Sink For Ozone Research Articles

What is causing high ozone at Summit, Greenland?

Causes for the unusually high and seasonally anomalous ozone concentrations at Summit, Greenland were investigated. Surface data from continuous monitoring, ozone sonde data, tethered balloon vertical profiling data, correlation of ozone with the radionuclide tracers 7Be and 210Pb, and synoptic transport analysis were used to identify processes that contribute to sources and sinks of ozone at Summit. Northern Hemisphere (NH) lower free troposphere ozone mixing ratios in the polar regions are ∼20 ppbv higher than in Antarctica. Ozone at Summit, which is at 3212 m above sea level, reflects its altitude location in the lower free troposphere. Transport events that bring high ozone and dry air, likely from lower stratospheric/higher tropospheric origin, were observed ∼40% of time during June 2000. Comparison of ozone enhancements with radionuclide tracer records shows a year-round correlation of ozone with the stratospheric tracer 7Be. Summit lacks the episodic, sunrise ozone depletion events, which were found to reduce the annual, median ozone at NH coastal sites by up to ∼3 ppbv. Synoptic trajectory analyses indicated that, under selected conditions, Summit encounters polluted continental air with increased ozone from central and western Europe. Low ozone surface deposition fluxes over long distances upwind of Summit reduce ozone deposition losses in comparison to other NH sites, particularly during the summer months. Surface-layer photochemical ozone production does not appear to have a noticeable influence on Summit's ozone levels.

Foliage surface ozone deposition: a role for surface moisture?

Abstract. This paper addresses the potential role of surface wetness in ozone deposition to plant foliage. We studied Scots pine foliage in field conditions at the SMEARII field measurement station in Finland. We used a combination of data from flux measurement at the shoot (enclosure) and canopy scale (eddy covariance), information from foliage surface wetness sensors, and a broad array of ancillary measurements such as radiation, precipitation, temperature, and relative humidity. Environmental conditions were defined as moist during rain or high relative humidity and during the subsequent twelve hours from such events, circumstances that were frequent at this boreal site. From the measured fluxes we estimated the ozone conductance using it as the expression of the strength of ozone removal surface sink or total deposition. Further, we estimated the stomatal contribution and the remaining deposition was interpreted and analysed as the non-stomatal sink. The combined time series of measurements showed that both shoot and canopy-scale ozone total deposition were enhanced when moist conditions occurred. On average, the estimated stomatal deposition accounted for half of the measured removal at the shoot scale and one third at the canopy scale. However, during dry conditions the estimated stomatal uptake predicted the behaviour of the measured deposition, but during moist conditions there was disagreement. The estimated non-stomatal sink was analysed against several environmental factors and the clearest connection was found with ambient relative humidity. The relationship disappeared under 70% relative humidity, a threshold that coincides with the value at which surface moisture gathers at the foliage surface according to the leaf surface wetness measurements. This suggests the non-stomatal ozone sink on the foliage to be modulated by the surface films. We attempted to extract such potential modulation with the estimated film formation via the theoretical expression of adsorption. Whereas this procedure could predict the behaviour of the non-stomatal sink, it implied a chemical sink that was not accountable as simple ozone decomposition. We discuss the existence of other mechanisms whose relevance in the removal of ozone needs to be clarified, in particular: a significant nocturnal stomatal aperture neglected in the estimations, and a potentially large chemical sink offered by reactive biogenic organic volatile compounds.

Multiphase Decomposition of Novel Oxygenated Organics in Aqueous and Organic Media

Prior to the massive use of new oxygenated solvents, data on their multiphase reactivity must be obtained to assess their environmental fate and impact on water and air quality. For this, the kinetics and mechanisms of the photochemical and photocatalytic degradation of selected oxygenated solvents by common tropospheric oxidants (such as OH and ozone) must be characterized. We studied the oxidation kinetics of new oxygenated solvents as pure organic liquids and in an aqueous medium by ozone and bythe OH radical, respectively. The studied chemicals are all unsaturated compounds, having none, one, or two ether groups. The results indicate that the OH reaction proceeds atthe diffusion limit by addition to the double bond. The reactive uptake coefficients associated with the reaction initiated by ozone are of the order of 10(-3). The reactions of compounds with two double bonds are very fast and probably occur at the surface. This kinetic information demonstrates that organic solvents in an organic medium or in an aqueous droplet will be oxidized rapidly by these oxidation reactions. These reactions, however, are not significant sinks for ozone and OH radicals.

What are the Implications of Nonequilibrium in the O+OH and O+HO2 Reactions?

Vibrationally excited O2, OH, and HO2 species have been suggested (J. Phys. Chem. A 2004, 108, 758) to provide clues for explaining the "ozone deficit problem" and "HOx dilemma" in the middle atmosphere under conditions of local thermodynamic disequilibrium (LTD), but the question arises of how much LTD will affect the title ozone sink reactions. Besides providing novel kinetic results, it is shown that LTD tends to disfavor ozone depletion relative to traditional atmospheric modeling under Boltzmann equilibration, which is partly due to competition between the various reactive channels. The calculations also suggest that the title LTD processes can be important sources of highly vibrationally excited O2 in the middle atmosphere. Moreover, LTD is shown to offer an explanation for the fact that some down revision of the O + HO2 rate constant, or the ratio of the O + HO2 to O + OH rate constants, is required to improve agreement between the predictions of traditional modeling and observation. This, in turn, provides significant evidence supporting LTD at such altitudes.

Latitudinal gradients in O3 and CO during INDOEX 1999

Measurements of ozone and carbon monoxide (CO) from the INDOEX 1999 experiment show large differences in concentrations of ozone and CO between the Northern Hemisphere and the Southern Hemisphere. These measurements confirm the theory that the Intertropical Convergence Zone (ITCZ) serves as a barrier to mixing over the Indian Ocean, effectively separating the polluted air in the Northern Hemisphere from cleaner air in the Southern Hemisphere. In spite of CO levels similar to those observed off the coast of North America, there is relatively little ozone off the coast of India. The ozone‐to‐CO ratio in air coming from India is 0.14, lower than 0.3 or 0.4 in air being transported to Bermuda from North America. Diurnal cycles are observed in both CO and ozone. INDOEX data taken onboard the R/V Ronald H. Brown show an average diurnal cycle in ozone of 20%, while data from the island of Kaashidhoo in the Republic of Maldives indicate a diurnal variation of 19%, consistent with our analyses of other experiments. Diurnal variations of this magnitude are larger than expected from ozone destruction by conventional HOx chemistry alone, implying that the sink of ozone in the remote marine boundary layer is likely considerably larger than had been assumed. Additional chemical cycles must be fairly substantial—large enough to rival HOx chemistry in ozone destruction.

On the "ozone deficit problem": what are Ox and HOx catalytic cycles for ozone depletion hiding?

Studies on the role of vibrational excitation in the reactants for the O2 + O2, OH + O2, and HO2 + O2 reactions show that they can be important sources of ozone in the stratosphere, particularly at conditions of local thermodynamic disequilibrium. The results suggest that the Ox and HOx cycles commonly viewed as catalytic sinks of ozone may actually lead to its production, and hence help to clarify the "ozone deficit problem". This paper also presents an explanation for the general overestimation of the OH abundance in the upper stratosphere and lower mesosphere through standard HOx chemistry.

Ozone Dynamics and Deposition Processes at a Deforested Site in the Amazon Basin

The goal of this study is to investigate the impact of deforestation on ozone dynamics and deposition in the Brazilian Amazon basin. This goal is accomplished through i) analyses of ozone levels and deposition rates at a deforested site during the rainy season; and ii) comparisons of these data with similar information derived at a forest. At the pasture site maximum ozone mixing ratios reach 20 parts per billion on a volume basis (ppbv) but about 6 ppbv prevail over the forest. Maximum ozone deposition velocities for pastures can reach 0.7 cm s-1, which is about threefold lower than values derived for forests. Combining ozone abundance and deposition velocities, pasture maximum ozone fluxes reach approximately 0.2 microgram (ozone) m-2 s-1. This flux represents approximately 70% of the deposition rates measured over the forest. Hence, this study suggests that conversion of rainforests to pastures could lead to a net reduction (30%) in the ozone sink in the Amazon.

Uncertainty in preindustrial abundance of tropospheric ozone: Implications for radiative forcing calculations

Recent model calculations of the global mean radiative forcing from tropospheric ozone since preindustrial times fall in a relatively narrow range, from 0.3 to 0.5 W m−2. These calculations use preindustrial ozone fields that overestimate observations available from the turn of the nineteenth century. Although there may be calibration problems with the observations, uncertainties in model estimates of preindustrial natural emissions must also be considered. We show that a global three‐dimensional model of tropospheric chemistry with reduced NOx emissions from lightning (1–2 Tg N yr−1) and soils (2 Tg N yr−1) and increased emissions of biogenic hydrocarbons can better reproduce the nineteenth century observations. The resulting global mean radiative forcing from tropospheric ozone since preindustrial times is 0.72–0.80 W m−2, amounting to about half of the estimated CO2 forcing. Reduction in the preindustrial lightning source accounts for two thirds of the increase in the ozone forcing. Because there is near‐total titration of OH by isoprene in the continental boundary layer of the preindustrial atmosphere, isoprene and other biogenic hydrocarbons represent significant ozone sinks. The weak or absent spring maximum in the nineteenth century observations of ozone is difficult to explain within our understanding of the natural ozone budget. Our results indicate that the uncertainty in computing radiative forcing from tropospheric ozone since preindustrial times is larger than is usually acknowledged.

Measurements of Ozone Deposition to Vegetation Quantifying the Flux, the Stomatal and Non-Stomatal Components

Ozone deposition to vegetation represents the major sink for boundary layer ozone and yet the underlying mechanism of reaction and uptake at the surface is poorly understood. While overall rates of O3 deposition are known, the fractions of the flux absorbed by stomata and deposited to non-stomatal surfaces in the field have been poorly quantified. This paper reports 4 years of continuous fluxes by micrometeorological methods to moorland vegetation in southern Scotland. The flux has been partitioned between stomatal and non-stomatal fluxes and shows over a seasonal scale that the non-stomatal deposition (50 kg O3 ha− y−1) dominates the overall flux (77 kg O3 ha−1 y−1) and represents 70% of the total deposition. The surface resistance for non-stomatal O3 deposition (rns) decreases with temperature from 400 s m−1 at 0°C to 200 s m−1 at 15°C in dry conditions and is consistent with thermal decomposition of ozone at the surface with an apparent activation energy of about 36 kJ mole−1. The 4 years of continuous flux measurements show that stomatal conductance, when O3 concentrations are 80 µg m−3, is substantially smaller than for smaller O3 concentrations, although whether this is a response to VPD or O3 remains unclear.

Tropospheric Ozone, Chemistry and Man-induced Trends

This paper presents an overview of the global large-scale aspects of tropospheric ozone. It describes and quantifies the major sources of tropospheric ozone, namely stratospheric influx and tropospheric photochemistry, which today contribute on the order of 500 Tg O3/yr and 4000 Tg O3/yr, respectively. Photochemistry also provides the largest sink of tropospheric ozone. The tropospheric production of ozone is regulated by the presence of nitrogen oxides. The ensuing impact of anthropogenic emissions on the tropospheric burden, distribution and temporal trend of O3 is discussed.

Sources and sinks of ozone in savanna and forest areas during EXPRESSO: Airborne turbulent flux measurements

An airborne study of ozone concentrations and fluxes in the lower layers of the atmosphere was conducted over the Central African Republic (CAR) and northern Congo in November/December 1996, within the framework of the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). The first 4 km of the atmosphere above savanna, rain forest, and the transitional area between them, were investigated with the French research aircraft Avion de Recherche Atmosphérique et de Télédétection (ARAT). Turbulent fluxes and deposition velocities of ozone were determined using the Eddy Correlation (EC) method. A specific methodology was developed to obtain accurate airborne turbulent flux measurements. This methodology is linked to the turbulence stationarity. The average values of ozone fluxes and ozone deposition velocities in the Atmospheric Boundary Layer (ABL) increase appreciably from savanna to forest. Near the ground, the ozone fluxes range between −0.115 +/−0.073 ppbv m/s above savanna and −0.350 +/−0.115 ppbv m/s above forest; for the deposition, the ranges are 0.0042 +/−0.0018 m/s and 0.015 +/−0.004 m/s. A simple empirical relationship between deposition velocity and Leaf Area Index (LAI) is proposed, giving an estimation of the deposition velocity for a whole latitudinal band. Vertical inputs of ozone to the ABL are estimated according to entrainment fluxes. The role of advection is neglected for horizontal transport of ozone in the ABL. The photochemical ozone production is deduced from the photo‐stationary state deviation, and compared to the net ozone increase in the ABL during the flights performed above the forest. A tentative ozone budget based on the aircraft measurements is proposed in the ABL of the rain forest. Around noon, the photochemical production dominates with a net production of about 10 ppbv/h.

Ozone in the remote marine boundary layer: A possible role for halogens

On the spring 1995 cruise of the National Oceanic and Atmospheric Administration research vessel Malcolm Baldrige, we measured very large diurnal variations in ozone concentrations in the marine boundary layer. Average diurnal variations of about 32% of the mean were observed over the tropical Indian Ocean. We simulated these observations with the Model of Chemistry in Clouds and Aerosols, a photochemical box model with detailed aerosol chemistry. The model was constrained with photolysis rates, humidity, aerosol concentrations, NO, CO, and O3 specified by shipboard observations and ozonesondes. Conventional homogeneous chemistry, where ozone photolysis to O(1D) and HOx chemistry dominate ozone destruction, can account for a diurnal variation of only about 12%. On wet sea‐salt aerosols (at humidities above the deliquescence point), absorption of HOBr leads to release of BrCl and Br2, which photolyze to produce Br atoms that may provide an additional photochemical ozone sink. After 8 days of simulation, these Br atoms reach a peak concentration of 1.2×107 cm−3 at noon and destroy ozone through a catalytic cycle involving BrO and HOBr. Reactive Br lost to HBr can be absorbed into the aerosol phase and reactivated. The model predicts a diurnal variation in O3 of 22% with aerosol‐derived Br reaction explaining much, but not all, of the observed photochemical loss. The lifetime of ozone under these conditions is short, about 2 days. These results indicate that halogens play an important role in oxidation processes and the ozone budget in parts of the remote marine boundary layer.

Impact of clouds on the tropospheric ozone budget: Direct effect of multiphase photochemistry of soluble organic compounds

A box model daytime study of the multiphase photochemistry (both gas and aqueous phases) of C 1 organic compounds within a non-precipitating cloud on a local scale was performed. In a situation where ozone accumulates in clear sky conditions, as soon as a cloud is formed, the tropospheric ozone budget changes drastically: the net production decreases by a factor of 2 or more, and, depending on the NO x concentrations and the pH values, can actually lead to a chemical net destruction. Both indirect and direct impacts caused by the presence of the liquid phase explain this. The indirect impact is the result of the much higher solubility of HO 2 and RO 2 radicals than NO and NO 2, resulting in lower gas-phase efficiency of the NO x cycles in producing ozone. The direct impact is caused by the very fast reaction of ozone towards O 2 − radicals within the liquid phase. The relative importance of these two impacts is determined for different NO x concentrations and pH values. When NO x =1 ppbv, the direct impact is of negligible importance at pH=3 (for a total decrease in ozone production of a factor of 2), but it accounts for 6% at pH=4.16 (for a total decrease in ozone production of a factor of 6.6), 16% at pH=5.2 (for a total net chemical destruction of ozone), and 28% at pH=6 (for a total net chemical destruction of ozone). At this NO x level and at pHs=5.2 and 6, despite its small contribution, the effect of the direct impact is the net chemical destruction of ozone. When NO x =0.1 ppbv, both the direct and the indirect impact contribute to the net chemical destruction at all pH values. However, the direct impact now contributes to a larger extent, accounting for 15% at pH=3, and up to 77% at pH=6. The direct impact involves the O 3+O 2 − reaction which leads to the production of aqueous phase OH radicals. The latter species in turn react with dissolved organic compounds to produce additional HO 2 and O 2 − radicals. Therefore, the aqueous phase sink of ozone is auto-catalytic in the presence of soluble organic compounds. The influence varying methanol concentrations has been tested.

Structural analysis and flux associations of CO 2, H 2O, heat and ozone over cotton and grape canopies

Micrometeorological tower data, collected over grape and cotton canopies as part of the California ozone deposition experiment (CODE) during the summer of 1991, are used to examine the temporal association between fluxes, and the physical characteristics of the coherent structures which dominate transport for both stable nighttime and unstable daytime conditions. Flux was calculated using the eddy covariance technique and the dominant modes of flux transport determined by quadrant analysis. The mean flux densities for both the cotton and grape site showed the surface acting as a sink for CO 2 and ozone and a source of heat and H 2O during the day, as would be expected, while during the night it became a source for CO 2 and a sink for heat, but remained a sink for ozone and a source of H 2O. The flux association results indicated a single vegetated ozone sink for the grape site, but a vegetated as well as a non-vegetated sink for the cotton site. For both sites, structures simultaneously transporting significant flux contributions of CO 2, H 2O, heat and ozone dominate during unstable conditions, but differed during stable conditions, where unmixed single flux structures dominated over cotton but not over grape. Structure sizes were less than 10 m during nighttime conditions and ranged from 3 to 69 m during the day. The results of this study contribute empirical evidence about the relationship between ozone uptake and the physical and physiological state of vegetation, as well as the limitations placed on eddy scales in simulation models.

Indoor Air Quality Impacts of Ventilation Ducts: Ozone Removal and Emissions of Volatile Organic Compounds

ABSTRACT The concentrations of contaminants in the supply air of mechanically ventilated buildings may be altered by pollutant emissions from and interactions with duct materials. We measured the emission rate of volatile organic compounds (VOCs) and aldehydes from materials typically found in ventilation ducts. The emission rate of VOCs per exposed surface area of materials was found to be low for some duct liners, but high for duct sealing caulk and a neo-prene gasket. For a typical duct, the contribution to VOC concentrations is predicted to be only a few percent of common indoor levels. We exposed selected materials to ~100-ppb ozone and measured VOC emissions. Exposure to ozone increased the emission rates of aldehydes from a duct liner, duct sealing caulk, and neoprene gasket. The emission of aldehydes from these materials could increase indoor air concentrations by amounts that are as much as 20% of odor thresholds. We also measured the rate of ozone uptake on duct liners and galvanized sheet metal to predict how much ozone might be removed by a typical duct in ventilation systems. For exposure to a constant ozone mol fraction of 37 ppb, a lined duct would initially remove ~9% of the ozone, but over a period of 10 days the ozone removal efficiency would diminish to less than 4%. In an unlined duct, in which only galvanized sheet metal is exposed to the air-stream, the removal efficiency would be much lower, ~0.02%. Therefore, ducts in ventilation systems are unlikely to be a major sink for ozone.

Ozone and meteorology during the 1994 Polar Sunrise Experiment

Observations of ozone mixing ratios in the lower troposphere of Arctic Canada in April 1994 are summarized. Except during a brief period of anomalous transport from high altitude, air depleted in ozone was always observed above sea ice and snow throughout the northern Ellesmere Island/Lincoln Sea region, or whenever air was sampled which had been in recent contact with sea ice and snow. Ozone mixing ratios observed at a camp on the sea ice north of Alert were consistent with a 1992 study. Observations at the ice camp confirmed that ozone was depleted more frequently (74% of all observations <5 ppbv) than at coastal and inland sites near Alert (10% of observations <5 ppbv). Mixing ratios briefly attained a maximum of 36 ppbv at the ice camp but were normally completely depleted or below typical free tropospheric levels of 35–45 ppbv observed elsewhere in the region. Aircraft measurements and vertical profiles of ozone and meteorological parameters from balloon sondes confirmed that ozone depletion existed in a layer above the sea ice, from the surface up to heights of 200–400 m. Some observations showed a very abrupt transition between depleted and nondepleted conditions at the upper boundary of the layer. The occurrence of a layer of fully depleted ozone was well correlated with surface high‐pressure systems. At Alert the appearance of air that was fully depleted in ozone was driven by advection from ocean areas to the north. A fortuitous set of meteorological conditions allowed the use of a simple model for testing general aspects of some proposed hypotheses for ozone destruction, during a period when it is believed the depleted layer was forming. It was found that the observed rate of ozone destruction would require levels of HO and Cl atoms much higher than would be expected for the prevailing conditions but reasonable concentrations of Br atoms. It was also found that an effective ozone deposition velocity of 0.1–0.2 cm s−1 could account for the observed depletion rate during this period. That is, the observed rate of ozone depiction during formation of a depleted layer was consistent with either a volume sink or a surface sink for ozone.

Flux association in coherent structures transporting CO 2, H 2O, heat and ozone over the code grid site

Aircraft-based eddy correlation flux data obtained at a height of 30 m above irrigated and non-irrigated agricultural land in southern California have been analyzed in terms of the coherent structures that dominate the turbulent exchange of energy and gases during daytime conditions. The analysis focused on transport of sensible heat, moisture, carbon dioxide and ozone in the gradient modes, i.e. excess up or deficit down for heat and moisture, and deficit up or excess down for carbon dioxide and ozone. Results are presented for composition and size of the dominant structures, over water-stressed and non-water-stressed surfaces, and on the relative frequency with which structures carrying only a single scalar, or given combinations of scalars, were encountered along the flight paths. Interpretation of results provides further evidence for the existence of a second (non-physiological) sink for ozone. The relative preponderance of structures that carry moisture, carbon dioxide and ozone simultaneously, particularly in the gradient up mode, reflects the importance of vegetation as co-located source/sink for these scalars. Surface characteristics resulting in thermal buoyancy and water vapour density gradients appears to be responsible for about 85% of gradient up transport. Finally, the detrending procedures described here may help to define more effective separation between local and mesoscale events in biosphere-atmosphere interactions.

Modelling cloud effects on ozone on a regional scale: A case study

We have investigated the influence of clouds on ozone on a regional scale (Europe) with a regional scale photochemical dispersion model (LOTOS). The LOTOS-model calculates ozone and other photo-oxidant concentrations in the lowest three km of the troposphere, using actual meteorologic data and emissions. We accounted for the altered radiative transfer in clouds and aqueous-phase chemistry (with and without iron and copper reactions in the aqueous phase). These cloud effects have been tested separately to distinguish their individual contribution to the ozone formation/degradation. Furthermore we estimated the effect of wet deposition parameterizing a upper-limit scavenging rate. Model results are shown for a cloudy period in August 1990. We find for this period that the radiative effect of clouds lead, locally, to ozone reductions in the planetary boundary layer by as much as 22%; overall to a reduction of ozone of 4%. This coincides with. a reduction of 14% of the net ozone formation. When aqueous-phase chemistry was added the net ozone formation and concentration were further reduced, up to 20% and 5%, respectively. Although aqueous-phase chemistry can lead to a strong reduction of ozone formation in the cloud (up to 110%), the overall effect is limited by the cloud volume fraction, cloud liquid water content and pH. The iron and copper reactions change the aqueous-phase reaction pathways substantially and render the droplet roughly a four times smaller sink for ozone.

Balloon‐borne measurements of stratospheric radicals and their precursors: Implications for the production and loss of ozone

Measurements of hydrogen, nitrogen and chlorine radicals from a balloon flight on 25 September 1993 from Ft. Sumner, NM provide an opportunity to quantify photochemical production and loss of stratospheric ozone. Ozone loss rates determined using measured radical concentrations agree fairly well with loss rates calculated using a photochemical model. Catalytic cycles involving OH and HO2 are shown to dominate photochemical loss of ozone for altitudes between 44 and 50 km. Reactions involving NO and NO2 are the dominant sink for ozone between 25 and 38 km. The total ozone loss rate determined from the measurements balances calculated production rates for altitudes between 30 and 40 km. However, loss of ozone exceeds production by ∼35% between 42 and 50 km. The imbalance between production and loss of ozone above 42 km is larger than the uncertainty of any one of the critical kinetic parameters or species concentrations. No single adjustment to any of these parameters can simultaneously resolve the imbalance and satisfy constraints imposed by measured OH, HO2, NO2 and ClO. Our results are consistent with an additional mechanism for ozone production above 40 km other than photolysis of ground state O2.

Long-term trends in concentrations of halocarbons and radiatively active trace gases in atlantic and european air masses monitored at mace head, Ireland from 1987–1994

Long-term trends in trace gas concentrations over the period 1987–1994 are reported here for air masses advected to the Mace Head monitoring station on the remote west coast of Ireland. The trace gases covered include the principal halocarbons: CFC-11, CFC-12, CFC-113, CCl 4 and methyl chloroform; the radiatively active trace gases: carbon dioxide, methane, nitrous oxide and ozone; together with carbon monoxide, the major photochemical ozone precursor. By careful sorting using two independent techniques, it is possible to distinguish air masses that arrive at Mace Head from over the North Atlantic Ocean and those that have recently travelled over polluted European continental land areas. Concentration trends have been derived for each trace gas in polluted European continental and baseline North Atlantic maritime air and they appear to be distinctly different. Using a simple long-range transport model, estimates have been made of the European source strengths required to sustain the observed concentrations of each trace gas and their recent trends. These are compared with published emission inventories where they are available. The European continent appears to be a significant source of methane, carbon dioxide and nitrous oxide, a net sink for ozone and a declining source of the principal halocarbons and carbon monoxide.