[1] In the laboratory, we have investigated the growth and composition of frost flowers. Their ionic composition has shown little difference from those of field measurements. Young frost flowers grown on sea ice are saline, leading us to speculate that wicking occurs continually during their growth on sea ice. The surface area of frost flowers is only a little larger than the area of ice underneath, consistent with recent field measurements from the Arctic. Time-lapse photography has allowed us to observe the extreme mobility of freshly forming sea ice, at the stage at which the mush has become rather solid, and continuing while the flowers grow. This mobility results in new brine being expelled to the surface, which therefore remains wet. During various stages of frost flower growth, we observed their freshly formed dendritic parts rapidly diminishing in size after contacting the surface, consistent with repeated wicking. Frost flowers proved to be very stable in the presence of wind, such that no aerosol was observed when wind was blown across them in the laboratory chamber. This is consistent with recent field observations of frost flowers coexisting with wind-blown snow.

The Compact Lightweight Aerosol Spectrometer Probe (CLASP) is an optical particle spectrometer capable of measuring size-resolved particle concentrations in 16 user-defined size bins spanning diameters in the range 0.24 Ͻ D Ͻ 18.5 m at a rate of 10 Hz. The combination of its compact nature and lightweight and robust build allows for deployment in environments and locations where the use of the larger, heavier, more traditional instrumentation would prove awkward or impossible. The high temporal resolution means it is particularly suited to direct measurements of aerosol fluxes via the eddy covariance technique. CLASP has been through an extended evolutionary development. This has resulted in an instrument whose performance characteristics are well established.

Aerosols are known to influence significantly the radiative budget of the Earth. Although the direct effect (whereby aerosols scatter and absorb solar and thermal infrared radiation) has a large perturbing influence on the radiation budget, the indirect effect (whereby aerosols modify the microphysical and hence the radiative properties and amounts of clouds) poses a greater challenge to climate modellers. This is because aerosols undergo chemical and physical changes while in the atmosphere, notably within clouds, and are removed largely by precipitation. The way in which aerosols are processed by clouds depends on the type, abundance and the mixing state of the aerosols concerned. A parametrization with sulphate and sea-salt aerosol has been successfully integrated within the Hadley Centre general circulation model (GCM). The results of this combined parametrization indicate a significantly reduced role, compared with previous estimates, for sulphate aerosol in cloud droplet nucleation and, consequently, in indirect radiative forcing. However, in this bicomponent system, the cloud droplet number concentration, N d (a crucial parameter that is used in GCMs for radiative transfer calculations), is a smoothly varying function of the sulphate aerosol loading. Apart from sea-salt and sulphate aerosol particles, biomass aerosol particles are also present widely in the troposphere. We find that biomass smoke can significantly perturb the activation and growth of both sulphate and sea-salt particles. For a fixed salt loading, N d increases linearly with modest increases in sulphate and smoke masses, but significant nonlinearities are observed at higher non-sea-salt mass loadings. This non-intuitive N d variation poses a fresh challenge to climate modellers.

Abstract. Most estimates of sea spray aerosol source functions have used indirect means to infer the rate of production as a function of wind speed. Only recently has the technology become available to make high frequency measurements of aerosol concentration suitable for direct eddy correlation determination of the particle flux. This was accomplished in this study by combining a newly developed fast aerosol particle counter with an ultrasonic anemometer which allowed for eddy covariance measurements of size-segregated particle fluxes. The aerosol instrument is the Compact Lightweight Aerosol Spectrometer Probe (CLASP) – capable of measuring 8-channel size spectra for mean radii between 0.15 and 0.35 μm at 10 Hz. The first successful measurements were made during the WASFAB (Waves, Air Sea Fluxes, Aerosol and Bubbles) field campaign in October 2005 in Duck (NC, USA). The method and results are presented and comparisons are made with recent sea spray source functions from the literature.

Abstract. The North Atlantic Marine Boundary Layer Experiment (NAMBLEX), involving over 50 scientists from 12 institutions, took place at Mace Head, Ireland (53.32 • N, 9.90 • W), between 23 July and 4 September 2002. A wide range of state-of-the-art instrumentation enabled detailed measurements of the boundary layer structure and atmospheric composition in the gas and aerosol phase to be made, providing one of the most comprehensive in situ studies of Correspondence to: D. E. Heard (dwayneh@chem.leeds.ac.uk) the marine boundary layer to date. This overview paper describes the aims of the NAMBLEX project in the context of previous field campaigns in the Marine Boundary Layer (MBL), the overall layout of the site, a summary of the instrumentation deployed, the temporal coverage of the measurement data, and the numerical models used to interpret the field data. Measurements of some trace species were made for the first time during the campaign, which was characterised by predominantly clean air of marine origin, but more polluted air with higher levels of NO x originating from continental regions was also experienced. ThisPublished by Copernicus GmbH on behalf of the European Geosciences Union. paper provides a summary of the meteorological measurements and Planetary Boundary Layer (PBL) structure measurements, presents time series of some of the longer-lived trace species (O 3 , CO, H 2 , DMS, CH 4 , NMHC, NO x , NO y , PAN) and summarises measurements of other species that are described in more detail in other papers within this special issue, namely oxygenated VOCs, HCHO, peroxides, organohalogenated species, a range of shorter lived halogen species (I 2 , OIO, IO, BrO), NO 3 radicals, photolysis frequencies, the free radicals OH, HO 2 and (HO 2 + RO 2 ), as well as a summary of the aerosol measurements. NAMBLEX was supported by measurements made in the vicinity of Mace Head using the NERC Dornier-228 aircraft. Using ECMWF windfields, calculations were made of the air-mass trajectories arriving at Mace Head during NAMBLEX, and were analysed together with both meteorological and trace-gas measurements. In this paper a chemical climatology for the duration of the campaign is presented to interpret the distribution of air-mass origins and emission sources, and to provide a convenient framework of air-mass classification that is used by other papers in this issue for the interpretation of observed variability in levels of trace gases and aerosols.