Methane Emissions in a Chemistry-Climate Model: Feedbacks and Climate Response.

Understanding the past, present, and future evolution of methane remains a grand challenge. Here we have used a hierarchy of models, ranging from simple box models to a chemistry-climate model (CCM), UM-UKCA, to assess the contemporary and possible future atmospheric methane burden. We assess two emission data sets for the year 2000 deployed in UM-UKCA against key observational constraints. We explore the impact of the treatment of model boundary conditions for methane and show that, depending on other factors, such as CO emissions, satisfactory agreement may be obtained with either of the CH4 emission data sets, highlighting the difficulty in unambiguous choice of model emissions in a coupled chemistry model with strong feedbacks. The feedbacks in the CH4-CO-OH system, and their uncertainties, play a critical role in the projection of possible futures. In a future driven by large increases in greenhouse gas forcing, increases in tropospheric temperature drive, an increase in water vapor, and, hence, [OH]. In the absence of methane emission changes this leads to a significant decrease in methane compared to the year 2000. However, adding a projected increase in methane emissions from the RCP8.5 scenario leads to a large increase in methane abundance. This is modified by changes to CO and NOx emissions. Clearly, future levels of methane are uncertain and depend critically on climate change and on the future emission pathways of methane and ozone precursors. We highlight that further work is needed to understand the coupled CH4-CO-OH system in order to understand better future methane evolution.

Delay in recovery of the Antarctic ozone hole from unexpected CFC-11 emissions.

The Antarctic ozone hole is decreasing in size but this recovery will be affected by atmospheric variability and any unexpected changes in chlorinated source gas emissions. Here, using model simulations, we show that the ozone hole will largely cease to occur by 2065 given compliance with the Montreal Protocol. If the unusual meteorology of 2002 is repeated, an ozone-hole-free-year could occur as soon as the early 2020s by some metrics. The recently discovered increase in CFC-11 emissions of ~ 13 Gg yr-1 may delay recovery. So far the impact on ozone is small, but if these emissions indicate production for foam use much more CFC-11 may be leaked in the future. Assuming such production over 10 years, disappearance of the ozone hole will be delayed by a few years, although there are significant uncertainties. Continued, substantial future CFC-11 emissions of 67 Gg yr-1 would delay Antarctic ozone recovery by well over a decade.

Quantifying the ozone and ultraviolet benefits already achieved by the Montreal Protocol.

Chlorine- and bromine-containing ozone-depleting substances (ODSs) are controlled by the 1987 Montreal Protocol. In consequence, atmospheric equivalent chlorine peaked in 1993 and has been declining slowly since then. Consistent with this, models project a gradual increase in stratospheric ozone with the Antarctic ozone hole expected to disappear by ∼2050. However, we show that by 2013 the Montreal Protocol had already achieved significant benefits for the ozone layer. Using a 3D atmospheric chemistry transport model, we demonstrate that much larger ozone depletion than observed has been avoided by the protocol, with beneficial impacts on surface ultraviolet. A deep Arctic ozone hole, with column values <120 DU, would have occurred given meteorological conditions in 2011. The Antarctic ozone hole would have grown in size by 40% by 2013, with enhanced loss at subpolar latitudes. The decline over northern hemisphere middle latitudes would have continued, more than doubling to ∼15% by 2013.

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.

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 NO(x) 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.

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

More than half the world's 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 (O(3)), an air pollutant that damages human health, plants, and materials, reduces crop productivity, and has effects on the Earth's climate. Our measurements show that, at present, O(3) 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 O(3) concentrations will reach 100 parts per billion (10(9)) 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.

Dynamical variability in the modelling of chemistry-climate interactions.

We have used a version of the Met Office's climate model, into which we have introduced schemes for atmospheric chemistry, to study chemistry-dynamics-climate interactions. We have considered the variability of the stratospheric polar vortex, whose behaviour influences stratospheric ozone loss and will affect ozone recovery. In particular, we analyse the dynamical control of high latitude ozone in a model version which includes an assimilation of the equatorial quasi-biennial oscillation (QBO), demonstrating the stability of the linear relation between vortex strength and high latitude ozone. We discuss the effect of interactive model ozone on polar stratospheric cloud (PSC) area/volume and winter-spring stratospheric ozone loss in the northern hemisphere. In general we find larger polar ozone losses calculated in those model integrations in which modelled ozone is used interactively in the radiation scheme, even though we underestimate the slope of the ozone loss per PSC volume relation derived from observations. We have also looked at the influence of changing stratosphere-to-troposphere exchange on the tropospheric oxidizing capacity and, in particular, have considered the variability of tropospheric composition under different climate regimes (El Niño/La Niña, etc.). Focusing on the UT/LS, we show the response of ozone to El Niño in two different model set-ups (tropospheric/ stratospheric). In the stratospheric model set-up we find a distinct signal in the lower tropical stratosphere, which shows an anti-correlation between the Niño 3 index and the ozone column amount. In contrast ozone generally increases in the upper troposphere of the tropospheric model set-up after an El Niño. Understanding future trends in stratospheric ozone and tropospheric oxidizing capacity requires an understanding of natural variability, which we explore here.

Role of sulphur photochemistry in tropical ozone changes after the eruption of Mount Pinatubo.

RECENT observations suggest that the eruption of Mount Pinatubo in June 1991 has had a considerable effect on ozone concentrations in the tropical stratosphere (refs 1, 2, and J. W. Waters, personal communication). Although stratospheric ozone losses following volcanic eruptions are generally attributed to the presence of sulphate aerosol3-7, we present model calculations which demonstrate that gas-phase sulphur chemistry may have played a part in the tropical ozone perturbations that followed the Pinatubo eruption. We find that in the first month or so after the eruption, the large amount of SO2 injected into the tropical atmosphere catalyses mid-stratospheric ozone production. On the other hand, the SO2 cloud absorbs solar radiation, thereby reducing the rate of O2 photolysis (and hence of ozone production) below it. These two effects cancel each other out at an altitude of about 25 kilometres. After one or two months, most of the SO2 has been oxidized to sulphate; the efficiency of these two mechanisms then becomes negligible (although ozone remains perturbed in the lower stratosphere because of its long photochemical lifetime in this region). The model features show good agreement with initial ozone measurements following the eruption, including both the mid-altitude switch from ozone loss to ozone gain1, and the increase and subsequent decrease in the total ozone column2,7.

A rabbit model of pulmonary hypertension induced by the synthetic platelet-activating factor acetylglyceryl ether phosphorylcholine.

Development of effective treatment for human pulmonary hypertension (PHT) has been hampered by an incomplete understanding of its pathogenesis. We present a rabbit model of PHT based on platelet-activating factor (PAF), a potent phospholipid autacoid synthesized by a variety of mammalian cells. PAF was intravenously infused into rabbits for 4 wk. After the infusion, rabbits underwent pulmonary arterial catheterization for hemodynamic evaluation, and lung tissue was morphometrically analyzed for changes in cross-sectional areas of intima and media, and alteration in number of small pulmonary arteries. The heart was evaluated by the method of Fulton for right ventricular hypertrophy. Mean pulmonary arterial pressure was 20 +/- 2 mm Hg in PAF-treated rabbits compared with 12 +/- 1 mm Hg in vehicle-treated control rabbits. PAF induced a trend toward loss of small muscular pulmonary arteries, measuring 50 to 200 microns in diameter, and right ventricular hypertrophy. There was a decrease in circumference of the internal elastic lamina in vessels accompanying alveolar ducts and in alveolar walls, and a relative increase in the intimal cross-sectional area of these vessels. These lesions were associated with a trend toward medial hypertrophy. No increase in lung water was found. Pressure changes occurred in the absence of alterations in hematocrit and arterial partial pressure of oxygen. We conclude that chronic intravenous infusion of PAF, a naturally synthesized substance, into rabbits provides a potentially useful model for the study of vascular changes associated with PHT.

Activation of human squamous cell carcinoma ornithine decarboxylase activity by guanosine triphosphate.

Previous studies have demonstrated the presence in mouse epidermal tumors of a structurally and functionally altered ornithine decarboxylase (ODC). In this report, the enzymatic properties of ODC from normal human skin and squamous cell carcinomas are examined. Some tumors contained a more heat stable ODC than the enzyme found in normal skin. GTP stimulated enzyme activity in four of seven tumor extracts tested but had no effect on normal skin ODC. Kinetic analyses indicated that GTP either lowered the apparent Km of tumor ODC for L-ornithine, increased the Vmax, or had both effects, depending on the tumor examined. Gel filtration chromatography of crude tumor extracts indicated the existence of multiple molecular weight forms of ODC, some of which can be activated by GTP and some of which are unaffected by GTP. Some tumors contain both a GTP-activatable and -nonactivatable form of the enzyme. Immunolocalization studies demonstrated the presence within squamous cell carcinomas of cells with a constitutively high level of immunoreactive ODC, a situation never observed in normal skin tissue. These results suggest that some human squamous cell carcinomas contain a functionally altered ODC that may be aberrantly regulated.

Ornithine decarboxylase from mouse epidermis and epidermal papillomas: differences in enzymatic properties and structure.

The properties of ornithine decarboxylase (OrnDCase) from mouse epidermis and benign epidermal tumors (papillomas) induced by the initiation-promotion protocol were compared. When crude extracts from each tissue were incubated at 55 degrees C, epidermal OrnDCase was rapidly inactivated, but the papilloma OrnDCase was more heat stable. Each of five individual papilloma extracts contained OrnDCase activity that was considerably more resistant to heat inactivation than was epidermal OrnDCase. Mixing of a papilloma and epidermal extract produced an intermediate heat-inactivation profile, suggesting that the differences in OrnDCase heat stability are not due to non-OrnDCase components of the extracts. Kinetic analyses indicated that the papilloma OrnDCase has an altered affinity for its substrate, L-ornithine, compared to epidermal OrnDCase. The apparent Km for L-ornithine for the epidermal enzyme was 0.07 mM while the Km values for the individual papilloma OrnDCases clustered around two higher values, 0.3 mM and 1.0 mM. The papilloma OrnDCases, but not epidermal OrnDCase, were activated by GTP and to a lesser extent by CTP. Immunoblot analysis showed the existence of multiple forms of OrnDCase in both epidermis and papilloma that differed in isoelectric point but not subunit molecular weight. None of the species of OrnDCase present in the epidermal extract coincided with the species present in papilloma. These results suggest that one consequence of neoplastic transformation in this in vivo system is the presence of an OrnDCase protein in benign tumors that differs structurally and functionally from the OrnDCase present in normal epidermis. The possible mechanisms responsible for these results and their significance for neoplastic development in this tissue are discussed.