M. C. Barth

Sulfur chemistry in the National Center for Atmospheric Research Community Climate Model: Description, evaluation, features, and sensitivity to aqueous chemistry

Sulfur chemistry has been incorporated in the National Center for Atmospheric Research Community Climate Model in an internally consistent manner with other parameterizations in the model. The model predicts mixing ratios of dimethylsulfide (DMS), SO 2 , SO 4 2 , H 2 O 2 . Processes that control the mixing ratio of these species include the emissions of DMS and SO 2 , transport of each species, gas- and aqueous-phase chemistry, wet deposition, and dry deposition of species. Modeled concentrations agree quite well with observations for DMS and H 2 O 2 , fairly well for SO 2 , and not as well for SO 4 2 The modeled SO 4 2- tends to underestimate observed SO 4 2- at the surface and overestimate observations in the upper troposphere. The SO 2 and SO 4 2- species were tagged according to the chemical production pathway and whether the sulfur was of anthropogenic or biogenic origin. Although aqueous-phase reactions in cloud accounted for 81% of the sulfate production rate, only ∼50-60% of the sulfate burden in the troposphere was derived from cloud chemistry. Because cloud chemistry is an important source of sulfate in the troposphere, the importance of H 2 O 2 concentrations and pH values was investigated. When prescribing H 2 O 2 concentrations to clear-sky values instead of predicting H 2 O 2 , the global-averaged, annual-averaged in-cloud production of sulfate increased. Setting the pH of the drops to 4.5 also increased the in-cloud production of sulfate. In both sensitivity simulations, the increased in-cloud production of sulfate decreased the burden of sulfate because less SO 2 was available for gas-phase conversion, which contributes more efficiently to the tropospheric sulfate burder than does aqueous-phase conversion.

Radiative forcing due to sulfate aerosols from simulations with the National Center for Atmospheric Research Community Climate Model, Version 3

The direct and indirect radiative forcing due to sulfate aerosols is calculated in a version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3). This model includes a sulfur chemistry model and predicts the mass of sulfate. New optical properties are presented that account for the hygroscopic growth effects on both extinction optical depth and asymmetry parameter. These new properties enhance the sulfate direct forcing for relative humidities above 90% compared to previous results. The global annual mean forcing is -0.56 W m -2 . The forcing due to the indirect cloud albedo effect is studied using four different methods to relate cloud drop number concentration to sulfate mass. One method assumes the presence of background aerosols that can also act as a source of cloud condensation nuclei. This effect reduces the magnitude of the indirect effect by 40% to -0.4 W m -2 , This sensitivity study indicates the importance of the presence of other aerosols that can nucleate cloud drops. The seasonal cycle of the indirect effect is different from that of the direct effect, as the maximum of the indirect effect occurs in the Northern Hemisphere springtime, while that of the direct effect is largest in the Northern Hemisphere summer. The four different methods of accounting for the indirect effect result in a large uncertainty in the global annual mean net forcing due to sulfates and greenhouse gases, 0.05 to 1.42 W m -2 . It is argued that a less empirical and more physically based approach is required to account for the indirect effect in climate models.

A description of the global sulfur cycle and its controlling processes in the National Center for Atmospheric Research Community Climate Model, Version 3

We examine the balance between processes that contribute to the global and regional distributions of sulfate aerosol in the Earths atmosphere using a set of simulations from the National Center for Atmospheric Research Community Climate Model, Version 3. The analysis suggests that the seasonal cycle of SO2 and SO42− are controlled by a complex interplay between transport, chemistry and deposition processes. The seasonal cycle of these species is not strongly controlled by temporal variations in emissions but by seasonal variations in volume of air processed by clouds, mass of liquid water serving as a site for aqueous chemistry, amount of oxidant available for the conversion from SO2 to SO42−, vertical transport processes, and deposition. A tagging of the sulfate by emission region (Europe, North America, Asia, and rest of world [ROW]), chemical pathway (gaseous versus in-cloud), and type of emissions (anthropogenic versus biogenic) is used to differentiate the balance of processes controlling the production and loading from this material. Significant differences exist in the destiny of SO2 molecules emitted from the several regions. An SO2 molecule emitted from the ROW source region has a much greater potential to form sulfate than one emitted from, for example, Europe. A greater fraction of the SO2 molecules is oxidized that originate from ROW compared with other areas, and once formed, the sulfate has a longer residence time (that is, it is not readily scavenged). The yield of sulfate from ROW sources of SO2 is a factor of 4 higher than that of Europe. A substantially higher fraction of the SO2 emitted over Europe is oxidized to sulfate through the ozone pathway compared to other regions. The analysis suggests that there are significant differences in the vertical distribution, and horizontal extent, of the propagation of sulfate emitted from the several source regions. Sulfate from Asian source regions reaches the farthest from its point of origin and makes a significant contribution to burdens in both hemispheres, primarily from plumes reaching out in the upper troposphere. Sulfate from other source regions tends to remain trapped in their hemisphere of origin.

Impact of anthropogenic aerosols on Indian summer monsoon.

Using an interactive aerosol-climate model we find that absorbing anthropogenic aerosols, whether coexisting with scattering aerosols or not, can significantly affect the Indian summer monsoon syst ...

Large-Eddy Simulation of Flow and Scalar Transport in a Modeled Street Canyon

This study uses large-eddy simulation (LES) to illustrate the flow and turbulence structure and to investigate the mechanism of passive scalar transport in a street canyon. Calculations for a modeled street canyon with building-height-to-street-width ratio of unity at Reynolds number equal to 12 000 are conducted. When the approaching wind is perpendicular to the street axis, the calculation produces a primary vortex in the street canyon, similar to previous studies. An evaluation of the LES results with wind-tunnel measurements reveals good agreement for both mean and turbulence parameters of the flow and scalar fields. The computed primary vortex is confined to the street canyon and is isolated from the free stream flow such that the removal of a scalar emitted at the street level is accomplished by turbulent diffusion at the roof level. It is determined from the calculations that very little scalar is removed from the street canyon, and 97% of the scalar is retained. The scalar mixing at the roof level occurs primarily on the leeward side of the street canyon. In addition to the primary vortex, three secondary vortices are located in the corners of the street canyon at which scalar mixing is enhanced. An examination of additional simulations shows how the location of the scalar source affects the distribution of the scalar.

The Deep Convective Clouds and Chemistry (DC3) Field Campaign

AbstractThe Deep Convective Clouds and Chemistry (DC3) field experiment produced an exceptional dataset on thunderstorms, including their dynamical, physical, and electrical structures and their impact on the chemical composition of the troposphere. The field experiment gathered detailed information on the chemical composition of the inflow and outflow regions of midlatitude thunderstorms in northeast Colorado, west Texas to central Oklahoma, and northern Alabama. A unique aspect of the DC3 strategy was to locate and sample the convective outflow a day after active convection in order to measure the chemical transformations within the upper-tropospheric convective plume. These data are being analyzed to investigate transport and dynamics of the storms, scavenging of soluble trace gases and aerosols, production of nitrogen oxides by lightning, relationships between lightning flash rates and storm parameters, chemistry in the upper troposphere that is affected by the convection, and related source character...

An overview of the Stratospheric‐Tropospheric Experiment: Radiation, Aerosols, and Ozone (STERAO)‐Deep Convection experiment with results for the July 10, 1996 storm

The Stratospheric-Tropospheric Experiment: Radiation, Aerosols and Ozone (STERAO)-Deep Convection Field Project with closely coordinated chemical, dynamical, electrical, and microphysical observations was conducted in northeastern Colorado during June and July of 1996 to investigate the production of NOx by lightning, the transport and redistribution of chemical species in the troposphere by thunderstorms, and the temporal evolution of intracloud and cloud-to-ground lightning for evolving storms on the Colorado high plains. Major observations were airborne chemical measurements in the boundary layer, middle and upper troposphere, and thunderstorm anvils; airborne and ground-based Doppler radar measurements; measurement of both intracloud (IC) and cloud-to-ground (CG) lightning flash rates and locations; and multiparameter radar and in situ observations of microphysical structure. Cloud and mesoscale models are being used to synthesize and extend the observations. Herein we present an overview of the project and selected results for an isolated, severe storm that occurred on July 10. Time histories of reflectivity structure, IC and CG lightning flash rates, and chemical measurements in the boundary layer and in the anvil are presented showing large spatial and temporal variations. The observations for one period of time suggest that limited mixing of environmental air into the updraft core occurred during transport from cloud base to the anvil adjacent to the storm core. We deduce that the most likely contribution of lightning to the total NOx observed in the anvil is 60–90% with a minimum of 45%. For the July 10 storm the NOx produced by lightning was almost exclusively from IC flashes with a ratio of IC to total flashes >0.95 throughout most of the storms lifetime. It is argued that in this storm and probably others, IC flashes can be major contributors to NOx production. Superposition of VHF lightning source locations on Doppler retrieved air motion fields for one 5 min time period shows that lightning activity occurred primarily in moderate updrafts and weak downdrafts with little excursion into the main downdraft. This may have important implications for the vertical redistribution of NOx resulting from lightning production, if found to be true at other times and in other storms.

Distribution and direct radiative forcing of carbonaceous and sulfate aerosols in an interactive size-resolving aerosol–climate model

[1] A multimode, two-moment aerosol model has been incorporated in the NCAR CAM3 to develop an interactive aerosol–climate model and to study the impact of anthropogenic aerosols on the global climate system. Currently, seven aerosol modes, namely three for external sulfate and one each for external black carbon (BC), external organic carbon (OC), sulfate/BC mixture (MBS; with BC core coated by sulfate shell), and sulfate/OC mixture (MOS; a uniform mixture of OC and sulfate) are included in the model. Both mass and number concentrations of each aerosol mode, as well as the mass of carbonaceous species in the mixed modes, are predicted by the model so that the chemical, physical, and radiative processes of various aerosols can be formulated depending on aerosol’s size, chemical composition, and mixing state. Comparisons of modeled surface and vertical aerosol concentrations, as well as the optical depth of aerosols with available observations and previous model estimates, are in general agreement. However, some discrepancies do exist, likely caused by the coarse model resolution or the constant rates of anthropogenic emissions used to test the model. Comparing to the widely used mass-only method with prescribed geometric size of particles (one-moment scheme), the use of prognostic size distributions of aerosols based on a two-moment scheme in our model leads to a significant reduction in optical depth and thus the radiative forcing at the top of the atmosphere (TOA) of particularly external sulfate aerosols. The inclusion of two types of mixed aerosols alters the mass partitioning of carbonaceous and sulfate aerosol constituents: about 35.5%, 48.5%, and 32.2% of BC, OC, and sulfate mass, respectively, are found in the mixed aerosols. This also brings in competing effects in aerosol radiative forcing including a reduction in atmospheric abundance of BC and OC due to the shorter lifetime of internal mixtures (cooling), a mass loss of external sulfate to mixtures (warming), and an enhancement in atmospheric heating per BC mass due to the stronger absorption extinction of the MBS than external BC (warming). The combined result of including a prognostic size distribution and the mixed aerosols in the model is a much smaller total negative TOA forcing (� 0.12 W m �2 ) of all carbonaceous and sulfate aerosol compounds compared to the cases using one-moment scheme either excluding or including internal mixtures (� 0.42 and � 0.71 W m �2 , respectively). In addition, the global mean all-sky TOA direct forcing of aerosols is significantly more positive than the clear-sky value due to the existence of low clouds beneath the absorbing (external BC and MBS) aerosol layer, particularly over a dark surface. An emission reduction of about 44% for BC and 38% of primary OC is found to effectively change the TOA radiative forcing of the entire aerosol family by � 0.14 W m �2 for clear-sky and � 0.29 W m �2 for

Numerical simulations of the July 10, 1996, Stratospheric-Tropospheric Experiment: Radiation, Aerosols, and Ozone (STERAO)-Deep Convection experiment storm: Redistribution of soluble tracers

By using a three-dimensional convective cloud model to simulate the July 10, 1996, Stratospheric-Tropospheric Experiment: Radiation, Aerosols, and Ozone-Deep Convection experiment storm, we investigate the fate of tracers of varying solubilities in midlatitude convection. The tracer distribution resulting from the interactions of the soluble tracers with the cloud hydrometeors is illustrated for two cases. The first case assumes that the dissolved tracer in the cloud water or rain completely degasses when the parent hydrometeor is converted to ice, snow, or hail through microphysical processes. The second case assumes that the dissolved tracer is retained in the ice, snow, or hail. We find that when soluble tracers are degassed, both low- and high-solubility tracers are transported to the upper troposphere. When tracers are retained in ice hydrometeors, the highly soluble tracers are not ultimately transported to the upper troposphere but, instead, are precipitated out of the upper troposphere by snow and hail. Tracers of low solubility are transported upward, similar to passive tracer transport. The key microphysical processes that control these results are the accretion of cloud water by snow and hail. For the simulation in which retention of tracers in ice was considered, highly soluble scalars (105 M atm−1) have a scavenging efficiency >55% and have a mass change in the upper troposphere (8–15 km mean sea level) of −0.5×105 kg to 0 for a 3-hour period, while a passive scalar has a mass change of 2.3×105 kg.

Large-Eddy Simulation of Flow and Pollutant Transport in Street Canyons of Different Building-Height-to-Street-Width Ratios

This study employs a large-eddy simulation technique to investigate the flow, turbulence structure, and pollutant transport in street canyons of building-height-to-street-width (aspect) ratios of 0.5, 1.0, and 2.0 at a Reynolds number of 12 000 and a Schmidt number of 0.72. When the approaching wind is perpendicular to the street axis, a single primary recirculation is calculated for the street canyons of aspect ratios 0.5 and 1.0, and two vertically aligned, counterrotating primary recirculations are found for the street canyon of aspect ratio 2.0. Two to three secondary recirculations are also calculated at the corners of the street canyons. A ground-level passive pollutant line source is used to simulate vehicular emission. The turbulence intensities, pollutant concentration variance, and pollutant fluxes are analyzed to show that the pollutant removal by turbulent transport occurs at the leeward roof level for all aspect ratios. Whereas the ground-level pollutant concentration is greatest at the leeward corner of the street canyons of aspect ratios 0.5 and 1.0, the ground-level pollutant concentration in a street canyon of aspect ratio 2.0 occurs at the windward corner and is greater than the peak concentrations of the other two cases. Because of the smaller ground-level wind speed and the domination of turbulent pollutant transport between the vertically aligned recirculations, the ground-level air quality is poor in street canyons of large aspect ratios. The retention of pollutant in the street canyons is calculated to be 95%, 97%, and 99% for aspect ratios of 0.5, 1.0, and 2.0, respectively.