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The concentration of cloud condensation nuclei (CCN) in the marine boundary layer (MBL) was estimated from dimethyl sulfide (DMS) flux, sea salt (SS) emission, and aerosols entrained from the free troposphere (FT). Only under clean air conditions, did the nucleation of DMS derived sulfur (DMS CCN) contribute significantly to the MBL CCN. The accommodation coefficient for sulfuric acid mass transfer was found to be a very important parameter in the modeling the contribution of DMS to MBL CCN. The relationship between seawater DMS and MBL CCN was found to be non-linear mainly due to the transfer processes of sulfuric acid onto aerosols. In addition, sea salt derived CCN (SS CCN) and entrained aerosol from the FT (FT CCN) affected the MBL CCN directly, by supplying CCN, and indirectly, by behaving as an efficient sink for sulfuric acid. The SS CCN explained more than 50% of the total predicted MBL CCN when wind speeds were moderate and high. Sea salt and FT aerosol may often be more efficient sources of MBL CCN than DMS.
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We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree increase in sea surface temperature produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback is small. Although not mentioned by Lindzen et al. (2001), tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al. (2001), we obtain a feedback factor in the range of -0.15 to -0.51, compared to their larger negative feedback factor of -0.45 to -1.03. It is noted that our feedback factor could still be overestimated due to the assumption of constant low cloud cover in the simple radiative-convective model.
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A 1-D marine stratocumulus cloud model has been supplemented with a comprehensive and up-to-date aqueous phase chemical mechanism for the purpose of assessing the impact that the presence of clouds has on gas phaseHO<sub>x</sub>, NO<sub>x</sub> and O<sub>3</sub> budgets in the marine boundary layer. The simulations presented here indicate that cloud may act as a heterogeneous source of HONO<sub>g</sub>. The conversion of HNO<sub>4(g)</sub> at moderate pH (~ 4.5) is responsible for this, and, to a lesser extent, the photolysis of nitrate (NO<sub>3</sub><sup>-</sup>). The effect of introducing deliquescent aerosol on the simulated increase of HONO<sub>g</sub> is negligible. The most important consequences of this elevation in HONO<sub>g</sub> are that, in the presence of cloud, gas phase concentrations of NO<sub>x</sub> species increase by a factor of 2, which minimises the simulated decrease in O<sub>3(g)</sub>, and results in a regeneration of OH<sub>g</sub>. This partly compensates for the removal of OH<sub>g</sub> by direct phase transfer into the cloud and may have important implications regarding the oxidising capacity of the marine boundary layer.
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An Ultrafine Tandem Differential Mobility Analyser (UF-TDMA) has been used in several field campaigns over the last few years. The investigations were focused on the origin and properties of nucleation event aerosols, which are observed frequently in various environments. This paper gives a summary of the results of 10 nm and 20 nm particle hygroscopic properties from different measurement sites: an urban site, an urban background site and a forest site in Finland and a coastal site in western Ireland. The data can be classified in four hygroscopic growth classes: hydrofobic, less-hygroscopic, more-hygroscopic and sea-salt. Similar classification has been earlier presented for Aitken and accumulation mode particles. In urban air, the summertime 10 nm particles showed varying less-hygroscopic growth behaviour, while winter time 10 nm and 20 nm particles were externally mixed with two different hygroscopic growth modes. The forest measurements revealed diurnal behaviour of hygroscopic growth, with high growth factors at day time and lower during night. The urban background particles had growth behaviour similar to the urban and forest measurement sites depending on the origin of the observed particles. The coastal measurements were strongly affected by air mass history. Both 10 nm and 20 nm particles were hygroscopic in marine background air. The 10 nm particles produced during clean nucleation burst periods were hydrofobic. Diurnal variation and higher growth factors of 10 nm particles were observed in air affected by other source regions. External mixing was occasionally observed at all the sites, but incidents with more than two growth modes were extremely rare.
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Due to the exponential positive feedback between sea surface temperature and saturated water vapour concentration, dependence of the planetary greenhouse effect on atmospheric water content is critical for stability of a climate with extensive liquid hydrosphere.<br> <br> In this paper on the basis of the law of energy conservation we develop a simple physically transparent approach to description of radiative transfer in an atmosphere containing greenhouse substances. It is shown that the analytical solution of the equation thus derived coincides with the exact solution of the well-known radiative transfer equation to the accuracy of 20% for all values of atmospheric optical depth. The derived equation makes it possible to easily take into account the non-radiative thermal fluxes (convection and latent heat) and obtain an analytical dependence of the greenhouse effect on atmospheric concentrations of a set of greenhouse substances with arbitrary absorption intervals.<br> <br> The established dependence is used to analyse stability of the modern climate of Earth. It is shown that the modern value of global mean surface temperature, which corresponds to the liquid state of the terrestrial hydrosphere, is physically unstable. The observed stability of modern climate over geological timescales is therefore likely to be due to dynamic singularities in the physical temperature-dependent behaviour of the greenhouse effect. We hypothesise that such singularities may appear due to controlling functioning of the natural global biota and discuss major arguments in support of this conclusion.
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Tropospheric NO<sub>2</sub> plays a variety of significant roles in atmospheric chemistry. In the troposphere it is one of the most significant precursors of photochemical ozone (O<sub>3</sub>) production and nitric acid (HNO<sub>3</sub>). In this study tropospheric NO<sub>2</sub> columns were calculated by the fully coupled chemistry-climate model ECHAM4.L39(DLR)/CHEM. These have been compared with tropospheric NO<sub>2</sub> columns, retrieved using the tropospheric excess method from measurements by the Global Ozone Monitoring Experiment (GOME) of up-welling earthshine radiance and the extraterrestrial irradiance. GOME is part of the core payload of the second European Research Satellite (ERS-2). For this study the first five years of GOME measurements have been used. The period of five years of observational data is sufficiently long to facilitate for the first time a comparison based on climatological averages with global coverage, focussing on the geographical distribution of the tropospheric NO<sub>2.</sub><br> <br> A new approach of analysing regional differences (i.e. on continental scales) by calculating individual averages for different environments provides more detailed information about specific NO<sub>x</sub> sources and of their seasonal variations. The results obtained enable the validity of the model NO<sub>2</sub> source distribution and the assumptions used to separate tropospheric and stratospheric parts of the NO<sub>2</sub> column amount from the satellite measurements to be investigated.
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The uptake and reaction of HOBr with frozen salt surfaces of variable NaCl / NaBr composition and temperature were investigated with a coated wall flow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOBr is efficiently taken up onto the frozen surfaces at temperatures between 253 and 233 K where it reacts to form the di-halogens BrCl and Br<sub>2</sub>, which are subsequently released into the gas-phase. The uptake coefficient for HOBr reacting with a frozen, mixed salt surface of similar composition to sea-spray was <approx> 10<sup>-2</sup>. The relative concentration of BrCl and Br<sub>2</sub> released to the gas-phase was found to be strongly dependent on the ratio of Cl<sup>-</sup> to Br <sup>-</sup> in the solution prior to freezing / drying. For a mixed salt surface of similar composition to sea-spray the major product at low conversion of surface reactants (i.e. Br <sup>-</sup> and Cl<sup>-</sup>) was Br<sub>2</sub>.<br> <br> Variation of the pH of the NaCl / NaBr solution used to prepare the frozen surfaces was found to have no significant influence on the results. The observations are explained in terms of initial formation of BrCl in a surface reaction of HOBr with Cl<sup>-</sup>, and conversion of BrCl to Br<sub>2</sub> via reaction of surface Br <sup>-</sup>. Experiments on the uptake and reaction of BrCl with frozen NaCl / NaBr solutions served to confirm this hypothesis. The kinetics and products of the interactions of BrCl, Br<sub>2</sub> and Cl<sub>2</sub> with frozen salt surfaces were also investigated, and lower limits to the uptake coefficients of > 0.034, >0.025 and >0.028 respectively, were obtained. The uptake and reaction of HOBr on dry salt surfaces was also investigated and the results closely resemble those obtained for frozen surfaces. During the course of this study the gas diffusion coefficients of HOBr in He and H<sub>2</sub>O were also measured as (273 ± 1) Torr cm<sup>2</sup> s<sup>-1</sup> and (51 ± 1) Torr cm<sup>2</sup> s<sup>-1</sup>, respectively, at 255 K. The implications of these results for modelling the chemistry of the Arctic boundary layer in springtime are discussed.
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Since November 1993 up to present from Benegas Station, Mendoza, Argentina (site of IEMA Institute) and from high locations in the Andes region, ground based radiometric measurements of stratospheric ozone and tropospheric water vapor have been achieved. Ozone measurements are performed by using a radiometer-spectrometer tuned at 142 GHz and tropospheric water vapor by means of a 92 GHz radiometer. In this paper two case studies of large stratospheric ozone variations due to dynamical processes will be presented. These processes are very likely associated to gravity waves, generated by airflow over the Andes Mountains, or due to Zonda wind effect.
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Polar stratospheric clouds (PSCs) of type 1a or 1a-enh containing high number densities of nitric acid trihydrate (NAT) particles, can act as mother clouds for extremely large NAT particles, termed NAT-rocks, provided the air below the clouds is supersaturated with respect to NAT. Individual NAT particles at the cloud base fall into undepleted gas phase and rapidly accelerate due to a positive feedback between their growth and sedimentation. The resulting reduction in number density is further enhanced by the strong HNO<sub>3</sub> depletion within a thin layer below the mother cloud, which delays subsequent particles. This paper introduces the basic microphysical principles behind this mother cloud/NAT-rock mechanism, which produces 10<sup>-4</sup> cm<sup>-3</sup> NAT-rocks with radii around 10 <font face="Symbol" >mm</font> some kilometers below the mother cloud. The mechanism does not require selective nucleation and works even for a monodisperse particle size distribution in the mother cloud.
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In assessing the iris effect suggested by Lindzen et al. (2001), Fu et al. (2002) found that the response of high-level clouds to the sea surface temperature had an effect of reducing the climate sensitivity to external radiative forcing, but the effect was not as strong as LCH found. The approach of FBH to specifying longwave emission and cloud albedos appears to be inappropriate, and the derived cloud optical properties may not have real physical meaning. The cloud albedo calculated by FBH is too large for cirrus clouds and too small for boundary layer clouds, which underestimates the iris effect.
