Doug Johnson from the UK's Met Office reflects on the lessons Europe has learnt from the volcanic ash cloud crisis that closed most of its airports last year.

Observations from the Meteorological Research Flight's Hercules C-130 aircraft of the microphysical characteristics of warm stratocumulus clouds have been analyzed to investigate the variation of the effective radius of cloud droplets in layer clouds. Results from experiments in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and the sea areas around the British Isles are presented. In situations where entrainment effects are small the (effective radius)3 is found to be a linear function of the (volume-averaged radius)3 in a given cloud and can thus be parameterized with respect to the liquid water content and the droplet number concentration in the cloud. However, the shape of the droplet size spectrum is very dependent on the cloud condensation nuclei (CCN) characteristics below cloud base, and the relationship between effective radius and volume-averaged radius varies between maritime air masses and continental air masses. This study also details comparisons that have been made in stratocumulus between the droplet number concentrations and (a) aerosol concentrations below cloud base in the size range 0.1 to 3.0 mum and (b) CCN supersaturation spectra in the boundary layer. A parameterization relating droplet concentration and aerosol concentration is suggested. The effects of nonadiabatic processes on the parameterization of effective radius are discussed. Drizzle is found to have little effect near cloud top, but in precipitating stratocumulus clouds the parameterization breaks down near cloud base. Comparisons are made between this parameterization of effective radius and others used currently or in the past.

Aircraft measurements are presented of the interaction of aerosols and stratocumulus cloud within the effluent of ships powered by partial combustion of low‐grade diesel fuel. Two case studies are shown where aerosol is modified by cloud processing over times of 1–2 hours, or <1 hour accumulatively spent within cloud. One case shows a good example of the Twomey effect, whereby the cloud droplet effective radius reduced and the extinction coefficient increased quite dramatically for a given liquid water content. The other cloudy case shows relatively little cloud perturbation due to the ship aerosol. In both cases a Hoppel dip appears in the dehydrated aerosol spectrum between 0.1 and 0.2 μm diameter and a new mode grows out to about 0.45 μm. Scattering coefficients of particles between 0.1–0.7 μm show that the modified spectra have greater efficiency at scattering solar radiation. A modeling study of one of these cases indicates that aqueous‐phase sulfur chemistry within cloud can explain the aerosol features. By comparison with the concomitant processing of the background aerosol, it is shown that the Hoppel dip and hence critical size for droplet activation lies at larger particle sizes in polluted clouds. A third case study is presented where the plume evolves in a cloud‐free boundary layer. Here no modal growth of the aerosol was seen and the aerosol was diluted primarily by mixing with background air. The aerosol was highly deliquesced, which suggests that haze particle chemistry was probably insignificant in modifying the aerosol in any of the cases.

Preliminary experimental results are presented from an aircraft-mounted probe designed to provide in situ data on cloud particle shape, size, and number concentration. In particular, the probe has been designed to facilitate discrimination between super-cooled water droplets and ice crystals of 1–25μm size within mixed-phase clouds and to provide information on cloud interstitial aerosols. The probe acquires spatial light scattering data from individual particles at throughput rates of several thousand particles per second. These data are logged at 100ms intervals to allow the distribution and number concentration of each particle type to be determined with 10m spatial resolution at a typical airspeed of 100ms−1. Preliminary results from flight data recorded in altocumulus castellanus, showing liquid water phase, mixed phase, and ice phase are presented to illustrate the probe's particle discrimination capabilities.

This paper reports on measurements made during the INTACC (INTeraction of Aerosol and Cold Clouds) experiment. Observations are shown from six wave cloud flights in the temperature range −12°C to −40 °C. In nearly all cases ice nucleation does not occur until after droplets have formed. Parcel trajectories that experience temperatures below −35°C are dominated by homogeneous nucleation, while parcel trajectories that stay above −35°C are affected by heterogeneous nucleation. Ice particles appear to be nucleated after the parent droplets reach their largest size and coldest temperature at the wave peak. The most likely heterogeneous nucleation mechanisms are immersion and/or contact nucleation.

CLOUDYCOLUMN is one of the 6 ACE-2 projects which took place in June-July 1997, between Portugal and the Canary Islands.It was specifically dedicated to the study of changes of cloud radiative properties resulting from changes in the properties of those aerosols which act as cloud condensation nuclei.This process is also refered to as the aerosol indirect effect on climate.CLOUDYCOLUMN is focused on the contribution of stratocumulus clouds to that process.In addition to the basic aerosol measurements performed at the ground stations of the ACE-2 project, 5 instrumented aircraft carried out in situ characterization of aerosol physical, chemical and nucleation properties and cloud dynamical and microphysical properties.Cloud radiative properties were also measured remotely with radiometers and a lidar.11 case studies have been documented, from pure marine to significantly polluted air masses.The simultaneity of the measurements with the multi-aircraft approach provides a unique data set for closure experiments on the aerosol indirect effect.In particular CLOUDYCOLUMN provided the 1st experimental evidence of the existence of the indirect effect in boundary layer clouds forming in polluted continental outbreacks.This paper describes the objectives of the project, the instrumental setup and the sampling strategy.Preliminary results published in additional papers are briefly summarized.

ACE-2. the second Aerosol Characterization Experiment of the International Global Atmospheric Chemistry Project (IGAC), was conducted in the area between Portugal, Tenerife and Madeira from IS June to 25 July 1997. We determined the concentration of SO2 and the soluble ion composition of the atmospheric aerosol in 113 samples collected by aircraft. Comparison between aircraft and ground-based samples collected from the same or similar airmasses showed good agreement (better than 40%) for the fine fraction of the aerosol, but suggests that, for the coarse fraction. the sampling efficiency of the aircraft inlet is only about 35%. During periods when trajectory analysis suggested no recent contact of the airmass with Europe or North America, SO2 and aerosol ions were at levels comparable to those found over remote ocean regions. The composition of airmasses originating from Europe showed signatures characteristic of the source regions and suggested rapid oxidation of SO2 during transport over the ocean. The first Lagrangian experiment was conducted in an unpolluted airmass and showed the physical and chemical evolution of a marine boundary layer traversing over increasingly warmer ocean waters. The sulfur cycle in this airmass could be explained based on the emission of DMS From the sea surface. In three other Lagrangian experiments, we investigated the evolution of boundary layers with increasing age since having left the European continent. SO2 was removed rapidly with lifetimes on the order of half a day in cloud-topped boundary layers. The production of nss-sulfate ceased after SO2 had declined to background levels, and the aerosol approached a nearly constant composition, with concentrations dominated by physical removal and dilution processes. Aerosol nitrate concentrations suggested that gaseous HNO3 was taken up by seasalt aerosol and subsequently removed by dry deposition.

Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks. This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.

A review is given of the climatological and actual meteorological conditions in the sub-tropical northeast Atlantic, during June−July 1997, when the 2nd Aerosol Characterization Experiment (ACE-2) took place. Surface pressure maps, trajectory calculations and in-situ measurements show how the outflow of European pollution into the marine boundary layer of this area is determined by the location of the Azores high pressure cell. Observations during ACE-2 and 3 preceding summers show that pollution outbreaks both from the Iberian peninsula and from northern or central Europe can occur during such situations. During ACE-2, an unusually low number of strong North African dust outbreaks were recorded at the free tropospheric station of Izaña (Tenerife, 2360 m asl), although dust was recorded aloft the station.

Three Lagrangian experiments were conducted during IGAC's second aerosol characterization experiment (ACE-2) in the area between Portugal, Tenerife and Madeira in June/July 1997. During each Lagrangian experiment, a boundary layer air mass was followed for about 30 h, and the temporal evolution of its chemical and aerosol composition was documented by a series of vertical profiles and horizontal box pattern flown by the Meteorological Research Flight research aircraft Hercules C130. The wealth of observational data that has been collected during these three Lagrangian experiments is the basis for the development and testing of a one-dimensional Lagrangian boundary layer model with coupled gas, aqueous, and aerosol phase chemistry. The focus of this paper is on current model limitations and strengths. We show that the model is able to represent the dynamical and chemical evolution of the marine boundary layer, in some cases requiring adjustments of the subsidence velocity and of the surface heat fluxes. Entrainment of a layer rich in ozone and carbon monoxide from a residual continental boundary layer into the marine boundary layer as well as in-cloud oxidation of sulphur dioxide by hydrogen peroxide are simulated, and coherent results are obtained, concerning the evolution of the small, presumably sulphate–ammonia aerosol mode.

Significant changes were observed in the sub-micron aerosol size distribution during a clean and a polluted Lagrangian study of marine boundary layer (MBL) aerosol and meteorological evolution during ACE-2. These changes were accompanied by significant alterations in boundary layer meteorology and structure. The clean case (LAG1) shows a reduction in the fine mode aerosol from 1050 cm-3 to 750 cm-3 and an increase in the accumulation mode concentration from 76 cm-3 to 162 cm-3 over 26 h. Dominant meteorological features during the same period comprised a reduction in boundary layer height from δ1500 m to δ800 m and an increase in the surface layer wind speed from 5 m s-1 to 15 m s-1. A detailed time-scale analysis, based upon measured data and including processes such as coagulation, condensation, deposition, chemical processing, sea-salt flux and entrainment, suggests that the dominant loss process for fine mode aerosol is coagulation, while the enhancement of accumulation mode aerosol can be almost totally ascribed to enhanced sea-salt aerosol flux into the reduced mixed layer volume. Aerosol size distributions from the polluted Lagrangian (LAG2) indicated little growth in particle diameter, and both fine and accumulation mode were observed to decrease in concentration from 2700 cm-3 to 1150 cm-3 and from 670 cm-3 to 430 cm-3 in 26 h, respectively. Dilution with cleaner free tropospheric air as the boundary layer height increased from δ500 m to >1000 m is suggested to be the primary factor relating to reduced aerosol concentrations in this case. To a smaller extent, coagulation and precipitation scavenging were calculated to be of some importance. For both Lagrangian case studies, meteorological changes, followed by physical aerosol-cloud interactions, appear to have the greatest influence on the MBL aerosol size distribution and number concentration over the given time-scale.

The LES model is applied for studying ship track formation under various boundary layer conditions observed during the Monterey Area Ship Track experiment. Simulations in well-mixed and decoupled boundary layers show that ship effluents are easily advected into the cloud layer in the well-mixed convective boundary layer, whereas their transport may be suppressed by the subcloud transitional layer in the decoupled case. The clear difference between the well-mixed and decoupled cases suggests the important role of diurnal variation of solar radiation and consequent changes in the boundary layer stability for ship track formation. The authors hypothesize that, all other conditions equal, ship track formation may be facilitated during the morning and evening hours when the effects of solar heating are minimal. In a series of experiments, the authors also studied the effects of additional buoyancy caused by the heat from the ship engine exhaust, the strength of the subcloud transitional layer, and the subcloud layer saturation conditions. The authors conclude that additional heat from ship engine and the increase in ship plume buoyancy may indeed increase the amount of the ship effluent penetrating into the cloud layer. The result, however, depends on the strength of the stable subcloud transitional layer. Another factor in the ship effluent transport is the temperature of the subcloud layer. Its decrease will result in lowering the lifting condensation level and increased ship plume buoyancy. However, the more buoyant plumes in this case have to overcome a larger potential barrier. The relation between all these parameters may be behind the fact that ship tracks sometimes do, and sometimes do not, form in seemingly similar boundary layer conditions.

Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as ‘‘ship tracks.’’ Airborne measurements are used to derive emission factors of SO 2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ;15‐89 and ;2‐25 g kg21 of fuel burned, respectively. By contrast a steam turbine‐powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ; 6gk g 21. Various types of ships, burning both MFO and navy distillate fuel, emitted from ;4 3 1015 to 2 3 1016 total particles per kilogram of fuel burned (;4 3 1015‐1.5 3 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (;0.03‐0.05 mm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ;0.02 mm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine‐powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis. The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO 2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN. The fluxes of heat and water vapor from ships are estimated to be ;2‐22 MW and;0.5‐1.5 kg s21, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.

The second Aerosol Characterisation Experiment (ACE-2) was aimed at investigating the physical, chemical and radiative properties of aerosol and their evolution in the North Atlantic region. In the 2nd "Lagrangian" experiment, an air mass was tracked over a 30-h period during conditions of extensive stratocumulus cover. Boundary-layer measurements of the aerosol size distribution obtained with a passive cavity aerosol spectrometer probe (PCASP) during the experiment show a gradual growth in size of particles in the 0.1-0.2 mu m diameter mode. Simultaneously, SO2 concentrations were found to decrease sharply from 800 to 20 ppt. The fraction of sulphate in aerosol ionic mass increased from 0.68 +/- 0.07 to 0.82 +/- 0.09 for small particles (diameter below 1.7 mu m) and from 0.21 +/- 0.04 to 0.34 +/- 0.03 for large particles (diameter above 1.7 mu m). The measurements were compared with a multicyclic parcel model of gas phase diffusion into cloud droplets and aqueous phase chemical reactions. The model was able to broadly reproduce the observed transformation in the aerosol spectra and the timescale for the transformation of SO2 to sulphate aerosol. The modelled SO2 concentration in the boundary layer fell to below half its initial value over a 6.5-h time period due to a combination of the entrainment of cleaner tropospheric air and cloud chemical reactions. NH3 and HCl gas were also found to play an important role in cloud processing in the model.

We present observations from the 2nd Aerosol Characterisation Experiment where over a 29-h period between 16−18 July 1997 a tagged column of air was followed by a fully instrumented aircraft. The Lagrangian framework this offered made it possible to measure the evolution of the aerosol size distribution, the cloud structure and microphysics, and the dynamic and thermodynamic structure of the marine boundary layer within a polluted airmass advecting off northwest Europe over the sub-tropical North Atlantic Ocean. The salient observations are presented and analysed. Processes responsible for the evolution are suggested, but quantification of their respective rates must be taken up by future modelling studies. Stratocumulus capped the boundary layer throughout the period that produced negligible washout of aerosol. This implies that the conversion of a continental to a maritime airmass within the cloud-capped sub-tropical marine boundary layer is not controlled by the drizzle process but by entrainment from the free troposphere. We find evidence of processing of aerosol particles by stratocumulus cloud, in particular by aqueous-phase reactions. The processing of the aerosol, realised by modification of the aerosol size distribution in the particle diameter range 0.1−0.5 μm, was complicated by rapid changes in boundary layer height and structure, and also by entrainment of both polluted and relatively clean aerosol from the free troposphere. The cloud microphysics was affected by these changes in the boundary layer aerosol through changes in the cloud condensation nuclei activation spectra. The cloud microphysics was also strongly affected by changes in the dynamics of the boundary layer which included variations (e.g., diurnal) in cloud thickness and an increase in vertical wind speed. Thermodynamic changes within the boundary layer included decoupling due to an increasing sea-surface temperature and a change in the subsidence rate in the free troposphere superimposed on diurnal decoupling. Hypotheses have been devised so that future modellers can focus their efforts to either validate or invalidate potentially important processes.

Exchange rates of aerosol particles and vapor species between layers in the atmosphere allow us to estimate the lifetime of particles in the lower troposphere. This work analyzes data obtained during the ACE-2 campaign to calculate exchange using two independent methods, divergence and flux. The net entrainment rates obtained from the divergence method are based on spatially-integrated horizontal winds as well as on the average boundary layer height change. The flux method is based on an eddy correlation approach, but relies in this case on a point measurement of concentration change across an atmospheric interface. The thermodynamic structure in these three experiments included well-mixed layers in addition to overlying, more-stratified buffer layers, between which we have studied the net entrainment of air between adjacent layers. The range of entrainment rate magnitudes reported from both methods was from 0.000 m s-1 to 0.050 m s-1(with the exception of a few outlying values). Since both methods have significant uncertainties, we believe the best estimates are the average net entrainment rates for both methods, which were 0.007, 0.007, and 0.006 m s-1 at the subsidence inversion, for Lagrangians 1, 2, and 3, respectively. The uncertainties were high for both methods, involving a factor of two uncertainty for entrainment rates below 0.020 m s-1. This high uncertainty suggests that continued use of multiple independent methods for measuring entrainment, preferably with the aid of improved instrumentation for fast measurement of conserved tracers and well-designed sampling strategies, is essential for improving models of the sources and sinks for chemical evolution. In some cases, net entrainment rates calculated from the 2 methods were comparable, but in others the spatial inhomogeneity and sampling limitations led to significant discrepancies in the predicted rates.

Airborne measurements from the Meteorological Research Flight's Hercules C-130 and the University of Washington's Convair C-131A during the Monterey Area Ship Track field project are used to evaluate Twomey's analytic expression for cloud susceptibility, which describes the sensitivity of cloud albedo to changes in droplet concentrations, This expression incorporates assumptions about cloud physics, such as the independence of the cloud liquid water content and the width of the droplet size distribution on droplet concentrations. Averaged over all 69 ship track penetrations, cloud liquid water content decreased slightly and the droplet size distributions broadened from the ambient values. For the 17 cases for which albedos were measured during overflights, Twomey's parameterization represents the trend of albedo changes with droplet concentrations remarkably well, passing through the midpoints of the considerable spread in the data. The fortuitous agreement results from compensating changes in cloud properties. To ether with the albedo changes, the changes in cloud liquid water content and droplet size distributions imply that cloud thickness usually increased in the ship tracks. Such an increase was observed on the occasions that changes in cloud thickness were recorded (in the Sanko Pence ship track during very clean ambient condilions). Unfortunately systematic measurements of cloud thickness were not made for most of the ship tracks observed. The greatest outlier in the data corresponds to measurements made under horizontally inhomogeneous ambient conditions; possible explanations for its divergence include an increase in cloud thickness or an error in matching above-cloud. albedo measurements with in-cloud microphysics measurements.

One of the primary aims of the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) was to quantify the physical and chemical processes affecting the evolution of the major aerosol types over the North Atlantic. The best, practical way of doing this is in a Lagrangian framework where a parcel of air is sampled over several tens of hours and its physical and chemical properties are intensively measured. During the intensive observational phase of ACE-2, between 15 June 1997 and 24 July 1997, 3 cloudy Lagrangian experiments and 3 cloud-free, Lagrangian experiments were undertaken between the south west tip of the Iberian Peninsula and the Canary Islands. This paper gives an overview of the aims and logistics of all of the Lagrangian experiments and compares and contrasts them to provide a framework for the more focused Lagrangian papers in this issue and future process modelling studies and parametrisation development. The characteristics of the cloudy Lagrangian experiments were remarkably different, enabling a wide range of different physical and chemical processes to be studied. In the 1st Lagrangian, a clean maritime air mass was sampled in which salt particle production, due to increased wind speed, dominated the change in the accumulation mode concentrations. In the 2nd Lagrangian, extensive cloud cover resulted in cloud processing of the aerosol in a polluted air mass, and entrainment of air from the free troposphere influenced the overall decrease in aerosol concentrations in the marine boundary layer (MBL). Very little change in aerosol characteristics was measured in the 3rd Lagrangian, where the pollution in the MBL was continually being topped up by entraining air from a residual continental boundary layer (CBL) above. From the analysis of all the Lagrangian experiments, it has been possible to formulate, and present here, a generalised description of a European continental outbreak of pollution over the sub-tropical North Atlantic.

The effects of anthropogenic particulate emissions from ships on the radiative, microphysical, and chemical properties of moderately polluted marine stratiform clouds are examined. A case study of two ships in the same air mass is presented where one of the vessels caused a discernible ship track while the other did not. In situ measurements of cloud droplet size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to measurements of cloud radiative properties. The differences between the aerosol in the two ship plumes are discussed; these indicate that combustion-derived particles in the size range of about 0.03-0.3-mu m radius were those that caused the microphysical changes in the clouds that were responsible for the ship track.The authors examine the processes behind ship track formation in a moderately polluted marine boundary layer as an example of the effects that anthropogenic particulate pollution can have in the albedo of marine stratiform clouds.

During the 1st Lagrangian experiment of the North Atlantic Regional Aerosol Characterisation Experiment (ACE-2), a parcel of air was tagged by releasing a smart, constant level balloon into it from the Research Vessel Vodyanitskiy. The Meteorological Research Flight’s C-130 aircraft then followed this parcel over a period of 30 h characterising the marine boundary layer (MBL), the cloud and the physical and chemical aerosol evolution. The air mass had originated over the northern North Atlantic and thus was clean and had low aerosol concentrations. At the beginning of the experiment the MBL was over 1500 m deep and made up of a surface mixed layer (SML) underlying a layer containing cloud beneath a subsidence inversion. Subsidence in the free troposphere caused the depth of the MBL to almost halve during the experiment and, after 26 h, the MBL became well mixed throughout its whole depth. Salt particle mass in the MBL increased as the surface wind speed increased from 8 m s-1 to 16 m s-1 and the accumulation mode (0.1μm to 3.0 μm) aerosol concentrations quadrupled from 50 cm-3 to 200 cm-3. However, at the same time the total condensation nuclei (>3 nm) decreased from over 1000 cm-3 to 750 cm-3. The changes in the accumulation mode aerosol concentrations had a significant effect on the observed cloud microphysics. Observational evidence suggests that the important processes in controlling the Aitken mode concentration which, dominated the total CN concentration, included, scavenging of interstitial aerosol by cloud droplets, enhanced coagulation of Aitken mode aerosol and accumulation mode aerosol due to the increased sea salt aerosol surface area, and dilution of the MBL by free tropospheric air.