In order to study the growth/shrinking of a hygroscopic nanoparticle during hydration/dehydration in an atmosphere of water vapour, we have employed a thermodynamic approach proposed by Shchekin et al. (2008). This approach uses the mechanic and thermodynamic concept of disjoining pressure of thin films and allows, among others, the prediction of the humidity growth factor of both (i) a homogeneous solution droplet with completely dissolved residual core and (ii) a heterogeneous solution droplet with partially dissolved residual core as a function of the ambient relative humidity. For application to a nanometric sodium chloride particle we have extended the original approach by (i) considering the nonideality of the solution through the dependence of molecular volumes of the solvent and solute molecules and the solute and solvent activities on the solution concentration, (ii) deriving an equation for the estimation of the efflorescence properties of a homogeneous solution droplet, and (iii) combining the empirical power law fittings for the size dependence of the deliquescence and efflorescence relative humidity values by Biskos et al. (2006a). It was demonstrated how the solution/solute interface energy and the correlation length of a thin solution film can be determined from a combination of experimentally determinable efflorescence and deliquescence humidities with the present calculus. The solution/solute interface energy was found to be in close agreement with some previous values reported in the literature, while it strongly differs from data of some other sources. The calculated deliquescence humidity shows a low sensitivity to the choice of the numerical value for the film correlation length. The estimated film correlation length of 1 nm for a nanometric sodium chloride particle with dry particle radius of 5 nm was found to be reconcilable with available a priori estimates of the correlation length from the literature when the measurement uncertainty of the deliquescence humidity is considered. Considering the combination of an extensive calculus, a comprehensive set of thermophysical constraints, and independent measurements of the deliquescence and efflorescence humidities as functions of dry particle radius, the obtained values of the solution/solute interface energy and the correlation length are in close agreement with previous estimations. The humidification of sodium chloride particles in the initial hydration stages was found to be very sensitive to the specification of the disjoining pressure. The enhancement of the wettability of the particle surface leads to an earlier onset of hygroscopic growth.

During the SAMUM field campaign in southern Morocco in May and June 2006 density currents generated by evaporative cooling after convective precipitation were frequently observed at the Sahara side of the Atlas Mountain chain. The associated strong surface cold‐air outflow during such events has been observed to lead to dust mobilization in the foothills. Here a regional model system is used to simulate a density current case on 3 June 2006 and the subsequent dust emission. The model studies are performed with different parameterization schemes for convection, and with different horizontal model grid resolutions to examine to which extent the model system can be used for reproducing dust emissions in this region. The effect of increasing the horizontal model grid resolution from 14 km to 2.8 km on the strength on the density currents and thus on dust emission is smaller than the differences due to different convection parameterization schemes in this case study. While the results in reproducing the observed density current at the Atlas Mountain varied with different convection parameterizations, the most realistic representation of the density current is obtained at 2.8 km grid resolution at which no parameterization of deep convection is needed.

Results are reported from a ground-based measurement campaign conducted in a highly polluted region in southeast of China in October-November 2004. The experiment focused on absorbing aerosol particles and their effects on the solar radiation field and local meteorology. A Raman lidar in conjunction with Sun photometer data measured profiles of particle extinction; ground-based in situ data of aerosol optical properties were collected by nephelometer and absorption photometer. Exceptionally high values of aerosol optical depth of up to 1.5 were observed.The measurements were input to a radiative transfer model, which simulated high solar radiative forcing values for the aerosol particles of up to -160 W m(-2) at the ground (daily average) for the observed particle single-scattering albedo of 0.85. Maximum solar heating rates of 7-8 K day(-1) were simulated at the top of the aerosol layer.The radiative simulations were used to drive a dynamic model of the planetary boundary layer (PBL). With this model the temporal course of the height of the PBL was simulated and compared with respective lidar data. The results show that the height of the PBL is significantly decreased due to the warming of the aerosol particles at the top of the PBL. In this way, the stabilizing effect of absorbing aerosol particles within the PBL was confirmed by a combination of experimental and modeling means. (c) 2008 Elsevier Ltd. All rights reserved.

Mineral dust radiative effects and feedbacks upon Saharan boundary layer meteorology are estimated by means of regional dust simulations for a one‐week period in May 2006. The regional dust model system LM‐MUSCAT is used, which allows online interaction of the computed dust load with the solar and thermal radiation and consequently with the model dynamics. For the Bodélé depression, a mechanism is proposed that describes positive and negative dust radiative feedbacks on surface wind speeds and dust emission in dependence on atmospheric stratification, dust‐induced stabilization, and baroclinity. These effects are attributed to the formation and breakdown of the low‐level jet (LLJ), which is a common phenomenon in the Bodélé. However, such processes are likely also relevant for other Saharan regions.

A new regional model system was developed for simulation of emission, transport, deposition, and radiative effects of Saharan desert aerosol within the framework of the Saharan Mineral Dust Experiment (SAMUM). For this the mesoscale meteorological model LM, a dust emission scheme and a transport model were coupled. To test the model performance, two major Saharan dust outbreaks directed to Europe in August and October 2001 are simulated. Comparisons with sounding data and 10‐m wind speeds from north African sites show that the LM provides reliable meteorological fields to describe the emission and near‐source transport of dust. As shown by comparisons with satellite observations, lidar profiles, and Sun photometer measurements at selected stations, the spatiotemporal evolution of the dust plume is reasonably well reproduced by the model. The predicted dust interacts with the LM radiation at solar and thermal wavelengths. Saharan dust causes a negative effect on the net radiative budget at the top of the atmosphere in the source regions and accounts for a reduction in 10‐m wind speeds. Thus it is responsible for a reduction in the dust production of up to about 50% during the October 2001 event.

This paper gives an overview of the set up, methodology and the obtained results of the CityDelta (phase 1 and 2) project. In the context of the Clean Air For Europe programme of the European Commission, the CityDelta project was designed to evaluate the impact of emission-reduction strategies on air quality at the European continental scale and in European cities. Ozone and particulate matter (PM) are the main components that have been studied. To achieve this goal, a model intercomparison study was organized with the participation of more than 20 modelling groups with a large number of modelling configurations. Two following main topics can be identified in the project. First, in order to evaluate their strengths and weaknesses, the participating models were evaluated against observations in a control year (1999). An accompanying paper will discuss in detail this evaluation aspect for four European cities. The second topic is the actual evaluation of the impact of emission reductions on levels of ozone and PM, with particular attention to the differences between large-scale and fine-scale models. An accompanying paper will discuss this point in detail. In this overview paper the main input to the intercomparison is described as well as the use of the ensemble approach. Finally, attention is given to the policy relevant issue on how to implement the urban air quality signal into large-scale air quality models through the use of functional relationships.

Tethered‐balloon borne observations show a correlation between a significant peak of ultrafine particle concentration and increased SO2 advected by a nocturnal low‐level jet. The maximum of ultrafine particles is found at the lower edge of the jet in a region where the advected SO2 is mixed with colder and more humid air masses from below the jet. Model results corroborate that kinetic nucleation is the key mechanism for this nucleation event.

On the basis of a new regional dust model system, the sensitivity of radiative forcing to dust aerosol properties and the impact on atmospheric dynamics were investigated. Uncertainties in optical properties were related to uncertainties in the complex spectral refractive index of mineral dust. The climatological‐based distribution of desert‐type aerosol in the radiation scheme of the nonhydrostatic regional model LM was replaced by dust optical properties from spectral refractive indices, derived from in situ measurements, remote sensing, bulk measurements, and laboratory experiments, employing Mie theory. The model computes changes in the solar and terrestrial irradiance from a spatially and temporally varying atmospheric dust load for five size classes. A model study of a Saharan dust outbreak in October 2001 was carried out when large amounts of Saharan dust were transported to Europe. The dust optical thickness computed from the simulation results in values of about 0.5 in large regions of the Saharan desert but can be larger than 5.0 near large dust sources (for example, Bodélé depression). During the dust outbreak, the aerosol in the southern Sahara causes a daytime reduction in 2‐m temperature of 3 K in average with differences of 10% depending on used dust optical properties. The simulations indicated that the large variability in radiative properties due to different mixture of clay aggregates in Saharan dust can lead in regional average to differences of up to 48% in net forcing efficiency at top of the atmosphere.

The manuscript presents a detailed description of the meteorological and chemical code of Malte – a model to predict new aerosol formation in the lower troposphere. The aerosol dynamics are achieved by the new developed UHMA (University of Helsinki Multicomponent Aerosol Model) code with kinetic limited nucleation as responsible mechanism to form new clusters. First results indicate that the model is able to predict the on- and offset of new particle formation as well as the total aerosol number concentrations that were in good agreement with the observations. Further, comparison of predicted and measured H2SO4 concentrations showed a satisfactory agreement. The simulation results indicated that at a certain transitional particle diameter (2–7 nm), organic molecules can begin to contribute significantly to the growth rate compared to sulphuric acid. At even larger particle sizes, organic molecules can dominate the growth rate on days with significant monoterpene concentrations. The intraday vertical evolution of newly formed clusters and particles in two different size ranges resulted in two maxima at the ground. These particles grow around noon to the detectable size range and agree well with measured vertical profiles.

Abstract. In part I to III of the present paper a revised columnar high-order modelling approach to model gas-aerosol interactions in the convective boundary layer (CBL) was proposed, and simulation results of two nucleation scenarios (binary vs. ternary) on new particle formation (NPF) in the anthropogenically influenced CBL were presented. It was demonstrated that both scenarios strongly differ with respect to the amplitude and phase of the NPF burst detectable in the Prandtl layer, as well as with respect to the time-height evolution of turbulent vertical fluxes and double correlation terms of physico-chemical and aerosoldynamical variables. In the present part, an attempt is made to re-evaluate previous observations of NPF bursts in the CBL in view of the scenario simulations discussed in part III. Special attention is payed to the role of CBL turbulence in NPF burst evolution. At first, a compilation of empirical findings and hypothesis on NPF in the CBL derived from a number of field experiments, is performed. Secondly, it is demonstrated, that the binary scenario simulated in part III corresponds well to a number of NPF burst events observed in Hyytiälä (Finland) and Melpitz (Eastern Germany). Here, one of the key hypothesis on the role of turbulence in NPF is confirmed. Other NPF events, such as those observed at Hohenpeissenberg, a mountain site (Southern Germany), can not yet be conclusively explained. To note, that the results of previous box modelling studies to explain NPF events at Hohenpeissenberg are not unambiguous. Nonetheless, based on only two simulated scenarios it is demonstrated, that a columnar high-order model is a helpful tool to elucidate the genesis of NPF bursts frequently observed in the CBL. A comprehensive verification/validation study using observed high-order moments as well as further scenario simulations remain to be performed.

Conceptual study on nucleation burst evolution in the convective boundary layer Part I: Modelling approach O. Hellmuth Atmos. Chem. Phys. Discuss., 5, 11413-11487, 2005 Paper I General comments: (These are for the whole 4-paper set together) In the paper "Conceptual study on nucleation burst evolution in the convective boundary layer" the author proposes a modelling approach to interprete anthropogenically

During the SATURN experiment, which took place from 27 May to 14 June 2002, new particle formation in the continental boundary layer was investigated. Simultaneous ground-based and tethered-balloon-borne measurements were performed, including meteorological parameters , particle number concentrations and size distributions, gaseous precursor concentrations and SODAR and LIDAR observations. Newly formed particles were observed inside the residual layer, before the break-up process of the nocturnal inversion, and inside the mixing layer throughout the break-up of the nocturnal inversion and during the evolution of the planetary boundary layer.

The physical and chemical processes in the atmosphere are very complex. They occur simultaneously, Coupled and in a wide range of scales. These facts have to be taken into account in the numerical methods for the solution of the model equations. The numerical techniques should allow the use of different resolutions in space and also In time. Air quality models base on mass balances described by systems of time-dependent, three-dimensional advection-diffusion-reaction equations. A parallel version of the multiscale chemistry-transport code MUSCAT is presented which is based on multiblock grid techniques and implicit-explicit (IMEX) time integration schemes. The meteorological fields are Generated simultaneously by the non-hydrostatic meteorological model LM. Both codes run in parallel mode on a predefined number of processors and-exchange informations by an implemented coupler interface. The ability and performance of the model system are discussed for a "Berlioz" ozone episode.