K.M. Beswick

Measurements of aerosol fluxes to Speulder forest using a micrometeorological technique

Abstract It has often been stated that micrometeorological and throughfall measurements of dry deposition differ by an order of magnitude with the results being highly variable and difficult to interpret or reconcile. We present measurements by the eddy correlation method of sub-micron aerosol deposition to a forest and show that they are large, typically 1 cm s −1 or more. We compare the measurements with literature values obtained by throughfall and related techniques. The results, rather than being irreconcilable, show a clear and consistent behaviour in deposition velocity across the aerosol size spectrum, despite the very different techniques involved. There would appear to be a contradiction with previously assumed model predictions of aerosol deposition velocity to forests and rough vegetated surfaces particularly for particles in the size range 0.1–1.0 μm where collection efficiencies appear to be significantly underestimated. A simple deposition velocity parameterisation as a function of stability and size is presented.

Direct measurements and parameterisation of aerosol flux, concentration and emission velocity above a city

Articles have recently been published on aerosol size distributions and number concentrations in cities, however there have been no studies on transport of these particles. Eddy covariance measurements of vertical transport of aerosol in the size range 11 nm<Dp<3 μm are presented here. The analysis shows that typical average aerosol number fluxes in this size range vary between 9000 and 90,000 cm−2 s−1. With concentrations between 3000 and 20,000 cm−3 this leads to estimates of particle emission velocity between 20 and 75 mm s−1. The relationships between number flux and traffic activity, along with emission velocity and boundary layer stability are demonstrated and parameterised. These are used to derive an empirical parameterisation for aerosol concentration in terms of traffic activity and stability. The main processes determining urban aerosol fluxes and concentrations are discussed and quantified where possible. The difficulties in parameterising urban activity are discussed

The great dun fell cloud experiment 1993: An overview

The 1993 Ground-based Cloud Experiment on Great Dun Fell used a wide range of measurements of trace gases, aerosol particles and cloud droplets at five sites to study their sources and sinks especially those in cloud. These measurements have been interpreted using a variety of models. The conclusions add to our knowledge of air pollution, acidification of the atmosphere and the ground, eutrophication and climate change. The experiment is designed to use the hill cap cloud as a flow-through reactor, and was conducted in varying levels of pollution typical of much of the rural temperate continental northern hemisphere in spring-time.

Experimental evidence for in-cloud production of aerosol sulphate

Abstract The modification of physical and chemical properties of aerosols passing through clouds has received considerable attention over recent years. Some of these transformations are related to in-cloud chemical reactions, particularly the oxidation of sulphur dioxide (SO 2 ) to sulphate (SO 4 2− . The Great Dun Fell experiment provided an opportunity to investigate the connection between the chemistry within cloud droplets and the processing of an aerosol population. We have noted significant increases in SO 4 2− in the aerosol population downstream of the cloud compared to the aerosol entering the cloud. These increases are connected to both S(IV) oxidation in the liquid phase and to the entrainment of new air into the cloud, supplying reactants such as H 2 O 2 to the system. The addition of SO 4 2− mass to the aerosol is also associated with changes in the NH 4 + aerosol concentrations, possibly as a result of neutralisation of the acidified cloud droplets by NH 3 . The study was performed taking into account dynamical mixing of air masses as well as possible sampling artefacts.

Cloud processing of soluble gases

Abstract Experimental data from the Great Dun Fell Cloud Experiment 1993 were used to investigate interactions between soluble gases and cloud droplets. Concentrations of H 2 O 2 , SO 2 , CH 3 COOOH, HCOOH, and HCHO were monitored at different sites within and downwind of a hill cap cloud and their temporal and spatial evolution during several cloud events was investigated. Significant differences were found between in-cloud and out-of-cloud concentrations, most of which could not be explained by simple dissolution into cloud droplets. Concentration patterns were analysed in relation to the chemistry of cloud droplets and the gas/liquid equilibrium. Soluble gases do not undergo similar behaviour: CH 3 COOH simply dissolves in the aqueous phase and is outgassed upon cloud dissipation; instead, SO 2 is consumed by its reaction with H 2 O 2 . The behaviour of HCOOH is more complex because there is evidence for in-cloud chemical production. The formation of HCOOH interferes with the odd hydrogen cycle by enhancing the liquid-phase production of H 2 O 2 . The H 2 O 2 concentration in cloud therefore results from the balance of consumption by oxidation of SO 2 in-cloud production, and the rate by which it is supplied to the system by entrainment of new air into the clouds.

The size-dependent chemical composition of cloud droplets

Abstract Size-dependent cloud droplet solute concentrations were measured using a two-stage fog water impactor at the summit station of Great Dun Fell (GDF) in the north of England. The measurements showed mostly higher concentrations in the small-droplet fraction. During one cloud event, however, higher solute concentrations were found in the larger-droplet fraction. In order to identify the factors governing the size dependence of cloud droplet solute concentrations, sensitivity studies by means of a diffusional growth model were performed. The time available for the droplets to grow was identified to be of great importance for the size dependence of solute concentrations. In cases when higher solute concentrations were found in the fraction containing the bigger droplets, the cloud droplets were relatively young having been formed by orographic lifting of the air at the GDF summit. For the other events the evidence indicates that the cloud was already formed far upwind from the summit site. Our experimental and model results imply that, after an initially strong decrease of solute concentrations with droplet size we would observe: • ⊎|increasing solute concentrations with increasing diameters during the initial stage of a cloud, e.g. near cloud base where the droplets have just been formed. The primary factors contributing to this behaviour are high peak supersaturations, large numbers of coarse aerosol particles, and high solubility of the aerosol particles. • ⊎|decreasing solute concentrations with increasing diameters in aged cloud parcels, such as those which can be observed high above the cloud base in cumuliform clouds or are advected to the observation point in the case of stratiform clouds. The primary factors contributing to this behaviour are low peak supersaturations, low numbers of coarse particles, and low solubility of the aerosol particles.

Observations and modelling of the processing of aerosol by a hill cap cloud

Abstract Observations are presented of the aerosol size distribution both upwind and downwind of the Great Dun Fell cap cloud. Simultaneous measurements of the cloud microphysics and cloud chemistry, and of the chemical composition of the aerosol both upwind and downwind of the hill were made along with measurements of sulphur dioxide, hydrogen peroxide and ozone. These observations are used for initialisation of, and for comparison with the predictions of a model of the air flow, cloud microphysics and cloud chemistry of the system. A broad droplet size distribution is often observed near to the hill summit, seemingly produced as a result of a complex supersaturation profile and by mixing between parcels with different ascent trajectories. The model generates several supersaturation peaks as the airstream ascends over the complex terrain, activating increasing numbers of droplets. In conditions where sulphate production in-cloud (due to the oxidation of S(IV) by hydrogen peroxide and ozone) is observed, there is a marked effect on the chemical evolution of the aerosol particles on which the droplets form. When sulphate production occurs, a significant modification of the aerosol size distribution and hygroscopic properties is both predicted and observed. The addition of sulphate mass to those aerosol particles nucleation scavenged by the cloud generally increases the ease with which they are subsequently able to act as cloud condensation nuclei (CCN). Often, this will lead to an increase in the number of CCN available for subsequent cloud formation, although this latter effect is shown to be strongly dependent upon the activation history of the droplets and the concentration of pollutant gases present in the interstitial air. Situations are also identified where cloud processing could lead to a reduction in the capacity of smaller aerosol to act as CCN.

METEOROLOGY OF THE GREAT DUN FELL CLOUD EXPERIMENT 1993

Synoptic and local meteorological conditions during the Spring 1993 Ground-based Cloud Experiment on Great Dun Fell are described, including cloud microphysics, general pollution levels and sources of air, especially for five case studies selected for detailed analysis. Periods when air was flowing across the hill are identified and the extent to which air mixed into the cloud from above reached the ground is estimated. To aid the interpretation of cloud chemistry and microphysics measurements, the horizontal and vertical extent of the cloud are used to estimate droplet lifetimes and to comment on the influence of complex terrain on peak supersaturation.

Behavior of ultrafine particles in continental and marine air masses at a rural site in the United Kingdom

Particle size distribution measurements were made at a coastal site in the United Kingdom. These are presented, and the behavior of recently formed ultrafine particles is discussed. No ultrafine particles were observed in maritime air masses; however, 3 to 7 nm particles were frequently observed at enhanced concentrations when the wind direction was from the land. Their formation was favored at lower temperatures, when 1 ppbv or more of SO2 was present and in air masses that had not been aged extensively. On days when enhanced ultrafine particle concentrations were observed, 3 nm particles increased sharply in the morning, approximately 30 to 90 min after the UV solar flux first increased. By early afternoon the ultrafine particle concentration had returned to background levels. Rapid measurements of 5 nm particles showed no correlation with turbulence parameters, although the boundary layer mixing scales were similar to growth times of freshly nucleated particles to 5 nm diameter. However, ultrafine particle concentrations do correlate with the availability of sulphuric acid vapor. A delay of approximately an hour between the increase of H2SO4 in the morning and a large increase in ultrafine particle concentrations is due to the growth of particles to observable sizes, not the nucleation process itself. An analysis of the timescales for growth showed that coagulation may be important immediately after the particles have nucleated but its effectiveness reduces as number concentration falls. Conversely, growth by condensation is initially slow due to the Kelvin effect but increases in importance as the particles reach observable sizes.

Methane emissions on large scales

Two separate studies have been undertaken to improve estimates of methane emissions on a landscape scale. The first study took place over a palsa mire in northern Finland in August 1995. A tethered balloon and a tunable diode laser were used to measure profiles of methane in the nocturnal boundary layer. Using a simple box method or the flux gradient technique fluxes ranging from 18.5 to 658 mu mol m(-2) h(-1) were calculated. The large fluxes may be caused by advection of methane pockets across the measurement site, reflecting the heterogeneous nature of methane source strengths in the surrounding area. Under suitable conditions, comparison with nearby ground-based eddy-correlation results suggested that the balloon techniques could successfully measure fluxes on field scales. The second study was carried out by the NERC Scientific Services Atmospheric Research Airborne Support Facility using the Hercules C130 operated by the United Kingdom Meteorological Research Flight. A flight path around the northern coastline of Britain under steady West-East wind conditions enabled the measurement of methane concentrations up- and down-wind of northern Britain. Using a simple one-dimensional, constant-source diffusion model, the difference between the upwind and downwind concentrations was accounted for by methane emission from the surface. The contribution to methane emissions from livestock was also modelled. Modelled non-agricultural methane emissions decreased with increasing latitude with fluxes in northern England being a factor of 4 greater than those in northern Scotland. Since the only major methane source in northern Scotland was peat bogs, these results indicated that emissions over northern England were dominated by anthropogenic sources. Emissions from livestock accounted for 12% of the total flux over northern England, decreasing to 4% in southern Scotland and becoming negligible in northern Scotland. The total methane flux over northern Scotland was consistent with previous results from the area, indicating that this method of data analysis provided good estimates of large scale methane emissions