Chemistry of Halogen Oxides in the Troposphere: Comparison of Model Calculations with Recent Field Data

Reactive halogen species (RHS = X, XO, HOX, OXO; X = Cl, Br, I) are known to have an important influence on the chemistry in the polar boundary layer (BL), where they are responsible for ozone depletion events in spring. Recent field campaigns at Mace Head, Ireland, and the Dead Sea, Israel, identified for the first time iodine oxide (IO) at mixing ratios of up to 6.6 ppt and 90 ppt bromine oxide (BrO), respectively, by DOAS also at lower latitudes. These results intensified the discussion about the role of the RHS in the mid-latitude BL.Photochemical box model calculations show that the observed IO mixing ratios can destroy ~0.45 ppb ozone per hour. This is comparable to the rates of the known O₃-loss processes in the boundary layer. The model studies also reveal that IO, at these levels, has a strong influence on the BL photochemistry, increasing the OH/HO₂- and the NO₂/NO - ratios. In combination these changes lead to a reduction of the photochemical ozone formation, which - in addition - reduces ozone mixing ratios by up to 0.15 ppb/h.The studies for the Dead Sea case give no information on the heterogeneous process responsible for the bromine release, but they show that a total of 2 – 4 ppb of total bromine have to be released to explain the observed complete depletion of 60 ppb ozone in 2 – 3 hours.     

Ozone Production Efficiency in Savanna and Forested Areas during the EXPRESSO Experiment

NO, NO_x, NO_y and O₃ have been measuredduring the airborne EXPRESSO experiment, November 96,which took place near Bangui (Central Africa) at thebeginning of the dry season. This period correspondsto an intense burning activity. Chemical andphotochemical characteristics of the planetaryboundary layer, which corresponds most of the time tothe monsoon layer, and the Harmattan layer, which issituated above the latter, have been studied forsavanna as well as rain forest areas. These two layersare very different when considering the chemicalcomposition (especially for ozone and NO_z) andthe photochemical age.The relationship of photochemical ozone productionversus photochemical NO_x oxidation products hasbeen investigated. Results indicate an ozoneproduction efficiency (OPE) ranging from 6.3 to 14.8in the planetary boundary layer. Thus, this layer ischaracteristic of a photochemically young and activeair mass. In this layer, the ozone potentialproduction increases with the air mass photochemicalage. On the other hand, the Harmattan layer shows anOPE ranging from 4.6 to 7.4. These values arecharacteristic of photochemically well-aged airmasses. In this layer, the ozone potential productionseems to be exhausted with values around 4.0 (i.e., 4ozone molecules produced for each NO_x moleculeemitted).     

Trace Gas Measurements Between Moscow and Vladivostok Using the Trans-Siberian Railroad

Using a laboratory wagon traveling along the Trans-Siberian railroad, O₃, NO, NO₂, CO, CH₄, SF₆ and black carbon aerosol have been measured during the summer of 1996. The expedition from Niznij Novgorod (500 km east of Moscow) to Vladivostok (and back to Moscow) has shown the great potential of the train method; here the first results are presented and discussed. A wealth of boundary layer air data was obtained during the over 18000 km travel without serious contamination problems from the electric train itself. The diurnal O₃ cycle peaked generally below 50 nmole/mole, showed the effects of changes in J(NO₂), and often dropped to a few nmole/mole at night time during inversions. Over the vast Siberian lowlands situated between the Ural mountains and the river Yenisey, CH₄ levels were consistently elevated at around 1.95 µmole/mole, which we mainly attribute to wetland emissions. Over eastern Siberia, however, CH₄ levels were generally lower at 1.85 µmole/mole. In contrast, over the west Siberian lowlands, CO levels were relatively low, often reaching values of only 110 nmole/mole, whereas over eastern Siberia CO levels were higher. Very high CO levels were detected over a 2000 km section east of Chita, along the river Amur, which represented an enormous polluted air mass. ¹⁴C analysis performed on several CO samples confirms that the origin was biomass burning. SF₆, which was measured as a general conserved tracer, showed an eastward attenuation from 4.0 to 3.9 pmole/mole, with peaks in a number of places due to local Russian emissions.     

Photochemistry in the Arctic Free Troposphere: Ozone Budget and Its Dependence on Nitrogen Oxides and the Production Rate of Free Radicals

Local ozone production and loss rates for the arctic free troposphere (58–85° N, 1–6 km, February–May) during the TroposphericOzone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1–3 km layer and a factor of 7 in the 3–6 kmlayer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1–3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NO_x mixing ratiosup to 300 pptv in February and for NO_x mixing ratios up to 500 pptv in May. These NO_x limits are an order of magnitude higher thanmedian NO_x levels observed, illustrating the strong dependence ofgross ozone production rates on NO_x mixing ratios for the majority of theobservations. The threshold NO_x mixing ratio needed for netpositive ozone production was also calculated to increase from NO_x 10pptv in February to 25 pptv in May, suggesting that the NO_x levels needed to sustain net ozone production are lower in winter than spring. This lower NO_x threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NO_x neededto produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1–3 km layerand net ozone production for the 3–6 km layer throughout the campaign.     

In Situ Trace Gas and Particle Measurements in the Summer Lower Stratosphere during STREAM II: Implications for O3 Production

In situ aircraft measurements of O₃, CO,HNO₃, and aerosol particles are presented,performed over the North Sea region in the summerlower stratosphere during the STREAM II campaign(Stratosphere Troposphere Experiments by AircraftMeasurements) in July 1994. Occasionally, high COconcentrations of 200-300 pbbv were measured in thelowermost stratosphere, together with relatively highHNO₃ concentrations up to 1.6 ppbv. The particlenumber concentration (at standard pressure andtemperature) between 0.018-1 m decreased acrossthe tropopause, from >1000 cm^-3 in the uppertroposphere to <500 cm^-3 in the lowermoststratosphere. Since the CO sources are found in thetroposphere, the elevated CO mixing ratios areattributed to mixing of polluted tropospheric air intothe lowermost extratropical stratosphere. Further wehave used a chemical model to illustrate that nitrogenoxide reservoir species (mainly HNO₃) determinethe availability of NO_x (=NO + NO₂) andtherefore largely control the total net O₃production in the lower kilometers of thestratosphere. Model simulations, applying additionalNO_x perturbations from aircraft, show that theO₃ production efficiency of NO_x is smallerthan previously assumed, under conditions withrelatively high HNO₃ mixing ratios, as observedduring STREAM II. The model simulations furthersuggest a relatively high O₃ productionefficiency from CO oxidation, as a result of therelatively high ambient HNO₃ and NO_xconcentrations, implying that upward transport of COrich air enhances O₃ production in the lowermoststratosphere. Analysis of the measurements and themodel calculations suggest that the lowermoststratosphere is a transition region in which thechemistry deviates from both the upper troposphere andlower stratosphere.     

Isoprene and Its Oxidation Products Methyl Vinyl Ketone, Methacrolein, and Isoprene Related Peroxides Measured Online over the Tropical Rain Forest of Surinam in March 1998

Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0–12 km altitude,2°–7° N, 54°–58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1–3 × 10⁵molecules cm^-3 averaged over 24 hours were calculated due to lossof OH and HO₂ in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer.     

Micrometeorological Measurements in a Street Canyon during the Joint ATREUS-PICADA Experiment

In order to investigate the microclimatic conditions in a street canyon, a physical model was used to conduct the Joint ATREUS-PICADA Experiment (JAPEX) in situ experimental campaign. Four lines of buildings simulated by steel containers were installed to form three parallel street canyons at 1:5 scale, with width/height aspect ratio approximately 0.40. The reference wind and atmospheric conditions were measured, as well as the flow velocity and direction in the street. Preliminary results concern street canyon ventilation and thermal effects on in-canyon airflow, and show that vortical motions appear for reference wind directions perpendicular to the street axis. The presence of adjacent rows of buildings did not appear to significantly influence the flow character within the canyon for the case of a low aspect ratio corresponding to a skimming flow regime. The flow structure was not significantly affected by the thermal effects although some slight interference occurred in the lower part of the canyon. An analysis of horizontal temperature gradients indicated that a thin boundary layer develops near the heated facade. These facts imply that the thermal effects are considerable only very close to the wall.     

An Assessment of HOx Chemistry in the Tropical Pacific Boundary Layer: Comparison of Model Simulations with Observations Recorded during PEM Tropics A

Reported are the results from a comparison of OH,H₂O₂CH₃OOH, and O₃ observationswithmodel predictions based on current HO_x–CH₄reaction mechanisms. The field observations are thoserecorded during the NASA GTE field program, PEM-Tropics A. The major focus ofthis paper is on thosedata generated on the NASA P-3B aircraft during a mission flown in the marineboundary layer (MBL) nearChristmas Island, a site located in the central equatorial Pacific (i.e.,2° N, 157° W). Taking advantage of thestability of the southeastern trade-winds, an air parcel was sampled in aLagrangian mode over a significantfraction of a solar day. Analyses of these data revealed excellent agreementbetween model simulated andobserved OH. In addition, the model simulations reproduced the major featuresin the observed diurnalprofiles of H₂O₂ and CH₃OOH. In the case ofO₃, the model captured the key observational feature whichinvolved an early morning maximum. An examination of the MBL HO_xbudget indicated that the O(¹D) + H₂Oreaction is the major source of HO_x while the major sinks involveboth physical and chemical processes involving the peroxide species,H₂O₂ and CH₃OOH. Overall, the generally goodagreement between modeland observations suggests that our current understanding ofHO_x–CH₄ chemistry in the tropical MBL isquite good; however, there remains a need to critically examine this chemistrywhen both CH₂O and HO₂are added to the species measured.     

Natural and anthropogenic C2 to C6 hydrocarbons in the central-eastern Venezuelan atmosphere during the rainy season

The levels of low molecular weight hydrocarbons were measured at pristine sites and rural locations affected by hydrocarbon emissions from oil and gas producing fields in Venezuela. At the clean sites, lower concentrations of C₂ to C₆ alkanes were observed, whereas, in comparison with remotes sites, very much higher levels were measured at the polluted sites. Alkenes present relatively high concentrations, with isoprene being the most abundant, all over the study region. The main sources of alkenes are likely to be natural, mainly from vegetation. The levels of alkanes recorded at the clean sites and the alkene levels found everywhere in the region are in agreement with the values reported for other clean sites in the tropics. The increase of ozone production capacity due to the anthropogenic emissions of alkanes from oil and gas fields was estimated. Due to the presence in the atmosphere of important amounts of naturally emitted isoprene, ethene and propene, which makes a substantial contribution to the reactivity of the hydrocarbon mixture, a small increase (<5%) was estimated to occur in the capacity of the ozone production at a regional scale during the rainy season.     

Measurements of gas and aerosol for two weeks in northern China during the winter–spring period of 2000, 2001 and 2002

Intensive measurements of gas and aerosol for 2 weeks were carried out at Qingdao (gas and aerosol in 2000, 2001 and 2002), Fenghuangshan (gas and aerosol in 2000 and 2001), and Dalian (aerosol in 2002) in the winter–spring period. High SO₂ episodes were observed on 18 January 2000 at both Qingdao and Fenghuangshan. According to back trajectory calculations and analysis of gaseous species, high SO₂ episodes were caused by local pollution and transport.Nitrate, sulfate and ammonium were the major species in PM_2.5. Mass fractions of NO₃⁻, nss-SO₄²⁻ and NH₄⁺ at Qingdao in 2002 were 10%, 12% and 5.5% for PM_2.5, respectively, which were higher than that of nss-Ca²⁺ (1%). Chemical compositions observed at Dalian and Fenghuangshan were similar to those at Qingdao. The mass ratio of nss-SO₄²⁻/SO₂ at Qingdao in winter was low (< 1.2), indicating that sulfate was probably produced by the slow oxidation of SO₂ in the gas phase and/or was transported from outside of Qingdao in winter. The equivalent ratio of NH₄⁺ to nss-SO₄²⁻ was 1.39, suggesting that ammonium sulfate was one of the major chemical compositions in PM_2.5. The NO₃⁻/SO₄²⁻ ratio at Qingdao was higher than that at remote places in East Asia. Gas and aerosol data obtained at Fenghuangshan were similar to data at Qingdao, suggesting that emissions from small cities may have a great influence on pollution in northern China.     

The partitioning of nitrogen oxides in the lower Arctic troposphere during spring 1988

Surface observations of several nitrogen oxides in the Canadian high Arctic during the period March-April 1988 are reported. These include data on NO₂, the inorganic nitrates HNO₃ and particulate nitrate, and the organic nitrates PAN and C₃–C₇ alkyl-nitrates. It is found that the organic nitrates make up 70–80% of the sum of the measured nitrogen oxides. Based on concurrently measured sulphur oxides, the period of observation was divided into two halves with the first half representing less polluted, more aged air than the second. The preponderance of the organic nitrates was less in the first period than the second. In contrast, there was little difference in the inorganic nitrates and NO₂ concentrations. The dominant inorganic nitrate shifted from particulate nitrate in the first period towards gaseous HNO₃ in the second. No correlation between the nitrates (inorganic or organic) and O₃ was observed; although some indication of a positive correlation between NO₂ and O₃ has been reported earlier (Bottenheimet al., 1990). Possible explanations for these observations are proposed. A survey of other potential nitrogen oxides that may be present in the Arctic air but not measured in these experiments suggests that the nitrogen oxides not measured here constitute a minor fraction of the total reactive nitrogen (NO _y ).Paper submitted to the 7th International Symposium of the Commission for Atmospheric Chemistry and Global Pollution on the Chemistry of the Global Atmosphere held in Chamrousse, France, from 5 to 11 September 1990.     

Measurements of Trace Substances in the Arctic Troposphere as Potential Precursors and Constituents of Arctic Haze

During two measuring campaigns in early spring 1994 and 1995 (March/April) and one campaign in summer 1994, measurements of ozone, PAN, sulfur dioxide, nitric acid, and particulate nitrate, sulfate, and ammonium (only 1995) were recorded in the Arctic. Observations were made by aircraft at various sites in the eastern and western Arctic. Ozone concentrations showed a steady increase with altitude both in spring and summer. During five flights in springtime, low ozone events (LOEs) could be observed near the surface and up to altitudes of 2000 m. SO₂ background concentrations, ranging from detection limit (0.5 nmol/m³) to 5 nmol/m³, were observed during both spring and summer. Distinct maxima up to 55 nmol/m³ in lower altitudes were only obtained in springtime. Concentrations of the organic nitrate PAN were within a similar range as those of the inorganic nitrate HNO₃ during spring campaigns. In contrast, concentrations of particulate nitrate were one half an order of magnitude lower. HNO₃ concentrations increased significantly with altitude. Evidently, HNO₃ was intruded from the stratosphere into the troposphere. Sulfate concentrations ranged between 5 and 30 nmol/m³; ammonium concentrations were obtained within a range from 10 to 50 nmol/m³.     

Simultaneous Measurements of Black Carbon, PM10, Ozone and NOx Variability at a Locally Polluted Island in the Southern Tropics

This paper shows a comparative study of particle and surface ozone concentration measurements undertaken simultaneously at two distinct semi-urban locations distant by 4 km at Saint-Denis, the main city of La Réunion island (21.5° S, 55.5° E) during austral autumn (May 2000). Black carbon (BC) particles measured at La Réunion University, the first site situated in the suburbs of Saint-Denis, show straight-forward anti-correlation with ozone, especially during pollution peaks ( 650 ng/m³ and 15 ppbv, for BC and ozone respectively) and at night-time (90 ng/m³ and 18.5 ppbv, for BC and ozone respectively). NO_x (NO and NO₂) and PM₁₀ particles were also measured in parallel with ozone at Lislet Geoffroy college, a second site situated closer to the city centre. NO_x and PM₁₀ particles are anti-correlated with ozone, with noticeable ozone destruction during peak hours (mean 6 and 9 ppbv at 7 a.m. and 8 p.m. respectively) when NO_x and PM₁₀ concentrations exhibit maximum values. We observe a net daytime ozone creation (19 ppbv, O₃ +4.5 ppbv), following both photochemical and dynamical processes. At night-time however, ozone recovers (mean 11 ppbv) when anthropogenic activities are lower ([BC] 100 ng/m³). BC and PM₁₀ concentration variation obtained during an experiment at the second site shows that the main origin of particles is anthropogenic emission (vehicles), which in turn influences directly ozone variability. Saint-Denis BC and ozone concentrations are also compared to measurements obtained during early autumn (March 2000) at Sainte-Rose (third site), a quite remote oceanic location. Contrarily to Saint-Denis observations, a net daytime ozone loss (14.5 ppbv at 4 p.m.) is noticed at Sainte-Rose while ozone recovers (17 ppbv) at night-time, with however a lower amplitude than at Saint-Denis. Preliminary results presented here are handful data sets for modelling and which may contribute to a better comprehension of ozone variability in relatively polluted areas.     

Production of N2O,CH4, and CO2 from soils in the tropical savanna during the dry season

Emissions of N₂O, CH₄, and CO₂ from soils at two sites in the tropical savanna of central Venezuela were determined during the dry season in February 1987. Measured arithmetic mean fluxes of N₂O, CH₄, and CO₂ from undisturbed soil plots to the atmosphere were 2.5×10⁹, 4.3×10¹⁰, and 3.0×10¹³ molecules cm^-2 s^-1, respectively. These fluxes were not significantly affected by burning the grass layer. Emissions of N₂O increased fourfold after simulated rainfall, suggesting that production of N₂O in savanna soils during the rainy season may be an important source for atmospheric N₂O. The CH₄ flux measurements indicate that these savanna soils were not a sink, but a small source, for atmospheric methane. Fluxes of CO₂ from savanna soils increased ninefold two hours after simulated rainfall, and remained three times higher than normal after 16 hours. More research is needed to clarify the significance of savannas in the global cycles of N₂O, CH₄, CO₂, and other trace gases, especially during the rainy season.