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The heterogeneous freezing temperatures of single binary sulfuric acid solution droplets were measured in dependency of acid concentration down to temperatures as low as -50°C. In order to avoid influence of supporting substrates on the freezing characteristics, a new technique has been developed to suspend the droplet by means of an acoustic levitator. The droplets contained immersed particles of graphite, kaolin or montmorillonite in order to study the influence of the presence of such contamination on the freezing temperature. The radii of the suspended droplets spanned the range between 0.4 and 1.1mm and the concentration of the sulfuric acid solution varied between 5 and 14 weight percent. The presence of the particles in the solution raises the freezing temperature with respect to homogeneous freezing of these solution droplets. The pure solution droplets can be supercooled up to 40 degrees below the ice-acid solution thermodynamic equilibrium curve. Depending on the concentration of sulfuric acid and the nature of the impurity the polluted droplets froze between -11°C and -35°C. The new experimental set-up, combining a deep freezer with a movable ultrasonic levitator and suitable optics, proved to be a useful approach for such investigations on individual droplets.
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First results concerning the retrieval of tropospheric carbon monoxide (CO) from satellite solar backscatter radiance measurements in the near-infrared spectral region (~2.3µm) are presented. The Weighting Function Modified (WFM) DOAS retrieval algorithm has been used to retrieve vertical columns of CO from SCIAMACHY/ENVISAT nadir spectra. We present detailed results for three days from the time periode January to October 2003 selected to have good overlap with the daytime CO measurements of MOPITT onboard EOS Terra. Because the WFM-DOAS Version 0.4 CO columns presented in this paper are scaled by a constant factor of 0.5 to compensate for an obvious overestimation we focus on the variability of the retrieved columns rather than on their absolute values. It is shown that plumes of CO resulting from, e.g. biomass burning in Africa, are detectable with single overpass SCIAMACHY data. Globally, the SCIAMACHY CO columns are in reasonable agreement with the Version 3 CO column data product of MOPITT. For example, for measurements over land, where the quality of the data is typically better than over ocean due to higher surface reflectivity, the standard deviation of the difference with respect to MOPITT is in the range 0.4-0.6x10<sup>18</sup> molecules/cm<sup>2</sup> and the linear correlation coefficient is between 0.4 and 0.7. The level of agreement between the data of both sensors depends on time and location but is typically within 30% for most latitudes. In the southern hemisphere outside Antarctica SCIAMACHY tends to give systematically higher values than MOPITT. More studies are needed to find out what the reasons for the observed differences with respect to MOPITT are and how the algorithm can be modified to improve the quality of the CO columns as retrieved from SCIAMACHY.
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A new chemical scheme is developed for the multiphase photochemical box model SEAMAC (size-SEgregated Aerosol model for Marine Air Chemistry) to investigate photochemical interactions between volatile organic compounds (VOCs) and reactive halogen species in the marine boundary layer (MBL). Based primarily on critically evaluated kinetic and photochemical rate parameters as well as a protocol for chemical mechanism development, the new scheme has achieved a near-explicit description of oxidative degradation of up to C<sub>3</sub>-hydrocarbons (CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, C<sub>3</sub>H<sub>8</sub>, C<sub>2</sub>H<sub>4</sub>, C<sub>3</sub>H<sub>6</sub>, and C<sub>2</sub>H<sub>2</sub>) initiated by reactions with OH radicals, Cl- and Br-atoms, and O<sub>3</sub>. Rate constants and product yields for reactions involving halogen species are taken from the literature where available, but the majority of them need to be estimated. In particular, addition reactions of halogen atoms with alkenes will result in forming halogenated organic intermediates, whose photochemical loss rates are carefully evaluated in the present work. Model calculations with the new chemical scheme reveal that the oceanic emissions of acetaldehyde (CH<sub>3</sub>CHO) and alkenes (especially C<sub>3</sub>H<sub>6</sub>) are important factors for regulating reactive halogen chemistry in the MBL by promoting the conversion of Br atoms into HBr or more stable brominated intermediates in the organic form. The latter include brominated hydroperoxides, bromoacetaldehyde, and bromoacetone, which sequester bromine from a reactive inorganic pool. The total mixing ratio of brominated organic species thus produced is likely to reach 10-20% or more of that of inorganic gaseous bromine species over wide regions over the ocean. The reaction between Br atoms and C<sub>2</sub>H<sub>2</sub> is shown to be unimportant for determining the degree of bromine activation in the remote MBL. These results imply that reactive halogen chemistry can mediate a link between the oceanic emissions of VOCs and the behaviors of compounds that are sensitive to halogen chemistry such as dimethyl sulfide, NO<sub>x</sub>, and O<sub>3</sub> in the MBL.
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The first Rayleigh lidar observation of a stratopause warming over a tropical site, Gadanki (13.5° N; 79.2° E), is presented in this paper. The warming event was observed on 22-23 February 2001, and found to occur in the stratopause region (~45km). The magnitude of the warming was found to be ~18K with respect to the winter-mean temperature profile derived from the lidar data collected over March 1998 to July 2001. The event observed by the lidar has also been seen in data from the Halogen Occultation Experiment (HALOE) on board the UARS satellite. The zonal-mean temperature at 80° N and the zonal-mean zonal wind at 60° N from the National Centre for Environmental Prediction (NCEP) reanalysis and the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis indicate that a major warming episode took place in the northern polar hemisphere a week before the day of the observation over Gadanki. Eliassen-Palm (E-P) flux calculations from ECMWF analysis show evidence of propagation of planetary-wave activity from high and mid- to low latitudes subsequent to the major warming episode over the pole. Our results support the view that the most likely source mechanism for the observed stratopause warming is the increase in planetary-wave activity.
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Polar stratospheric clouds (PSC) have been observed in early winter (December 2002) during the SOLVE II/Vintersol campaign, both from balloons carrying comprehensive instrumentation for measurements of chemical composition, size distributions, and optical properties of the particles, as well as from individual backscatter soundings from Esrange and Sodankylä. The observations are unique in the sense that the PSC particles seem to have formed in the early winter under synoptic temperature conditions and not being influenced by mountain lee waves. A sequence of measurements during a 5-days period shows a gradual change between liquid and solid type PSCs with the development of a well-known sandwich structure. It appears that all PSC observations show the presence of a background population of solid particles, occasionally mixed in with more optically dominating liquid particles. The measurements have been compared with results from a detailed microphysical and optical simulation of the formation processes. Calculated extinctions are in good agreement with SAGE-III measurements from the same period. Apparently the solid particles are controlled by the synoptic temperature history while the presence of liquid particles is controlled by the local temperatures at the time of observation. The temperature histories indicate that the solid particles are nucleated above the ice frost point, and a surface freezing mechanism for this is included in the model. Reducing the calculated freezing rates by a factor 10-20, the model is able to simulate the observed particle size distributions and reproduce observed HNO<sub>3</sub> gas phase concentrations.
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We report on measurements of the rate of homogeneous ice nucleation in supercooled water microdroplets levitated in an electrodynamic balance. By comparison of the freezing probability for droplets of radius 49µm and 19µm, we are able to conclude that homogeneous freezing is a volume-proportional process and that surface nucleation might only be important, if at all, for much smaller droplets.
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Explicit mechanisms describing the complex degradation pathways of atmospheric volatile organic compounds (VOCs) are important, since they allow the study of the contribution of individual VOCS to secondary pollutant formation. They are computationally expensive to solve however, since they contain large numbers of species and a wide range of time-scales causing stiffness in the resulting equation systems. This paper and the following companion paper describe the application of systematic and automated methods for reducing such complex mechanisms, whilst maintaining the accuracy of the model with respect to important species and features. The methods are demonstrated via application to version 2 of the Leeds Master Chemical Mechanism. The methods of Jacobian analysis and overall rate sensitivity analysis proved to be efficient and capable of removing the majority of redundant reactions and species in the scheme across a wide range of conditions relevant to the polluted troposphere. The application of principal component analysis of the rate sensitivity matrix was computationally expensive due to its use of the decomposition of very large matrices, and did not produce significant reduction over and above the other sensitivity methods. The use of the quasi-steady state approximation (QSSA) proved to be an extremely successful method of removing the fast time-scales within the system, as demonstrated by a local perturbation analysis at each stage of reduction. QSSA species were automatically selected via the calculation of instantaneous QSSA errors based on user-selected tolerances. The application of the QSSA led to the removal of a large number of alkoxy radicals and excited Criegee bi-radicals via reaction lumping. The resulting reduced mechanism was shown to reproduce the concentration profiles of the important species selected from the full mechanism over a wide range of conditions, including those outside of which the reduced mechanism was generated. As a result of a reduction in the number of species in the scheme of a factor of 2, and a reduction in stiffness, the computational time required for simulations was reduced by a factor of 4 when compared to the full scheme.
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During the calendar years 1998-2002, 147 clear 8nm diameter particle formation events have been identified at the SMEAR I station in Värriö, northern Finland. The events have been classified in detail according to the particle formation rate, growth rate, event starting time, different trace gas concentrations and pre-existing particle concentrations as well as various meteorological conditions. The frequency of particle formation and growth events was highest during the spring months between March and May, suggesting that increasing biological activity might produce the precursor gases for particle formation. The apparent 8nm particle formation rates were around 0.1 /cm<sup>3</sup>s, and they were uncorrelated with growth rates that varied between 0.5 and 10nm/h. The air masses with clearly elevated sulphur dioxide concentrations (above 1.6ppb) came, as expected, from the direction of the Nikel and Monschegorsk smelters. Only 15 formation events can be explained by the pollution plume from these sources.
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This paper presents a formal method of species lumping that can be applied automatically to intermediate compounds within detailed and complex tropospheric chemical reaction schemes. The method is based on grouping species with reference to their chemical lifetimes and reactivity structures. A method for determining the forward and reverse transformations between individual and lumped compounds is developed. Preliminary application to the Leeds Master Chemical Mechanism (MCMv2.0) has led to the removal of 734 species and 1777 reactions from the scheme, with minimal degradation of accuracy across a wide range of test trajectories relevant to polluted tropospheric conditions. The lumped groups are seen to relate to groups of peroxy acyl nitrates, nitrates, carbonates, oxepins, substituted phenols, oxeacids and peracids with similar lifetimes and reaction rates with OH. In combination with other reduction techniques, such as sensitivity analysis and the application of the quasi-steady state approximation (QSSA), a reduced mechanism has been developed that contains 35% of the number of species and 40% of the number of reactions compared to the full mechanism. This has led to a speed up of a factor of 8 in terms of computer calculation time within box model simulations.
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A quartz crystal microbalance apparatus has been used to measure the room temperature uptake of water vapour by thin films of oleic acid as a function of relative humidity, both before and following exposure of the films to various partial pressures of gas phase ozone. A rapid increase in the water-sorbing ability of the film is observed as its exposure to ozone is increased, followed by a plateau region in which additional water is taken up more gradually. In this fully-processed region the mass of water taken up by the film is about 4 times that of the unprocessed film. Infrared spectra of the films, measured after variable exposures to ozone, show dramatic increases in both the "free" and hydrogen-bonded O-H stretching regions, and a decrease in the intensity of olefinic features. These results are consistent with the formation of an oxygenated polymeric product or products, as well as the gas phase products previously identified.
