Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is approximately 50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.
Abstract. The lifetime of methane is controlled to a very large extent by the abundance of the OH radical. The tropics are a key region for methane removal, with oxidation in the lower tropical troposphere dominating the global methane removal budget (Bloss et al., 2005). In tropical forested environments where biogenic VOC emissions are high and NO x concentrations are low, OH concentrations are assumed to be low due to rapid reactions with sink species such as isoprene. New, simultaneous measurements of OH concentrations and OH reactivity, k OH , in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ∼2.5×10 6 molecule cm −3 (10 min average) observed around solar noon. Whilst j (O 1 D) and humidity were high, low O 3 concentrations limited the OH production from O 3 photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s −1 , corresponding to an OH lifetime of only 34 ms. To maintain the observed OH concentration given the measured OH reactivity requires a rate of OH production approximately 10 times greater than calculated using all measured OH sources. A test of our current understanding of the chemistry within a tropical rainforest was made using a detailed zero-dimensional model to compare with measurements. The model overpredicted the observed HO 2 concentrations and significantly under-predicted OH concentrations. Inclusion of an additional OH source formed as a recycled product of OH iniCorrespondence to: L. K. Whalley (l.k.whalley@leeds.ac.uk) tiated isoprene oxidation improved the modelled OH agreement but only served to worsen the HO 2 model/measurement agreement. To replicate levels of both OH and HO 2 , a process that recycles HO 2 to OH is required; equivalent to the OH recycling effect of 0.74 ppbv of NO. This recycling step increases OH concentrations by 88 % at noon and has wide implications, leading to much higher predicted OH over tropical forests, with a concomitant reduction in the CH 4 lifetime and increase in the rate of VOC degradation.
Abstract. Atmospheric composition and chemistry above tropical rainforests is currently not well established, particularly for south-east Asia. In order to examine our understanding of chemical processes in this region, the performance of a box model of atmospheric boundary layer chemistry is tested against measurements made at the top of the rainforest canopy near Danum Valley, Malaysian Borneo. Multivariate optimisation against ambient concentration measurements was used to estimate average canopy-scale emissions for isoprene, total monoterpenes and nitric oxide. The excellent agreement between estimated values and measured fluxes of isoprene and total monoterpenes provides confidence in the overall modelling strategy, and suggests that this method may be applied where measured fluxes are not available, assuming that the local chemistry and mixing are adequately understood. The largest contributors to the optimisation cost function at the point of best-fit are OH (29%), NO (22%) and total peroxy radicals (27%). Several factors affect the modelled VOC chemistry. In particular concentrations of methacrolein (MACR) and methyl-vinyl ketone (MVK) are substantially overestimated, and the hydroxyl radical (OH)Correspondence to: T. A. M. Pugh (t.pugh@lancs.ac.uk) concentration is substantially underestimated; as has been seen before in tropical rainforest studies. It is shown that inclusion of dry deposition of MACR and MVK and wet deposition of species with high Henry's Law values substantially improves the fit of these oxidised species, whilst also substantially decreasing the OH sink. Increasing OH production arbitrarily, through a simple OH recycling mechanism , adversely affects the model fit for volatile organic compounds (VOCs). Given the constraints on isoprene flux provided by measurements, a substantial decrease in the rate of reaction of VOCs with OH is the only remaining option to explain the measurement/model discrepancy for OH. A reduction in the isoprene+OH rate constant of 50%, in conjunction with increased deposition of intermediates and some modest OH recycling, is able to produce both isoprene and OH concentrations within error of those measured. Whilst we cannot rule out an important role for missing chemistry, particularly in areas of higher isoprene flux, this study demonstrates that the inadequacies apparent in box and global model studies of tropical VOC chemistry may be more strongly influenced by representation of detailed physical and micrometeorological effects than errors in the chemical scheme.
A highly sensitive chemiluminescence instrument has been deployed to measure nitric oxide (NO) and nitrogen dioxide (NO2) at the Cape Verde Atmospheric Observatory in the remote tropical North Atlantic marine boundary layer (MBL). Using two different methods, the instrument was assessed to have a detection limit of around 1.8 parts per trillion by volume (pptv) for NO and 5.5 pptv for NO2 for hour‐long integration periods. The overall accuracy was estimated at ∼18% for NO and 30% for NO2. Measurements of NO, NO2, and ozone (O3) over a period of 12 months in 2007 show very low levels of NOx (typically <30 pptv) and net daytime ozone destruction on most days of the measurement period. Air originating over Africa exhibited the highest levels of NOx (∼35 pptv) and reduced daily O3 destruction, with O3 production observed on a few days. Air that had not originated over Africa showed lower NOx levels (∼25 pptv), with greater observed O3 destruction. A dependence of the observed O3 destruction on NO mixing ratios, averaged over all air masses, was observed and reproduced using a simple box model. The model results imply that the presence of between 17 and 34 pptv of NO (depending on the month) would be required to turn the tropical North Atlantic from an O3 destroying to an O3 producing regime.
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. air quality ͉ land use change ͉ sustainable development ͉ biofuel G round-level ozone (O 3 ) is a priority air pollutant that damages human health, plants, and materials, reduces crop productivity, and has direct and indirect effects on the Earth's climate system (1). It is formed in the atmosphere by reactions involving oxides of nitrogen (NO x ) and volatile organic compounds (VOCs) in the presence of sunlight. The terrestrial biosphere is a major source of both these families of trace gases; in fact, the great majority of reactive VOCs globally are of biogenic origin (2). Here we show, using integrated and fully comprehensive measurements of biosphere-to-atmosphere trace gas fluxes and atmospheric composition, together with atmospheric chemistry modeling, that conversion of tropical rainforest to oil palm plantations results in much greater emissions of these reactive trace gases that lead to O 3 formation. Increased NO x emissions will cause severe ground-level O 3 pollution (Ͼ 100 ppbv), but this pollution could be prevented by strict control of emissions of reactive nitrogen species to the atmosphere. Our study shows the importance of quantifying the current and future effects of land use change on air quality when assessing the ''environmental friendliness'' of palm oil and other biofuel crops. Of course, air quality is only a single consideration; in assessing the consequences of biofuel production, effects o...
Abstract. In April-July 2008, intensive measurements were made of atmospheric composition and chemistry in Sabah, Malaysia, as part of the "Oxidant and particle photochemical processes above a South-East Asian tropical rainforCorrespondence to: C. N. Hewitt (n.hewitt@lancaster.ac.uk) est" (OP3) project. Fluxes and concentrations of trace gases and particles were made from and above the rainforest canopy at the Bukit Atur Global Atmosphere Watch station and at the nearby Sabahmas oil palm plantation, using both ground-based and airborne measurements. Here, the measurement and modelling strategies used, the characteristics of the sites and an overview of data obtained are described. Composition measurements show that the rainforest Published by Copernicus Publications on behalf of the European Geosciences Union. 170 C. N. Hewitt et al.: The OP3 project: introduction, rationale, location characteristics and tools site was not significantly impacted by anthropogenic pollution, and this is confirmed by satellite retrievals of NO 2 and HCHO. The dominant modulators of atmospheric chemistry at the rainforest site were therefore emissions of BVOCs and soil emissions of reactive nitrogen oxides. At the observed BVOC:NO x volume mixing ratio (∼100 pptv/pptv), current chemical models suggest that daytime maximum OH concentrations should be ca. 10 5 radicals cm −3 , but observed OH concentrations were an order of magnitude greater than this. We confirm, therefore, previous measurements that suggest that an unexplained source of OH must exist above tropical rainforest and we continue to interrogate the data to find explanations for this.
Observations of the tropical atmosphere are fundamental to the understanding of global changes in air quality, atmospheric oxidation capacity and climate, yet the tropics are under-populated with long-term measurements. The first three years (October 2006 -September 2009) of meteorological, trace gas and particulate data from the global WMO/Global Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory Humberto Duarte Fonseca (CVAO; 16° 51' N, 24° 52' W) are presented, along with a characterisation of the origin and pathways of air masses arriving at the station using the NAME dispersion model and simulations of dust deposition using the COSMO-MUSCAT dust model. The observations show a strong influence from Saharan dust in winter with a maximum in super-micron aerosol and particulate iron and aluminium. The dust model results match the magnitude and daily variations of dust events, but in the region of the CVAO underestimate the measured aerosol optical thickness (AOT) because of contributions from other aerosol. The NAME model also captured the dust events, giving confidence in its ability to correctly identify air mass origins and pathways in this region. Dissolution experiments on collected dust samples showed a strong correlation between soluble Fe and Al and measured solubilities were lower at high atmospheric dust concentrations.Fine mode aerosol at the CVAO contains a significant fraction of non-sea salt components including dicarboxylic acids, methanesulfonic acid and aliphatic amines, all believed to be of oceanic origin. A marine influence is also apparent in the year-round presence of iodine and bromine monoxide (IO and BrO), with IO suggested to be confined mainly to the surface few hundred metres but BrO well mixed in the boundary layer. Enhanced CO 2 and CH 4 and depleted oxygen concentrations are markers for air-sea exchange over the nearby northwest African coastal upwelling area. Long-range transport results in generally higher levels of O 3 and anthropogenic non-methane hydrocarbons (NMHC) in air originating from North America. Ozone/CO ratios were highest (up to 0.42) in European air masses that contain relatively less well-aged air. In air heavily influenced by Saharan dust the O 3 /CO ratio was as low as 0.13, possibly indicating O 3 uptake to dust. Nitrogen oxides (NO x and NO y ) show generally higher concentrations in winter when air mass origins are predominantly from Africa. High photochemical activity at the site is shown by maximum spring/summer concentrations of OH and HO 2 of 9 × 10 6 molecule cm -3 and 6 × 10 8 molecule cm -3 , respectively. After the primary photolysis source, the chemistry of IO and BrO, the abundance of HCHO, and aerosol uptake are important for the HO x budget in this region.3
Low cost pollution sensors have been widely publicized, in principle offering increased information on the distribution of air pollution and a democratization of air quality measurements to amateur users. We report a laboratory study of commonly-used electrochemical sensors and quantify a number of cross-interferences with other atmospheric chemicals, some of which become significant at typical suburban air pollution concentrations. We highlight that artefact signals from co-sampled pollutants such as CO2 can be greater than the electrochemical sensor signal generated by the measurand. We subsequently tested in ambient air, over a period of three weeks, twenty identical commercial sensor packages alongside standard measurements and report on the degree of agreement between references and sensors. We then explore potential experimental approaches to improve sensor performance, enhancing outputs from qualitative to quantitative, focusing on low cost VOC photoionization sensors. Careful signal handling, for example, was seen to improve limits of detection by one order of magnitude. The quantity, magnitude and complexity of analytical interferences that must be characterised to convert a signal into a quantitative observation, with known uncertainties, make standard individual parameter regression inappropriate. We show that one potential solution to this problem is the application of supervised machine learning approaches such as boosted regression trees and Gaussian processes emulation.
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