A. R. MacKenzie

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.

Effects of fluid‐dynamical stirring and mixing on the deactivation of stratospheric chlorine

We quantify the sensitivity to mixing of chlorine deactivation and ozone depletion in simplified models of the northern hemisphere, middle-latitude lower stratosphere. A photochemical box model augmented by a volume exchange model of mixing was used to identify situations for which effects of ClO x deactivation through mixing with NO x -rich air are at least as important as photochemical effects alone. In one simulation representative of a filament of high-latitude, ClO x -activated air mixing with low-latitude air, the O 3 depletion rate was found to be 0.287%, 0.127%, or 0.08% day -1 , for a volume exchange rate of 0, 0.06, or 0.48 day -1 , respectively. If we take the first rate as representative of Lagrangian models, the second rate as typical of the lower stratosphere, and the third rate as typical of those grid-based models that do not resolve the real mixing length scales, then our results suggest that Lagrangian models that do not represent mixing processes can, in certain circumstances, be in error by as much or more than grid-based models. The box model results allowed the formulation of a simplified, yet reasonably accurate reaction scheme for mixing-induced chlorine deactivation which was then implemented in a two-dimensional model of quasi-horizontal transport along isentropic surfaces. The two-dimensional model represents mixing by an effective horizontal diffusivity that accounts for vertical diffusivity and horizontal strain, and for advective transport it uses winds from lower stratospheric observational analyses. Mixing-induced chlorine deactivation was found to exhibit substantial sensitivity to the effective diffusivity of the two-dimensional model. The associated O 3 depletion from the Molina and Molina [1987] ClO-dimer cycle was found to exhibit sensitivity in a number of different regimes. The sensitivity increases with time and also depends on other details of the wind fields and the NO x concentration field. We suggest that O 3 loss in low-resolution models is sensitive to perturbations in the NO 2 field, whereas, by contrast, O 3 loss in the lower stratosphere is probably far less sensitive to such perturbations. To obtain estimates of O 3 loss over 11 days that are not sensitive to the diffusivity employed, it was necessary to use effective horizontal diffusivities D H < 10 5 m 2 s -1 , corresponding to horizontal features of about 200 km and hence requiring spatial resolution of about 40 km, which is much higher resolution than routinely employed. The results from the box model and the two-dimensional model are combined to assess the importance of mixing-induced chlorine deactivation relative to photolysis-induced deactivation.

BIOGENIC VOLATILE ORGANIC COMPOUND (VOC) EMISSION ESTIMATES FROM AN URBAN TREE CANOPY

Biogenic emissions of the volatile organic compounds isoprene and monoterpenes (BVOCs) can contribute to tropospheric ozone and secondary particle formation and have indirect effects on climate change. While there are few studies of BVOC emissions from European towns and cities, several studies in North America indicate that the urban tree canopy may be a significant source of BVOC compounds, contributing to ozone and particle formation in the urban air-shed. Here, BVOC emissions from the U.K. West Midlands (UKWM) metropolitan area were estimated and compared with anthropogenic VOC emission estimates, and with BVOC emission estimates for other urban and U.K. regions. Monoterpene and isoprene emission potential estimates for the UKWM urban land-use classes spanned as much as two orders of magnitude, from 17–104 g·km–2·h–1 and from 42–1570 g·km–2·h–1, respectively. Isoprene emission potential estimates for the UKWM urban land classes (42–530 g·km–2·h–1) were of the same order of magnitude as isoprene emissi...

The atmospheric chemistry of trace gases and particulate matter emitted by different land uses in Borneo

We report measurements of atmospheric composition over a tropical rainforest and over a nearby oil palm plantation in Sabah, Borneo. The primary vegetation in each of the two landscapes emits very different amounts and kinds of volatile organic compounds (VOCs), resulting in distinctive VOC fingerprints in the atmospheric boundary layer for both landscapes. VOCs over the Borneo rainforest are dominated by isoprene and its oxidation products, with a significant additional contribution from monoterpenes. Rather than consuming the main atmospheric oxidant, OH, these high concentrations of VOCs appear to maintain OH, as has been observed previously over Amazonia. The boundary-layer characteristics and mixing ratios of VOCs observed over the Borneo rainforest are different to those measured previously over Amazonia. Compared with the Bornean rainforest, air over the oil palm plantation contains much more isoprene, monoterpenes are relatively less important, and the flower scent, estragole, is prominent. Concentrations of nitrogen oxides are greater above the agro-industrial oil palm landscape than over the rainforest, and this leads to changes in some secondary pollutant mixing ratios (but not, currently, differences in ozone). Secondary organic aerosol over both landscapes shows a significant contribution from isoprene. Primary biological aerosol dominates the super-micrometre aerosol over the rainforest and is likely to be sensitive to land-use change, since the fungal source of the bioaerosol is closely linked to above-ground biodiversity.

Airborne polar experiment - polar ozone, leewaves, chemistry, and transport (APE-POLECAT): Rationale, road map and summary of measurements

The Airborne Polar Experiment-Polar Ozone, Leewaves, Chemistry and Transport (APE-POLECAT) mission took place between December 19, 1996, and January 16, 1997. APE-POLECAT comprised the inaugural mission of the high-altitude research aircraft, the M-55 Geophysica, flights by the DLR Falcon, measurements from a number of Arctic ground stations, and atmospheric modeling. Both aircraft flew out of Rovaniemi in Finland. The Geophysica was equipped with a payload designed to probe the chemistry and microphysics of polar stratospheric clouds (PSCs) at, or above, the aircraft altitude (up to 20 km geometric altitude). The Deutsches Zentrum fur Luft und Raumfahrt (DLR) Falcon was fitted with an aerosol lidar called OLEX, which looked upward. Ground-based measurements included aerosol lidar, meteorological sondes, and ozone sondes, from both sides of the Scandinavian Mountains, and from the southern Arctic Ocean. The original primary aim of the mission, to study PSC processes in situ, was modified in the light of unfavorable meteorological conditions. Flights concentrated on studies of transport and chemistry around the polar vortex, and on remote sensing of very high, mountain-wave-induced, PSCs. Here we report the objectives and rationale of the mission, provide basic descriptions of the conditions of the stratosphere at the time of each flight, and give a summary of the measurements made.

The impact of local surface changes in Borneo on atmospheric composition at wider spatial scales: Coastal processes, land-use change and air quality

We present results from the OP3 campaign in Sabah during 2008 that allow us to study the impact of local emission changes over Borneo on atmospheric composition at the regional and wider scale. OP3 constituent data provide an important constraint on model performance. Treatment of boundary layer processes is highlighted as an important area of model uncertainty. Model studies of land-use change confirm earlier work, indicating that further changes to intensive oil palm agriculture in South East Asia, and the tropics in general, could have important impacts on air quality, with the biggest factor being the concomitant changes in NOx emissions. With the model scenarios used here, local increases in ozone of around 50 per cent could occur. We also report measurements of short-lived brominated compounds around Sabah suggesting that oceanic (and, especially, coastal) emission sources dominate locally. The concentration of bromine in short-lived halocarbons measured at the surface during OP3 amounted to about 7 ppt, setting an upper limit on the amount of these species that can reach the lower stratosphere.

What role do type I polar stratospheric cloud and aerosol parameterizations play in modelled lower stratospheric chlorine activation and ozone loss

The chlorine activation and subsequent ozone loss of the northern winter lower stratosphere have been modelled using different schemes for type I polar stratospheric clouds (PSCs) and sulphate aerosols. Type I PSCs were assumed to consist of either nitric acid trihydrate (NAT) at equilibrium, supercooled ternary solutions (STS) at equilibrium, or to follow a hysteresis cycle between frozen and liquid particles depending on the temperature history. The sulphate aerosol was assumed to be present as either liquid binary H2SO4/H2O aerosol (LBA) or as solid sulphuric acid tetrahydrate (SAT). Our box model integrations show that NAT and STS, representing the upper and lower limits of lower stratospheric chlorine activation, respectively, appear to destroy ozone equally efficiently after a cold PSC event (Tmin ≤ 190K at 50 mbar). For higher minimum temperatures, up to the equilibrium NAT point, there is significantly more ozone loss in the NAT scheme than in the STS scheme. On NAT, chlorine is activated directly by ClONO2 + HCl → 2Cl + HNO3, whereas on STS, indirect activation by ClONO2 + H2O → HOCl + HNO3 followed by HOCl + HCl → 2Cl + H2O, dominates. During the processing period, the indirect activation on STS will produce a temporary peak in HOCl. Box model integrations also show that direct chlorine activation is faster on SAT than on LBA, yielding significantly more ozone loss in air parcels which remain below the SAT melting point (215–220 K). Our single-layer chemical transport model simulations (θ = 465K) of the lower stratospheric chlorine activation during Arctic winter 1994/1995 show that chlorine is activated more quickly on NAT than on STS. However, in mid December 1994, when temperatures are low enough for substantial STS particle growth, maximum active chlorine becomes similar in both schemes and remains similar until the end of January 1995. A model integration which includes SAT produces up to 200 parts per trillion by volume more ClOx, inside the polar vortex during Arctic winter 1994/1995, than a model integration which includes LBA. The high melting point of SAT means that it may contribute to midlatitude ozone loss when filaments of processed air are shed by the vortex. For example, a model integration shows that air peeling off the Arctic vortex on February 14, 1995, contains 10% more ClOx at middle latitudes in an integration that includes SAT formation in a hysteresis scheme, than in an integration that includes LBA. The major differences in ozone loss predicted by the model PSC schemes occur inside the polar vortex. The largest differences in ozone at 465 K inside the vortex at the end of March 1995, up to 500 parts per billion by volume, are found between the equilibrium NAT and STS schemes. Ozone values in the hysteresis schemes are intermediate between those of the NAT and STS schemes. The inclusion of SAT in a hysteresis scheme des not have a major global effect.

Coordinated Airborne Studies in the Tropics (CAST)

This is the final version of the article. It first appeared from the American Meteorological Society via http://dx.doi.org/10.1175/BAMS-D-14-00290.1

Spatially-varying surface roughness and ground-level air quality in an operational dispersion model

Urban form controls the overall aerodynamic roughness of a city, and hence plays a significant role in how air flow interacts with the urban landscape. This paper reports improved model performance resulting from the introduction of variable surface roughness in the operational air-quality model ADMS-Urban (v3.1). We then assess to what extent pollutant concentrations can be reduced solely through local reductions in roughness. The model results suggest that reducing surface roughness in a city centre can increase ground-level pollutant concentrations, both locally in the area of reduced roughness and downwind of that area. The unexpected simulation of increased ground-level pollutant concentrations implies that this type of modelling should be used with caution for urban planning and design studies looking at ventilation of pollution. We expect the results from this study to be relevant for all atmospheric dispersion models with urban-surface parameterisations based on roughness.

Ultrathin subvisible cirrus clouds at the tropical tropopause

Subvisible cirrus clouds with a vertical thickness of only 100–300 m but horizontal extent of thousands of square kilometers have been detected at the tropical tropopause around 17 km altitude during the European-Union-funded APE-THESEO campaign. The cloud layers have been characterized by measurements on board of two aircraft: the Russian high-flying research aircraft Geophysica, which performed in situ measurements of the cloud layers; and a German Falcon research aircraft flying up to 13 km altitude and directing the Geophysica into these clouds, which remained invisible for the Geophysical pilot even during level flight within the layer. Both in situ and remote measurements suggest that the condensed phase volume ranges between 1 and 5 μm3 cm−3. If the particles consisted of water ice, this would correspond to 10–40 ppbv condensed water. Concerning the condensed mixing ratio this would correspond to the thinnest ice cloud ever observed. As a matter of fact, to condense only 10–40 ppbv of H2O in equili...