Radiative Forcing Of Sulfate Aerosols Research Articles

Global and regional trends of atmospheric sulfur

The profound changes in global SO2 emissions over the last decades have affected atmospheric composition on a regional and global scale with large impact on air quality, atmospheric deposition and the radiative forcing of sulfate aerosols. Reproduction of historical atmospheric pollution levels based on global aerosol models and emission changes is crucial to prove that such models are able to predict future scenarios. Here, we analyze consistency of trends in observations of sulfur components in air and precipitation from major regional networks and estimates from six different global aerosol models from 1990 until 2015. There are large interregional differences in the sulfur trends consistently captured by the models and observations, especially for North America and Europe. Europe had the largest reductions in sulfur emissions in the first part of the period while the highest reduction came later in North America and East Asia. The uncertainties in both the emissions and the representativity of the observations are larger in Asia. However, emissions from East Asia clearly increased from 2000 to 2005 followed by a decrease, while in India a steady increase over the whole period has been observed and modelled. The agreement between a bottom-up approach, which uses emissions and process-based chemical transport models, with independent observations gives an improved confidence in the understanding of the atmospheric sulfur budget.

Quantifying uncertainty from aerosol and atmospheric parameters and their impact on climate sensitivity

Abstract. Climate sensitivity in Earth system models (ESMs) is an emergent property that is affected by structural (missing or inaccurate model physics) and parametric (variations in model parameters) uncertainty. This work provides the first quantitative assessment of the role of compensation between uncertainties in aerosol forcing and atmospheric parameters, and their impact on the climate sensitivity of the Community Atmosphere Model, Version 4 (CAM4). Running the model with prescribed ocean and ice conditions, we perturb four parameters related to sulfate and black carbon aerosol radiative forcing and distribution, as well as five atmospheric parameters related to clouds, convection, and radiative flux. In this experimental setup where aerosols do not affect the properties of clouds, the atmospheric parameters explain the majority of variance in climate sensitivity, with two parameters being the most important: one controlling low cloud amount, and one controlling the timescale for deep convection. Although the aerosol parameters strongly affect aerosol optical depth, their impacts on climate sensitivity are substantially weaker than the impacts of the atmospheric parameters, but this result may depend on whether aerosol–cloud interactions are simulated. Based on comparisons to inter-model spread of other ESMs, we conclude that structural uncertainties in this configuration of CAM4 likely contribute 3 times more to uncertainty in climate sensitivity than parametric uncertainties. We provide several parameter sets that could provide plausible (measured by a skill score) configurations of CAM4, but with different sulfate aerosol radiative forcing, black carbon radiative forcing, and climate sensitivity.

Differences in Sulfate Aerosol Radiative Forcing between the Daytime and Nighttime over East Asia Using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) Model

The effect of aerosols is an important indicator of climate change. Sulfate aerosols, as the major scattering aerosols, which have attracted more and more attention in recent years. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) were utilized to investigate the spatial distribution of sulfate aerosols and their radiative forcing characteristics over East Asia in 2010. Results showed that sulfate aerosols were mainly distributed over eastern China (24–43° N, 101–126° E), especially in the Sichuan Basin. The concentration of sulfate aerosols decreased with increasing altitude over East Asia. It also exhibited obvious seasonal variations, where the largest range of sulfate aerosol concentrations was found in summer, with a maximum of 2.4 μg kg−1 over eastern China. Although sulfate aerosol concentrations varied slightly during day and night, there was still a significantly difference in the sulfate aerosol radiative forcing. Specifically, the magnitude of the direct radiative forcing induced by sulfate aerosols at the surface was approximately −3.02 W m−2 in the daytime, while that was +0.24 W m−2 in the nighttime. This asymmetric change that was caused by the radiative forcing of sulfate aerosols between day and night would have significant impacts on climate change at the regional scale.

Does Humidity’s Seasonal Cycle Affect the Annual-Mean Tropical Precipitation Response to Sulfate Aerosol Forcing?

Abstract Sulfate aerosol radiative forcing alters the distribution of tropical precipitation in climate model simulations. The annual-mean tropical precipitation change is typically described as arising from an annual-mean change in the mean atmospheric circulation. However, the seasonality of the climatology of tropical humidity may modulate the annual-mean precipitation response. Here, the role of seasonality of tropical humidity is assessed using reanalysis and idealized atmospheric general circulation model (GCM) simulations perturbed by sulfate aerosol radiative forcing. When coupled to an aquaplanet slab ocean with low thermal inertia, the seasonal cycle in GCM simulations of the “continental” regime is large and the annual-mean precipitation change depends on both the seasonally varying perturbation mean meridional circulation and the seasonally varying climatological specific humidity. When coupled to an aquaplanet slab ocean with a higher thermal inertia, the seasonal cycle in GCM simulations of the “oceanic” regime is smaller and the annual-mean precipitation change can be approximated by considering the perturbation convergence of the water vapor flux of the annual-mean perturbation mean meridional circulation and the annual-mean climatological specific humidity. The results of the aquaplanet simulations taken together with the magnitude of the seasonality of humidity in an atmospheric reanalysis suggest that the simplest forms of energetic arguments for the tropical precipitation response to perturbations in the atmospheric energy budget, which neglect an explicit role for the seasonality of the radiative forcing and the climatological specific humidity, are not quantitatively accurate for Earth’s tropical land regions.

Parametric uncertainty effects on aerosol radiative forcing

[1] Among the known radiative forcings, the fourth IPCC assessment report estimates the aerosol radiative forcing to harbor the widest range of uncertainty extending from −1.8 to −0.3 W/m2. The IPCC estimates focus mainly on structural uncertainties, including uncertainties in aerosol sources. Here, we study the uncertainty of the sulfate aerosol radiative forcing due to parametric uncertainty in a state-of-the-art general circulation model (GCM). Numerical experiments were carried out by perturbing seven cloud parameters in the model. We find that the uncertainty due to a single one of these parameters can be as large as 0.5 W/m2, and the uncertainty due to combinations of these parameters can reach more than 1 W/m2. These numbers should be compared with the sulfate aerosol forcing of −1.9 W/m2 for the year 2000, obtained using the default values of the parameters. The uncertainty results from a high sensitivity of cloud optical properties to aerosol concentrations, which can be amplified by changing cloud parameter setting.

A GCM study of effects of radiative forcing of sulfate aerosol on large scale circulation and rainfall in East Asia during boreal spring

The effect of sulfate aerosol radiative forcing on spring rainfall in East Asia are studied based on numerical simulations with the NASA finite‐volume General Circulation Model (fvGCM) forced with monthly varying three‐dimensional aerosol distribution from the Goddard Ozone Chemistry Aerosol Radiation and Transport model (GOCART). Result shows that radiative forcing of sulfate aerosol leads to cooling of the land surface and reduction in rainfall over central East Asia. The maximum reduction in precipitation is shifted northward relative to the maximum aerosol loading region as a result of dynamical feedback. The anomalous thermal gradient by aerosol cooling near the land surface, reduces the baroclinicity of the atmosphere, leading to a deceleration of the upper level westerly flow. The westerly deceleration induces, through ageostrophic wind adjustment, anomalous meridional secondary circulation at the entrance region of the East Asian jetstream, with strong sinking motion and suppressed precipitation near 30°N, coupled to weak rising motion and moderately enhanced precipitation over southern China and the South China Sea. These results suggest that the radiative forcing of aerosol through induced dynamical feedback with the atmospheric water cycle, may be a causal factor in the observed spring precipitation trend over East Asia.

Direct shortwave radiative forcing of sulfate aerosol over Europe from 1900 to 2000

On the basis of historical simulations of the atmospheric distribution of sulfate aerosol over Europe, we have estimated the evolution of the direct shortwave radiative forcing due to sulfate aerosol from 1900 to the present day. Following the trend of atmospheric sulfate burden, the radiative forcing reaches its peak in the 1980s. Since then, environmental policies regulating SOx emissions successfully reduced the atmospheric load. On average, the forcing of the year 2000, representing present day, equals that of the 1950s. Spatially, the forcing maxima experienced a shift from the northwest to the southeast during the century. The ship emissions of sulfur keep increasing since the 1980s, hence their relative contribution to the sulfate load and radiative forcing constantly increased, from 3% in the 1980s to over 10% in the year 2000. Forcing is strongest during summertime, with a seasonal mean of −2.7 W m−2 in the 1980s and −1.2 W m−2 in summer 2000. The mean forcing efficiency is slightly reduced from −246 W (g sulfate)−1 in the 1900s to −230 W (g sulfate)−1 in the year 2000, and it declines with changed geographical distribution of sulfur emissions.

Sulfate aerosols forcing: An estimate using a three-dimensional interactive chemistry scheme

The tropospheric sulfate radiative forcing has been calculated using an interactive chemistry scheme in LMD-GCM. To estimate the radiative forcing of sulfate aerosol on climate, a consistent interaction between atmospheric circulation and radiation computation has been allowed in LMD-GCM. The model results indicate that the change in the sulfate aerosols number concentration is negatively correlated to the indirect radiative forcing. The model simulated annual mean direct radiative forcing ranges from −0.1 to −1.2 W m −2, and indirect forcing ranges from −0.4 to −1.6 W m −2. The global annual mean direct effect estimated by the model is −0.48 W m −2, and that of indirect is −0.68 W m −2.

Organic reactions increasing the absorption index of atmospheric sulfuric acid aerosols

Unlike most environments present at Earth's surface atmospheric aerosols can be favorable to organic reactions. Among them, the acid‐catalyzed aldol condensation of aldehydes and ketones produces light‐absorbing compounds. In this work the increase of the absorption index of sulfuric acid solutions 50–96 wt. % resulting from the uptake of gas‐phase acetaldehyde, acetone, and 2‐butanone (methyl ethyl ketone), has been measured in the near UV and visible range. Our results indicate that the absorption index between 200 and 500 nm for stratospheric sulfuric aerosols exposed to 100 pptV of acetaldehyde (1 pptV = 10−12 v/v) would increase by four orders of magnitude over a two‐year lifetime. Rough estimates based on previous radiative calculations suggest that this reaction could result in an increase of the radiative forcing of sulfate aerosols of the order of 0.01 W m−2, and that these processes are worth further investigation.

Seasonal variations of anthropogenic sulfate aerosol and direct radiative forcing over China

Regional climate model (RegCM2) and sulfur transport model (NJUADMS) were combined to simulate the distribution of anthropogenic sulfate aerosol burden over China, where a “look up table” method was applied to illustrate sulfate formation from SO2-oxidation. Direct radiative forcing of sulfate aerosol was further estimated using the scheme suggested by Charlson et al (1991). Investigations show that the annual average total sulfate column over mainland China is 2.01 mg/m2 with high value in East and Central areas (more than 7 mg/m2). The annual average direct radiative forcing of China is about −0.85 W/m2. The forcing can reach −7 W/m2 in Central and East China during the winter season. Total sulfate column shows significant seasonal variations with winter maximum-summer minimum in the Southern part of China and spring maximum-autumn minimum in the northern part of China. Strong seasonal cycles of direct radiative forcing are also found due to the influence of total sulfate column, cloud, relative humidity and the reflectivity of underlying surface

Why is the climate forcing of sulfate aerosols so uncertain?

Sulfate aerosol particles have strong scattering effect on the solar radiation transfer which results in increasing the planet albedo and, hence, tend to cool the earth-atmosphere system. Also, aerosols can act as the cloud condensation nuclei (CCN) which tend to increase the albedo of clouds and cool the global warming. The ARPEGE-Climat version 3 AGCM with FMR radiation scheme is used to estimate the direct and indirect radiative forcing of sulfate aerosols. For minimizing the uncertainties in assessing this kind of cooling effect, all kinds of factors are analyzed which have been mixed in the assessment process and may lead to the different results of the radiative forcing of aerosols. It is noticed that one of the uncertainties to assess the climate forcing of aerosols by GCM results from the different definition of radiative forcing that was used. In order to clarify this vague idea, the off-line case for considering no feedbacks and on-line case for including all the feedbacks have been used for assessment. The direct forcing of sulfate aerosols in off-line case is −0.57 W/ m2 and −0.38 W/ m2 for the clear sky and all sky respectively. The value of on-line case appears to be a little larger than that in off-line case chiefly due to the feedback of clouds. The indirect forcing of sulfate aerosols in off-line case is −1.4 W/ m2 and −1.0 W/ m2 in on-line case. The radiative forcing of sulfate aerosols has obvious regional characteristics. There is a larger negative radiative forcing over North America, Europe and East Asia. If the direct and indirect forcing are added together, it is enough to offset the positive radiative forcing induced by the greenhouse gases in these regions.

Abrupt change of the mid-summer climate in central east China by the influence of atmospheric pollution

Following the great flooding of summer 1998, the mid-lower Yangtze Basin further suffered from another large flooding in summer 1999. Successive droughts through 3 recent summers (1997–1999) appeared in north China in addition, leading to an abnormal summer climate pattern of “north drought with south flooding”. Such southward move of the summer monsoon rainy belt in east China started in the late 1970s–early 1980s. Its main cause may not be a purely natural climate change, but the acceleration of industrialization in east China could play a major role by emitting large volumes of SO 2, especially from the rapidly growing rural factories of east China. The annual release of SO 2 in China exceeded 20 Tg during 1992–1998, so dense sulfate aerosols covered the central east China which significantly reduced the sunlight. Although present estimates for the changes of clear sky global solar radiation may include some error, they show that the negative radiative forcing of sulfate aerosols in central east China by far exceeds the effect of greenhouse warming in summer. Hence the mid-summer monsoon rainy belt of east China has a trend moving southward in 21 recent years (1979–1999), showing the very sensitive characteristic of the summer monsoon system to the change in heat equilibrium of the land surface. The occurrence rate of summer climate pattern of “north drought with south flooding” in east China during 21 recent years is the largest since AD 950; such anomalous climate has brought large losses to China. The only possible way to reverse this southward trend of summer monsoon rainy belt is to significantly reduce air pollution by using more clean energy. Recently, the PRC has paid serious attention to this problem by adopting a series of countermeasures.

Radiative forcing of climate by sulfate aerosols as determined by a regional circulation chemistry transport model

H 2SO 4/SO 2- 4 aerosols have the potential to modify the radiation budget of the atmosphere. Under clear-sky condition they scatter solar radiation back to space, reducing absorption of solar irradiance (direct effect). The capability of sulfate particles to act as cloud condensation nuclei, thus influencing cloud droplet number concentration, cloud albedo and the development of precipitation is referred to as indirect effect. Evidence has been presented that sulfate aerosol climate forcing is sufficiently large to reduce significantly the positive forcing by anthropogenic greenhouse gases regionally, especially in the Northern Hemisphere. Until now, only coarse grid global models with rather simple chemistry modules have been applied to estimate the radiative forcing of sulfate aerosols. In this paper we would like to ascertain the short wave sulfate forcing over Europe, one of the main anthropogenic source regions. For this purpose the three-dimensional European sulfate distribution was generated by a regional climate model in combination with a complex chemistry transport model. Then a computationally efficient radiation transfer model was applied. It estimates the direct and indirect short wave forcing of sulfate aerosols on the basis of the variable sulfate mass distribution and meteorological input data. For comparison coarse grid global model results will be shown. Regional and global model results are comparable concerning the distribution and amount of sulfate burden and radiative forcing over Europe. Hence, for the estimation of the European budgets, the global model produces sufficiently precise information. As expected, the advantage of the higher resolution regional model is to show smaller scale phenomena, which cannot be resolved by the global model. Different predictions of the amount of clouds by the regional and global model modify the forcing significantly, emphazising the role of clouds in estimating the direct (and, of course, the indirect) short wave forcing of sulfate aerosols. It is further interesting to note that in the global model results about 30% of the sulfate burden and its short wave radiative forcing over Europe, one of the main anthropogenic source regions, is caused by sulfate from natural sources (DMS and volcanoes). Thus, the long-range transport of primary and secondary pollutants from outside the regional model domain contributes significantly to the limited area model’s atmospheric load. Therefore, the initial and boundary chemical composition of the atmosphere for the limited area model should be investigated in more detail in the future.

The contribution of Earth degassing to the atmospheric sulfur budget

Volcanic gases have contributed to the development of the Earth's atmosphere to a large extent. Today, the sulfur budget of the atmosphere is still much influenced by volcanic degassing. Even though the total flux of sulfur gases from quiescent and small eruptive volcanic sources is small compared with anthropogenic release, the geographical distribution and the height of the main sources lead to comparable effects in total atmospheric sulfur budget and also for the important radiative effects. In the upper troposphere the volcanic sources even determine the sulfur content of the atmosphere. When estimated volcanic sulfur emissions are processed and transported in an atmosphere chemistry–climate model, where the contribution of different sources is trace marked, one can estimate the relative contribution of the emittents to total sulfate aerosol radiative forcing. In the global mean the volcanic (−0.15 W/m 2) is comparable with the anthropogenic (−0.17 W/m 2) direct backscatter effect of sulfate aerosol. Only in the Northern hemisphere winter, when anthropogenic emissions have their maximum, do they have a stronger (−0.25 W/m 2) effect than volcanic contributions (−0.15 W/m 2).

Reply [to “Comments on ‘A limited‐area‐model case study of the effects of sub‐grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol’”

Reply [to “Comments on ‘A limited‐area‐model case study of the effects of sub‐grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol’”

Comments on “A limited‐area‐model case study of the effects of sub‐grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol”

Comments on “A limited‐area‐model case study of the effects of sub‐grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol”

Radiative forcing of sulfate aerosols as determined by a regional circulation-chemistry transport model

Radiative forcing of sulfate aerosols as determined by a regional circulation-chemistry transport model

Volcanic sulfur emissions: Estimates of source strength and its contribution to the global sulfate distribution

Anthropogenic emission of SO2 and conversion into SO42− is argued to be the most important factor damping and modulating the global greenhouse effect. Recent estimates of the relative strength of the three important sources of volatile sulfur (SO2 from fossil fuel combustion ∼78 Tg S/yr, from biomass burning ∼2 Tg S/yr, and from natural sources ∼25 Tg S/yr) suggest an over‐whelming effect of the anthropogenic emissions for climate forcing. However, the radiauvely relevant product SO42− may have different patterns due to the distribution of the sources (some very dense areas near the surface for anthropogenic SO2, formation of SO2 from dimethylsulfide in the marine boundary layer, and emission of volcanic SO2 mostly in the free atmosphere in rural areas). In this paper we study the relative contribution of volcanic SO2 emissions to the atmospheric sulfur budget applying an atmospheric general circulation model including a full sulfur cycle and prescribed source distributions. An off‐line analysis tool is applied to determine the radiative forcing of sulfate aerosols. The results show that natural S sources are at least as important as the anthropogenic ones, even though their source strength is much smaller. The reasons are different lifetimes due to different production and emission processes. Therefore, we should improve our knowledge about the volcanic volatile sources and their time‐space variability.

A limited‐area‐model case study of the effects of sub‐grid scale Variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol

A limited‐area non‐hydrostatic model with a horizontal spatial resolution of 2km by 2km is used to assess the importance of sub‐grid scale variations in relative humidity and cloud upon the direct radiative forcing (DRF) by tropospheric sulfate aerosols. The DRF from the limited‐area model for both clear and cloudy regions is analyzed and the results compared against those obtained using general circulation model (GCM) parameterizations that perform the computations over coarse horizontal grids. In this idealized model study, the GCM calculations underestimate the clear sky DRF by approximately 73% and the cloudy sky DRF by approximately 60%. These results indicate that, for areas where the relative humidity is high and where there is substantial spatial variability in relative humidity and cloud, GCM calculations may considerably underestimate the DRF.