Martin Gallagher

Measurements and parameterizations of small aerosol deposition velocities to grassland, arable crops, and forest: Influence of surface roughness length on deposition

New micrometeorological measurements of small (0.1-0.2 μ diameter) aerosol particle fluxes using the eddy correlation technique are presented for moorland and also for grassland vegetation, the latter measurements being made both before and after cutting of the grassland to observe the resultant change in particle deposition velocity. These data are considered together with previously reported and reanalyzed micrometeorological measurements, again using the eddy correlation technique, for a number of different surface types, including arable crops and forest. Differences in observed surface deposition velocities, vds, due to the different surface roughnesses are highlighted. It was found that the various data sets showed a wholly consistent behavior when ensemble averages over the typical range of atmospheric stability ranges are considered in order to reduce the scatter inherent in these types of measurements. A working parameterization of surface deposition velocity in terms of the surfaces roughness length, z0, is presented. This is then extended for different atmospheric stabilities, using the parameterization suggested by Lamaud et al. [1994c], to yield vds/u* = k1 + k2 (-300 z/L 2/3, where k1 = k1 = 0.001222 log(z0) + 0.003906, k2 = 0.0009, where z is the measurement height, L is the Obukhov stability length, and u* is the local friction speed. The new data are finally compared to current analytical model descriptions of the deposition process, highlighting deficiencies in our understanding of the surface collection efficiency even for these small particles. Copyright 2002 by the American Geophysical Union.

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.

Nitrogen management is essential to prevent tropical oil palm plantations from causing ground-level ozone pollution

More than half the worlds rainforest has been lost to agriculture since the Industrial Revolution. Among the most widespread tropical crops is oil palm (Elaeis guineensis): global production now exceeds 35 million tonnes per year. In Malaysia, for example, 13% of land area is now oil palm plantation, compared with 1% in 1974. There are enormous pressures to increase palm oil production for food, domestic products, and, especially, biofuels. Greater use of palm oil for biofuel production is predicated on the assumption that palm oil is an “environmentally friendly” fuel feedstock. Here we show, using measurements and models, that oil palm plantations in Malaysia directly emit more oxides of nitrogen and volatile organic compounds than rainforest. These compounds lead to the production of ground-level ozone (O3), an air pollutant that damages human health, plants, and materials, reduces crop productivity, and has effects on the Earths climate. Our measurements show that, at present, O3 concentrations do not differ significantly over rainforest and adjacent oil palm plantation landscapes. However, our model calculations predict that if concentrations of oxides of nitrogen in Borneo are allowed to reach those currently seen over rural North America and Europe, ground-level O3 concentrations will reach 100 parts per billion (109) volume (ppbv) and exceed levels known to be harmful to human health. Our study provides an early warning of the urgent need to develop policies that manage nitrogen emissions if the detrimental effects of palm oil production on air quality and climate are to be avoided.

A review of measurement and modelling results of particle atmosphere–surface exchange

Atmosphere–surface exchange represents one mechanism by which atmospheric particle mass and number size distributions are modified. Deposition velocities (vd) exhibit a pronounced dependence on surface type, due in part to turbulence structure (as manifest in friction velocity), with minima of approximately 0.01 and 0.2 cm s-1 over grasslands and 0.1–1 cm s-1 over forests. However, as noted over 20 yr ago, observations over forests generally do not support the pronounced minimum of deposition velocity (vd) for particle diameters of 0.1–2 μm as manifest in theoretical predictions. Closer agreement between models and observations is found over less-rough surfaces though those data also imply substantially higher surface collection efficiencies than were originally proposed and are manifest in current models. We review theorized dependencies for particle fluxes, describe and critique model approaches and innovations in experimental approaches, and synthesize common conclusions of experimental and modelling studies. We end by proposing a number of research avenues that should be pursued in to facilitate further insights and development of improved numerical models of atmospheric particles.

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

Particle deposition to forests : Summary of results and application

Abstract Particle deposition to forest was studied at Speulder forest using experimental and modelling results. In this paper a short overview of the main results is given and they are applied in generalisation of deposition in The Netherlands. The results of the Aerosol project show that the deposition of particles to forests has been underestimated until now. Particle deposition makes a reasonable contribution to the total deposition of acidifying components and base cations to forests. Parametrisations of the deposition processes were derived from the experiments and model developed for particle deposition to Speulder forest. The parametrisations were used together with concentrations, meteorological observations and land-use data to determine the deposition on a 5 x 5 km basis in The Netherlands. Results of the canopy exchange experiments at Speulder forest and of the comparison of atmospheric deposition estimates and throughfall estimates at Speuld and other locations in The Netherlands show that throughfall fluxes and atmospheric deposition estimates are reasonably in agreement for all components, except nitrate. There is still large uncertainty in canopy exchange processes of oxidised nitrogen and in deposition estimates of the different gases contributing to the total oxidised nitrogen flux. The contribution of aerosol deposition to the total deposition in The Netherlands is 7% for SO42−, 11% for NH4+ and 9% for NO3−; for deciduous forests these numbers are 14,15 and 17%, and for coniferous forests 10, 17 and 20%, respectively. Dry deposition is estimated to amount to about 21% of total deposition for Na+, 20% for K+, 27% for Mg2+ and 25% for Ca2+.

Anatomy of cirrus clouds: Results from the Emerald airborne campaigns

The Emerald airborne measurement campaigns have provided a view of the anatomy of cirrus clouds in both the tropics and mid-latitudes. These experiments have involved two aircraft that combine remote sensing and in-situ measurements. Results are presented here from two separate flights: one in frontal cirrus above Adelaide, Australia, the other in the cirrus outflow from convection above Darwin. Recorded images of ice crystals are shown in relation to the cloud structure measured simultaneously by an airborne lidar. In mid-latitude frontal cirrus, columnar and irregular ice crystals were observed throughout the cloud while rosettes were found only at the top. The cirrus outflow from a tropical thunderstorm extended for hundreds of kilometres between the heights of 12.2 and 15.8 km. This was composed mainly of hexagonal plates, columns, and large crystal aggregates that originated from within the main core region of the convection. A small number of bullet rosettes were found at the top of the outflow cirrus and this is interpreted as an indication of in-situ crystal formation. It was found that the largest aggregates fell to the lower regions of the outflow cirrus cloud while the single crystals and small aggregates remained at the top.

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.

Measurements of CH4 and N2O fluxes at the landscape scale using micrometeorological methods

Flux gradient, eddy covariance and relaxed eddy accumulation methods were applied to measure CH4 and N2O emissions from peatlands and arable land respectively. Measurements of N2O emission by eddy covariance using tunable diode laser spectroscopy provided fluxes ranging from 2 to 60 µ mol N2O m-2 h-1 with a mean value of 22 µ mol N2O m-2 h-1 from 320 h of continuous measurements. Fluxes of CH4 measured above peatland in Caithness (U.K.) during May and June 1993 by eddy covariance and relaxed eddy accumulation methods were in the range 70 to 120 µ mol CH4 m-2 h-1 with means of 14.7 µ mol CH4 m -2 h-1 and 22.7 µ mol CH4 m-2 h-1 respectively. Emissions of CH4 from peatland changed with water table depth and soil temperature; increasing from 25 |Amol CH4 m-2 h-1 at 5% pool area to 50 p.mol CH4 m-2 h-1 with 30% within the flux footprint occupied by pools. A temperature response of 4.9 (xmol CH4 m-2 h-1 °C-1 in the range 6-12 °C was also observed. The close similarity in average CH4 emission fluxes reported for wetlands in Caithness, Hudson Bay and Alaska in the range 11 to 40 jamol CH4 m-2 h-1 suggests that earlier estimates of CH4 emission from high latitude wetlands were too large or that the area of high latitudes contributing to CH4 emission has been seriously underestimated.

Calibration of the Cloud Particle Imager Probes Using Calibration Beads and Ice Crystal Analogs: The Depth of Field

Abstract This paper explains and develops a correction algorithm for measurement of cloud particle size distributions with the Stratton Park Engineering Company, Inc., Cloud Particle Imager (CPI). Cloud particle sizes, when inferred from images taken with the CPI, will be oversized relative to their “true” size. Furthermore, particles will cease to be “accepted” in the image frame if they lie a distance greater than the depth of field from the object plane. By considering elements of the scalar theory for diffraction of light by an opaque circular disc, a calibration method is devised to overcome these two problems. The method reduces the error in inferring particle size from the CPI data and also enables the determination of the particles distance from the object plane and hence their depth of field. These two quantities are vital to enable quantitative measurements of cloud particle size distributions (histograms of particle size that are scaled to the total number concentration of particles) in the atm...