A. Robert MacKenzie

Effectiveness of Green Infrastructure for Improvement of Air Quality in Urban Street Canyons

Street-level concentrations of nitrogen dioxide (NO(2)) and particulate matter (PM) exceed public health standards in many cities, causing increased mortality and morbidity. Concentrations can be reduced by controlling emissions, increasing dispersion, or increasing deposition rates, but little attention has been paid to the latter as a pollution control method. Both NO(2) and PM are deposited onto surfaces at rates that vary according to the nature of the surface; deposition rates to vegetation are much higher than those to hard, built surfaces. Previously, city-scale studies have suggested that deposition to vegetation can make a very modest improvement (<5%) to urban air quality. However, few studies take full account of the interplay between urban form and vegetation, specifically the enhanced residence time of air in street canyons. This study shows that increasing deposition by the planting of vegetation in street canyons can reduce street-level concentrations in those canyons by as much as 40% for NO(2) and 60% for PM. Substantial street-level air quality improvements can be gained through action at the scale of a single street canyon or across city-sized areas of canyons. Moreover, vegetation will continue to offer benefits in the reduction of pollution even if the traffic source is removed from city centers. Thus, judicious use of vegetation can create an efficient urban pollutant filter, yielding rapid and sustained improvements in street-level air quality in dense urban areas.

In situ measurements of background aerosol and subvisible cirrus in the tropical tropopause region

In situ aerosol measurements were performed in the Indian Ocean Intertropical Convergence Zone (ITCZ) region during the Airborne Polar Experiment-Third European Stratospheric Experiment on Ozone (APE-THESEO) field campaign based in Mahe, Seychelles between 24 February and 6 March 1999. These are measurements of particle size distributions with a laser optical particle counter of the Forward Scattering Spectrometer Probe (FSSP)-300 type operated on the Russian M-55 high-altitude research aircraft Geophysica in the tropical upper troposphere and lower stratosphere up to altitudes of 21 km. On 24 and 27 February 1999, ultrathin layers of cirrus clouds were penetrated by Geophysica directly beneath the tropical tropopause at 17 km pressure altitude and temperatures below 190 K. These layers also were concurrently observed by the Ozone Lidar Experiment (OLEX) lidar operating on the lower-flying German DLR Falcon research aircraft. The encountered ultrathin subvisual cloud layers can be characterized as (1) horizontally extending over several hundred kilometers, (2) persisting for at least 3 hours (but most likely much longer), and (3) having geometrical thicknesses of 100–400 m. These cloud layers belong to the geometrically and optically thinnest ever observed. In situ particle size distributions covering diameters between 0.4 and 23 μm obtained from these layers are juxtaposed with those obtained inside cloud veils around cumulonimbus (Cb) anvils and also with background aerosol measurements in the vicinity of the clouds. A significant number of particles with size diameters around 10 μm were detected inside these ultrathin subvisible cloud layers. The cloud particle size distribution closely resembles a background aerosol onto which a modal peak between 2 and 17 μm is superimposed. Measurements of particles with sizes above 23 μm could not be obtained since no suitable instrument was available on Geophysica. During the flight of 6 March 1999, upper tropospheric and lower stratospheric background aerosol was measured in the latitude band between 4°S and 19°S latitude. The resulting particle number densities along the 56th meridian exhibit very little latitudinal variation. The concentrations for particles with sizes above 0.5 μm encountered under these background conditions varied between 0.1 and 0.3 particles/cm3 of air in altitudes between 17 and 21 km.

A numerical simulation of kinetic constraints upon achievement of the ammonium nitrate dissociation equilibrium in the troposphere.

Abstract Various mechanisms and associated rate expressions for the reversible evaporation of volatile ammonium salts are presented, and experimental evidence for them discussed. The literature concerned with the existence of stable NH 4 + salt monomers in the gas phase in highlighted. Four kinetic mechanisms are presented: monomer formation; adsorption of one precursor species on the particle surface; biomolecular surface reaction; and transport-limited particle growth. No mechanism emerges as the rate-limiting process for all experimental studies. Rate constant ratios for the first two mechanisms are calculated from the literature and shown to differ greatly from the thermodynamic equilibrium constant. A simple box model is developed to determine the importance of kinetic constraints on aerosol growth and evaporation in the tropospheric boundary layer. No attempt is made to model actual tropospheric processes other than the daytime oxidation of NO 2 and the kinetics of aerosol growth itself, so that model output is representative rather than definitive. Substantial departures from equilibrium are seen, giving gas concentration products ranging from −200 to 5000% of the theoretical value, Ke . At low temperatures and high relative humidities (r.h.) a large proportion of the potential condensable NO 3 − remains in the gas phase. The behaviour is general over a range of initial aerosol loadings, temperatures, r.h. and model formulations. Both pure NH 4 NO 3 aerosol and a hypothetical equimolar mixture with (NH 4 ) 2 SO 4 , conform to the general case.

The role of biogenic hydrocarbons in the production of ozone in urban plumes in southeast England

Abstract First order approximations of the rates of emission of isoprene and α-pinene are made for the region around London. Isoprene is found to be the dominant biogenic non-methane hydrocarbon species by mass in the region. Chemical degradation schemes for isoprene and α-pinene are added to an existing urban plume model which has previously been applied to London. The influence of each hydrocarbon is investigated by comparing model runs which include biogenic chemistry to a base case scenario. Oveerall, the effect is small: ozone levels in the plume are increased by the addition of the new schemes by up to 88 ppb. A model run with biogenic hydrocarbon and NO x emissions only demonstrates the VOC-limited behaviour of the urbans plume. Conversely, removall of anthropenic VOC emissions from the rural/suburban model results in a decrease of only 5 ppb in ozone production over one day. The reason for this difference in behaviour is the different NMHC/NO x ratios within and outside the urban plume. The inclusion of biogenic NMHC chemistry in the model results in a negligible change to the calculated ‘urban ozone effect’ defined as as the difference between ozone concentrations within and outsside of the plume. Substantial enhancement to present biogenic NMHC database are required if our understanding of urban plume chemistry is to be made more accurate.

A climatological study of polar stratospheric clouds (1989-1997) from LIDAR measurements over Dumont d’Urville (Antarctica).

Backscatter lidar data from the French Antarctic base in Dumont d’Urville (66.40°S, 140.01°E), including aerosol background and observations of polar stratospheric clouds (PSCs), have been collected since 1989. In the present work we present a climatological study of PSCs, using a data base consisting of almost 90 observations. The seasonal evolution of PSCs, their optical classification, and their relationship with the observation temperature were studied. The first PSC was observed on day number 175 (15 June) and the last on day number 260 (7 September). The characteristic mid-cloud altitude decreases through the season at a rate of 2.5 km/month. Type Ia, Ib, and II PSCs — identified by their optical properties — have been observed. External mixtures of these types have also been observed. These observations have been related to the local temperature measured by radiosondes. The relationship between PSC type and the period of the winter season was also investigated. Mixed (solid and liquid) type I clouds are mostly observed at the beginning of the winter. Type II clouds are observed only during the coldest period around midwinter, although temperatures below the frost point begin earlier and persist longer than this. Type Ia, solid-particle, clouds are observed mostly at the end of the winter.

Clouds at the tropical tropopause: A case study during the APE‐THESEO campaign over the western Indian Ocean

In this paper, we report a detailed description of a thin cirrus at the tropopause above a cumulonimbus (Cb) convective cluster observed during the Airborne Platform for Earth Observation–Third European Stratospheric Experiment for Ozone (APE-THESEO) campaign in February–March 1999 in the western Indian Ocean. The thin cirrus (Ci) has an optical depth at 532 nm below 0.1, with extended subvisible stretches, and is located directly below the tropopause, which was supersaturated with respect to ice. A direct comparison between the optical depth retrieved by Meteosat and that obtained by means of the hygrometers installed on the M55-Geophysica aircraft is discussed showing discrepancies ranging from 10 to 20%. Combining satellite and aircraft data, we show that the observed Ci is not due to cirrus outflow from Cb anvils. In the absence of any deeply convective clouds reaching altitudes above 15 km, we propose a possible mechanism of Ci formation based on a net mesoscale transport of water vapor from altitudes above 16 km to the tropopause region around 18 km. This transport could be driven by the critical layer and turbulence induced by gravity waves that could have been generated by lower level Cb cluster activity. The proposed mechanism for high-altitude Ci formation corroborates the new paradigm of a tropical tropopause layer (TTL) or “substratosphere,” several kilometers thick, which is decoupled from the convection-dominated lower troposphere.

The influence of surface kinetics on the growth of stratospheric ice crystals

A model of the growth of polar stratospheric ice crystals is described, taking account of the interstitial supersaturation, the effect of radiative heat transfer, heat conduction, mass diffusion and surface kinetic processes. This last effect has commonly been neglected in treatments of solid particle growth derived from models of liquid droplet aerosols. Numerical solutions for a wide range of conditions indicate that surface kinetic effects can be the dominant rate—limiting process for small particles and low supersaturations. Outside of these conditions the surface resistance remains a significant fraction of the total resistance to growth. Limits to the application of our formula, including its application to type I particles, are outlined.

The Turnbull correlation and the freezing of stratospheric aerosol droplets

An empirical correlation that is important in the calculation of homogeneous freezing probabilities, the “Turnbull correlation” for interfacial tensions, has been reevaluated and applied to systems of interest as possible components of polar stratospheric clouds (PSCs). The systems studied were: sulphuric acid solutions freezing to water ice and sulphuric acid tetrahydrate (SAT); and nitric acid solutions freezing to nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD). The calculations have been compared to experimental data: agreement is generally good, although aerosol freezing experiments, which would rigorously test the theory, have not been made for NAT. Of the three measurements of aerosol freezing to NAD, the calculations are closer to those measurements showing a lower freezing temperature, of about 175–177 K. The comparison substantially improves our confidence in our understanding of the mechanisms of PSC formation. Freezing of stratospheric aerosol to water ice remains the most plausible first step in solid-particle PSC formation if homogeneous freezing is the mechanism by which solid-particle PSC formation occurs.

Correction to “On the theories of type 1 polar stratospheric cloud formation” by A. R. MacKenzie, et al.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 Mass fraction of acid Figure 1. Changing composition of stratospheric aerosol overlaid on the freezing diagram for nitric acid and sulphuric acid solutions. The thin isolines are the temperature at which the

In Spite of Our "Own" Best Interests: Lessons from an Interdisciplinary Project on Urban Sustainability.

To contribute effectively to academic discourse on urban sustainability, disciplines need to think outside their silos and work together more collaboratively. Although straightforward to posit in theory, the practical realities of bringing together people with different worldviews, languages and skills can be frustrating and lead to loss of motivation. This paper presents the findings from a qualitative study of interdisciplinary team working on an urban sustainability project. Through analysis of archival materials, observations and interviews, a major theme of motivation was uncovered and explored. We conclude with some suggestions for working more effectively interdisciplinary, urban sustainability projects.