Catherine C. Chuang

A global three‐dimensional model study of carbonaceous aerosols

We have developed detailed emission inventories for the amount of both black and organic carbon particles from biomass burning sources (wood fuel, charcoal burning, dung, charcoal production, agricultural, savanna and forest fires). We have also estimated an inventory for organic carbon particles from fossil fuel burning and urban activities from an existing inventory for fossil fuel sources of black carbon. We also provide an estimate for the natural source of organic matter. These emissions have been used together with our global aerosol model to study the global distribution of carbonaceous aerosols. The accuracy of the inventories and the model formulation has been tested by comparing the model simulations of carbonaceous aerosols in the atmosphere and in precipitation with observations reported in the literature. For most locations and seasons, the predicted concentrations are in reasonable agreement with the observations, although the model underpredicts black carbon concentrations in polar regions. The predicted concentrations in remote areas are extremely sensitive to both the rate of removal by wet deposition and the height of injection of the aerosols. Finally, a global map of the aerosol single scattering albedo was developed from the simulated carbonaceous particle distribution and a previously developed model for aerosol sulfates. The computed aerosol single scattering albedos compare well with observations, suggesting that most of the important aerosol species have been included in the model. For most locations and seasons, the single scattering albedo is larger than 0.85, indicating that these aerosols, in general, lead to a net cooling.

A New Model for the Equilibrium Shape of Raindrops

Abstract The equilibrium shape of raindrops has been determined from Laplaces equation using an internal hydrostatic pressure with an external aerodynamic pressure based on measurements for a sphere but adjusted for the effect of distortion. The drop shape was calculated by integration from the upper pole with the initial curvature determined by iteration on the drop volume. The shape was closed at the lower pole by adjusting either the pressure drag or the drop weight to achieve an overall force balance. Model results provide bounds on the axis ratio of raindrops with an uncertainty of about 1% and very good agreement with extensive wind tunnel measurements for moderate to large water drops. The model yields the peculiar asymmetric shape of raindrops: a singly curved surface with a flattened base and a maximum curvature just below the major axis. A close match was found between model shapes and profiles obtained from photos of water drops for diameters up to 5 mm. Coefficients are provided for computing...

Prediction of the number of cloud droplets in the ECHAM GCM

In this paper a prognostic equation for the number of cloud droplets (CDNC) is introduced into the ECHAM general circulation model. The initial CDNC is based on the mechanistic model of Chuang and Penner [1995], providing a more realistical prediction of CDNC than the empirical method previously used. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and a shape parameter, which takes the aerosol composition and size distribution into account. The total number of aerosol particles is obtained as the sum of marine sulfate aerosols produced from dimethyl sulfide, hydrophylic organic and black carbon, submicron dust, and sea-salt aerosols. Anthropogenic sulfate aerosols only add mass to the preexisting aerosols but do not form new particles. The simulated annual mean liquid water path, column CDNC, and effective radius agree well with observations, as does the frequency distributions of column CDNC for clouds over oceans and the variations of cloud optical depth with effective radius.

An assessment of the radiative effects of anthropogenic sulfate

We use a coupled climate/chemistry model with cloud nucleation processes parameterized in terms of local aerosol number, anthropogenic sulfate mass concentration, and updraft velocity to investigate both direct and indirect anthropogenic sulfate radiative forcings. We estimate that the global direct radiative forcing is about {minus}0.4Wm{sup {minus}2} with a maximum over Europe where the strongest anthropogenic sulfur emissions occur. With different approaches for the formation of anthropogenic sulfate and its relation to aerosol size distribution, we estimate that the indirect forcing may range from {minus}0.6 to {minus}1.6Wm{sup {minus}2}. This range reduces to {minus}0.4 to {minus}1.1Wm{sup {minus}2} if a prescribed marine background particle number concentration is universally applied over the ocean. Contrary to the direct effect which is more significant over continents, the calculated maximum of indirect forcing is located over the Atlantic Ocean near the coastline of North America. Our simulations indicate that anthropogenic sulfate may result in important increases in reflected solar radiation, which would mask locally the warming from increased greenhouse gases. We also compare the simulated cloud drop effective radii with those retrieved from satellite data to validate the accuracy of our cloud drop parameterization.{copyright} 1997 American Geophysical Union

Cloud susceptibility and the first aerosol indirect forcing: Sensitivity to black carbon and aerosol concentrations

[1] Present-day global anthropogenic emissions contribute more than half of the mass in submicron particles primarily due to sulfate and carbonaceous aerosol components derived from fossil fuel combustion and biomass burning. These anthropogenic aerosols increase cloud drop number concentration and cloud albedo. Here, we use an improved version of the fully coupled climate/chemistry models to investigate cloud susceptibility and the first indirect effect of anthropogenic aerosols (the Twomey effect). We examine the correspondence between the model simulation of cloud susceptibility and that inferred from satellite measurements to test whether our simulated aerosol concentrations and aerosol/cloud interactions give a faithful representation of these features. This comparison provides an overall measure of the adequacy of cloud cover and drop concentrations. We also address the impact of black carbon absorption in clouds on the first indirect forcing and examine the sensitivity of the forcing to different representations of natural aerosols. We find that including this absorption does not change the global forcing by more than 0.07 W m � 2 , but that locally it could decrease the forcing by as much as 0.7 W m � 2 in regions where black carbon emissions are pronounced. Because of the nonlinear relationship between cloud drop number and aerosol number concentrations, the total forcing does not equal the sum of the forcing from each individual source. Our estimated total first indirect forcing is � 1.85 W m � 2 , with � 0.30 W m � 2 associated with anthropogenic sulfate, � 1.16 W m � 2 associated with carbonaceous aerosols from biomass burning, and � 0.52 W m � 2 associated with carbonaceous aerosols from fossil fuel combustion. Estimates of forcing by sulfate and total carbonaceous aerosols increase to � 0.31 and � 1.67 W m � 2 , respectively, if natural emissions of organic aerosols are only 8.4 Tg yr � 1 , but decrease to � 0.26 and � 1.27 W m � 2 if they are as large as 42 Tg yr � 1 . Even larger estimates of forcing are derived if dust and sea-salt emissions are not included. The effect of aerosol abundance on cloud life cycle may be important but is not treated in this study. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 1610 Global Change: Atmosphere (0315, 0325)

A Comparison of Model- and Satellite-Derived Aerosol Optical Depth and Reflectivity

The determination of an accurate quantitative understanding of the role of tropospheric aerosols in the earth’s radiation budget is extremely important because forcing by anthropogenic aerosols presently represents one of the most uncertain aspects of climate models. Here the authors present a systematic comparison of three different analyses of satellite-retrieved aerosol optical depth based on the Advanced Very High Resolution Radiometer (AVHRR)-measured radiances with optical depths derived from six different models. Also compared are the model-derived clear-sky reflected shortwave radiation with satellite-measured reflectivities derived from the Earth Radiation Budget Experiment (ERBE) satellite. The three different satellite-derived optical depths differ by between 20.10 and 0.07 optical depth units in comparison to the average of the three analyses depending on latitude and month, but the general features of the retrievals are similar. The models differ by between 20.09 and 10.16 optical depth units from the average of the models. Differences between the average of the models and the average of the satellite analyses range over 20.11 to 10.05 optical depth units. These differences are significant since the annual average clear-sky radiative forcing associated with the difference between the average of the models and the average of the satellite analyses ranges between 23.9 and 0.7 W m22 depending on latitude and is 21.7 W m22 on a global average annual basis. Variations in the source strengths of dimethylsulfide-derived aerosols and sea salt aerosols can explain differences between the models, and between the models and satellite retrievals of up to 0.2 optical depth units. The comparison of model-generated reflected shortwave radiation and ERBE-measured shortwave radiation is similar in character as a function of latitude to the analysis of modeled and satellite-retrieved optical depths, but the differences between the modeled clear-sky reflected flux and the ERBE clear-sky reflected flux is generally larger than that inferred from the difference between the models and the AVHRR optical depths, especially at high latitudes. The difference between the mean of the models and the ERBE-analyzed clear-sky flux is 1.6 W m22. The overall comparison indicates that the model-generated aerosol optical depth is systematically lower than that inferred from measurements between the latitudes of 108 and 308S. It is not likely that the shortfall is due to small values of the sea salt optical depth because increases in this component would create modeled optical depths that are larger than those from satellites in the region north of 30 8N and near 508S. Instead, the source strengths for DMS and biomass aerosols in the models may be too low. Firm conclusions, however, will require better retrieval procedures for the satellites, including better cloud screening procedures, further improvement of the model’s treatment of aerosol transport and removal, and a better determination of aerosol source strengths.

A Numerical Model for the Equilibrium Shape of Electrified Raindrops

Abstract The model Beard Chuang, using the differential form of Laplaces formula, has been extended to raindrop shapes under the influence of vertical electric fields and drop charges. A finite volume method was used with a boundary-fitted coordinate system to calculate the shape-dependent electric field. The distorted shape was obtained by numerical integration from the upper to lower pole by iteration to achieve the appropriate drop volume and force balance using shape-dependent stresses. The model prediction of the critical electric field for instability is within a few percent of previous models for a stationary drop, but stability was found to be considerably enhanced for raindrops because of the counteracting aerodynamic distortion. The predicted critical fields for larger raindrops, however, are about 2 kV cm−1 higher than found in the wind tunnel measurements of Richards and Dawson. Model raindrop shapes in a strong, electric field show a pronounced extension of the upper pole, and a flattened ba...

Modeling the spectral optical properties of ammonium sulfate and biomass burning aerosols: parameterization of relative humidity effects and model results

The importance of including the global and regional radiative effects of aerosols in climate models has increasingly been realized. Accurate modeling of solar radiative forcing due to aerosols from anthropogenic sulfate and biomass burning emissions requires adequate spectral resolution and treatment of spatial and temporal variability. The variation of aerosol spectral optical properties with local relative humidity and dry aerosol composition must be considered. Because the cost of directly including Mie calculations within a climate model is prohibitive, parameterizations from offline calculations must be used. Starting from a log-normal size distribution of dry ammonium sulfate, we developed optical properties for tropospheric sulfate aerosol at 15 relative humidities up to 99 percent. The resulting aerosol size distributions were then used to calculate bulk optical properties at wavelengths between 0.175 {micro}m and 4 {micro}m. Finally, functional fits of optical properties were made for each of 12 wavelength bands as a function of relative humidity. Significant variations in optical properties occurred across the total solar spectrum. Relative increases in specific extinction and asymmetry factor with increasing relative humidity became larger at longer wavelengths. Significant variation in single-scattering albedo was found only in the longest near-IR band. This is also the band with the lowest albedo. A similar treatment was done for aerosols from biomass burning. In this case, size distributions were taken as having two carbonaceous size modes and a larger dust mode. The two carbonaceous modes were considered to be humidity dependent. Equilibrium size distributions and compositions were calculated for 15 relative humidities and five black carbon fractions. Mie calculations and Chandrasekhar averages of optical properties were done for each of the resulting 75 cases. Finally, fits were made for each of 12 spectral bands as functions of relative humidity and black carbon fraction.

A Simple Perturbation Model for the Electrostatic Shape of Falling Drops

Abstract A perturbation model for the shape of falling drops in the presence of electric fields and charges was developed by extension of previous methods that includes aerodynamic effects in the pressure balance equation of Laplace. Use of a consistent first-order perturbation equation and spherical harmonies helped to reconcile apparent inconsistencies in the cosine series method originated by Savic and modified by Pruppacher and Pitter. The electric stress was included in the first-order model using a sphere in a uniform vertical field, with the effect of a net charge approximated by a reduction in surface tension for a spherical drop. The perturbation drop shapes for electric distortion were found to be appreciably altered by coupling between shape and fall speed. A simple axis ratio formula, valid for small perturbation was based on the principal shape coefficient, containing term for the aerodynamic distortion, electric field and charge: α = 1 − (0.1 98 We-0.281X2)FQ. This equation, when coupled wit...

The contribution of carbonaceous aerosols to climate change

Contribution of aerosols to climate change results from two effects: clear-sky and cloudy-sky forcing. The clear-sky climate forcing by carbonaceous aerosols from biomass burning and fossil fuel burning depends on the relative contribution of scattering and absorption by the aerosols which in turn depends on the fraction of aerosol mass associated with black carbon and its size distribution. This paper reviews estimates for the emission of carbonaceous aerosols, placing these estimates in the context of estimates for the emissions of anthropogenic and natural sulfate aerosols and natural sources of organic particulate matter. The cloudy-sky forcing from carbonaceous aerosols is difficult to estimate because, among other factors, it depends on the amount of absorption by the aerosols in the cloud. It is also highly sensitive to the assumed pre-existing, natural aerosol abundance. An upper limit for this cloudy-sky forcing is -4.4 W/m{sup 2}, but may range as low as -2.4 W/m{sup 2}, depending on background aerosol concentrations. These estimates do not yet account for absorption of radiation by black carbon associated with cloud or the presence of pre-existing dust particles.