so thatonly particles with radius larger than R* are activated.The total number of aerosols activated can be obtainedby integrating the distribution function from R* toRmax. Nucleation of ice crystals can occur both by ho-mogeneous-freezing nucleation of liquid particles andby heterogeneous nucleation caused by aerosol par-ticles, but it is unclear which process is dominating inthe atmosphere (e.g., Cziczo et al. 2004; Haag andKärcher 2004; DeMott et al. 2003; Haag et al. 2003).Both of the above nucleation processes are included inthe CRM. Only pure sulfate aerosols and mixed aero-

sols are considered to constitute CCN or IN. An addi-tional path for nucleation of ice crystals, that is, directfreezing of aerosols to form ice crystals, has been pro-posed recently (e.g., Kärcher and Lohmann 2003). Thisprocess is not included in our model.

Another path for scavenging of aerosols is throughcollision with falling raindrops, graupel, or ice crystals,that is, the precipitation (impaction) scavenging. In themodel, the collision efficiency E varies with size and isprescribed for the different aerosol bins (as in Ekmanet al. 2004). The removal is efficient for small and largeparticles whereas the collision efficiency for particles inthe size range from 0.1 to 1.0 �m is relatively small.Resuspension is not treated in the CRM; that is, theaerosols are assumed to be scavenged when they are indroplets or ice crystals.

c. Simulated case

The selected case to simulate is a cumulonimbuscloud with an extended anvil over northern Germany,observed during the Stratosphere–Troposphere Experi-ment by Aircraft Measurements (STREAM) on 29 July1994 (Ström et al. 1999, hereafter S99). This case is thesame as simulated in Ekman et al. (2004), but in thepresent study the 3D version of the model is utilizedinstead of the 2D version. At the observed location,several smaller groups of thunderstorms were formedalong a cold front, and aircraft measurements of aero-sols, cloud water, relative humidity, carbon monoxide,and ozone were conducted along a cross sectionthrough the center of the anvil of one of these stormclouds. The research aircraft entered the cumulonim-bus cloud at approximately 1436 UTC at an altitude of�10 400 m. The aircraft traveled a distance of �260 kmacross a frontal zone before leaving the cloudy air atabout 1503 UTC. In situ data from this level are pre-sented in Fig. 5 in S99.

The meteorological part of the CRM simulation isinitialized using analyzed 3D initial data fields of pres-sure, temperature, winds, and specific humidity ob-tained from the National Centers for EnvironmentalPrediction (NCEP). Horizontally interpolated fields ofNO2, O3, and SO2 obtained from surface observationsconducted by the Cooperative Program for Monitoringand Evaluation of the Long-Range Transmission of AirPollutants in Europe (EMEP) (Hjellbrekke and Hans-sen 1998) are used to initialize the chemistry module.There are no observations of gaseous HNO3 or H2SO4

available from EMEP. For HNO3, the initial concen-tration is instead obtained by combining results fromprevious simulations using the CRM (Wang and Chang1993a; Wang and Crutzen 1995) and measured particu-


late concentrations of NO�3 from EMEP. For H2SO4,

the initial concentration is assumed to be equal to zero.Vertical profiles of all chemical compounds are pre-scribed as to decrease with height except for O3, whichis based on previous work (for initial profiles of SO2,CO, and O3, cf. Ekman et al. 2004).

For black carbon and mixed mode aerosols, a hori-zontally constant surface concentration of 500 cm�3 isassumed initially, corresponding to a mass concentra-tion of 100 ng m�3. The surface concentration of Aitkenmode and accumulation mode aerosols is set to be50 000 and 3000 cm�3, respectively. These aerosol con-centrations are representative for what may be ob-served in urban continental air (R. Krejci 2004, per-sonal communication; Seinfeld and Pandis 1998). Allaerosol concentrations are initially prescribed to de-crease with height as a function of air density (Fig. 2).This type of vertical dependence is in fairly good agree-ment with the observations by Schröder et al. (2002)and Petzold et al. (2002). The initial mass concentrationfor each mode is calculated by assuming spherical par-ticles with a density of 1.7 g cm�3 and a radius of 6.29nm for the Aitken mode and 48.5 nm for the accumu-lation, mixed, and black carbon modes. The nucleationmode aerosol concentration is assumed to be zero atthe beginning of the simulation.

3. Comparison with observations

Several complications arise when comparing thesimulated and observed properties of aerosols andgases within the convective cloud. The observed andmodeled clouds are not identical. They can only beconsidered to represent different “samples” along the

line of convection. Hence, the time scale of the clouddevelopment and the location of the anvil may slightlydiffer. In addition, the measurements are conductedalong one cross section within the cloud, whereas themodel output may be collected in all cloudy grid pointsat a certain level. Nevertheless, the general character-istics of the modeled and observed cloud should besimilar. A comparison should give us an indication ofpotential shortcomings of the chemistry and aerosolmodules.

After 3 hours of simulation, the cloud has reached itsdecaying state, which is also the time reported for theobservations. General features of the cloud during thesimulation are summarized in Table 1. The modeledcloud anvil covers an area of approximately 100 � 100km2 (Fig. 3), which is somewhat smaller than what wasobserved by the aircraft measurements (the aircraft wasestimated to fly 150 km through cloudy air). In addi-tion, the observed cloud appeared to have two majorbodies, whereas the modeled cloud only consists of onebody (cf. Fig. 4 in the present study and Fig. 5 in S99).

FIG. 2. Vertical profiles of initial aerosol concentrations.

TABLE 1. General features of the modeled convective cloud.

Maximum vertical velocity (m s�1) 30.22 (at t � 3 h 30 min)Maximum cloud-top height (km) 15.6 (at t � 2 h 20 min)Maximum cloud water content

(g m�3)3.1 (at t � 0 h 40 min)

Maximum precipitation (kg m�2 s�1) 0.05 (at t � 2 h 30 min)

FIG. 3. Isosurface for the mass mixing ratio of total condensedwater � 0.01 g kg�1 after a 3-h simulation. The full line, alignedwith the direction of the spreading of the anvil and close to thecenter of the convective core, indicates the location of the crosssections displayed in Figs. 4 and 9.


a. Chemical compounds

Carbon monoxide is in general a good indicator ofvertical transport of air from a polluted boundary layerto the free troposphere, as CO is emitted mainly fromcombustion processes and it is not affected by chemicalreactions on a time scale of a few hours. High concen-trations of O3 in the free troposphere may, on the otherhand, both be an indicator of vertical transport of pol-luted boundary layer air (photochemical smog) as wellas downward transport of ozone-rich stratospheric air.The modeled minimum, average, and maximum con-centrations of CO and O3 along the cross section at10.4-km altitude agree fairly well with the observed cor-responding values (approximately 20% difference, cf.Fig. 5). There is a tendency to underestimate the COconcentration and overestimate the O3 concentration,which could be a sign of too little transport of air fromthe boundary layer. Before the convective event on 29July, a buildup of high concentrations of O3, CO, SO2,and other pollutants in the boundary layer occurreddue to several weeks of clear weather and weak winds.According to S99, a strong correlation between O3 andCO was noted as high as 10.4 km within the cloud. Thecorrelation between CO and O3 within the modeledcloud is high, just as in S99 (0.65, cf. Fig. 4). Carbonmonoxide is also well correlated with SO2 and Aitken

FIG. 4. Simulated variables at t � 3 h along a cross section withinconvective cloud ( y � 120 km, z � 10.4 km) at 10.4-km altitude:(top) ice hydrometeor number (light gray line) and ice water con-tent (black line), (middle) carbon monoxide mixing ratio (lightgray line) and O3 mixing ratio (black line), and (bottom) Aitkenmode aerosol concentration (light gray line) and temperature(black line) at standard pressure and temperature (STP).

FIG. 5. Modeled and observed minimum, average, and maxi-mum (a) CO and (b) O3 concentrations along the cross section ofconvective clouds at 10.4-km altitude. Sample numbers are indi-cated within brackets.


mode aerosols, which indicates that all these variablesoriginate from the polluted boundary layer.

b. Meteorological variables

The modeled minimum, average, and maximum tem-perature are in good agreement with observations (lessthan 3% difference, cf. Fig. 6a). For the ice water con-tent and ice crystal number, the variation in their valuesoccurs over several orders of magnitude (both in modeland in observations); hence it is more appropriate tocompare median and percentile values instead of aver-age values (Figs. 6b,c). Note that for this comparison,all cloudy grid points at the 10.4-km level (instead of

only along the cross section) are compared with obser-vations in order to increase the number of samplepoints. The modeled ice water content and ice crystalnumber are, in general, in good agreement (within afactor of 2) with the observations.

c. Aerosols

A large number of small- (diameter �7 nm) and me-dium-sized (d � 18 nm) aerosols were observed at 10.4-km altitude by S99. The model predicts small- and me-dium-size aerosol concentrations up to 1.6 � 104 cm�3

and 8.3 � 103 cm�3, respectively at 10 km, which is 52%and 68% lower, respectively, than the observed valuesof 3.3 � 104 cm�3 and 2.6 � 104 cm�3 (Fig. 7). Themodel simulates the distribution of aerosol concentra-tions at 10.4 km fairly well (Fig. 7), but the variability islower. There are several possible explanations for thediscrepancy: the observations are performed along onlyone cross section whereas the model output is taken inall cloudy grid points; difference in location of the ob-served cross section and modeled surface within thecloud; difference in time point chosen for the compari-son; difference in how the selection of a cloudy gridpoint is made; and the assumption of a lognormal dis-tribution for the modeled aerosols.

FIG. 6. Modeled and observed (a) temperature along the crosssection of convective clouds, (b) ice water content in all cloudygrid points, and (c) ice hydrometeor concentration in all cloudygrid points at 10.4-km altitude. STP values shown are in (a) mini-mum, mean, and maximum; (b) median, 75% tile, 90% tile, andmaximum; (c) 10% tile, 25% tile, median, 75% tile, 90% tile, andmaximum. Sample numbers are indicated within brackets.

FIG. 7. Modeled and observed number of aerosols (a) (d � 7nm) and (b) (d � 18nm) at 10.4-km altitude in all cloudy gridpoints. STP values shown are minimum, 10% tile, 25% tile, me-dian, 75% tile, 90% tile, and maximum. Sample numbers are in-dicated within brackets.


S99 also performed measurements at the absolutetop of the cloud, at 12.2-km altitude. High concentra-tions of aerosols, up to 20 000 cm�3, could be observedat this level. S99 suggested that these particles had beentransported from the boundary layer up to the cloudanvil, but did not exclude the possibility that particleproduction might have occurred at the top of the cloudwhere evaporating crystals humidify the cold air. Themodel simulations also display high concentrations ofAitken mode aerosols at 12-km altitude (up to 14 � 103

cm�3, not shown). The Aitken mode aerosol concen-tration at this altitude is highly correlated with CO andSO2 concentrations (correlation coefficient equal to0.98 and 0.78, respectively), again indicating transportof aerosols from the polluted boundary layer to the topof the cloud.

4. Aerosol transport within the convective cloud

Figure 8 shows the distribution of nucleation, Aitken,and BC mode aerosols for isosurfaces equal to 0.1,6000, and 50 cm�3, respectively, after 3 hours of simu-lation. The vertical transport of nucleation, Aitken, andBC particles is also illustrated by the vertical cross sec-tions shown in Figs. 9a,b,d (the location in the y direc-tion of the cross section is the same as shown in Fig. 3,y � 120 km). Accumulation mode and mixed modeaerosols are efficient as CCN and are hence almostcompletely scavenged by precipitation, mainly throughnucleation scavenging (cf. Fig. 9c and Ekman et al.2004).

During the simulation, SO2 is oxidized to H2SO4 andhence a possibility for new particle formation is pro-vided. As displayed by Figs. 8a and 9a, a small amountof nucleation mode aerosols are actually formed withinthe cloud. This type of formation was not observed inthe 2D modeling study by Zhang et al. (1998). Nucle-ation mode aerosols are subjected to impaction scav-enging by precipitation and may also be coagulated toform larger aerosols. However, some of the particlesreach the top of the convective cloud. After 3 hours ofsimulation, the maximum concentration is 3.4 cm�3 inthe cloud anvil. The transport of aerosols can also bestudied by calculating the aerosol budget at differentlevels of the cloud as well as below the cloud. There isa small supply of nucleation mode aerosols both belowand within the cloud during the simulation (Fig. 10a).Not investigated in this study, due to a lack of inputdata, is a potential nucleation mechanism via organicvapors. This type of nucleation process could generatehydrophobic particles that would have a better chanceto survive the cloud processing.

Aitken mode aerosols are to some extent washed out

by precipitation, but a substantial amount of this type ofparticles is transported toward the top of the convectivecloud (Figs. 8b, 9b, and 10b). These aerosols are notlarge enough to be effective CCN and not small enoughto be efficiently scavenged through impaction scaveng-ing (cf. Ekman et al. 2004). As the formation of newparticles within the cloud is small, most Aitken modeparticles observed at the top of the cloud are likely tooriginate from transport. This theory is also supportedby Fig. 10b. After a 3-h simulation, the average con-centration within the cloud anvil is approximately 10%of the surface concentration. This corresponds to anincrease of the aerosol concentration in the upper partsof the cloud by 36% (from 2.4 � 103 to 3.3 � 103 cm�3).Zhang et al. (1998) noted a similar increase (�1000cm�3) of the small aerosol number concentration in

FIG. 8. (a) Modeled isosurfaces for nucleation mode aerosolconcentration equal to 0.1 cm�3 (dark gray) and Aitken modeaerosol concentration equal to 6000 cm�3 (light gray) after 3-hsimulation. (b) Modeled isosurface for a black carbon aerosolconcentration equal to 50 cm�3.


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FIG. 10. (a) Time evolution of average nucleation modeaerosol concentration at different levels below and withinthe convective cloud (full lines). Below the cloud, all gridpoints within the square (x, y) � (100–140 km, 100–140 km)are considered. Within the cloud, only grid points withCWC � 0.01 g kg�1 are considered. The initial averageconcentration for the grid box below the cloud is alsoshown for comparison (dashed lines). (b) As in (a) but forAitken mode aerosols. (c) As in (a) but for accumulationmode aerosols. (d) As in (a) but for BC mode aerosols. (e)As in (a) but for mixed mode aerosols.


their 2D simulations of a convective cloud. In theboundary layer, the average Aitken mode aerosol con-centration is equal to 36% of the initial surface concen-tration. The removal of Aitken mode aerosols belowthe cloud occurs both through impaction scavenging byprecipitating raindrops and through transfer to the ac-cumulation mode aerosol size bin.

Accumulation mode and mixed mode aerosols areefficiently transported from the boundary layer to thecloud base and subsequently nucleation scavengedwithin the cloud. The average concentration of accu-mulation mode aerosols in the boundary layer at theend of the simulation is approximately 30% of the ini-tial average surface concentration, and the averagemixed mode concentration is about 50% (Figs. 10c,d).Within the anvil level, the average concentration ofmixed mode aerosols is about 40% of the initial value inthe upper troposphere, whereas the accumulationmode aerosol concentration is close to zero.

As the cloud develops, BC aerosols in the boundarylayer are transported to the middle and high parts ofthe cloud by the convective updraft (Figs. 8b, 9d, and10e). Although BC aerosols are hydrophobic, they maystill be subjected to impact scavenging by falling pre-cipitation. At the end of the simulation, the average BCaerosol concentration in the boundary layer is 57% ofthe initial value. In the cloud anvil, the average concen-tration is 9% of the initial boundary layer concentra-tion, an increase by 28% (from 24 to 31 cm�3) com-pared to the initial anvil concentration. The maximumconcentration of BC aerosols at the cloud top is 105 cm�3,corresponding to a mass concentration of 20 ng m�3.

5. Aerosol processing in the free troposphere

After 3 hours of simulation using the CRM, the maxi-mum SO2 and H2SO4 concentration at 10.4 km is 62pptm (approximately 30% of the initial surface con-centration) and 0.02 pptm, respectively. The averageSO2 and H2SO4 concentrations within and below thecloud are displayed in Table 2. There is an overall in-crease of sulfur compounds in the free troposphere dueto the convective activity. According to Wang andPrinn (2000), there are three factors contributing to theincomplete dissolving of SO2 into cloud water and thus

the transport of SO2 to the cloud anvil: the much lowersolubility of SO2 relative to H2O2, the conversion ofwater to ice phase particles that terminates the aqueousreactions, and the relatively short lifetime and limitedcoverage of the liquid phase portions of the cloud.

The peak in the Aitken mode aerosol concentrationat the top of the convective cloud could be attributedboth to direct transport from the lower troposphere aswell as to newly formed aerosols in the upper tropo-sphere. The latter process, if proved to be an importantfactor, could enable enhanced new aerosol formation tolast longer than the convective core. Due to the limitedmodel domain the current 3D simulation cannot fullycover this time period. To study the importance of par-ticle nucleation in the free troposphere after the cloud’sdissipation, a box model has been designed. Using thebox model, we also examine the possibility of the newlyformed nucleation mode aerosols and transported Ait-ken mode aerosols to grow to suitable CCN or IN size(i.e., approximately the accumulation mode size bin ofthe model).

The box model includes aerosol microphysical andchemical processes such as H2SO4–H2O nucleation, co-agulation, condensation of H2SO4 and oxidation of SO2

by OH to form H2SO4. Sulfur dioxide is assumed to bedepleted as the oxidation to form H2SO4 occurs. Out-put from the CRM at t � 3 h and z � 10.4 km are usedfor initialization of the box model (Table 3; Fig. 11).Because the radiation model is not included in the boxmodel, we need to make some assumptions about theOH concentration. Two model simulations, one withOH being depleted as SO2 is oxidized and one withconstant OH concentration are thus introduced. Thesetwo simulations provide the lower and upper bound forSO2 oxidation. A box model simulation using a diur-nally varying OH concentration has also been con-

TABLE 2. Average initial (init) and after 4 hours of simulation (end) concentrations of trace gases below the cloud (0–3.2 km), andin low (3.2–5.2 km), middle (5.2–8 km), and high parts (8–14 km) of the convective cloud.

Below cloud (init/end) Low cloud (init/end) Middle cloud (init/end) High cloud (init/end)

SO2 (pptm) 179/68 19/24 0/11 0/5H2SO4 (pptm) 0/4.1 � 10�4 0/1.3 � 10�4 0/3.2 � 10�4 0/4.0 � 10�4

TABLE 3. Initial values applied for the box model simulations.

Initial value

Temperature (K) 225.2Pressure (hPa) 262.5Specific humidity (kg kg�1) 1.3 � 10�4

SO2 (pptm) 65OH (molecules cm�3) 3.6 � 106


ducted for comparison with the constant OH concen-tration simulation. This simulation yielded similar re-sults to the constant OH concentration simulation.

Figure 11a displays the results from the first run withOH being depleted. The 60-h time evolution of all sul-fate aerosol modes as well as the H2SO4 concentrationin the grid point of the box model with highest initialH2SO4 concentration is shown. Note that for a simula-tion time of this length the results should be interpretedwith some caution, as there is no meteorological orradiative processes included in the box model (e.g., dif-fusion, cloud formation, etc.). The formation of nucle-ation mode aerosols is substantial; up to 1.0 � 104 cm�3

new particles are formed. In contrast to the CRM simu-

lation, these aerosols cannot be impaction scavenged byprecipitation. The number aerosols simulated is inagreement both with the model study by Zhang et al.(1998) and by several observations (Lee et al. 2004;Twohy et al. 2002; Clarke et al. 1999), which indicatenew particle concentrations downwind of convectiveanvils in the range from 1.0 � 104 cm�3 to 5 � 104 cm�3.

Aitken mode aerosols do grow; mostly due to coagu-lation (as a result, the particle number concentration isreduced whereas the mass increases). However, in oursimulation none of these aerosols can reach the pre-scribed accumulation size limit of the model (31 nm).The wind speed at 10.4-km altitude is on average9 m s�1 in the CRM. This is in good agreement withradiosonde data reported from the same area (moreinformation available online at http://www.weather.uwyo.edu/upperair). A 60-h simulation would corre-spond to a transport distance of 2000 km assuming aconstant wind speed.

The main reason that the formation of new particlesin the first box model run cease to exist with time is thatthe OH concentration rapidly approaches zero. Hence,there is no more production of H2SO4. When a constantOH concentration (� 6.1 � 106 molecules cm�3) is as-sumed, as in the second box model simulation (Fig.11b), H2SO4 is continuously formed and there is thus acontinuous formation of nucleation mode aerosols. Asa result (and different from the box model simulationwith OH being depleted), not only the mass within theAitken mode increases, but also the number concentra-tion as aerosols from the nucleation mode size bin growand are transferred to the Aitken mode.

The maximum concentration of nucleation modeparticles is 2.7 � 104 cm�3. When the aerosols withinthe Aitken mode reach a critical size (at �0.5 h), thecondensation coefficient increases substantially. Thisincrease results in that the H2SO4 concentration dropsand that the formation of new particles becomes muchslower. Aerosols within both the nucleation and Aitkenmode grow through coagulation and condensation, butthe growth rate of the Aitken mode aerosols is low.After 24 hours of simulation, or an approximate trans-port distance of 800 km, no aerosols have reached theaccumulation mode size bin of the model. After 40hours of simulation the formation of new particles isapproximately zero. Aerosols within the nucleationmode are continuously transferred to the Aitken modeand after 46 hours of simulation (or 1500 km), there issome formation of accumulation mode aerosols. Noteagain, that this formation occurs assuming the condi-tions within the grid box remain the same during thewhole simulation period.

The amount of available H2SO4 for nucleation is not

FIG. 11. Time development of aerosol number concentrationand H2SO4 concentration at grid point x � 114 km, y � 180 km;10.4 km calculated using the box model (a) with and (b) withoutOH depletion. The discontinuities seen in the figures are a resultof the “remapping” procedure of aerosols between the differentsize bins.


only dependent on the oxidation rate of SO2, but alsoon the initial (and thus continuously simulated) Aitkenmode aerosol concentration. If the initial Aitken modenumber concentration is doubled, less H2SO4 is avail-able for nucleation, and the maximum concentration ofnucleation mode aerosols is reduced to 7.0 � 103 cm�3.At the same time, the growth of the Aitken mode aero-sol particles becomes faster and accumulation modeaerosols are formed already after 24 hours of simula-tion. In an equivalent manner, if the initial Aitkenmode aerosol concentration is halved, more nucleationmode aerosols are formed (maximum concentration�3.2 � 104 cm�3), the growth of the Aitken mode aero-sols is slower and accumulation mode aerosols areformed later during the simulation.

6. Summary and conclusions

Several observational studies have indicated highconcentrations of small aerosols in the vicinity of anvilsof convective clouds. In the present study, we examinethe possible origin of these aerosols using a 3D cloud-resolving model including an explicit aerosol module.We have first evaluated the performance of the modelby comparing the model results with aircraft observa-tions. Five different types of aerosols are considered:nucleation mode sulfate aerosols (here defined by 0 �

d � 5.84 nm), Aitken mode sulfate aerosols (here de-fined by 5.84 nm � d � 31.0 nm), accumulation modesulfate aerosols (here defined by d � 31.0 nm), mixedaerosols, and black carbon aerosols.

The simulated average values and variability of me-teorological and chemical variables are in general ingood agreement with measurements. Polluted air istransported from the boundary layer during the con-vective event, and relatively high concentrations ofchemical compounds such as CO and SO2 can be foundat the top of the cloud. A plume of O3-rich air is trans-ported from the surface to the cloud anvil, which is alsonoted in the observations.

Nucleation mode, accumulation mode, and mixedmode aerosols are efficiently scavenged by the heavyprecipitation within the convective cloud through im-paction scavenging (nucleation mode) and nucleationscavenging (accumulation and mixed mode) and onlysmall numbers of these types of aerosols can reach thecloud anvil. However, and in agreement with the resultsobtained by Ekman et al. (2004) and Zhang et al.(1998), a substantial part of the Aitken mode and blackcarbon aerosol populations are transported to the up-per troposphere. As the cloud reaches its decayingstate, �10% of the initial surface concentrations of Ait-ken mode and black carbon aerosols are present at the

top of the convective cloud. The average number ofsmall aerosols simulated at 10.4-km altitude is of thesame order of magnitude as in the observations. Theresults strongly suggest the critical role of vertical con-vective transport in the redistributions of sulfate aero-sols in Aitken size range and for noncoated black car-bon aerosols.

During the 3-h CRM simulation, new aerosols areformed within the convective cloud through binaryH2O–H2SO4 nucleation, but the formation is rathersmall. At the end of the simulation a maximum con-centration of approximately 3–4 cm�3 is noted in thecloud anvil. A different pathway to form more newparticles within the cloud could be via organic vapors.Due to a lack of input data, this type of nucleationprocess has not been considered in the present study.

Using a box model including all the major micro-physical and chemical processes of aerosols, additionalsimulations have been carried out to study the forma-tion of new aerosols at the top of the cloud after thecloud has evaporated and the possibility of these aero-sols to grow to suitable CCN or IN size. The concen-tration of SO2 in the anvil is relatively high and a sub-stantial amount of new particles are formed (up to1–3 � 104 cm�3). This number is in agreement bothwith observations downwind of convective clouds (Leeet al. 2004; Twohy et al. 2002; Clarke 1993) and withprevious 2D simulations by Zhang et al. (1998). Nucle-ation and Aitken mode aerosols grow through coagu-lation and condensation, but the growth rate is low.Provided that there is enough OH available to oxidizethe SO2, and that the ambient conditions within the boxmodel remain unchanged during the simulation, someof the aerosols (concentrations �400 cm�3) can reachthe accumulation mode size bin of the box model after46 hours of simulation.

Altogether, the CRM and box model simulationssuggest that the impact of convective transport on up-per-tropospheric aerosol concentrations can last severaldays and extend over areas as wide as 1000 km2. Duringthe active stage of convection direct transport throughconvective activity is responsible for injecting largenumber concentrations of small aerosols to the uppertroposphere. In addition, after the cloud has dissipated,increased aerosol precursors in the upper tropospherecan lead to formation of small aerosols in concentra-tions very close to that of the directly transported ones.

The fate of black carbon aerosols in the free tropo-sphere is still quite unclear and so is the climate impactof these aerosols. There is a possibility that these aero-sols could become “aged” with time and thus convertinto large hygroscopic aerosols, interesting from a liq-uid cloud formation point of view. In addition, aged


black carbon aerosols may be efficient as IN throughheterogeneous freezing below water saturation. Theseissues need to be further investigated and a parameter-ization of heterogeneous freezing of aerosols (e.g.,Khvorostyanov and Curry 2004) is planned to be in-cluded in the CRM.

Acknowledgments. The first author would like tothank the Knut and Alice Wallenberg foundation, Swe-den, postdoctoral fellowship program on sustainabilityand the environment for research funding. This workwas also partially supported by the NOAA Climate andGlobal Change Program Grant GC97-474, by NSFGrant ATM-0329759, by the Ford–MIT Alliance, andby the industrial consortium of the MIT Joint Programon the Science and Policy of Global Change.

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Explicit Simulation of Aerosol Physics in a Cloud-Resolving Model: Aerosol Transport and Processing in the Free Troposphere