Surface O3 Concentrations Research Articles

Calculation of the Energetics for the Oligomerization of Gas Phase HgO and HgS and for the Solvolysis of Crystalline HgO and HgS

Recent experimental studies indicate that gaseous elemental Hg (GEM) is rapidly oxidized to Hg(II) compounds, known collectively as reactive gaseous Hg (RGM), in Arctic and Antarctic regions after polar sunrise. The reduction in GEM is correlated with a reduction in surface O(3) concentration, which is thought to be caused by photochemically initiated catalytic reactions involving halogen species, particularly Br and BrO. Initially, the reaction of Hg(0) and BrO to produce HgO and Br was thought to be the dominant reaction, but recent theoretical studies have decisively shown that this reaction is highly endoergic due to the low stability of monomeric gas-phase HgO. This result is in conflict with experimental data on the energetics of the species existing in the vapor over heated HgO (s). One possible explanation for this discrepancy is the existence of highly stable oligomers formed from HgO. Recent high-level quantum calculations on the dimers of HgO and HgS support this concept. In the present work, we systematically examine the structures, stabilities, and other properties of closed (HgX)(n)() ring-type oligomers, n = 2, 3, 4, and 6, X = O, S, as well as infinite one-dimensional (1D) polymers of HgX (studied by using the periodic boundary condition DFT implementation in GAUSSIAN03). We find that the HgX ring oligomers become systematically more stable (per HgX unit) as n increases but that this stability levels off around n = 4-6. We also find that the 1D chain polymers are only marginally more stable than the n = 6 oligomers. To estimate the energies of interaction between the chains in the 3-dimensional (3D) crystal structures of HgX (s), we adopt a cluster model and use the MP2 method to describe the interchain dispersion interactions. We have also obtained optimized geometries for open chain triplets for the dimers, finding them to be substantially more stable than the closed ringlike dimeric species previously described. Trends in relative energies and structures indicate that the higher n oligomers are fairly normal Hg(II) compounds that can be accurately described at low computational levels, as opposed to the monomer and dimer, which possess highly unusual bonding properties and require high-level methods for their description. Nonetheless, even the high n ring, oligomers show close approach of Hg atoms, consistent with a metallophilic-type stabilization. Calculated free energies for the interaction of HgO with H(2)O and with simple models for silicate surfaces are highly favorable, indicating that hydration and surface effects will greatly promote the formation of such species. Molecular cluster models of the HgX surface such as Hg(2)X(XH)(2) are used to calculate the energetics for solvolysis reactions with H(2)O or H(2)S, obtaining good agreement with experiment for the energetics of the dissolution reaction of HgS (s, cinnabar) with H(2)S.

Temporal and spatial variability of surface ozone at Delhi and Antarctica

This study deals with the temporal and spatial variability of 365 days; hourly mean surface ozone data for three different locations, New Delhi (a site of intense anthropogenic activity), Syowa and McMurdo stations in Antarctica (sites of clean background air). The analysis not only shows the seasonal and diurnal variation of O3 over New Delhi and Antarctica, but also provides statistics of means and variability on a temporal scale. Eight-hour (9-17 h) surface O3 concentration at New Delhi (2001) was recorded, which was 59 of the WHO (80 ppb) ambient air quality standard for ozone. Likewise, the monthly mean of daily maximum O3 during April and November (2001) was observed, which was 97 of the WHO ambient air quality standard for ozone that indicates the serious ozone pollution in New Delhi. Mean rate of daytime photochemical production of surface O3 at Delhi (2001) has been observed around 7.1 ppb h-1 between 6 and 12 h, while, Syowa and McMurdo stations showed photochemical loss of surface O3 during daylight hours of about 1 ppb and 0.2 ppb, respectively. The diurnal trend observed during summer at Syowa station shows a sign of daytime photochemical depletion. The photochemical loss of surface O3 during summer months with respect to the winter maximum ozone was observed to be about 56 and 64 at Syowa and McMurdo stations, respectively. The day-to-day variability at Syowa and McMurdo station suggests that the major loss term is a process in the sea ice zone.

Aerosol Lidar and in situ ozone observations of the planetary boundary layer over Bulgaria during the solar eclipse of 11 August 1999

A complex investigation of the planetary boundary layer (PBL) is presented. Observations were carried out on 11 August 1999 during the solar eclipse over Bulgaria using a Light Detection and Ranging Device (Lidar), ozone meters and ground meteorological stations. The Lidar was used to measure the height of the mixing layer before, during and after the solar eclipse in Sofia city; the ozone meters measured the surface O3 concentrations during the phenomenon, while the ground stations took meteorological parameters of the atmospheric ground layer. Weather conditions in all the regions (Sofia, Shabla, Ahtopol and Rozhen peak) were favourable for the observations. The data of the three types of measurements demonstrate with certainty that the solar eclipse affects the meteorological parameters of the atmosphere near the ground, the ozone concentration and the height of the mixing layer. It was found that a certain time delay exists in the solar eclipse's impact on the meteorological parameters, the ozone concentration and the mixing layer height and that this delay was different for each of the different parameters.

Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States

Reducing surface ozone (O3) to concentrations in compliance with the national air quality standard has proven to be challenging, despite tighter controls on O3 precursor emissions over the past few decades. New evidence indicates that isoprene emissions changed considerably from the mid‐1980s to the mid‐1990s owing to land‐use changes in the eastern United States (Purves et al., 2004). Over this period, U.S. anthropogenic VOC (AVOC) emissions decreased substantially. Here we apply two chemical transport models (GEOS‐CHEM and MOZART‐2) to test the hypothesis, put forth by Purves et al. (2004), that the absence of decreasing O3 trends over much of the eastern United States may reflect a balance between increases in isoprene emissions and decreases in AVOC emissions. We find little evidence for this hypothesis; over most of the domain, mean July afternoon (1300–1700 local time) surface O3 is more responsive (ranging from −9 to +7 ppbv) to the reported changes in anthropogenic NOx emissions than to the concurrent isoprene (−2 to +2 ppbv) or AVOC (−2 to 0 ppbv) emission changes. The estimated magnitude of the O3 response to anthropogenic NOx emission changes, however, depends on the base isoprene emission inventory used in the model. The combined effect of the reported changes in eastern U.S. anthropogenic plus biogenic emissions is insufficient to explain observed changes in mean July afternoon surface O3 concentrations, suggesting a possible role for decadal changes in meteorology, hemispheric background O3, or subgrid‐scale chemistry. We demonstrate that two major uncertainties, the base isoprene emission inventory and the fate of isoprene nitrates (which influence surface O3 in the model by −15 to +4 and +4 to +12 ppbv, respectively), preclude a well‐constrained quantification of the present‐day contribution of biogenic or anthropogenic emissions to surface O3 concentrations, particularly in the high‐isoprene‐emitting southeastern United States. Better constraints on isoprene emissions and chemistry are needed to quantitatively address the role of isoprene in eastern U.S. air quality.

NO emissions from large point sources: variability in ozone production, resulting health damages and economic costs

We present a proof-of-concept analysis of the measurement of the health damage of ozone (O3) produced from nitrogen oxides (NOx=NO+NO2) emitted by individual large point sources in the eastern United States. We use a regional atmospheric model of the eastern United States, the Comprehensive Air quality Model with Extensions (CAMx), to quantify the variable impact that a fixed quantity of NOx emitted from individual sources can have on the downwind concentration of surface O3, depending on temperature and local biogenic hydrocarbon emissions. We also examine the dependence of resulting O3-related health damages on the size of the exposed population. The investigation is relevant to the increasingly widely used “cap and trade” approach to NOx regulation, which presumes that shifts of emissions over time and space, holding the total fixed over the course of the summer O3 season, will have minimal effect on the environmental outcome. By contrast, we show that a shift of a unit of NOx emissions from one place or time to another could result in large changes in resulting health effects due to O3 formation and exposure. We indicate how the type of modeling carried out here might be used to attach externality-correcting prices to emissions. Charging emitters fees that are commensurate with the damage caused by their NOx emissions would create an incentive for emitters to reduce emissions at times and in locations where they cause the largest damage.

Charging Nox Emitters for Health Damages: An Exploratory Analysis

We present a proof-of-concept analysis of the measurement of the health damage of ozone (O3) produced from nitrogen oxides (NOx = NO + NO2) emitted by individual large point sources in the eastern United States. We use a regional atmospheric model of the eastern United States, the Comprehensive Air Quality Model with Extensions (CAMx), to quantify the variable impact that a fixed quantity of NOx emitted from individual sources can have on the downwind concentration of surface O3, depending on temperature and local biogenic hydrocarbon emissions. We also examine the dependence of resulting ozone-related health damages on the size of the exposed population. The investigation is relevant to the increasingly widely used ?cap and trade? approach to NOx regulation, which presumes that shifts of emissions over time and space, holding the total fixed over the course of the summer O3 season, will have minimal effect on the environmental outcome. By contrast, we show that a shift of a unit of NOx emissions from one place or time to another could result in large changes in the health effects due to ozone formation and exposure. We indicate how the type of modeling carried out here might be used to attach externality-correcting prices to emissions. Charging emitters fees that are commensurate with the damage caused by their NOx emissions would create an incentive for emitters to reduce emissions at times and in locations where they cause the largest damage.

An ozone episode in the Philadelphia metropolitan area

In July and August 1999 a Northeast Oxidant and Particle Study field campaign was conducted in the Philadelphia metropolitan area to determine causes for episodically high levels of O3 and particulate matter ≤2.5 μm in aerodynamic diameter. We report emission estimates, weather information, surface O3 monitoring data, and aircraft observations, with a focus on 31 July, the last day of an O3 episode in which concentrations in Philadelphia reached 165 ppb, the highest level observed there in the past 11 years. As is common in the northeastern states, this O3 episode started with the development of a broad ridge over the central United States and ended with the northeast corridor under the influence of an Appalachian lee trough with airflow from the SW in an along‐corridor direction. For a portion of the morning of 31 July, winds were nearly stagnant, allowing local emissions to accumulate. In contrast to typical O3 episodes in the northeast, transport on 31 July was limited, and O3 hot spots occurred close to NOx and volatile organic compound (VOC) emission sources. High O3 was observed downwind of Baltimore and Philadelphia, both major urban areas. High O3 was also observed in a less likely region near the Delaware‐Pennsylvania border, downwind of Wilmington, Delaware, but near utility and industrial emission sources. Surface O3 monitoring data and morning aircraft observations show that the residual layer aloft contained 80–100 ppb of O3 but almost no O3 precursors. Same‐day photochemistry on 31 July caused surface O3 concentrations to increase by 60–80 ppb. Photochemical model calculations indicate O3 production rates in excess of 20 ppb h−1 in regions with NOx > 5 ppb. High NOx concentrations are a consequence of poor ventilation. Peroxide observations and calculations indicate that O3 production is VOC limited in the high‐NOx portions of the Philadelphia urban plume. Our results are contrasted against a severe O3 episode that occurred in 1995. While the 1999 episode had stagnation conditions leading to local O3 hot spots, there were mesoscale meteorological features in 1995 that favored interregional transport.

Sensitivity of regional ozone concentrations to temporal distribution of emissions

Temporal representations of emissions bear large uncertainty. Before any costly efforts are undertaken to improve the accuracy of temporal emission profiles, however, the impact of their uncertainty on predictions of surface O3 concentrations should be examined. In this study, a 3-D air quality modeling system was used to probe the sensitivity of regional surface O3 concentrations to temporal allocation of emissions over the continental US. The raw emissions inventory was processed using SMOKE and was segmented into hourly intervals using both “time-varying” and “uniform” temporal profiles of anthropogenic sources. Our simulation results show that, on average and with the grid resolution (90 km) used, regional daytime O3 concentrations are not sensitive to changes in the temporal allocation of emissions, while nighttime O3 concentrations are lower under “uniform” profiles than under “time-varying” profiles.

Characterizing distributions of surface ozone and its impact on grain production in China, Japan and South Korea: 1990 and 2020

Abstract Using an integrated assessment approach, we evaluate the impact that surface O3 in East Asia had on agricultural production in 1990 and is projected to have in 2020. We also examine the effect that emission controls and the enforcement of environmental standards could have in increasing grain production in China. We find that given projected increases in O3 concentrations in the region, East Asian countries are presently on the cusp of substantial reductions in grain production. Our conservative estimates, based on 7- and 12-h mean (M7 or M12) exposure indices, show that due to O3 concentrations in 1990 China, Japan and South Korea lost 1–9% of their yield of wheat, rice and corn and 23–27% of their yield of soybeans, with an associated value of 1990US$ 3.5, 1.2 and 0.24 billion, respectively. In 2020, assuming no change in agricultural production practices and again using M7 and M12 exposure indices, grain loss due to increased levels of O3 pollution is projected to increase to 2–16% for wheat, rice and corn and 28–35% for soybeans; the associated economic costs are expected to increase by 82%, 33%, and 67% in 2020 over 1990 for China, Japan and South Korea, respectively. For most crops, the yield losses in 1990 based on SUM06 or W126 exposure indices are lower than yield losses estimated using M7 or M12 exposure indices in China and Japan but higher in South Korea; in 2020, the yield losses based on SUM06 or W126 exposure indices are substantially higher for all crops in all three countries. This is primarily due to the nature of the cumulative indices which weight elevated values of O3 more heavily than lower values. Chinese compliance with its ambient O3 standard in 1990 would have had a limited effect in reducing the grain yield loss caused by O3 exposure, resulting in only US$ 0.2 billion of additional grain revenues, but in 2020 compliance could reduce the yield loss by one third and lead to an increase of US$ 2.6 (M7 or M12) –27 (SUM06) billion in grain revenues. We conclude that East Asian countries may have tremendous losses of crop yields in the near future due to projected increases in O3 concentrations. They likely could achieve substantial increases in future agricultural production through reduction of surface O3 concentrations and/or use of O3 resistant crop cultivars.

Seasonal and Diurnal Variation of Surface Ozone and a Preliminary Analysis of Exceedance of its Critical Levels at a Semi-arid Site in India

The mixing ratios of surface O3 were measured at St. John's College, Agra, an urban and traffic influenced area for the period of 2000–2002. The monthly averaged O3 mixing ratios ranged between 8 to 40 ppb with an annual average of 21 ppb. Strong diurnal and seasonal variations in O3 mixing ratios were observed throughout the year except for monsoon season. The mixing ratios of O3 follow the surface temperature cycle and solar radiation (r = 0.72 and r = 0.65 with temperature and solar radiation, respectively). Concentrations were higher with winds associated with NE and NW direction indicating the impact of pollution sources on surface O3 concentration. Exceedance of ozone critical level was calculated using the AOT 40 index and found to be 840 ppb.h and 2430 ppb.h for summer and winter seasons, respectively. The present O3 exposures are lower than the critical level of O3 and suggest that the present level of O3 does not have any impact on reduction in crop yields.

Variability in surface ozone background over the United States: Implications for air quality policy

The U.S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background O3 level as a baseline in its O3 risk assessments. This background is defined as those concentrations that would exist in the absence of North American emissions. Lefohn et al. [2001] have argued that frequent occurrences of O3 concentrations above 50–60 ppbv at remote northern U.S. sites in spring are of stratospheric origin, challenging the EPA background estimate and implying that the current O3 standard (84 ppbv, 8‐hour average) may be unattainable. We show that a 3‐D global model of tropospheric chemistry reproduces much of the observed variability in U.S. surface O3 concentrations, including the springtime high‐O3 events, with only a minor stratospheric contribution (always <20 ppbv). We conclude that the previous interpretations of a stratospheric source for these events underestimated the role of regional and hemispheric pollution. While stratospheric intrusions might occasionally elevate surface O3 at high‐altitude sites, our results indicate that these events are rare and would not compromise the O3 air quality standard. We find that the O3 background is generally 15–35 ppbv, with some incidences of 40–50 ppbv in the west in spring at high‐elevation sites (>2 km). It declines from spring to summer and further decreases during O3 pollution episodes. The 40 ppbv background assumed by EPA thus actually underestimates the risk associated with O3 during polluted conditions. A better definition would represent background as a function of season, altitude, and total surface O3 concentration. Natural O3 levels are typically 10–25 ppbv and never exceed 40 ppbv. International controls to reduce the hemispheric pollution background would facilitate compliance with an AOT40‐type standard (cumulative exposure to O3 above 40 ppbv) in the United States.

Development and application of a state‐of‐the‐science plume‐in‐grid model

We describe the development, evaluation, and application of a new plume‐in‐grid model for investigating the subgrid‐scale effects, associated with NOx emissions from large elevated point sources, on O3 formation. Traditional Eulerian air quality models cannot resolve the strong concentration gradients created by plumes emitted from large point sources. Although several plume‐in‐grid approaches have been used in the past to address this issue, they have been limited by their simplistic simulation of plume dispersion and/or chemistry and their lack of treatment of the effect of turbulence on plume chemistry. In the plume‐in‐grid model presented here, the embedded reactive plume model combines a state‐of‐the‐science puff model with a gas‐phase chemistry mechanism that is consistent with that used in the host grid model. The puff model uses a second‐order closure scheme, allowing for a more accurate treatment of dispersion and the influence of turbulent concentration fluctuations on chemical rates. It also allows the splitting and merging of puffs to account for wind shear effects, varying chemistry across the plume, and interplume and intraplume interactions. The combined puff/chemistry model is embedded into an Eulerian grid model. Results from the application of this model to the northeastern United States, a domain containing some of the largest NOx‐emitting power plants in the United States, show that the plume‐in‐grid treatment leads to significant differences in surface O3 and HNO3 concentrations.

PROTECTING AGRICULTURAL CROPS FROM THE EFFECTS OF TROPOSPHERIC OZONE EXPOSURE: Reconciling Science and Standard Setting in the United States, Europe, and Asia

▪ Abstract Ozone (O3) is well documented as the air pollutant most damaging to agricultural crops and other plants. Most crops in developed countries are grown in summer when O3 concentrations are elevated and frequently are sufficiently high to reduce yields. This article examines the difficulties in scientifically determining the reduction in yield that results from the exposure of agricultural crops to surface O3 and then transforming that knowledge into efficient and effective regulatory standards. The different approaches taken by the United States and Europe in addressing this issue as well as the few studies that have been conducted to date in developing countries are examined and summarized. Extensive research was conducted in the United States during the 1980s but has not been continued. During the 1990s, the European community forged ahead with scientific research and innovative proposals for air-quality standards. These efforts included the development of a “critical level” (CL) for O3 based on a cumulative exposure above a cutoff concentration below which only an acceptable level of harm is incurred. Current research focuses on estimating O3 dosage to plants and incorporating this metric into regulatory standards. The US regulatory community can learn from current European scientific research and regulatory strategies, which argue strongly for a separate secondary standard for O3 to protect vegetation. Increasing impacts of O3 on crops are likely in developing countries as they continue to industrialize and their emissions of air pollutants increase. More research is needed on surface O3 concentrations in developing countries, on their projected increase, and on the sensitivity that crop cultivars used in developing countries have to O3. The threat of reduced agricultural yields due to increasing O3 concentrations may encourage developing countries to increase their energy efficiency and to use different energy sources. This could simultaneously achieve a local benefit through improved regional air quality and a global benefit through a reduction in the emission of greenhouse gases.

Perturbation to global tropospheric oxidizing capacity due to latitudinal redistribution of surface sources of NOx, CH4 and CO

Economic and social projections indicate that during next several decades there will be major geographical redistribution of surface emissions of O3 precursors, such as NOx, CH4 and CO. A net decrease in their emissions from northern hemispheric mid‐latitudes will be accompanied by substantial increases from the tropics. We have investigated a hypothetical scenario of currently underway transition of such emission patterns using a global two‐dimensional photochemical model. With overall O3 precursor releases held constant, a simultaneous transfer of their emissions by 25% from the latitude belt 75°N–35°N to 5°S–35°N increases tropospheric oxidizing capacity such that the methane global lifetime and concentrations fall by more than 3%. Seasonally dependent changes in surface O3 concentrations are also calculated. In influencing OH concentration, redistribution of surface NOx emissions is 2–3 orders of magnitude more efficient per unit mass than CO emissions. Shifts in methane sources have insignificant effects on global photochemistry, but lead to a decrease in its interhemispheric gradient.

The atmospheric budget of oxidized nitrogen and its role in ozone formation and deposition

Emissions of reactive oxidized nitrogen (NO and NO2), collectively known as NOx, from human activities are c. 21 Tg N annually, or 70% of global total emissions. They occur predominantly in industrialized regions, largely from fossil fuel combustion, but also from increased use of N fertilizers. Soil emissions of NO not only make an important contribution to global totals, but also play a part in regulating the dry deposition of NO and NO2 (NOx) to plant canopies. Soil microbial production of NO leads to a soil ‘compensation point’ for NO deposition or emission, which depends on soil temperature, N and water status. In warm conditions, the net emission of NOx from plant canopies contributes to the photochemical formation of ozone. Moreover, the effect of NOx emissions from soil is to reduce net rates of NO2 deposition to terrestrial surfaces over large areas.Increasing anthropogenic emissions of NOx have led to an approximate doubling in surface O3 concentrations since the last century. NOx acts as a catalyst for the production of O3 from volatile organic compounds (VOCs). Paradoxically, emission controls on motor vehicles might lead to increases in O3 concentrations in urban areas.Removal of NO and NO2 by dry deposition is regulated to some extent by soil production of NO; the major sink for NO2 is stomatal uptake. Long‐term flux measurements over moorland in Scotland show very small deposition rates for NO2 at night and before mid‐day of 1–4 ng NO2‐N m−2 s−1, and similar emission rates during afternoon. The bi‐directional flux gives 24‐h average deposition velocities of only 1–2 mm s−1, and implies a long life‐time for NOx due to removal by dry deposition.Rates of removal of O3 at the ground are also influenced by stomatal uptake, but significant non‐stomatal uptake occurs at night and in winter. Measurements above moorland showed 40% of total annual flux was stomatal, with 60% non‐stomatal, giving nocturnal and winter deposition velocities of 2–3 mm s−1 and daytime summer values of 10 mm s−1. The stomatal uptake is responsible for adverse effects on vegetation. The critical level for O3 exposure (AOT40) is used to derive a threshold O3 stomatal flux for wheat of 0·5 μg m−2 s−1. Use of modelled stomatal fluxes rather than exposure might give more reliable estimates of yield loss; preliminary calculations suggest that the relative grain yield reduction (%) can be estimated as 38 times the stomatal ozone flux (g m−2) above the threshold, summed over the growing season.

CART Decision-Tree Statistical Analysis and Prediction of Summer Season Maximum Surface Ozone for the Vancouver, Montreal, and Atlantic Regions of Canada

Prediction of daily maximum surface ozone (O3) concentration was begun by Environment Canada in the spring of 1993 for the Vancouver, Montreal, and Atlantic regions in order to advise the public of expected air quality. Forecasts have been issued for southern Ontario for many years by the province of Ontario, but this is a new undertaking in other parts of the country, where air quality has become a concern in recent years. There is a need for guidance to prepare the forecasts, particularly for prediction of surface O3 concentration levels near or exceeding the Canadian 1-h maximum acceptable concentration of 82 ppb. Such occurrences are episodic and relatively rare in southern Canada. Probability of occurrence is in the range 0.00–0.08 at the sites in the regions studied here, thus, reliable prediction is difficult without guidance. Mesoscale numerical meteorological–photochemical models are not currently available for routine use in operations, but the capability exists for development and use of sophisticated multivariate statistical techniques for prediction of daily maximum O3 concentration. Most statistical ozone forecast procedures to date in Canada have been based on multiple linear regression with a limited number of predictors mainly drawn from surface meteorology and subjective classification of the synoptic meteorological flow pattern. Since the relationship between surface O3 and meteorology is nonlinear, tree-based statistical models with several predictors are appropriate for developing objective forecast guidance. Surface and upper-air meteorological predictors and other predictors were matched with several years of observed daily maximum O3 concentrations for the months of May–September at air-monitoring sites in the three regions. A recent nonparametric data-driven tree-based analysis method known as CART (classification and regression trees) was used to analyze the data at each site. The decision trees built by CART were found to fit the data reasonably well, and the rules for node splitting were found to be physically realistic. Some of the important aspects of the analyses are noted. One interesting result was that moisture content of the air plays a limiting role on the maximum surface O3 concentration that can be achieved when other factors point to occurrence of high values. The decision trees can be used to predict maximum surface O3 concentrations if the predictor variables are forecast, thus providing an inexpensive site-specific model for forecasts and climate impact analysis. An estimation of performance with independent data was conducted for the Vancouver–lower Fraser River valley and Montreal regions for each of the five years 1988–92. Verification of the ensemble of forecasts in the two regions shows the technique would have reasonably good skill in forecasting surface O3 concentrations near or exceeding acceptable 1-h limits. A computer version of the technique has been provided for use in the regional forecast offices.

The atmospheric boundary layer

Measurements of surface O3 and carbon monoxide (CO) were made from September 2009 to August 2011 at Dangxiong (30.48°N, 91.10°E, 4187 m a.s.l.), a remote highland site in a southern valley of the Nyainqêntanglha Mountains in the Tibetan Plateau, China. The monthly mean O3 mixing ratio ranged from 29.1 to 51.4 ppb, with an average of 38.5 ppb, and the maximum value was observed in May. The average diurnal cycle of O3 concentration showed a minimum in early morning and a maximum in the afternoon, with a broader “high platform” from the late morning to the late afternoon, and resembled that of surface wind speed. The concentration of surface O3 was highly significantly correlated with tropospheric column O3 over the regions surrounding Dangxiong and with that of surface O3 observed at a site north of the Nyainqêntanglha Mountains, suggesting a good regional representativeness of surface O3 at Dangxiong. In the afternoon when stronger winds blew, surface air showed distinct features of free-atmospheric air, with higher O3, lower CO, and lower relative humidity (RH). The negative O3–CO and O3–RH correlations in most months indicate a significant influence of air masses from the free troposphere. Trajectory analysis suggests that air masses originating from the south of the site make a negative net contribution to surface O3 and a positive contribution to CO and humidity, and those from the northwest sector contribute conversely to the respective quantities.

Reaction of N2O5 on tropospheric aerosols: Impact on the global distributions of NOx, O3, and OH

Using a three‐dimensional global model of the troposphere, we show that the heterogeneous reactions of NO3 and N2O5 on aerosol particles have a substantial influence on the concentrations of NOx, O3, and OH. Due to these reactions, the modeled yearly average global NOx burden decreases by 50% (80% in winter and 20% in summer). The heterogeneous removal of NOx in the northern hemisphere (NH) is dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller aerosol concentrations, liquid water clouds can provide an additional sink for N2O5 and NO3. During spring in the NH subtropics and at mid‐latitudes, O3‐concentrations are lowered by 25%. In winter and spring in the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric average O3 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet deposition patterns in North America and Europe. O3 concentrations in springtime smog situations are shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions resulting from NOx and SO2 emissions. However, a preliminary analysis shows that under present conditions a change in aerosol concentrations due to limited SO2 emission control strategies (e.g., reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on O3 concentrations. Much larger reductions in SO2 emissions may cause larger increases in surface O3 concentrations, up to a maximum of 15%, if they are not accompanied by a reduction in NOx or hydrocarbon emission.

Ozone and Aitken nuclei over equatorial Africa: Airborne observations during DECAFE 88

We determined the distribution of ozone (O3) and Aitken condensation nuclei (CN) over the rain forest in equatorial Africa during February 12–25, 1988. A pronounced O3 maximum with levels up to 70 ppbv was present in a layer between 1 and 3 km altitude throughout the period. It coincided with a CO maximum and with high levels of CO2 and gaseous organic acids. In general, the vertical distribution of CN was similar to that of O3, with number densities ranging up to approximately 3000 cm−3. Dense haze was visible within this layer. O3 and CN decreased sharply above the haze layer to values typical of the remote troposphere. Survey flights showed little change in levels of O3 and CN or in their vertical distribution over distances of hundreds of kilometers. Meteorological observations suggest that this ozone and particulate enriched layer is formed from air masses which originate in northern Africa and subsequently advect over dry tropical regions where biomass burning emits large amounts of aerosols, CO, NO, and hydrocarbons. These air masses then become trapped in the equatorial region between the near‐surface monsoon flow from the southeast and the permanent easterly flow above 3–4 km. Differences in the vertical distribution of O3 and CN result from the removal of O3 by surface uptake and reactions with NO and hydrocarbons, leading to surface O3 concentrations near zero and a steep O3 gradient through the subcloud layer at night. During the day this gradient is reduced by convective mixing. CN concentrations showed no pronounced gradients in the subcloud layer, consistent with the absence of a strong sink for CN at the ground. CN gradients near the surface suggest emission of particles from the forest vegetation or from biomass burning.

Ground-Level Ozone and Regional Transport of Air Pollutants

Photochemical air pollution was studied in the lower Great Lakes region of southern Ontario, Canada, using O3 data for 1973–76 inclusive. High O3 concentrations in the boundary layer are correlated primarily with the several meteorological variables in warm seasons: pressure, temperature, radiation, wind speed and direction. Fluctuations of O3 are highly dependent on the synoptic-, regional- and small-scale air flows at low levels. In general, O3 concentrations were observed to increase when the ridge of a surface anticyclone was passing over the region. Relatively high values (>80 ppb) were observed on the rear sides of high pressure centers and in the warm sectors of cyclones (well ahead of cold fronts). In turn, high concentrations of surface O3 usually occurred during periods of high temperature with southerly and/or southwesterly airflows. After the passage of a cold front, when strong north-northwest flows developed in the low levels (e.g., at the forward side of a “new” anticyclone with general subsidence), O3 concentrations decreased considerably. These observations were also based on results obtained from using a regional-scale trajectory model. Also, temperature changes due to cold air advection, local rainshowers and thunderstorm activity, as well as the direct import of NOx, were associated with decreases in O3. This study also suggests that lake effects influence concentrations of photochemical air pollution. Frequently, high O3 values were associated with lake breezes on the rear sides of anticyclones. In addition, it was found that nocturnal O3 maxima were present in the diurnal cycles for urban stations in Toronto and Montreal only. For high O3 situations the daily maxima occurred about 30 km downwind of a large urban area, Toronto (population: 2.5 millions). On the average, this city appeared to produce ground-level O3 downstream in the range 40–60 ppb. Air pollutants crossing the Canada-United States international boundary from the northeastern Midwest were also found to be important factors for high O3 situations. In these cases, O3 in the order of ∼100 ppb was readily contained in air parcels arriving in southern Ontario. The results of this study do not agree with earlier theories which assumed that the peaks in surface O3 are due to transport downward from the stratosphere. In many cases, high surface O3 concentrations were mainly the result of photochemically active precursors (of anthropogenic origin) which were transported over medium (10–100 km) and long distances (∼1000 km). The time for the photochemical production of surface O3 required a few hours under favorable meteorological conditions.s