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Discusses the role of sulfate aerosol cooling in climatic change. Difficulty in measuring the amount of cooling; Clear-sky and in-cloud processes of creating sulfate aerosols; Discrepancy between observed global mean temperature changes and the predictions from climate models; Comparison of aerosol cooling with greenhouse effect. INSETS: Sulfur, acid rain and the ozone layer.;How much light do aerosols reflect away?.

AbstractThis paper describes the effect of the presence of water‐soluble organic compounds (WSOC) in aerosol particles on the aerosol critical supersaturation as defined by the Köhler theory and on cloud condensation nuclei (CCN) number concentration. Taking into account both the soluble mass increase and the surface tension depression due to WSOC, we calculated a substantial decrease of the aerosol critical supersaturation, which results in a large increase in CCN number concentration. CCN supersaturation spectra were computed for three different aerosol types: marine, rural and urban. The increase of CCN number concentration in the presence of WSOC (with respect to the case when only the inorganic aerosol compounds are considered) varies with aerosol type, with an increase up to 13% in the marine case, up to 97% in the rural case, and up to 110% in the urban case, for the supersaturation range typical of atmospheric conditions.

Organic aerosols are gaining increasing attention because of their importance in studies of radiative forcing of climate. However, there is a potential for large and unquantified biases in most of the published data, which has been derived using variations on the evolved gas analysis method (EGA). We argue that the magnitude of these putative uncertainties makes it impossible both to test hypotheses concerning the variation of organic carbon (OC) with altitude and to develop realistic models of radiative climate forcing.

AbstractAerosol particles can act as cloud condensation nuclei and thereby influence the number and size of droplets in clouds. Consequently, anthropogenic particles have the potential to influence global climate by increasing cloud albedo and decreasing precipitation efficiencies. Enhanced cloud reflectances associated with increases in panicle number have been observed, but our understanding of these interactions has been hindered by incomplete empirical studies and models of limited scope.In this study, aerosol and droplet size distributions were measured on 13 research flights in stratiform clouds within 300 km west of the northern California coast. The chemical composition of the droplet solute was also assessed. Microphysical and chemical properties indicated that most of the clouds were influenced by pollution from the North American continent, but pristine marine clouds were sampled on one flight during westerly flow conditions. Data from this flight and another, representing a pristine and polluted environment, were compared with high-resolution satellite observations.In the polluted case, particle and droplet number concentrations decreased, mean droplet size increased, and satellite-derived reflectance at 3.7 μm decreased with increasing distance from the northern California urban region. Relative to the unpolluted stratiform cloud, the polluted cloud had, on average, a sulfate concentration that was higher by an order of magnitude, droplet number concentrations higher by a factor of 6, droplet sizes smaller by a factor of 2, and 3.7-μm reflectance that was higher by a factor of 2. However, no significant difference in the visible reflectance was detected between the two cases, probably a result of differences in liquid water path.

An instrument is described that samples cloud droplets by removing them from the surrounding air and small unactivated particles through inertial impaction. The sampled droplets are then evaporated, leaving behind the material dissolved or suspended in the droplets as residue particles or gases. The instrument is capable of sampling droplets as a function of their size; it has an adjustable cut size in the range between about 9 and 30 μm in diameter, rejects droplets and particles smaller than the cut size, and captures droplets larger than the cut size. Details of the instrumental design and construction are discussed, as well as a relative calibration of the collection efficiency. Results from the calibration experiments indicate that the counterflow virtual impactor probe behaves in accordance with theoretical predictions using Stoke's number calculations. A complete description of the calibration methodology is presented.

A relationship is developed between the visibility of distant mountains seen from an aircraft and a level of background aerosol pollution for a model atmosphere. It is found that the distance at which Mt. Rainier can be seen on clean-air days, which are typical of background aerosol levels, is consistent with the level of aerosol light-scattering measurements in other background situations.

Laser radar (lidar) can be used to estimate atmospheric extinction coefficients that are due to aerosols if the ratio between optical extinction and 180° backscatter (the lidar ratio) at the laser wavelength is known or if Raman or high spectral resolution data are available. Most lidar instruments, however, do not have Raman or high spectral resolution capability, which makes knowledge of the lidar ratio essential. We have modified an integrating nephelometer, which measures the scattering component of light extinction, by addition of a backward-pointing laser light source such that the detected light corresponds to integrated scattering over 176–178° at a common lidar wavelength of 532 nm. Mie calculations indicate that the detected quantity is an excellent proxy for 180° backscatter. When combined with existing techniques for measuring total scattering and absorption by particles, the new device permits a direct determination of the lidar ratio. A four-point calibration, run by filling the enclosed sample volume with particle-free gases of a known scattering coefficient, indicates a linear response and calibration reproducibility to within 4%. The instrument has a detection limit of 1.5 × 10^-7 m^-1 sr^-1 (∼10% of Rayleigh scattering by air at STP) for a 5-min average and is suitable for ground and mobile/airborne surveys. Initial field measurements yielded a lidar ratio of ∼20 for marine aerosols and ∼60–70 for continental aerosols, with an uncertainty of ∼20%.

Unlike traditional virtual impactors, which suffer from a fixed level of small particle contamination in the large particle sample stream, the counterflow virtual impactor (CVI; Ogren et al., 1985) offers the potential for contamination-free sampling of large aerosol particles. Here we present the design and calibration of a CVI intended for studying aerosol effects on cloud microphysics. Such studies require that a large and well-characterized fraction of total cloud droplet number be sampled and that the interstitial aerosol be excluded. The new CVI provides cut sizes (i.e., 50% collection efficiency diameters) down to 7 μm, which is too small for calibration with previous methods (Noone et al., 1988a). Therefore, a new calibration method has been developed which is capable of measuring both cut size and cut sharpness at small diameters. This new method, which uses a polydispersion of glass beads, represents the first set of CVI collection efficiency results for diameters well below 10 μm. The new CVI is shown to have excellent size-selectivity, comparable to other well-designed impactors, and to be well suited for number-based studies in marine stratiform clouds

Monthly mean maximum and minimum temperatures for over 50 (10) of the Northern (Southern) Hemisphere landmass, accounting for 37 of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951–90 (0.84°C versus 0.28°C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied.The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diurnal temperature range is clearly important, both scientifically and practically.

Calculations of direct climate forcing by anthropogenic aerosols commonly use radiative transfer parameters, including asymmetry parameter g. One method of obtaining the asymmetry parameter of a particle population is to convert measured values of the hemispheric-to-total-scatter ratio (backscatter ratio b) into their corresponding g values. We compare a conversion derived from Mie calculations with one derived from the Henyey–Greenstein (HG) phase function to show that the HG method systematically overestimates g for typical size distributions of accumulation-mode aerosols. A delta-Eddington radiative transfer calculation is used to show that a 10% overestimation of g can systematically reduce climate forcing as a result of aerosols by 12% or more. Mie computations are used to derive an empirical relationship between backscatter ratio and asymmetry parameter for log-normal accumulation-mode aerosols. This relationship can be used to convert the backscatter ratio to the asymmetry parameter, independent of geometric mean diameter D_gv or complex refractive index m, but the conversion requires knowledge of the breadth σg of the size distribution.

The major source of cloud-condensation nuclei (CCN) over the oceans appears to be dimethylsulphide, which is produced by planktonic algae in sea water and oxidizes in the atmosphere to form a sulphate aerosol Because the reflectance (albedo) of clouds (and thus the Earth's radiation budget) is sensitive to CCN density, biological regulation of the climate is possible through the effects of temperature and sunlight on phytoplankton population and dimethylsulphide production. To counteract the warming due to doubling of atmospheric CO2, an approximate doubling of CCN would be needed.

Aerosol particles serve as cloud condensation nuclei worldwide, and they affect the Earth's radiation budget both directly and indirectly. These particles consist mostly of sulfate compounds. Ice core measurements can be used to infer past variations of atmospheric sulfate concentration, but to do so requires knowledge of the deposition mechanisms. Significant “dry” deposition may occur by filtering when air moves through the snow due to changes in pressure caused by wind blowing over a rough surface (wind pumping). The filtering efficiency of snow was measured at South Pole Station, using an optical particle counter and a condensation nucleus counter. The number size‐distribution of ambient aerosol peaks at a dry particle diameter of 0.13 μm, the volume size‐distribution at 0.17 μm. Less than 5% of the particles have diameters >0.3 μm. Diffusion from interstitial air to snow grains appears to be the primary mechanism of dry deposition for particles <0.6 μm in diameter, but another mechanism, probably impaction, becomes significant for larger particles. Aerosol deposition by filtering occurs with an e‐folding time of 1–3 s depending on particle size, corresponding to an e‐folding depth of 0.5–1 cm for an estimated air velocity of 0.4 cm s−1within the surface snow. Even for long residence times, a small number of particles (<0.1%) are found in filtered air, suggesting a small degree of new particle formation or reentrainment. However, both the e‐folding depth and the reentrainment rate are small enough that smoothing of the sulfate records in ice cores should be negligible. Three mathematical models for filters agree in describing filtering by snow as dominated by diffusion, but all underpredict the filter efficiency. Capture of aerosol particles is found to be 2–3 times as rapid as that assumed by Cunningham and Waddington[1993], supporting their conclusion of nearly total removal of particles from air entering the snow. Blowing snow might also be expected to collect aerosol particles; however, a calculation suggests that deposition to blowing snow on the Antarctic Plateau is insignificant. If wind pumping and diffusion contribute significantly to total deposition, the flux of air into the snow and the residence time of the air within the snow control the deposition rate. Both air flux and residence time are functions of wind speed and surface roughness, so that the aerosol flux to the snow depends on these factors as well as atmospheric concentration, complicating the interpretation of paleoclimate records for aerosol‐bound substances.

Recent models predict that the effect of atmospheric aerosol particles on global temperature depends in part on b_abs, or on n_2, where b_abs is the aerosol absorption coefficient, and n_2 is the imaginary part of the aerosol refractive index for radiation in visible wavelengths. Satisfactory techniques for measuring b_abs have yet to be developed. A method for measurement of b_abs is described and discussed. The apparatus uses a piece of opal glass to integrate light scattered by particles collected on Nuclepore filters. It is quite mobile, inexpensive, and can be calibrated. Analysis of the errors inherent in the measurement shows it is accurate to well within an order of magnitude for particles of radius larger than 0.1 μm.

Oceanic dimethylsulphide (DMS) emissions and atmospheric aerosol particle populations (condensation nuclei, CN), resolved by latitude and season, appear to be directly correlated, in that CN, as measured with a condensation nucleus counter, are high (or low) in regions where DMS fluxes and incident solar radiation are high (or low). Although it has been previously hypothesized that CN are produced from DMS1,2, we report the first attempt to correlate DMS flux and CN. As the population of cloud condensation nuclei (CCN) in marine air is a subset of the CN population3,4, and CCN in turn control the albedo of marine clouds5,6, DMS could be involved in climate control through a cloud albedo feedback mechanism.

Ammonia, being the dominant base other than sea salt in the remote marine troposphere, plays an important role in the acid–base chemistry of the atmosphere1–3. An understanding of the cycling of ammonia through the marine environment has, until now, been hampered by the lack of concurrent measurements of key acidic and basic atmospheric and seawater species. Reported here are the results of simultaneous concentration measurements of these species during May 1987 in the coastal north-east Pacific Ocean environment. Gas (g) and particulate (p) phase data suggest that low concentrations of NH3(g) in the marine atmosphere lead to partly neutralized H2SO4in aerosol particles and in rainwater. Gas-phase concentrations combined with seawater concentration measurements indicate that for the region and time period studied the ocean was a local source of atmospheric ammonia. These data, combined in a simple box model, suggest a marine boundary layer residence time of 3.6 h and 22 h for NH3(g) and NH+4(p), respectively.

Stellar extinction measurements made at three astronomical observatories showed that on 50% of the nights the extinction due to aerosol light scattering increased rather than decreased with increasing wavelength (anomalous extinction) for wavelengths close to 500 nm. This extinction behavior is analyzed in this paper and limits are established for the aerosol characteristics necessary for this phenomenon to exist, including geometric standard deviations, imaginary part of refractive index, mean radius, and gaseous NO_2.

Measurements of light-scattering made with an integrating nephelometer at Point Barrow, Alaska, and Mount Olympus, Washington, are in agreement in magnitude, and both sets of measurements also are in agreement with other measurements of light-scattering and turbidity made in remote locations. This agreement suggests that a background level of aerosol exists and that the concept of a background is applicable.

An improved method of humidity measurement based on the dielectric constant of a liquid-solid mixture has shown agreement with a simple theory. The hygrometer has shown no measurable hysteresis and has identical time-response characteristics for increasing or decreasing humidity.

ASTRONOMICAL observatories are particularly suited for studies of the background aerosol content of the atmosphere for four reasons. (1) The observatories are usually in remote locations away from the lights and smoke of cities. (2) Most observatories have standard equipment designed to correct the measured light fluxes of stars for the loss of brightness in the passage through the atmosphere (in other words, to correct for atmospheric extinction). (3) There is good global coverage of observatories. (4) Observations of the extinction of starlight by the atmosphere have been made for nearly forty years and are available for re-analysis. The last point is particularly important because there are no other reliable records of the condition of the atmosphere in remote locations.

Particles generated by combustion of fossil fuels contribute to climate forcing by absorbing and scattering visible light. Residential combustion takes place in homes for heating or cooking purposes and is thought to contribute a large fraction of the global burden of anthropogenic primary particles. We present optical properties and size distributions of particulate matter emitted from three types of coal burned in residential combustors: bituminous coal, hard coal briquettes, and lignite. Emissions from these coals differ significantly and can be partially explained by differences in coal composition. For bituminous coal, particulate matter emission factors are somewhat greater than those used in current emission inventories. We observe particles for which the light absorption is weak and has a strong spectral dependence. For hard coal briquettes and lignite, emitted light absorption is low, and based on our measurements, current inventories of light‐absorbing aerosols significantly overestimate the contribution from these sources. Hard coal briquettes produce very few particles in the optically active size range. For all coals tested the size distributions required to represent the average of the emitted particles are broader than atmospheric size distributions, with geometric standard deviations between 2.2 and 3.0.