Changes In Aerosol Concentrations Research Articles

Offsetting effects of aerosols on Arctic and global climate in the late 20th century

Abstract. We examine the impacts of atmospheric aerosols on Arctic and global climate using a series of 20th century transient simulations from Community Climate System Model version 4 (CCSM4). We focus on the response of surface air temperature to the direct radiative forcing driven by changes in sulfate and black carbon (BC) concentrations from 1975 to 2005 and we also examine the response to changes in sulfate, BC, and organic carbon (OC) aerosols collectively. The direct forcing from sulfate dominates the aerosol climate effect. Globally averaged, simultaneous changes in all three aerosols produce a cooling trend of 0.015 K decade−1 during the period 1975–2005. In the Arctic, surface air temperature has large spatial variations in response to changes in aerosol concentrations. Over the European Arctic, aerosols induce about 0.6 K decade−1 warming, which is about 1.8 K warming over the 30-year period. This warming is triggered mainly by the reduction in sulfate and BC emissions over Europe since the 1970s and is reinforced by sea ice loss and a strengthening in atmospheric northward heat transport. Changes in sulfate concentrations account for about two thirds of the warming and BC for the remaining one third. Over the Siberian and North American Arctic, surface air temperature is likely influenced by changes in aerosol concentrations over Asia. An increase in sulfate optical depth over Asia induces a large cooling while an increase in BC over Asia causes a significant warming.

Cluster Analysis for Daily Patterns of SO2 and NO2 Measured by the DOAS System in Xiamen

Daily patterns of air pollutants are important to improve measurement retrievals and to model the regimes of local air quality. In this study, the daily patterns of SO2 and NO2 as well as their association with visibility and meteorological conditions in a suburban area of Xiamen are investigated. To achieve this goal, continuous field measurements were collected with a Differential Optical Absorption Spectroscopy (DOAS) system in 2011. The K-means clustering is used to classify the daily variation cycles of these measurements associated with different visibility and meteorological conditions such as temperature, relative humidity, wind speed and direction. The Davies-Bouldin index strategy is used to determine the optimal number of clusters. The regime of each cluster associated with visibility and meteorological conditions was then explored and compared. The comparative analyses show that both the maximum hourly average concentrations and the maximum daily average concentrations of SO2 and NO2 occurred in spring. Only 0.04 percent and 3.19 percent of the days with SO2 and NO2, respectively, did not comply with the latest national ambient air quality standards of China (GB 3095-2012). Moreover, the clustering results highlighted three representative patterns of daily SO2 concentrations and four representative patterns of daily NO2 concentrations. Both similarities and differences were presented among these clusters. The consistent changes in aerosol concentration with the changes in the measurements of NO2 and SO2 in each cluster provided supplemental evidence for the presence of the daily patterns of SO2 and NO2.

Relative humidity and its effect on aerosol optical depth in the vicinity of convective clouds

The hygroscopic growth of aerosols is controlled by the relative humidity (RH) and changes the aerosols’ physical and hence optical properties. Observational studies of aerosol–cloud interactions evaluate the aerosol concentration using optical parameters, such as the aerosol optical depth (AOD), which can be affected by aerosol humidification. In this study we evaluate the RH background and variance values, in the lower cloudy atmosphere, an additional source of variance in AOD values beside the natural changes in aerosol concentration. In addition, we estimate the bias in RH and AOD, related to cloud thickness. This provides the much needed range of RH-related biases in studies of aerosol–cloud interaction. Twelve years of radiosonde measurements (June–August) in thirteen globally distributed stations are analyzed. The estimated non-biased AOD variance due to day-to-day changes in RH is found to be around 20% and the biases linked to cloud development around 10%. Such an effect is important and should be considered in direct and indirect aerosol effect estimations but it is inadequate to account for most of the AOD trend found in observational studies of aerosol–cloud interactions.

Projected effect of 2000–2050 changes in climate and emissions on aerosol levels in China and associated transboundary transport

Abstract. We investigate projected 2000–2050 changes in concentrations of aerosols in China and the associated transboundary aerosol transport by using the chemical transport model GEOS-Chem driven by the Goddard Institute for Space Studies (GISS) general circulation model (GCM) 3 at 4° × 5° resolution. Future changes in climate and emissions projected by the IPCC A1B scenario are imposed separately and together through sensitivity simulations. Accounting for sulfate, nitrate, ammonium, black carbon (BC), and organic carbon (OC) aerosols, concentrations of individual aerosol species change by −1.5 to +0.8 μg m−3, and PM2.5 levels are projected to change by about 10–20% in eastern China as a result of 2000–2050 change in climate alone. With future changes in anthropogenic emissions alone, concentrations of sulfate, BC, and OC are simulated to decrease because of assumed reductions in emissions, and those of nitrate are predicted to increase because of higher NOx emissions combined with decreases in sulfate. The net result is a predicted reduction of seasonal mean PM2.5 concentrations in eastern China by 1–8 μg m−3 (or 10–40%) over 2000–2050. It is noted that current emission inventories for BC and OC over China are judged to be inadequate at present. Transboundary fluxes of different aerosol species show different sensitivities to future changes in climate and emissions. The annual outflow of PM2.5 from eastern China to the western Pacific is estimated to change by −7.0%, −0.7%, and −9.0% over 2000–2050 owing to climate change alone, changes in emissions alone, and changes in both climate and emissions, respectively. The fluxes of nitrate and ammonium aerosols from Europe and Central Asia into western China increase over 2000–2050 in response to projected changes in emissions, leading to a 10.5% increase in annual inflow of PM2.5 to western China with future changes in both emissions and climate. Fluxes of BC and OC from South Asia to China in spring contribute a large fraction of the annual inflow of PM2.5. The annual inflow of PM2.5 from South Asia and Southeast Asia to China is estimated to change by −8%, +281%, and +227% over 2000–2050 owing to climate change alone, changes in emissions alone, and changes in both climate and emissions, respectively. While the 4° × 5° spatial resolution is a limitation of the present study, the direction of predicted changes in aerosol levels and transboundary fluxes still provides valuable insight into future air quality.

Impact of aerosols on tropical cyclones: An investigation using convection‐permitting model simulation

AbstractThe role of aerosols' effect on two tropical cyclones over the Bay of Bengal is investigated using a convection‐permitting model with a two‐moment mixed‐phase bulk cloud microphysics scheme. The simulation results show the role of aerosol on the microphysical and dynamical properties of the cloud and bring out the change in efficiency of the clouds in producing precipitation. The tracks of the tropical cyclones (TCs) are hardly affected by the changing aerosol concentrations, but the intensity exhibits significant sensitivity due to the change in aerosol concentration. It is also clearly seen from the analyses that higher heating in the middle troposphere within the cyclone center is in response to latent heat release as a consequence of greater graupel formation. Greater heating in the middle level is particularly noticeable for the clean aerosol regime which causes enhanced divergence in the upper level, which, in turn, forces lower level convergence. As a result, the cleaner aerosol perturbation is more unstable within the cyclone core and produces a more intense cyclone as compared to the other two aerosol perturbations. This study, along with previous simulations, shows the robustness of the concept of TC weakening by storm ingestion of high concentrations of cloud condensation nuclei (CCN). The consistency of these model results gives us confidence in stating that there is a high probability that ingestion of high CCN concentrations in a TC will lead to weakening of the storm but has little impact on storm direction. Moreover, as pollution is increasing over the Indian subcontinent, this study suggests that pollution may be weakening TCs over the Bay of Bengal.

Impact of Chinese anthropogenic emissions on submicrometer aerosol concentration at Mt. Tateyama, Japan

Abstract. Rapid Asian economic development might engender secondary impacts of atmospheric aerosol particles over the western Pacific after conversion of gaseous pollutants such as SO2. To elucidate changes in aerosol concentrations in leeward areas undergoing remarkable industrialization, the number-size distributions of submicrometer (0.3–1.0 μm) aerosols were measured at Murododaira (36.6° N, 137.6° E, 2450 m a.s.l.) on the western flank of Mount Tateyama in central Japan during January 1999–February 2009. Nighttime data obtained from 2400 to 0500 were used to analyze free-tropospheric aerosol concentration. Monthly average volume concentrations were calculated for months with >50% daily data coverage. Volume concentrations of submicrometer aerosols were high in spring to early summer and low in winter. Significant increasing trends at 95% confidence levels were found for volume concentrations in winter–spring. Simulated monthly anthropogenic aerosol concentrations at Mt. Tateyama from results of regional aerosol modeling with emission inventory up to 2005 showed seasonal variation and winter–spring increasing trends similar to those of observed aerosol concentration. According to the model analyses, the contribution of anthropogenic aerosol concentrations derived from China was high during winter–spring (60–80% of total anthropogenic aerosols at Mt. Tateyama). This accords with the increasing trend observed for winter–spring. Because SO42− is the dominant component of total anthropogenic aerosols, these results suggest that increasing anthropogenic emissions, especially for SO2, in China, engender enhancement of submicrometer-diameter aerosols over Japan during winter–spring.

A six year satellite-based assessment of the regional variations in aerosol indirect effects

Abstract. Aerosols act as cloud condensation nuclei (CCN) for cloud water droplets, and changes in aerosol concentrations have significant microphysical impacts on the corresponding cloud properties. Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud properties are combined with NCEP Reanalysis data for six different regions around the globe between March 2000 and December 2005 to study the effects of different aerosol, cloud, and atmospheric conditions on the aerosol indirect effect (AIE). Emphasis is placed in examining the relative importance of aerosol concentration, type, and atmospheric conditions (mainly vertical motion) to AIE from region to region. Results show that in most regions, AIE has a distinct seasonal cycle, though the cycle varies in significance and period from region to region. In the Arabian Sea (AS), the six-year mean anthropogenic + dust AIE is −0.27 Wm−2 and is greatest during the summer months (<−2.0 Wm−2) during which aerosol concentrations (from both dust and anthropogenic sources) are greatest. Comparing AIE as a function of thin (LWP<20 gm−2) vs. thick (LWP≥20 gm−2) clouds under conditions of large scale ascent or decent at 850 hPa showed that AIE is greatest for thick clouds during periods of upward vertical motion. In the Bay of Bengal, AIE is negligible owing to less favorable atmospheric conditions, a lower concentration of aerosols, and a non-alignment of aerosol and cloud layers. In the eastern North Atlantic, AIE is weakly positive (+0.1 Wm−2) with dust aerosol concentration being much greater than the anthropogenic or sea salt components. However, elevated dust in this region exists above the maritime cloud layers and does not have a hygroscopic coating, which occurs in AS, preventing the dust from acting as CCN and limiting AIE. The Western Atlantic has a large anthropogenic aerosol concentration transported from the eastern United States producing a modest anthropogenic AIE (−0.46 Wm−2). Anthropogenic AIE is also present off the West African coast corresponding to aerosols produced from seasonal biomass burning (both natural and man-made). Interestingly, atmospheric conditions are not particularly favorable for cloud formation compared to the other regions during the times where AIE is observed; however, clouds are generally thin (LWP<20 gm−2) and concentrated very near the surface. Overall, we conclude that vertical motion, aerosol type, and aerosol layer heights do make a significant contribution to AIE and that these factors are often more important than total aerosol concentration alone and that the relative importance of each differs significantly from region to region.

Clean air policy and Arctic warming

The enhanced Arctic warming over the past three decades is attracting much attention. Combining forward and inverse models with observations suggests that regional changes in aerosol concentrations have contributed significantly.

A new empirical relation to estimate atmospheric electrical columnar resistance, effect of changing aerosol concentration and cosmic ray ionization on it: A quantitative study

A new empirical relation is found among atmospheric electrical columnar resistance, aerosol concentration, and eddy diffusion coefficient. The methodology is reported for the first time. This empirical relation can be used regularly for estimating columnar resistance. As solar modulation of cosmic rays and consequent changes in columnar resistance of global atmospheric electric circuit convey solar influence on lower atmospheric meteorological phenomena, a quantitative comparison of percent change in columnar resistance due to change in cosmic ray ionization and that due to change in aerosol concentration is made, which shows that 20% change in cosmic ray ionization causes more (about two times) change in columnar resistance over ocean but less (about 1/3) change over land as compared to that caused by 20% change in aerosol concentration. This confirms that over ocean, ionization due to cosmic rays and over land, atmospheric aerosol concentration mainly determine columnar resistance. This land‐ocean contrast gets reduced with atmospheric turbulence. It is found that, for small heights of the boundary layer (low eddy diffusivity), columnar resistance changes more with respect to change in aerosol concentration, and change in eddy diffusivity has negligible effect on it, but for large values of heights (large eddy diffusivity), there is substantial increase in columnar resistance. Furthermore, exceeding a certain critical value of conductivity, columnar resistance becomes independent of conductivity. The estimated values of columnar resistance using this method are very well consistent with the previous studies. Also, calculated percent change in global atmospheric electrical parameters agrees well with the previous results.

Retrieval of global atmospheric electrical activity at a polluted urban site

The atmospheric electrical Potential Gradient (PG) arises from global thunderstorm activity, but surface measurements of the atmospheric Potential Gradient (PG) are influenced by global thunderstorms and local aerosol concentration changes. The local aerosol change can be monitored independently, and in some cases the concentration changes are closely related to PG changes. For these circumstances, a general theory to remove the local aerosol influence on PG measurements has been developed. Continuous measurements of PG and aerosol mass concentration were made during 24–31 Dec, 2005 within an urban environment at Reading, UK. The average diurnal variation of PG showed a double diurnal cycle, with maxima in the early morning and evening hours. The aerosol concentration has similar double maxima. Removing the aerosol using from the PG and aerosol correlation returns a single diurnal cycle, suggestive of the more global PG diurnal cycle.

Seasonal variation in satellite‐derived effects of aerosols on clouds in the Arabian Sea

Aerosols act as cloud condensation nuclei for cloud water droplets, with changes in aerosol concentrations having significant impacts on the corresponding cloud properties. An increase in aerosol concentration may lead to an increase in CCN, with an associated decrease in cloud droplet size for a given cloud liquid water content. Smaller droplet sizes may then lead to a reduction in precipitation efficiency and an increase in cloud lifetimes. However, these effects are highly dependent on the aerosol concentration, aerosol species, and the meteorological conditions. In the Arabian Sea (10–20°N, 62–72°E), prevailing aerosol type transitions from mostly small‐mode anthropogenic aerosols during the winter months to mostly coarse‐mode mineral dust and sea salt during the summer due to a change in prevailing wind speed and direction, is likely to impart substantial variability on any associated indirect effects. To examine this variability, we use one year (2004) of MODIS derived aerosol optical thickness (AOT) and cloud products over the Arabian Sea to quantify aerosol indirect effects. Results show that indirect effects in the Arabian Sea are a strong function of season, which is a result of the changing aerosol and moisture (humidity) concentrations during the course of the year. During the winter months (DJF), cloud‐droplet size and AOT were found to have a weak positive correlation (r = 0.12), opposite of the expected effect. The low atmospheric humidity coupled with wide‐spread subsidence and other dynamical factors may prevent these aerosols from being activated. During the summer months (JJA), AOT increases with the addition of mineral dust and sea salt aerosols and the correlation between AOT and cloud droplet size becomes negative (r = −0.22). The magnitude of the first indirect effect corresponds to an increase in low level wind speeds, increasing the concentration of hygroscopic sea salt into the atmosphere. For both periods, a positive correlation (r = 0.16, 0.32) was found between AOT and LWP indicating a reduction in precipitation efficiency.

Examples of scale interactions in local, urban, and regional air quality modelling

Air quality modeling can help to improve understanding of scale interactions related to meteorology, transport, emissions, formation, removal, and other processes taking place at local, urban, and regional scales. For the local scale, we used the coupling of a street canyon model with a Gaussian dispersion model to study the interactions of emissions and concentrations in urban streets and surrounding urban neighborhoods. The model combination was applied to a city quarter in Ghent, Belgium, and showed that up to 40% of the PM 2.5 concentrations inside street canyons were caused by emissions from the surrounding streets. For the urban scale, the AURORA model has been used successfully in assessments of urban air quality for entire cities or urbanized areas. It has been applied to the Ruhr area in Germany to evaluate the impact of compact or polycentric cities versus the impact of urban sprawl developments. Results for ozone and PM 10 showed that compact city structures may have more adverse effects in terms of air pollution exposure. For the regional scale, the EUROS model was used to study the urban and regional-scale interactions that are important in simulating concentrations of ozone, PM 2.5, and PM 10. It has been applied to study seasonal changes in aerosol concentrations in Flanders. High secondary aerosol concentrations were found during summer. This contribution was related to large contributions from outside the region, showing the importance of the continental scale when studying regional air quality problems.

Capturing vertical profiles of aerosols and black carbon over the Indian Ocean using autonomous unmanned aerial vehicles

Abstract. Measurements of the vertical distribution of aerosol properties provide essential information for generating more accurate model estimates of radiative forcing and atmospheric heating rates compared with employing remotely sensed column averaged properties. A month long campaign over the Indian Ocean during March 2006 investigated the interaction of aerosol, clouds, and radiative effects. Routine vertical profiles of aerosol and water vapor were determined using autonomous unmanned aerial vehicles equipped with miniaturized instruments. Comparisons of these airborne instruments with established ground-based instruments and in aircraft-to-aircraft comparisons demonstrated an agreement within 10%. Aerosol absorption optical depths measured directly using the unmanned aircraft differed from columnar AERONET sun-photometer results by only 20%. Measurements of total particle concentration, particle size distributions, aerosol absorption and black carbon concentrations are presented along with the trade wind thermodynamic structure from the surface to 3000 m above sea level. Early March revealed a well-mixed layer up to the cloud base at 500 m above mean sea level (m a.s.l.), followed by a decrease of aerosol concentrations with altitude. The second half of March saw the arrival of a high altitude plume existing above the mixed layer that originated from a continental source and increased aerosol concentrations by more than tenfold, yet the surface air mass showed little change in aerosol concentrations and was still predominantly influenced by marine sources. Black carbon concentrations at 1500 m above sea level increased from 70 ng/m3 to more than 800 ng/m3 with the arrival of this polluted plume. The absorption aerosol optical depth increased from as low as 0.005 to as much as 0.035 over the same period. The spectral dependence of the aerosol absorption revealed an absorption Angstrom exponent of 1.0, which is typical of an aerosol with most of its absorption attributed to black carbon and generally indicates the absorbing component originated from fossil fuel sources and other high-temperature combustion sources. The results indicate that surface measurements do not represent the aerosol properties within the elevated layers, especially if these layers are influenced by long range transport.

On the climate forcing consequences of the albedo continuum between cloudy and clear air

It has been long understood that the partly cloudy atmosphere manifests a continuum of states between the end members ‘clear’ and ‘cloud.’ Nevertheless, many research methods are premised on a dichotomy of states—for example, those that use ‘cloud cover’ or ‘cloud-clearing.’ Here we consider the consequences of this practice for studies of aerosolclimate effects. Aerosols affect the Earth’s energy budget primarily by affecting albedo; therefore, we explore the nature of albedo variability in the partly cloudy marine boundary layer on scales down to a few tens of metres. We employ two diagnostic tools: a cloud resolving model and an albedo proxy derived from high altitude lidars. We show that a continuum of albedo values results from indeterminate and variable combinations of hydrated aerosol and wispy (including subvisible) clouds. Two consequences arise. First, cloud-clearing schemes employed by different observational methods are mutually inconsistent and are sensitive to concentrations of unactivated aerosol particles. Second, aerosol radiative forcing (the sensitivity of overall albedo to changes in aerosol concentration) is inaccurately calculated as the average of clear and overcast conditions. Together, these results imply that dividing the aerosol forcing problem into ‘direct’ and ‘indirect’ components may lead to substantial errors.

Aerosol modulation of small ion growth in coastal air

Molecular cluster ions are produced throughout the troposphere by radon isotopes and cosmic rays, causing the slight electrical conductivity ( σ ) of atmospheric air. In a field campaign at the Weybourne Atmospheric Observatory on the Norfolk coast of the UK (52°57′23′′N 1°7′40′′E), aerosol size spectra were obtained simultaneously with σ measurements and background radioactivity, from 11th to 17th May 2004. σ varied with wind direction, from the modulation of the total aerosol number concentration Z rather than changes in ion production from radioactivity. σ was at its greatest when the air originated over the sea. Cluster ion properties were deduced using the σ data and the ion balance equation. The ion mobility μ decreased as Z decreased, corresponding to an increase in average cluster ion size. Using the attachment timescale of ions to aerosol as the typical time during which the ions can grow, negative ion growth occurred at ∼3 nm hr −1 . Aerosol concentration changes modulate the time in which the ions can grow and the availability of condensable vapour.

Comment to “Twentieth century secular decrease in the atmospheric potential gradient” by Giles Harrison

Harrison [2002] has attributed the decline in surface potential gradient in the United Kingdom (UK) over the 20th century to the decline in cosmic radiation and the global electrical circuit. This paper addresses the supporting evidence for Harrison's explanation, and considers an alternative hypothesis for the observations based on local changes in aerosol concentration. This hypothesis is supported by the concurrent historical record of black carbon emission in the UK.

Verification of different techniques for the measurement of marine aerosols in coastal areas

processes as well as in the production and dynamics of marine aerosols. Marine aerosol concentrations are significantly higher in such areas. Aerosols, in the zone of direct interaction between the air and the ocean surface are characterized by rapid time-space changes of concentrations. The continual changes in aerosol concentrations are due both to external changes, such as variations in particle input, and internal changes like coalescence, coagulation and evaporation. These processes result in the variations of aerosol sizes, which also influences particle removal from the marine boundary layer. This means that changes in aerosol concentration in the marine boundary layer are connected to with the changes in the spectrum of marine aerosol size distribution functions. This spectrum is complex and depends strongly on weather conditions in the marine boundary layer. In order to determine aerosol size distribution and concentration six cascade impactors, a particle measurement system and a lidar were employed. The impactor is an inertial separator of particles whose separation effectiveness depends on the nozzle diameter and the Reynolds number. It was confirmed that the effectiveness of catching particles from the size range r E [ 1, 10 pm] and larger is loaded with an error of not greater than 24.5 %. A particle measurement system CSASP-100~HV(Sp) is a water droplet spectrometer which operates on the principle that the light scattered by a particle within a high intensity laser beam is directly a function of its size. Particles produce pulses of radiant energy during transit through the laser beam. These light pulses are sensed by detectors and are read by two peak readers. System CSASP- 1 00-HV provides means of resolving and sizing particles in an in-situ mode. The CSASP- 1 00-HV is capable of sizing particles into fifteen size classes (size ranges: 247 urn; 2-32 pm; 1-16 pm; 0.5-8 pm). In the lidar method, laser pulses are transmitted into the atmosphere and backscattered radiation is collected by an optical telescope and detected. Mie scattering from particles and Raleigh scattering from molecules provide strong signals observed in backscattering in lidar sounding of the free atmosphere. These scattering processes are elastic, i.e.. the scattered photons have the same frequency as the incoming light. Lidar method allows for remote measurements of vast areas which range is closely related to the platform arrangement for the

Large-Eddy Simulations of Strongly Precipitating, Shallow, Stratocumulus-Topped Boundary Layers

Large-eddy simulations that incorporate a size-resolving representation of cloud water are used to study the effect of heavy drizzle on PBL structure. Simulated surface precipitation rates average about 1 mm day 21. Heavily drizzling simulations are compared to nondrizzling simulations under two nocturnal PBL regimes—one primarily driven by buoyancy and the other driven equally by buoyancy and shear. Drizzle implies a net latent heating in the cloud that leads to sharp reductions in both entrainment and the production of turbulent kinetic energy by buoyancy (particularly in downdrafts). Drizzle, which evaporates below cloud base, promotes a cooler and moister subcloud layer that further inhibits deep mixing. The cooling and moistening is in quantitative agreement with some observations and is shown to favor the formation of cumuli rising out of the subcloud layer. The cumuli, which are local in space and time, are responsible for most of the heat and moisture transport. They also appear to generate a larger-scale circulation that differs dramatically from the regularity typically found in nonprecipitating stratocumulus. Time-averaged turbulent fluxes of heat and moisture increase in the presence of precipitation, suggesting that drizzle (and drizzle-induced stratification) should not necessarily be taken as a sign of decoupling. Because drizzle primarily affects the vertical distribution of buoyancy, shear production of turbulent kinetic energy mitigates some of the effects described above. Based on large-eddy simulation the authors hypothesize that shallow, well-mixed, radiatively driven stratocumulus cannot persist in the presence of heavy drizzle. In accord with some simpler models, the simulated case with heavy precipitation promotes a reduction in both liquid-water path and entrainment. However, the simulations suggest that timeintegrated cloud fraction may increase as a result of drizzle because thinner precipitating clouds may persist longer if the boundary layer does not deepen as rapidly. These somewhat more complicated dynamics have important implications for a number of hypotheses suggesting that changes in aerosol concentrations, when metabolized by stratocumulus, have a significant effect on climate.

On Modification of Global Warming by Sulfate Aerosols

There is increasing evidence that the response of climate to increasing greenhouse gases may be modified by accompanying increases in sulfate aerosols. In this study, the patterns of response in the surface climatology of a coupled ocean–atmosphere general circulation model forced by increases in carbon dioxide alone is compared with those obtained by increasing carbon dioxide and aerosol forcing. The simulations are run from early industrial times using the estimated historical forcing and continued to the end of the twenty-first century assuming a nonintervention emissions scenario for greenhouse gases and aerosols. The comparison is made for the period 2030–2050 when the aerosol forcing is a maximum. In winter, the cooling due to aerosols merely tends to reduce the response to carbon dioxide, whereas in summer, it weakens the monsoon circulations and reverses some of the changes in the hydrological cycle on increasing carbon dioxide. This response is in some respects similar to that found in simulations with changed orbital parameters, as between today and the middle Holocene. The hydrological response in the palaeosimulations is supported by palaeoclimatic reconstructions. The results of changes in aerosol concentrations of the magnitude projected in the scenarios would have a major effect on regional climate, especially over Europe and Southeast Asia.

Planetary boundary layer structure and air mass transport during the International Arctic Ocean Expedition 1991

During the International Arctic Ocean Expedition 1991 north of latitudes 70° N from August 1 to October 6 (late summer and early autumn), the dominant air mass origin was from the seas that surround the central Arctic Ocean, while land was a much less important source. This resulted in air temperatures near 0 °C, fogs and extensive stratus layers. In the autumn, long residence time of the air over the pack ice and subsidence became more common, accompanied by decreasing cloudiness, foginess and air temperature. The residence time of air over the pack ice and the time since land contact had medians of 52 and 91 h, respectively. The structure and evolution of the marine boundary layer above the pack ice is described. A stable layer, 300–1700 m high, grew in height proportionally to the square root of the distance from the ice edge, the fetch, up to 3000 km. After 400 km, a mixed layer formed, typically 100–250 m thick, related to the presence of low level jets in 60–90% of the profiles. Layers of subcritical Richardson number were present in 65–80% of the profiles above and below the jet centres. Cloud streets and roll vortices were observed above the pack ice with wavelengths of 4–12 km. The high aspect ratios of 7–9 of the few earlier roll observations above ice were confirmed. Cold air advection and weak convection caused the roll vortices in aged air that had been advected usually 2000 km or more over the ice. Breaking of Kelvin-Helmholtz waves, gravity waves or other reasons for intermittent planetary boundary layer breakdown combined with vertical gradients in concentration are suggested to have caused the observed sudden changes, in an hour or less, in aerosol and tracer concentrations and to have influenced fog formation. Roll vortices are suggested as the cause of periodic changes in aerosol concentrations and fog formation. DOI: 10.1034/j.1600-0889.1996.t01-1-00004.x