Eric Girard

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper

Microphysical parameterization of Arctic diamond dust, ice fog, and thin stratus for climate models

Abstract A parameterization is described for low-level clouds that are characteristic of the Arctic during winter. This parameterization simulates the activation of aerosols, the aggregation/coalescence, and the gravitational deposition of ice crystals/water droplets and the deposition/condensation of water vapor onto ice crystals/water droplets. The microphysics scheme uses four prognostic variables to characterize clouds: ice water content, liquid water content, and the mean diameter for ice crystals and for water droplets, and includes prognostic supersaturation. The parameterization simulates stable clouds where turbulence and entrainment are weak, like ice fogs, thin stratus, and diamond dust. The parameterization is tested into the Local Climate Model (LCM), which is the single column version of the Northern Aerosol Regional Climate Model (NARCM). NARCM is a regional model with an explicit representation of the aerosol physics and with the physics package of the Canadian Climate Center General Circu...

Simulation of arctic low‐level clouds observed during the FIRE Arctic Clouds Experiment using a new bulk microphysics scheme

A new bulk cloud microphysics scheme that accounts for aerosol microphysical properties and size distribution is implemented into the single-column version of the ARCSyM. This scheme is distinguished from other bulk microphysics schemes by its prognostic determination of cloud particle number concentration and saturation ratio. The new scheme is compared to a simpler bulk microphysics scheme and observations taken during the FIRE Arctic Clouds Experiment in May 1998. Qualitatively, the two microphysics schemes are generally in agreement with the observed cloud formation and evolution. Comparison with aircraft measurements at 3 times shows that the new scheme better discriminates cloud phase and reproduces reasonably well the observed liquid and ice water content for two cases. The better performance of the new scheme is attributed to its more elaborated treatment of the freezing process which is made possible by the prognostic determination of cloud particle number concentration and the assumption of a bimodal lognormal cloud size distribution. Sensitivity studies are performed to assess four aerosol microphysical properties on the evolution of cloud microphysical processes. Results show that the IFN concentration, the aerosol number concentration, the slope of the aerosol size distribution, and the aerosol solubility may impact substantially on cloud phase and total water content. The liquid water path and ice water path can vary by as much as 100 g m−2 locally as a result of the variation of these parameters related to aerosols.

Characterization of Arctic ice cloud properties observed during ISDAC

Extensive measurements from ground-based sites and satellite remote sensing (CloudSat and CALIPSO) reveal the existence of two types of ice clouds (TICs) in the Arctic during the polar night and early spring. The first type (TIC-2A), being topped by a cover of nonprecipitating very small (radar unseen) ice crystals (TIC-1), is found more frequently in pristine environment, whereas the second type (TIC-2B), detected by both sensors, is associated preferentially with a high concentration of aerosols. To further investigate the microphysical properties of TIC-1/2A and TIC-2B, airborne in situ and satellite measurements of specific cases observed during Indirect and Semi-Direct Aerosol Campaign (ISDAC) have been analyzed. For the first time, Arctic TIC-1/2A and TIC-2B microstructures are compared using in situ cloud observations. Results show that the differences between them are confined in the upper part of the clouds where ice nucleation occurs. TIC-2B clouds are characterized by fewer (by more than 1 order of magnitude) and larger (by a factor of 2 to 3) ice crystals and a larger ice supersaturation (of 15-20%) compared to TIC-1/2A. Ice crystal growth in TIC-2B clouds seems explosive, whereas it seems more gradual in TIC-1/2A. It is hypothesized that these differences are linked to the number concentration and the chemical composition of aerosols. The ice crystal growth rate in very cold conditions impinges on the precipitation efficiency, dehydration and radiation balance. These results represent an essential and important first step to relate previous modeling, remote sensing and laboratory studies with TICs cloud in situ observations.

Evaluation of the direct and indirect radiative and climate effects of aerosols over the western Arctic

From the observations of recent years, there is still not enough evidence to verify the Arctic warming as most global circulation models (GCMs) suggested. This study is dedicated to quantifying the aerosol effect on the Arctic climate change by Northern Aerosol Regional Climate Model (NARCM). The direct and indirect radiative and climate effects of aerosols such as Arctic haze sulfate, black carbon, sea salt, organics, and dust have been evaluated from our NARCM simulations. Within the Arctic Regional Climate Model Intercomparison Project (ARCMIP) our model simulations have been directly compared with the enhanced observation data sets such as the Surface Heat Budget of the Arctic Ocean (SHEBA) and the Atmospheric Radiation Measurement (ARM) in the time period from October 1997 to September 1998. Results show that the climate effects of aerosols strongly depend on the aerosol composition. The surface radiative forcing of pure sulfate aerosols which includes the direct and indirect components reaches up to −7.2 W/m2 in annual mean. The climate responses to radiative forcing of pure sulfate and five kinds of aerosols together are amazingly different. The impacts of aerosols present strong seasonal cycle. In comparison with observations we find the simulation with five kinds of aerosols can better represent the surface temperature from observation. The aerosol radiative and microphysical effects must be taken into account in order to better simulate and predict the change of energy and water cycle occurring in polar climate system.

An Evaluation of Arctic Cloud and Radiation Processes Simulated by the Limited-Area Version of the Global Multiscale Environmental Model (GEM-LAM)

Cloud and radiation processes simulated by the limited area version of the Global Environmental Multiscale Model (GEM-LAM) are evaluated for the period September 1997 to October 1998 over the western Arctic Ocean. This period coincides with the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. Surface downwelling solar and terrestrial radiation, surface albedo, vertically integrated water vapour, liquid water path, precipitation, cloud cover and cloud radiative forcing simulated by GEM-LAM are evaluated against the SHEBA observation dataset. GEM-LAM simulates the annual cycle of the downwelling shortwave (SWD) and longwave (LWD) radiation at the surface reasonably well, as well as precipitable water at monthly and daily time scales. Cloud fraction at daily and monthly time scales is not captured well by the model. During winter, GEM-LAM produces a large negative bias for the vertically integrated liquid water path and a positive bias for cloud fraction. As a result, cloud radiative forcing at the surface and LWD radiation are well reproduced but for the wrong reasons because these two biases have an opposing effect on their magnitudes. During summer, the model underestimates the surface albedo, thus resulting in a substantial overestimation of the cloud radiative forcing at the surface. Precipitation is underestimated during winter and overestimated during summer and spring. The sensitivity of the results to the effective radius of ice crystals and the parameterization of cloud phase is also discussed.

Relative importance of acid coating on ice nuclei in the deposition and contact modes for wintertime Arctic clouds and radiation

Aerosols emitted from volcanic activities and polluted mid-latitudes regions are efficiently transported over the Arctic during winter by the large-scale atmospheric circulation. These aerosols are highly acidic. The acid coating on ice nuclei, which are present among these aerosols, alters their ability to nucleate ice crystals. In this research, the effect of acid coating on deposition and contact ice nuclei on the Arctic cloud and radiation is evaluated for January 2007 using a regional climate model. Results show that the suppression of contact freezing by acid coating on ice nuclei leads to small changes of the cloud microstructure and has no significant effect on the cloud radiative forcing (CRF) at the top of the atmosphere when compared with the effect of the alteration of deposition ice nucleation by acid coating on deposition ice nuclei. There is a negative feedback by which the suppression of contact freezing leads to an increase of the ice crystal nucleation rate by deposition ice nucleation. As a result, the suppression of contact freezing leads to an increase of the cloud ice crystal concentration. Changes in the cloud liquid and ice water contents remain small and the CRF is not significantly modified. The alteration of deposition ice nucleation by acid coating on ice nuclei is dominant over the alteration of contact freezing.

Effects of sulfuric acid and ammonium sulfate coatings on the ice nucleation properties of kaolinite particles: FREEZING OF COATED KAOLINITE PARTICLES

[1] The onset conditions for ice nucleation on H 2 SO 4 coated, (NH 4 ) 2 SO 4 coated, and uncoated kaolinite particles at temperatures ranging from 233 to 246 K were studied. We define the onset conditions as the relative humidity and temperature at which the first ice nucleation event was observed. Uncoated particles were excellent ice nuclei; the onset relative humidity with respect to ice (RH i ) was below 110% at all temperatures studied, consistent with previous measurements. H 2 SO 4 coatings, however, drastically altered the ice nucleating ability of kaolinite particles, increasing the RH i required for ice nucleation by approximately 30%, similar to the recent measurements by Mohler et al. [2008b]. (NH 4 ) 2 SO 4 coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K. The differences between H 2 SO 4 and (NH 4 ) 2 SO 4 coatings may be explained by the deliquescence and efflorescence properties of (NH4) 2 SO 4 . These results support the idea that emissions of SO 2 and NH 3 may influence the ice nucleating properties of mineral dust particles.

Depenses publiques et cycles economiques

We analyze a stochastic general equilibrium model which incorporates three different types of government expenditure. We calibrate the model and estimate, using US data, the multivariate stochastic process generating the components of public expenditure and the Solow residual. These estimates allow us to evaluate the multiplier effects of fiscal spending in the neoclassical model, when the shocks are as persistent as those observed in the data. We also evaluate the degree to which the multiplier effects depend on how public spending is financed. Then, we simulate the model in order to analyze the impact of public expenditure shocks on the comovements of economic aggregates.