ALBERT

Description: A new algorithm is presented to retrieve the three-dimensional structure of clouds from airborne measurements of microphysical parameters. Data from individual flight legs are scanned for characteristic patterns, and the autocorrelation functions for several directions are used to extrapolate the observations along the flight path to a full three-dimensional distribution of the cloud field. Thereby, the mean measured profiles of microphysical parameters are imposed to the cloud field by mapping the measured probability density functions onto the model layers. The algorithm was tested by simulating flight legs through synthetic clouds (by means of Large Eddy Simulations (LES)) and applied to a stratocumulus cloud case measured during the first field experiment of the EC project INSPECTRO (INfluence of clouds on the SPECtral actinic flux in the lower TROposphere) in East Anglia, UK. The number and position of the flight tracks determine the quality of the retrieved cloud field. If they provide a representative sample of the entire field, the derived pattern closely resembles the statistical properties of the real cloud field.

Description: This paper describes the inclusion of organic particulate material within the Aerosol Diameter Dependent Equilibrium Model (ADDEM) framework described in the companion paper applied to inorganic aerosol components. The performance of ADDEM is analysed in terms of its capability to reproduce the behaviour of various organic and mixed inorganic/organic systems using recently published bulk data. Within the modelling architecture already described two separate thermodynamic models are coupled in an additive approach and combined with a method for solving the Köhler equation in order to develop a tool for predicting the water content associated with an aerosol of known inorganic/organic composition and dry size. For development of the organic module, the widely used group contribution method UNIFAC is employed to explicitly deal with the non-ideality in solution. The UNIFAC predictions for components of atmospheric importance were improved considerably by using revised interaction parameters derived from electro-dynamic balance studies. Using such parameters, the model was found to adequately describe mixed systems including 5–6 dicarboxylic acids, down to low relative humidity conditions. The additive approach for modelling mixed inorganic/organic systems worked well for a variety of mixtures. As expected, deviations between predicted and measured data increase with increasing concentration. Available surface tension models, used in evaluating the Kelvin term, were found to reproduce measured data with varying success. Deviations from experimental data increased with increased organic compound complexity. For components only slightly soluble in water, significant deviations from measured surface tension depression behaviour were predicted with both model formalisms tested. A Sensitivity analysis showed that such variation is likely to lead to predicted growth factors within the measurement uncertainty for growth factor taken in the sub-saturated regime. Greater sensitivity was found for the value of dry density used in the assumed form of the dried out aerosol. Comparison with a coupled thermodynamic approach showed that assumed values for interactions parameters may lead to erroneous results where a simple additive approach may provide more accurate results. However, where available, the use of coupled thermodynamics can better reproduce measured behaviour. Further work (and laboratory data) is required to assess whether this difference lies within the experimental uncertainty of observed hygroscopic behaviour for a variety of systems.

Description: A thermodynamic modelling framework to predict the equilibrium behaviour of mixed inorganic salt aerosols is developed, and then coupled with a technique for finding a solution to the Köhler equation in order to create a diameter dependent hygroscopic aerosol model (Aerosol Diameter Dependent Equilibrium Model – ADDEM). The model described here provides a robust and accurate inorganic basis using a mole fraction based activity coefficient model and adjusted energies of formation for treating solid precipitation. The model framework can accommodate organic components, though this added complexity is considered in a companion paper, whereas this paper describes the development of the modelling architecture to be used and predictions of an inorganic model alone. The modelling framework has been developed to flexibly use a combination of mixing rules and other potentially more accurate techniques where available to calculate the water content. Comparisons with other state-of-the-art general equilibrium models and experimental data are presented and show excellent agreement. The Kelvin effect can be considered in this scheme using a variety of surface tension models. Comparison of predicted diameter dependent phenomena, such as the increased relative humidity for onset of deliquescence with decreasing diameter, with another diameter dependent model is very good despite the different approach used. The model is subject to various sensitivities. For the inorganic systems studied here, the model is sensitive to choice of surface tension scheme used, which decreases for larger aerosol. Large sensitivities are found for the value of dry density used. It is thus likely that the history of the aerosol studied in a hygroscopic tandem differential mobility analyser (HTDMA), specifically the nature of the drying process that will influence the final crystalline form, will create systematic uncertainties upon comparisons with theoretical predictions. However, the magnitudes of all of the above sensitivities are potentially less than those introduced when using a semi ideal growth factor analogue for certain conditions.

Description: Air quality modeling associated with emission scenarios has become an important tool for air quality management. The set-up of realistic emission scenarios requires accurate emission inventories including the whole methodology used to calculate the emissions. This means a good description of the source characteristics including a detailed composition of the emitted fluxes. Two main approaches are used. The so-called bottom-up approach that relies on the modification of the characteristics of the sources and the top-down approach whose goal is generally to reach standard pollutant concentration levels. This paper is aimed at providing a general methodology for the elaboration of such emission scenarios and giving examples of applications at local and regional scales for air quality management. The first example concerns the impact of the installation of the urban tramway in place of the road traffic in the old centre of Strasbourg. The second example deals with the use of oxygenated and reformulated car fuels on local (Strasbourg urban area) and regional (upper Rhine valley) scales. Finally, we analyze in detail the impacts of the incoming European emission regulation for 2015 on the air quality of the upper Rhine valley.

Description: A polar stratospheric cloud (PSC) was observed on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (~10−4 cm−3) of nitric acid (HNO3) containing particles – probably NAT – with diameters up to 6 µm were measured at altitudes between 18 and 20 km. These particles have the potential to grow further and to remove HNO3 from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures T〉TNAT−3.5 K, just slightly below the NAT equilibrium temperature TNAT. This unique measurement of PSC formation at extremely low NAT saturation ratios (SNAT≤11) constrains current NAT nucleation theories. In particular, NAT formation on ice can for certain be excluded. Conversely, we suggest that meteoritic particles may be favorable candidates for triggering nucleation of NAT at the observed low number densities.

Description: To explore the relationship between tropospheric ozone and radiative forcing with changing emissions, we compiled two sets of global scenarios for the emissions of the ozone precursors methane (CH4), carbon monoxide (CO), non-methane volatile organic compounds (NMVOC) and nitrogen oxides (NOx) up to the year 2030 and implemented them in two global Chemistry Transport Models. The "Current Legislation" (CLE) scenario reflects the current perspectives of individual countries on future economic development and takes the anticipated effects of presently decided emission control legislation in the individual countries into account. In addition, we developed a "Maximum technically Feasible Reduction" (MFR) scenario that outlines the scope for emission reductions offered by full implementation of the presently available emission control technologies, while maintaining the projected levels of anthropogenic activities. Whereas the resulting projections of methane emissions lie within the range suggested by other greenhouse gas projections, the recent pollution control legislation of many Asian countries, requiring introduction of catalytic converters for vehicles, leads to significantly lower growth in emissions of the air pollutants NOx, NMVOC and CO than was suggested by the widely used IPCC (Intergovernmental Panel on Climate Change) SRES (Special Report on Emission Scenarios) scenarios (Nakicenovic et al., 2000). With the TM3 and STOCHEM models we performed several long-term integrations (1990–2030) to assess global, hemispheric and regional changes in CH4, CO, hydroxyl radicals, ozone and the radiative climate forcings resulting from these two emission scenarios. Both models reproduce realistically the observed trends in background ozone, CO, and CH4 concentrations from 1990 to 2002. For the "current legislation" case, both models indicate an increase of the annual average ozone levels in the Northern hemisphere by 5 ppbv, and up to 15 ppbv over the Indian sub-continent, comparing the 2020s with the 1990s. The corresponding higher ozone and methane burdens in the atmosphere increase radiative forcing by approximately 0.2 Wm−2. Full application of today's emissions control technologies, however, would bring down ozone below the levels experienced in the 1990s and would reduce the current radiative forcing of ozone and methane by approximately 0.1Wm−2. While methane reductions lead to lower ozone burdens and to less radiative forcing, further reductions of the air pollutants NO4 and NMVOC result in lower ozone, but at the same time increase the lifetime of methane. Control of methane emissions appears an efficient option to reduce tropospheric ozone as well as radiative forcing.

Description: SCIAMACHY onboard the European environmental research satellite ENVISAT is an UV/visible/near-infrared spectrometer providing 3 near infrared channels covering wavelengths from 1–1.75 µm, 1.94–2.04 µm and 2.26–2.38 µm with moderate spectral resolution (0.22–1.5 nm). From their structured absorption in these spectral regions, we can quantitatively determine the total column densities of the greenhouse gases CO2, CH4, N2O and H2O as well as of CO. A modified DOAS algorithm based on optimal estimation (IMAP-DOAS) has been developed at the University of Heidelberg to account for the peculiarities of these absorbers. CO is a relatively weak absorber whose spectral signature is overlapped by strong CH4 and H2O absorptions. Hence, retrieval of CO from SCIAMACHY spectra (within 2.26–2.38 µm) is a challenging task. Therefore, the calibration of the raw spectra with respect to dark current issues and nonlinearity were analysed in detail and substantially improved to enable reasonable retrieval of CO. This paper focusses on first results of the CO retrieval where various sources like biomass burning events and their seasonal variability can be clearly identified.

Description: Within the framework of the ENVISAT/-SCIAMACHY satellite validation, solar irradiance spectra are absolutely measured at moderate resolution in the UV/visible spectral range (in the UV from 316.7–418 nm and the visible from 400–652 nm at a full width half maximum resolution of 0.55 nm and 1.48 nm, respectively) from aboard the azimuth-controlled LPMA/DOAS balloon gondola at around 32 km balloon float altitude. After accounting for the atmospheric extinction due to Rayleigh scattering and gaseous absorption (O3, and NO2), the measured solar spectra are compared with previous observations. Our solar irradiance is +1.6% larger than the re-calibrated Kurucz et al. (1984) solar spectrum (Fontenla et al., 1999, called MODTRAN 3.5) in the visible spectral range (435–650 nm), +1.5% larger in the (370–415 nm) wavelength interval, but −4% smaller in the UV spectral range (316.7–370 nm), when the Kurucz spectrum is convolved to the spectral resolution of our instrument. The same comparison with the SOLSPEC solar spectrum (Thuillier et al., 1997, 1998a, b) confirms the somewhat larger solar irradiance (+1.7%) measured by the balloon instrument from 435–500 nm, but not from 500–650 nm, where the SOLSPEC is −1.3% lower than MODTRAN 3.5. Comparison of the SCIAMACHY solar spectrum from channels 1 to 4 (– re-calibrated by the University of Bremen –) with MODTRAN 3.5 indicates an agreement of +0.2% in the visible spectral range (435–585 nm). With this calibration, the SCIAMACHY solar spectrum is congruent with the balloon observations (−1%) in the 316.7–370 nm wavelength range, but both are up to −5%/−3% smaller than MODTRAN 3.5 and SOLSPEC, respectively. In agreement with findings of Skupin et al. (2002) our study emphasizes that the present ESA SCIAMACHY level 1 calibration is systematically +15% larger in the considered wavelength intervals when compared to all available other solar irradiance measurements.

Description: Ozone and nitrous oxide are measured at high spatial and temporal resolution by instruments flying on the ER-2 NASA research aircraft. Comparing the airborne transects to reconstructions by ensemble of diffusive backward trajectories allows to estimate the average vertical Lagrangian turbulent diffusion experienced by the air parcels. The resulting estimates show large Lagrangian diffusion of the order of 0.1 m2 s−1 outside the polar vortex in the surf zone and smaller values of the order of 0.01 m2 s−1 inside. Locally, large variation of Lagrangian diffusion occur over mesoscale distances. It is found that high temporal resolution (3 h or less) is required for off-line transport calculations and that the reconstructions are sensitive to spurious motion in standard analysed winds.

Description: A case study is presented on the formation and evolution of an ice-supersaturated region (ISSR) that was detected by a radiosonde in NE Germany at 06:00 UTC 29 November 2000. The ISSR was situated in the vicinity of the outflow region of a warm conveyor belt associated with an intense event of cyclogenesis in the eastern North Atlantic. Using ECMWF analyses and trajectory calculations it is determined when the air parcels became supersaturated and later subsaturated again. In the case considered, the state of air parcel supersaturation can last for longer than 24 h. The ISSR was unusually thick: while the mean vertical extension of ISSRs in NE Germany is about 500 m, the one investigated here reached 3 km. The investigated ice-supersaturated region was bordered both vertically and horizontally by strongly subsaturated air. Near the path of the radiosonde the ISSR was probably cloud free, as inferred from METEOSAT infrared images. However, at other locations within the ISSR it is probable that there were cirrus clouds. Relative humidity measurements are used to correct the negative bias of the ECMWF humidity and to construct two-dimensional maps of ice supersaturation over Europe during the considered period. A systematic backward trajectory analysis for the ISSRs on these maps shows that the ISSR air masses themselves experienced only a moderate upward motion during the previous days, whereas parts of the ISSRs were located just above strongly ascending air masses from the boundary layer. This indicates qualitatively that warm conveyor belts associated with mid-latitude cyclogenesis are disturbances that can induce the formation of ISSRs in the upper troposphere. The ISSR maps also lead us to a new perception of ISSRs as large dynamic regions of supersaturated air where cirrus clouds can be embedded at some locations while there is clear air at others.