Stephen E. Schwartz

Climate Forcing by Anthropogenic Aerosols

Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be –1 to –2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.

The Atmospheric Radiation Measurement (ARM) Program: Programmatic Background and Design of the Cloud and Radiation Test Bed

Abstract The Atmospheric Radiation Measurement (ARM) Program, supported by the U.S. Department of Energy, is a major new program of atmospheric measurement and modeling. The program is intended to improve the understanding of processes that affect atmospheric radiation and the description of these processes in climate models. An accurate description of atmospheric radiation and its interaction with clouds and cloud processes is necessary to improve the performance of and confidence in models used to study and predict climate change. The ARM Program will employ five (this paper was prepared prior to a decision to limit the number of primary measurement sites to three) highly instrumented primary measurement sites for up to 10 years at land and ocean locations, from the Tropics to the Arctic, and will conduct observations for shorter periods at additional sites and in specialized campaigns. Quantities to be measured at these sites include longwave and shortwave radiation, the spatial and temporal distributi...

Mass-Transport Considerations Pertinent to Aqueous Phase Reactions of Gases in Liquid-Water Clouds

Reactions of gases in liquid-water clouds are potentially important in the transformation of atmospheric pollutants affecting their transport in the atmosphere and subsequent removal and deposition to the surface. Such processes consist of the following sequence of steps: Mass-transport of the reagent gas or gases to the air-water interface; transfer across the interface and establishment of solubility equilibria locally at the interface; mass-transport of the dissolved gas or gases within the aqueous phase; aqueous-phase chemical reaction(s); mass-transport of reaction product(s) and possible subsequent evolution into the gas-phase. Description of the rate of the overall process requires identification of the rate-limiting step (or steps) and evaluation of the rate of such step(s). Identification of the rate-limiting step may be achieved by evaluation and comparison of the characteristic times pertinent to the several processes and may be readily carried out by methods outlined herein, for known or assumed reagent concentrations, drop size, and fundamental constants as follows: gas- and aqueous-phase diffusion coefficients; Henry’s law coefficient and other pertinent equilibrium constants; interfacial mass-transfer accommodation coefficient; aqueous-phase reaction rate constants(s). A graphical method is described whereby it may be ascertained whether a given reaction is controlled solely by reagent solubility and intrinsic chemical kinetic or is mass-transport limited by one or another of the above processes. In the absence of mass-transport limitation, reaction rates may be evaluated uniformly for the entire liquid-water content of the cloud using equilibrium reagent concentrations. In contrast, where appreciable mass-transport limitation is indicated, evaluation of the overall rate requires knowledge of and integration over the drop-size distribution characterizing the cloud.

Quantifying and minimizing uncertainty of climate forcing by anthropogenic aerosols

The clear-sky climate forcing by anthropogenic aerosols has been shown to be of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Anthropogenic aerosols are composed of a variety of aerosol types including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), organic condensed species, elemental or black carbon, and mineral dust. Estimates of the clear-sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols have been published previously. Here we estimate the uncertainty in the forcing by these aerosol types. Estimates of the clear-sky forcing by other anthropogenic aerosol types do not even exist though the forcing by these aerosol types is thought to be smaller than that by sulfate and biomass burning aerosols.

Production flux of sea spray aerosol

Knowledge of the size- and composition-dependent production flux of primary sea spray aerosol (SSA) particles and its dependence on environmental variables is required for modeling cloud microphysical properties and aerosol radiative influences, interpreting measurements of particulate matter in coastal areas and its relation to air quality, and evaluating rates of uptake and reactions of gases in sea spray drops. This review examines recent research pertinent to SSA production flux, which deals mainly with production of particles with r 80 (equilibrium radius at 80% relative humidity) less than 1 m and as small as 0.01 m. Production of sea spray particles and its dependence on controlling factors has been investigated in laboratory studies that have examined the dependences on water temperature, salinity, and the presence of organics and in field measurements with micrometeorological techniques that use newly developed fast optical particle sizers. Extensive measurements show that water-insoluble organic matter contributes substantially to the composition of SSA particles with r80 < 0.25 m and, in locations with high biological activity, can be the dominant constituent. Order-of-magnitude variation remains in estimates of the size-dependent production flux per white area, the quantity central to formulations of the production flux based on the whitecap method. This variation indicates that the production flux may depend on quantities such as the volume flux of air bubbles to the surface that are not accounted for in current models. Variation in estimates of the whitecap fraction as a function of wind speed contributes additional, comparable uncertainty to production flux estimates.

The whitehouse effect—Shortwave radiative forcing of climate by anthropogenic aerosols: an overview

Loadings of tropospheric aerosols have increased substantially over the past 150 yr as a consequence of industrial activities. These aerosols enhance reflection of solar radiation by the Earth-atmosphere system both directly, by scattering light in clear air and, indirectly, by increasing the reflectivity of clouds. The magnitude of the resultant decrease in absorption of solar radiation is estimated to be comparable on global average to the enhancement in infrared forcing at the tropopause due to increases in concentrations of CO2 and other greenhouse gases over the same time period. Estimates of the aerosol shortwave forcing are quite uncertain, by more than a factor of two about the current best estimates. This article reviews the atmospheric chemistry and microphysical processes that govern the loading and light scattering properties of the aerosol particles responsible for the direct effect and delineates the basis for the present estimates of the magnitude and uncertainty in the resultant radiative forcing. The principal sources of uncertainty are in the loading of anthropogenic aerosols, which is highly variable spatially and temporally because of the relatively short residence time of these aerosols (ca. 1 week) and the episodic removal in precipitation, and in the dependence of light scattering on particle size, and in turn on relative humidity. Uncertainty in aerosol forcing is the greatest source of uncertainty in radiative forcing of climate over the industrial period. At the high end of the uncertainty range, the aerosol forcing is comparable to the anthropogenic greenhouse forcing, and substantially greater in industrialized regions. Even at the low end of the range, the aerosol forcing cannot be neglected in considerations of influences on climate over the industrial period. This uncertainty greatly limits the ability to draw empirical inferences of climate sensitivity to radiative forcing.

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.

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.

Direct shortwave forcing of climate by the anthropogenic sulfate aerosol: Sensitivity to particle size, composition, and relative humidity

Recent estimates of global or hemispheric average forcing of climate by anthropogenic sulfate aerosol caused by scattering of shortwave radiation (“direct” effect) are uncertain by somewhat more than a factor of 2. The principal sources of this uncertainty are atmospheric chemistry properties (yield, residence time), and microphysical properties (scattering efficiency, upscatter fraction, and the dependence of these properties on particle size, composition, and relative humidity, (RH)). This paper examines the sensitivity of forcing to these microphysical properties to identify and improve understanding of the properties required to reduce the uncertainty in the forcing. The relations between aerosol loading and forcing developed here are suitable for comparing modeled and measured aerosol forcing at specific locations and for use in climate models, provided aerosol composition and microphysical properties are known, calculated, or assumed. Results are presented showing the dependence of scattering efficiency, upscatter fraction, and normalized forcing (W m−2/g(SO24−) m−2 or W g(SO24−)−1) on dry particle size (expressed as mole(sulfate) per particle), composition ((NH4)2SO4, NH4HSO4, H2SO4), solar zenith angle, latitude, and season. Forcing is strongly dependent on dry particle size and RH but is relatively insensitive to composition. The normalized forcing can be integrated over a known or assumed size distribution to evaluate the sulfate aerosol forcing. Global and annual average values of the normalized forcing are evaluated as a function of particle size and RH. Depending on values of these variables, normalized forcing may be less than, intermediate to, or greater than the range of previous estimates of sulfate aerosol forcing.

Heat capacity, time constant, and sensitivity of Earth's climate system

The equilibrium sensitivity of Earths climate is determined as the quotient of the relaxation time constant of the system and the pertinent global heat capacity. The heat capacity of the global ocean, obtained from regression of ocean heat content vs. global mean surface temperature, GMST, is 14 ± 6 W yr m -2 K -1 , equivalent to 110 m of ocean water; other sinks raise the effective planetary heat capacity to 17 ± 7 W yr m -2 K -1 (all uncertainties are 1-sigma estimates). The time constant pertinent to changes in GMST is determined from autocorrelation of that quantity over 1880-2004 to be 5 ± 1 yr. The resultant equilibrium climate sensitivity, 0.30 ± 0.14K/(W m -2 ), corresponds to an equilibrium temperature increase for doubled CO2 of 1.1 ± 0.5 K. The short time constant implies that GMST is in near equilibrium with applied forcings and hence that net climate forcing over the twentieth century can be obtained from the observed temperature increase over this period, 0.57 ± 0.08 K, as 1.9 ± 0.9 W m -2 . For this forcing considered the sum of radiative forcing by incremental greenhouse gases, 2.2 ± 0.3 W m -2 , and other forcings, other forcing agents, mainly incremental tropospheric aerosols, are inferred to have exerted only a slight forcing over the twentieth century of -0.3 ± 1.0 W m -2 .