Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: Comparison with observations

A comparison of large‐scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO₂, SO²⁻₄, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.     

Evolution of boundary‐layer aerosol particles due to in‐cloud chemical reactions during the 2nd Lagrangian experiment of ACE‐2

The second Aerosol Characterisation Experiment (ACE‐2) was aimed at investigating the physical, chemical and radiative properties of aerosol and their evolution in the North Atlantic region. In the 2nd "Lagrangian" experiment, an air mass was tracked over a 30‐h period during conditions of extensive stratocumulus cover. Boundary‐layer measurements of the aerosol size distribution obtained with a passive cavity aerosol spectrometer probe (PCASP) during the experiment show a gradual growth in size of particles in the 0.1–0.2 μm diameter mode. Simultaneously, SO₂ concentrations were found to decrease sharply from 800 to 20 ppt. The fraction of sulphate in aerosol ionic mass increased from 0.68±0.07 to 0.82±0.09 for small particles (diameter below 1.7 μm) and from 0.21±0.04 to 0.34±0.03 for large particles (diameter above 1.7 μm). The measurements were compared with a multicyclic parcel model of gas phase diffusion into cloud droplets and aqueous phase chemical reactions. The model was able to broadly reproduce the observed transformation in the aerosol spectra and the timescale for the transformation of SO₂ to sulphate aerosol. The modelled SO₂ concentration in the boundary layer fell to below half its initial value over a 6.5‐h time period due to a combination of the entrainment of cleaner tropospheric air and cloud chemical reactions. NH₃ and HCl gas were also found to play an important rôle in cloud processing in the model.     

Sulfur budget above the Eastern Mediterranean: relative contribution of anthropogenic and biogenic sources

To access the relative contribution of anthropogenic and biogenic sulfur sources to the sulfur budget in the Eastern Mediterranean, an area characterized by very high nss-     levels, measurements of both wet and dry deposition of sulfur were performed at a remote area on the island of Crete (Finokalia) during a 3-yr period (1996–1999). The estimation of dry deposition is based on both gaseous sulfur dioxide (SO₂) and particulate phase non-sea-salt sulfate (nss-     ) and methane sulfonate (MSA) measurements. During the dry period, deposition of SO₂ from long-range transport is the main component of anthropogenic sulfur deposition in the area. The results of the wet and dry deposition obtained at Finokalia have been compared with DMS emission from seawater obtained during two yearly surveys (1997–1998) in the Cretan Sea. Our results indicate that the contribution from biogenic sources to the sulfur budget in the Eastern Mediterranean, although negligible during winter, can account for up to 26% during summer.     

A pseudo-Lagrangian study of the sulfur budget in the remote Arctic marine boundary layer

The atmospheric sulfur cycle of the remote Arctic marine boundary layer is studied using trajectories and measurements of sulfur compounds from the International Arctic Ocean Expedition 1991, along with a pseudo-Lagrangian approach and an analytical model. The dimethyl sulfide [DMS(g)] turnover time was  h. Only  % of DMS(g) followed reaction paths to sulfur dioxide [SO₂(g)], sub-micrometre aerosol non-seasalt sulfate (nss-SO₄²⁻) or methane sulfonate (MSA). During the first 3 d of transport over the pack ice, fog deposition and drizzle resulted in short turnover times;  h for SO₂(g),  h for MSA and  h for nss-SO₄²⁻. Therefore, DMS(g) will, owing to its origin along or south of the ice edge and longer turnover time, survive the original sub-micrometre sulfur aerosol mass and gradually replace it with new biogenic sulfur aerosol mass. The advection of DMS(g) along with heat and moisture will influence the clouds and fogs over the Arctic pack ice through the formation of cloud condensation nuclei (CCN). If the pack ice cover were to decrease owing to a climate change, the total Arctic Ocean DMS production would change, and potentially there could be an ice–DMS–cloud–albedo climate feedback effect, but it would be accompanied by changes in the fog aerosol sink.     

Model analysis of the influence of gas diffusivity in soil on CO and H2 uptake

CO and H₂ uptake by soil was studied as a diffusion process. A diffusion model was used to determine how the surface fluxes (net deposition velocities) were controlled by in‐situ microbial uptake rates and soil gas diffusivity calculated from the 3‐phase system (solid, liquid, gas) in the soil. Analytical solutions of the diffusion model assuming vertical uniformity of soil properties showed that physical properties such as air‐filled porosity and soil gas diffusivity were more important in the uptake process than in the emission process. To incorporate the distribution of in‐situ microbial uptake, we used a 2‐layer model incorporating "a microbiologically inactive layer and an active layer" as suggested from experimental results. By numerical simulation using the 2‐layer model, we estimated the effect of several factors on deposition velocities. The variations in soil gas diffusivity due to physical properties, i.e., soil moisture and air‐filled porosity, as well as to the depth of the inactive layer and in‐situ microbial uptake, were found to be important in controlling deposition velocities. This result shows that the diffusion process in soil is critically important for CO and H₂ uptake by soil, at least in soils with higher in‐situ uptake rates and/or with large variation in soil moisture. Similar uptake rates and the difference in deposition velocity between CO and H₂ may be attributable to differences in CO and H₂ molecular diffusivity. The inactive layer is resistant to diffusion and creates uptake limits in CO and H₂ by soil. The coupling of high temperature and a thick inactive layer, common in arid soils, markedly lowers net CO deposition velocity. The temperature for maximum uptake of CO changes with depth of the inactive layer.     

Atmospheric concentrations and deposition of oxidised sulfur and nitrogen species at Petaling Jaya, Malaysia, 1993–1998

Wet‐only rainwater composition, acid‐precursor gas mixing ratios and aerosol loading were determined from weekly‐averaged samples at Petaling Jaya, Malaysia, over the five year period from March 1993 to March 1998. Annual deposition fluxes of acidic sulfur and nitrogen species estimated from these data show this site to be heavily impacted by acidic deposition, with total oxidised sulfur plus nitrogen deposition in the range 277–480 meq m⁻² yr⁻¹. Average contributions were 56% as sulfur species, 44% as nitrogen species, with wet deposition in this region of high rainfall accounting for 67% of total deposition. Thus total acid deposition fluxes were equivalent to levels that provided motivation for emissions reduction programs in both Europe and North America. The possibility of adverse environmental effects in Malaysia caused by acid deposition therefore merits serious consideration and assessment.     

欧拉型区域硫沉降模式研究

建立了一个三维硫沉降欧拉模式，模式中比较全面地考虑了硫沉降过程中的物理、化学机制。包括平流、扩散、干湿沉降和积云的垂直输送作用等物理过程，气相化学、液相化学和气溶胶表面的非均相化学等化学过程。其中非均相化学和积云的垂直输送参数化在国内外同类模式中尚不多见。模式结果与实测及其他模式结果的对比表明，该模式能够较好地模拟出SO2的水平和垂直分布及SO2-4在降水中的浓度。     

Boundary layer and aerosol evolution during the 3rd Lagrangian experiment of ACE‐2

Aircraft measurements are presented of the Lagrangian evolution of a marine boundary layer over a 30‐h period during the ACE‐2 field campaign. At the start of the observational period, a 500‐m deep polluted marine internal boundary layer (MIBL) was overlain by the remnants of a polluted continental boundary layer extending to around 2 km below a clean, dry free troposphere. The MIBL grew rapidly to a thickness of 900–1000 m in response to increasing sea surface temperatures. No significant aerosol spectral evolution was observed in the boundary layer. Low concentrations of SO₂ were observed in the MIBL suggesting that the air mass contained relatively aged aerosol. Aerosol spectra show a broad mode with a modal diameter of around 0.1μm. The polluted layer between the MIBL and the unpolluted free troposphere was only weakly and intermittently turbulent which prevented significant entrainment of clean air into the polluted layer from aloft. The polluted layer depth was thus controlled mainly by subsidence which as a result becomes shallower, decreasing from over 2000 m to around 1200 m during the observational period. The aerosol characteristics of the polluted layer were similar to those in the MIBL and so although the MIBL entrained considerable amounts of air from above the MIBL the aerosol characteristics underwent no significant change. This has important implications for the rate at which a polluted continental air mass is converted to a clean marine one. The dataset should prove useful in the validation of the modelling of continental pollution outbreaks.     

Sulfur (32S, 33S, 34S, 36S) and oxygen (16O,17O,18O) isotopic ratios of primary sulfate produced from combustion processes

The recent discovery of an anomalous enrichment in ¹⁷O isotope in atmospheric sulfate has opened a new way to investigate the oxidation pathways of sulfur in the atmosphere. From laboratory investigations, it has been suggested that the wet oxidation of sulfur in rain droplets was responsible for the excess ¹⁷O. In order to confirm this theory, sulfur and oxygen isotope ratios of different primary sulfates produced during fossil fuel combustion have been investigated and are reported. None of these samples exhibits any anomalous oxygen or sulfur isotopic content, as compared to urban sulfate aerosols. These results, in agreement with the laboratory investigations, reinforce the idea of an aqueous origin for the oxygen-17 anomaly found in tropospheric sulfates.     

Broadband extinction method to determine atmospheric aerosol optical properties

The equivalent wavelength ( λ _E), at which the aerosol optical depth (AOD) is equal to broadband AOD (BAOD), can change in a wide range from 0.619 μm to 1.575 μm in the usual aerosol conditions. By using the least squares technique and some empirical corrections, a parameterized relationship of λ _E with BAOD, Ångström wavelength exponent ( α ), solar zenith angle ( θ ₀) and H₂O amount is developed. Using this relationship, and based on the strong sensitivity of BAOD on θ ₀ when θ ₀>70°, the broadband extinction method to derive the spectral AOD and α is further proposed. As shown in comparative simulations to retrieve AOD by the present, Molineaux et al. and Gueymard methods, the present method has the best accuracy in most simulations using Junge, MODTRAN, log‐normal and Deirmendjian aerosol models. A key question of the pyrheliometer method to determine wavelength-dependent AODs is the effect of uncertainty in the aerosol size istribution. It is found that the AOD solution around λ _E is less sensitive to the uncertainty. The wavelength exponent α is derived using an assumption of the stable atmospheric turbidity. If the pyrheliometer data from θ ₀=85° to 70° are used and the change of the turbidity is ±10%, the error of solution α is usually within ±0.32. If the variation of the turbidity is random, the mean value of a lot of the measurements of α would be very reasonable.     

Feedback mechanisms and sensitivities of ocean carbon uptake under global warming

Global warming simulations are performed with a coupled climate model of reduced complexity to investigate global warming–marine carbon cycle feedbacks. The model is forced by emissions of CO₂ and other greenhouse agents from scenarios recently developed by the Intergovernmental Panel on Climate Change and by CO₂ stabilization profiles. The uptake of atmospheric CO₂ by the ocean is reduced between 7 to 10% by year 2100 compared to simulations without global warming. The reduction is of similar size in the Southern Ocean and in low‐latitude regions (32.5°S‐32.5°N) until 2100, whereas low‐latitude regions dominate on longer time scales. In the North Atlantic the CO₂ uptake is enhanced, unless the Atlantic thermohaline circulation completely collapses. At high latitudes, biologically mediated changes enhance ocean CO₂ uptake, whereas in low‐latitude regions the situation is reversed. Different implementations of the marine biosphere yield a range of 5 to 16% for the total reduction in oceanic CO₂ uptake until year 2100. Modeled oceanic O₂ inventories are significantly reduced in global warming simulations. This suggests that the terrestrial carbon sink deduced from atmospheric O₂/N₂ observations is potentially overestimated if the oceanic loss of O₂ to the atmosphere is not considered.     

Anthropogenic and biogenic winter sources of Arctic CO2—a model study

Long‐range transport of anthropogenic and biogenic CO₂ to a remote site in the Arctic is studied. A limited area, off‐line, Eulerian atmospheric transport model is used, and the results are compared to the observed CO₂ concentration at the "Ny‐Alesund International Arctic Research and Monitoring Facility". Inventories of anthropogenic CO₂ emissions and estimates of biogenic CO₂ emissions are used to investigate the respective impact of these emissions on Arctic CO₂ variations during 4 winter months. A direct comparison of the modelled and observed concentrations reveals remarkably good timing in the modelled variations as compared to the observed variations for most of the time. The correlation of observed versus modelled CO₂ concentration is significant at the 95% confidence level. The biogenic and the anthropogenic CO₂ emissions are shown to have approximately equal influence on Arctic CO₂ variations during winter. Europe is found to be the dominant source of anthropogenic CO₂ at the monitoring station, while Siberia and Northern America have little influence on Arctic CO₂, during the months studied. These results contradict Engardt and Holmén whose results indicate that the lower‐Ob region in western Siberia has a large impact on Arctic CO₂.     

Tropospheric carbon dioxide concentrations at a northern boreal site in Finland: basic variations and source areas

Diurnal and annual variations of CO₂, O_3, SO₂, black carbon and condensation nuclei and their source areas were studied by utilizing air parcel trajectories and tropospheric concentration measurements at a boreal GAW site in Pallas, Finland. The average growth trend of CO₂ was about 2.5 ppm yr⁻¹ according to a 4-yr measurement period starting in October 1996. The annual cycle of CO₂ showed concentration difference of about 19 ppm between the summer minimum and winter maximum. The diurnal cycle was most pronounced during July and August. The variation between daily minimum and maximum was about 5 ppm. There was a diurnal cycle in aerosol concentrations during spring and summer. Diurnal variation in ozone concentrations was weak. According to trajectory analysis the site was equally affected by continental and marine air masses. During summer the contribution of continental air increased, although the southernmost influences decreased. During daytime in summer the source areas of CO₂ were mainly located in the northern parts of the Central Europe, while during winter the sources were more evenly distributed. Ozone showed similar source areas during summer, while during winter, unlike CO₂, high concentrations were observed in air arriving from the sea. Sulfur dioxide sources were more northern (Kola peninsula and further east) and CO₂ sources west-weighted in comparison to sources of black carbon. Source areas of black carbon were similar to source areas of aerosols during winter. Aerosol source area distributions showed signs of marine sources during spring and summer.     

Chemical composition of the background aerosol at two sites in southwestern and northwestern China: potential influences of regional transport

Concentrations of organic carbon (OC), elemental carbon (EC), selected trace elements and water-soluble (WS) ions were determined for samples collected from August 2004 to February 2005 to assess the aerosol background at two remote sites in China. The OC and EC concentrations in PM₁₀ from near the Tibetan Plateau at Zhuzhang (ZUZ) were comparable with other background sites, averaging 3.1 and 0.34 μg m⁻³, respectively, with no pronounced seasonality. At Akdala (AKD) on northern margin of the Zhungaer Basin, the average concentrations were similar (mean OC = 2.9 μg m⁻³ and EC = 0.35 μg m⁻³), but the concentrations were higher in winter. The aerosol mass at both sites was dominated by OC and SO₄²⁻, but a stronger contribution from soil dust was observed at AKD. At ZUZ, NO₃⁻ showed a unique weather-related fluctuation in PM₁₀ with a periodicity of ∼1 week. Anthropogenic sources in the Sichuan Basin and southeastern Yunnan Province evidently influence ZUZ in summer and autumn while pollutants from Russia and the China–Mongolia border affect AKD nearly all year. The identification of these upwind sources demonstrates that transboundary transport needs to be taken into account when assessing air quality in remote parts of China.     

Aerosol modulation of atmospheric and surface solar heating over the tropical Indian Ocean

The major finding of this study is that aerosols over the tropical Indian Ocean enhance clear sky atmospheric solar heating significantly and decrease the surface solar heating by even a larger amount. The results presented here are based on aerosol chemical, microphysical, and optical and radiometric data collected at the island of Kaashidhoo (4.97°N, 73.47°E) during February and March of 1998, as part of the first field phase of the Indian Ocean experiment (INDOEX). The aerosol optical properties were integrated with a multiple scattering Monte Carlo radiative transfer model which was validated at the surface with broadband flux measurements and at the top of the atmosphere (TOA) with the clouds and earth's radiant energy system (CERES) radiation budget measurements. We consider both externally and internally mixed aerosol models with very little difference between the two models in the estimated forcing. For the February–March period, the aerosols increase the monthly mean clear sky atmospheric solar heating by about 12 W/m²(about 15% of the total atmospheric solar heating) and decrease the sea surface clear sky solar heating by about 16 W/m² with a daily range from 5 to 23 W/m². The net aerosol forcing at the top of the atmosphere is about −4 W/m² with a daily range from −2 to −6 W/m². Although the soot contributes only about 10% to the aerosol optical thickness, it contributes more than 50% to the aerosol induced atmospheric solar heating. The fundamental conclusion of this study is that anthropogenic aerosols over the tropical Indian Ocean are altering the clear sky radiation budget of the atmosphere and surface in a major manner.     

Future changes of the atmospheric composition and the impact of climate change

The development of the future atmospheric chemical composition is investigated with respect to NO _y and O₃ by means of the off‐line coupled dynamic‐chemical general circulation model ECHAM3/CHEM. Two time slice experiments have been performed for the years 1992 and 2015, which include changes in sea surface temperatures, greenhouse gas concentrations, emissions of CFCs, NO _x and other species, i.e., the 2015 simulation accounts for changes in chemically relevant emissions and for a climate change and its impact on air chemistry. The 2015 simulation clearly shows a global increase in ozone except for large areas of the lower stratosphere, where no significant changes or even decreases in the ozone concentration are found. For a better understanding of the importance of (A) emissions like NO _x and CFCs, (B) future changes of air temperature and water vapour concentration, and (C) other dynamical parameters, like precipitation and changes in the circulation, diabatic circulation, stratosphere‐troposphere‐exchange, the simulation of the future atmosphere has been performed stepwise. This method requires a climate‐chemistry model without interactive coupling of chemical species. Model results show that the direct effect of emissions (A) plays a major rôle for the composition of the future atmosphere, but they also clearly show that climate change (B and C) has a significant impact and strongly reduces the NO _y and ozone concentration in the lower stratosphere.     

Aircraft study of aerosol vertical distributions over Beijing and their optical properties

A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration ( N _a) reveals a high aerosol number density in this region with average surface level number concentration ( N ₀) of about 6600 cm⁻³ (0.12–3.0 μm). The average vertical profile of N _a approximately satisfies an exponential decline function with a scale height of 1419 m. The N _a profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of N _a profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.     

Determination of microbial versus root‐produced CO2 in an agricultural ecosystem by means of δ13CO2 measurements in soil air

The amounts of microbial and root‐respired CO₂ in a maize/winter wheat agricultural system in south western Germany were investigated by measurements of the CO₂ mixing ratio and the ¹³C/¹²C ratio in soil air. CO₂ fluxes at the soil surface for the period of investigation (1993–1995) were also determined. Root respired CO₂ shows a strong correlation with the plant mass above ground surface of the respective vegetation (R²≥0.88); the maximum CO₂ release from roots was in August for the maize (2.0±0.5 mmol m⁻² h⁻¹) and in June for winter wheat (1.5±0.5 mmol m⁻² h⁻¹). Maximum CO₂ production by roots correlate well with the maximum amount of plant root matter. Integrating the CO₂ production over the whole growing season and normalizing to the dry root matter yields, the CO₂ production per gram dry organic root matter (DORM) of maize was found to be 0.14±0.03 gC (g DORM)⁻¹. At the sites investigated, root‐produced CO₂ contributed (16±4)% for maize, and (24±4)% for winter wheat, respectively, to the total annual CO₂ production in the soil (450±50 gC m⁻² for maize, 210±30 gC m⁻² for winter wheat).     

Projections of SO2, NOx and carbonaceous aerosols emissions in Asia

Estimates of Asian emissions of air pollutants and carbonaceous aerosols and their mid-term projections have been changing significantly in the last years. The remote sensing community has shown that increase in NO _x in Central East Asia is much stronger than any of the emission inventories or projections indicated so far. A number of studies reviewing older estimates appeared. Here, we review the key contributions and compare them to the most recent results of the GAINS model application for Asia and to the SRES projections used in the IPPC work. The recent projections indicate that the growth of emissions of SO₂ in Asia should slow down significantly towards 2010 or even stabilize at the current level. For NO _x , however, further growth is projected although it will be most likely slower that in the last decade, owing to introduction of measures in transport. Emissions of carbonaceous aerosols (black carbon and organic carbon) are expected to decline after 2010, largely due to reduced use of biofuels in residential sector and efficiency improvements. The estimates of these emissions are burdened with significantly larger uncertainties than SO₂ and NO _x ; even for the year 2000 the differences in estimates between studies are up to a factor of 2.     

Development of particle size and composition distributions with a novel aerosol dynamics model

An aerosol dynamics model, AEROFOR2, is developed in the context of the BIOFOR project focussing on boreal forest aerosol. It is the second version of a Lagrangian type box model AEROFOR for investigating the formation and growth of particles under clear sky atmospheric conditions. Particles can consist of soluble and insoluble material and the particle population can be externally or internally mixed. AEROFOR2 includes gas phase chemistry and aerosol dynamics, and calculates the number and composition distributions of particles as functions of time. Observed growth rates of the nucleation mode particles after a typical nucleation event are 2–3 nm/h. The model simulations predict that 3·10⁷ molecules cm⁻³ of insoluble organic vapour and less than 6·10⁶ molecules cm⁻³ of soluble vapour condensing onto particles are enough to make them grow in good agreement with the observed growth rates. Then the source rate of the organic vapour must be an order of 10⁵ molecules cm⁻³ s⁻¹, and its saturation vapour density should be below 10⁶ molecules cm⁻³. If the aerosol was initially an internal mixture of soluble (70%) and insoluble (30%) constituents it transformed to an externally mixed aerosol during the simulation. By applying the externally‐mixed aerosol based on measured soluble volume fractions, it was concluded that the modelled soluble fraction of the nucleation mode was too low in comparison with the measurements, and thus, a part of the condensable organic vapour must be water soluble.