The influence of south foehn on the ozone mixing ratios at the high alpine site Arosa

Within 2 years of trace gas measurements performed at Arosa (Switzerland, 2030 m above sea level), enhanced ozone mixing ratios were observed during south foehn events during summer and spring (5–10 ppb above the median value). The enhancements can be traced back to ozone produced in the strongly industrialized Po basin as confirmed by various analyses. Backward trajectories clearly show advection from this region during foehn. NO_y versus O₃ correlation and comparison of O₃ mixing ratios between Arosa and Mt. Cimone (Italy, 2165 m asl) suggest that ozone is the result of recent photochemical production (+5.6 ppb on average), either directly formed during the transport or via mixing of air processed in the Po basin boundary layer. The absence of a correlation between air parcel residence times over Europe and ozone mixing ratios at Arosa during foehn events is in contrast to a previous analysis, which suggested such correlation without reference to the origin of the air. In the case of south foehn, the continental scale influence of pollutants emission on ozone at Arosa appears to be far less important than the direct influence of the Po basin emissions. In contrast, winter time displays a different situation, with mean ozone reductions of about 4 ppb for air parcels passing the Po basin, probably caused by mixing with ozone-poor air from the Po basin boundary layer.     

Surface ozone at four remote island sites and the preliminary assessment of the exceedances of its critical level in Japan

Analysis of the recent surface ozone data at four remote islands (Rishiri, Oki, Okinawa, and Ogasawara) in Japan indicates that East Asian anthropogenic emissions significantly influence the boundary layer ozone in Japan. Due to these regional-scale emissions, an increase of ozone concentration is observed during fall, winter, and spring when anthropogenically enhanced continental air masses from Siberia/Eurasia arrive at the sites. The O₃ concentrations in the “regionally polluted” continental outflow among sites are as high as 41–46 ppb in winter and 54–61 ppb in spring. Meanwhile, marine air masses from the Pacific Ocean show as low as 13–14 ppb of O₃ at Okinawa and Ogasawara in summer but higher O₃ concentrations, 24–27 ppb, are observed at Oki and Rishiri due to the additional pollution mainly from Japan mainland. The preliminary analysis of the exceedances of ozone critical level using AOT40 and SUM06 exposure indices indicates that the O₃ threshold were exceeded variously among sites and years. The highest AOT40 and SUM06 were observed at Oki in central Japan where the critical levels are distinctly exceeded. In the other years, the O₃ exposures at Oki, Okinawa, and Rishiri are about or slightly higher than the critical levels. The potential risk of crop yields reduction from high level of O₃ exposure in Japan might not be a serious issue during 1990s and at present because the traditional growing season in Japan are during the low O₃ period in summer. However, increases of anthropogenic emission in East Asia could aggravate the situation in the very near future.     

An analysis on abnormally low ozone in the upper troposphere over subtropical East Asia in spring 2004

Abnormally low ozone (O₃) mixing ratios were observed by electrochemical concentration cell (ECC) ozonesondes in the upper troposphere over subtropical East Asia in spring 2004, a season when high tropospheric O₃ is usually observed in the region. Low O₃ with a lowest mixing ratio of 13 ppbv, less than a fourth of the respective seasonal average of 60–100 ppbv, was observed at 11–18 km above ground over Hong Kong (22.31°N, 114.17°E), Sanya (18.23°N, 109.52°E) and Taipei (24.98°N, 121.43°E). The origin of the low O₃ was investigated using meteorological evidence, satellite imagery and three-dimensional backward air trajectory. We found for the first time that the low O₃ resulted from deep convective pumping of low O₃ maritime air masses near the center of typhoon Sudal from the boundary layer of the tropical region to the east of the Philippines to the upper troposphere. The low O₃ air masses were then transported to the higher latitudes far ahead of the typhoon following the long-range transport driven by the circulations associated with the typhoon and the northern Hadley cell. The findings of this study highlight that more research efforts are needed to understand the effect of the circulation associated with tropical cyclones on the distribution and budget of O₃ and other trace gases in the troposphere.     

Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models

Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O₃/NO_z and O₃/NO_y and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NO_x emission reductions were correlated with the base-case extent of reaction and ratios of O₃/NO_z and O₃/NO_y. Peak ozone values increased following NO_x control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O₃/NO_z ratios less than 5 : 1, O₃/NO_y less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NO_x reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O₃/NO_z exceeded 10 : 1, O₃/NO_y was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O₃/NO_z was between 5 : 1 and 10 : 1, O₃/NO_y was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.     

Flow patterns influencing the seasonal behavior of surface ozone and carbon monoxide at a coastal site near Hong Kong

Surface O₃ and CO were measured at Cape D’Aguilar, Hong Kong during the period of January 1994 to December1996 in order to understand the temporal variations of surface O₃ and CO in East Asia–West Pacific region. The isentropic backward trajectories were used to isolate different air masses reaching the site and to analyze the long-range transport and photochemical buildup of O₃ on a regional scale. The results show that the diurnal variation of surface O₃ was significant in all seasons with daily O₃ production being about 20 ppbv in fall and 10 ppbv in winter, indicating more active photochemical processes in the subtropical region. The distinct seasonal cycles of O₃ and CO were found with a summer minimum (16 ppbv)–fall maximum (41 ppbv) for O₃ and a summer minimum (116 ppbv)–winter maximum (489 ppbv) for CO. The isentropic backward trajectory cluster analyses suggest that the air masses (associated with regional characteristics) to the site can be categorized into five groups, which are governed by the movement of synoptic weather systems under the influence of the Asian monsoon. For marine-originated air masses (M-SW, M-SE and M-E, standing for marine-southwest, marine-southeast and marine-east, respectively) which always appear in summer and spring, the surface O₃ and CO have relatively lower mixing ratios (18, 16 and 30 ppbv for O₃, 127, 134 and 213 ppbv for CO), while the continental air masses (C-E and C-N, standing for continent-east and continent-north, respectively) usually arrive at the site in winter and fall seasons with higher O₃ (43 and 48 ppbv) and CO (286 and 329 ppbv). The 43 ppbv O₃ and 286 ppbv CO are representative of the regionally polluted continental outflow air mass due to the anthropogenic activity in East Asia, while 17 ppbv O₃ and 131 ppbv CO can be considered as the signature of the approximately clean marine background of South China Sea. The very high CO values (461–508 ppbv) during winter indicate that the long-range transport of air pollutants from China continent is important at the monitoring site. The fall maximum (35–46 ppbv) of surface O₃ was believed to be caused by the effects of the weak slowly moving high-pressure systems which underlie favorable photochemical production conditions and the long-range transport of aged air masses with higher O₃ and its precursors.     

Aromatic hydrocarbons at urban,sub-urban,rural (8°52′N; 67°19′W) and remote sites in Venezuela

Using the novel on-line proton transfer reaction mass spectrometry (PTR-MS) technique, atmospheric concentrations of benzene, toluene, xylenes, and C₉-benzenes were measured in Caracas (urban), Altos de Pipe (sub-urban), Calabozo (rural) and Parupa (remote), during various campaigns in 1999 and 2000.Average daytime mixing ratios measured in Caracas are 1.1, 3.2, 3.7, and 2.7 nmol/mol for benzene, toluene, xylenes, and C₉-benzenes. At the sub-urban site, located only few km from Caracas, relatively low levels (∼20% of the levels measured in Caracas) of these aromatic hydrocarbons were observed.At the rural site during the dry season, higher concentrations of benzene (0.15 nmol/mol) were recorded, whereas those of toluene (0.08 nmol/mol) were lower during that time. The aromatic hydrocarbon ratios in the wet season (benzene: 0.08 nmol/mol; toluene: 0.09 nmol/mol) are consistent with an aged urban plume, whereas biomass burning emissions dominate during the dry season. From rural and urban [benzene]/[toluene] ratios a mean HO concentration of 2.6×10⁶ molecules/cm³ was estimated during the wet season. This value must be considered an overestimate because it does not account for background concentrations which are likely for benzene and toluene.At the remote “La Gran Sabana” region (Parupa) very low mixing ratios (0.031 and 0.015 nmol/mol for benzene and toluene) are showing the pristine region to be unaffected by local sources. From the [benzene]/[toluene] ratio we deduced, that “urban” air arriving from the coastline (350 km) is likely mixed with air containing some background of benzene and toluene.Urban emissions (automobiles) should be the major source of aromatic compounds, however, during the dry season biomass burning seems to make an important contribution.     

Observational study of ozone pollution at a rural site in the Yangtze Delta of China

Ozone and related trace gases (CO, NO_x, and SO₂) were measured from June 1999 to July 2000 at a rural site in the Yangtze Delta of China, a region of intensive anthropogenic activity. Elevated ozone levels were frequently observed during the study period, with the highest frequency in late spring and early summer. Over a 1 yr period, 21 d were found to have ozone concentrations exceeding the new US 8-h 80 ppb health standard. Calculation of the “SUM06” exposure index also shows relatively high (>15 ppm h) values for each season except winter. At these levels ozone may have adverse effects on human health as well as agricultural crops. Analysis of meteorological data shows that the high ozone days were associated with large-scale stagnation, intense solar radiation, and minimum rainfall. Large-scale back trajectories indicate a slow-moving/re-circulating airmass during the episodic days. Examination of chemical data shows that the observed daytime high ozone concentrations were due to downward mixing of ozone-rich air, in situ photochemical formation, and in some cases, advection to the site of aged plumes. The very high CO levels (and high CO to NO_x ratios) were found to coincide with many of the ozone episodes, suggesting a contribution from sources of emission involving incomplete combustion. It is suggested that the burning of biomass (e.g., biofeuls and crop residues) may be an important source for the observed high CO and O₃ values.     

Transport of SO2 and aerosol over the Yellow sea

Aircraft measurements of air pollutants were made to investigate the characteristic features of long-range transport of sulfur compounds over the Yellow Sea for the periods of 26–27 April and 7–10 November in 1998, and 9–11 April and 19 June in 1999, together with aerosol measurements at the Taean background station in Korea. The overall mean concentrations of SO₂, O₃ and aerosol number in the boundary layer for the observation period ranged 0.1–7.4 ppb 32.1–64.1 ppb and 1.0–143.6 cm⁻³, respectively. It was found that the air mass over the Yellow Sea had a character of both the polluted continental air and clean background air, and the sulfur transport was mainly confined in the atmospheric boundary layer. The median of SO₂ concentration within the boundary layer was about 0.1–2.2 ppb. However, on 8 November, 1998, the mean concentrations of SO₂ and aerosol number increased up to 7.4 ppb and 109.5 cm⁻³, respectively, in the boundary layer, whereas O₃ concentration decreased remarkably. This enhanced SO₂ concentration occurred in low level westerly air stream from China to Korea. Aerosol analyses at the downstream site of Taean in Korea showed 2–3 times higher sulfate concentration than that of other sampling days, indicating a significant amount of SO₂ conversion to non sea-salt sulfate during the long-range transport.     

Ozone and meteorological boundary-layer conditions at Summit,Greenland, during 3–21 June 2000

The temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground.The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between ∼9.00 and 18.00 h local time with the formation of shallow mixing heights of ∼70–250 m above the surface.The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37–76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. A ∼0.1–3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime.     

The impact of an 8 h ozone air quality standard on ROG and NOx controls in Southern California

The new National Ambient Air Quality Standard for ozone in the US uses 8 h averaging for the concentration. Based on the 1993 ambient data for Southern California, 8 h averaging has a moderate tendency to move the location of the peak ozone concentration east of the location of the peak 1 h ozone concentration. Reducing the area-wide peak 8 h ozone concentration to 80 ppb would require an effective reduction of the area-wide peak 1 h ozone concentration to around 90 ppb. The Urban Airshed Model with improved numerical solvers, meteorological input based on a mesoscale model and an adjusted emissions inventory was used to study the effect of reactive organic gases (ROG) and NO_x controls on daily-maximum and peak 8 h ozone concentrations under the 26–28 August 1987 ozone episodic conditions in Southern California. The NO_x disbenefit remains prominent for the case of 8 h ozone concentration but is somewhat less prominent, especially when areal ozone exposure is considered, than the case for 1 h ozone concentration. The role of two indicators – O₃/NO_y and H₂O₂/HNO₃ – for NO_x- and ROG-sensitivity for 1 and 8 h ozone concentrations were also studied. In general, the indicator trends are consistent with model predictions, but the discriminating power of the indicators is rather limited.     

Trace gas variations at Cape Point,South Africa,during May 1997 following a regional biomass burning episode

During the continuous monitoring of atmospheric parameters at the station Cape Point (34°S, 18°E), a smoke plume originating from a controlled fire of 30-yr-old fynbos was observed on 6 May 1997. For this episode, which was associated with a nocturnal inversion and offshore airflow, atmospheric parameters (solar radiation and meteorological data) were considered and the levels of various trace gases compared with those measured at Cape Point in maritime air. Concentration maxima in the morning of 6 May for CO₂, CO, CH₄ and O₃ amounted to 370.3 ppm, 491 ppb, 1730 ppb and 47 ppb, respectively, whilst the mixing ratios of several halocarbons (F-11, F-12, F-113, CCl₄ and CH₃CCl₃) remained at background levels. In the case of CO, the maritime background level for this period was exceeded by a factor of 9.8. Differences in ozone levels of up to 5 ppb between air intakes at 4 and 30 m above the station (located at 230 m above sea level) indicated stratification of the air advected to Cape Point during the plume event. Aerosols within the smoke plume caused the signal of global solar radiation and UV–A to be attenuated from 52.4 to 13.0 mW cm⁻² and from 2.3 to 1.3 mW cm⁻², respectively, 5 h after the trace gases had reached their maxima. Emission ratios (ERs) calculated for CO and CH₄ relative to CO₂ mixing ratios amounted to 0.042 and 0.0040, respectively, representing one of the first results for fires involving fynbos. The CO ER is somewhat lower than those given in the literature for African savanna fires (average ER=0.048), whilst for CH₄ the ER falls within the range of ERs reported for the flaming (0.0030) and smouldering phases (0.0055) of savanna fires. Non-methane hydrocarbon (NMHC) data obtained from a grab sample collected during the plume event were compared to background levels. The highest ERs (ΔNMHC/ΔCH₄) have been obtained for the C₂–C₃ hydrocarbons (e.g. ethene at 229.3 ppt ppb⁻¹), whilst the C₄–C₇ hydrocarbons were characterised by the lowest ERs (e.g. n-hexane at 1.0 and n-pentane at 0.8 ppt ppb⁻¹).     

Estimating the effects of increased urbanization on surface meteorology and ozone concentrations in the New York City metropolitan region

Land use and pollutant emission changes can have significant impacts on air quality, regional climate, and human health. Here we describe a modeling study aimed at quantifying the potential effects of extensive changes in urban land cover in the New York City (NYC), USA metropolitan region on surface meteorology and ozone (O₃) concentrations. The SLEUTH land-use change model was used to extrapolate urban land cover over this region from “present-day” (ca. 1990) conditions to a future year (ca. 2050), and these projections were subsequently integrated into meteorological and air quality simulations. The development of the future-year land-use scenario followed the narrative of the “A2” scenario described by the Intergovernmental Panel on Climate Change (IPCC), but was restricted to the greater NYC area. The modeling system consists of the Penn State/NCAR MM5 mesoscale meteorological model; the Sparse Matrix Operator Kernal Emissions processing system; and the US EPA Community Multiscale Air Quality model, and simulations were performed for two 18-day episodes, one near-past and one future. Our results suggest that extensive urban growth in the NYC metropolitan area has the potential to increase afternoon near-surface temperatures by more than 0.6 °C and planetary boundary layer (PBL) heights by more than 150 m, as well as decrease water vapor mixing ratio by more than 0.6 g kg⁻¹, across the NYC metropolitan area, with the areal extent of all of these changes generally coinciding with the area of increased urbanization. On the other hand, the impacts of these land use changes on ozone concentrations are more complex. Simulation results indicate that future changes in urbanization, with emissions held constant, may lead to increases in episode-average O₃ levels by about 1–5 ppb, and episode-maximum 8 h O₃ levels by more than 6 ppb across much of the NYC area. However, spatial patterns of ozone changes are heterogeneous and also indicate the presence of areas with decreasing ozone concentrations. When anthropogenic emissions were increased to be consistent with the extensive urbanization in the greater NYC area, the O₃ levels increased in outer counties of the metropolitan region but decreased in others, including coastal Connecticut and the Long Island Sound area.     

Surface ozone at the Swiss Alpine site Arosa: the hemispheric background and the influence of large-scale anthropogenic emissions

An innovative and effective method using isentropic trajectory analysis based on the residence time of air masses over the polluted region of Europe was successfully applied to categorize surface ozone amounts at Arosa, Switzerland during 1996–1997. The “European representative” background ozone seasonal cycle at Arosa is associated with long-range transport of North Atlantic air masses, and displays the spring maximum–summer minimum with an annual average of 35 ppb. The photochemical ozone production due to the intense large-scale anthropogenic emission over Europe is estimated as high as 20 ppb in summer, whereas it is insignificant in winter. European sources contribute an annual net ozone production of 9–12 ppb at Arosa. Comparison with the selected regional representative site in Western Europe shows similar results indicating that the categorized ozone data at Arosa by this technique could be regarded as a representative for northern hemispheric mid-latitudes.     

Shouldn’t snowpacks be sources of monocarboxylic acids?

We report the first measurements of the mixing ratios of acetic (CH₃COOH) and formic (HCOOH) acids in the air filling the pore spaces of the snowpacks (firn air) at Summit, Greenland and South Pole. Both monocarboxylic acids were present at levels well above 1 ppbv throughout the upper 35 cm of the snowpack at Summit. Maximum mixing ratios in Summit firn air reached nearly 8 ppbv CH₃COOH and 6 ppbv HCOOH. At South Pole the mixing ratios of these acids in the top 35 cm of firn air were also generally >1 ppbv, though their maximums barely exceeded 2.5 ppbv of CH₃COOH and 2.0 ppbv of HCOOH. Mixing ratios of the monocarboxylic acids in firn air did not consistently respond to diel and experimental (fast) variations in light intensity, unlike the case for N oxides in the same experiments. Air-to-snow fluxes of CH₃COOH and HCOOH apparently support high mixing ratios (means of (CH₃COOH/HCOOH) 445/460 and 310/159 pptv at Summit and South Pole, respectively) in air just above the snow during the summer sampling seasons at these sites. We hypothesize that oxidation of carbonyls and alkenes (that are produced by photo- and OH-oxidation of ubiquitous organic compounds) within the snowpack is the source of the monocarboxylic acids.     

The influence of ozone on atmospheric emissions of gaseous elemental mercury and reactive gaseous mercury from substrates

Experiments were performed to investigate the effect of ozone (O₃) on mercury (Hg) emission from a variety of Hg-bearing substrates. Substrates with Hg(II) as the dominant Hg phase exhibited a 1.7 to 51-fold increase in elemental Hg (Hg^o) flux and a 1.3 to 8.6-fold increase in reactive gaseous mercury (RGM) flux in the presence of O₃-enriched clean (50 ppb O₃; 8 substrates) and ambient air (up to ∼70 ppb O₃; 6 substrates), relative to clean air (oxidant and Hg free air). In contrast, Hg^o fluxes from two artificially Hg^o-amended substrates decreased by more than 75% during exposure to O₃-enriched clean air relative to clean air. Reactive gaseous mercury emissions from Hg^o-amended substrates increased immediately after exposure to O₃ but then decreased rapidly. These experimental results demonstrate that O₃ is very important in controlling Hg emissions from substrates. The chemical mechanisms that produced these trends are not known but potentially involve heterogenous reactions between O₃, the substrate, and Hg. Our experiments suggest they are not homogenous gas-phase reactions. Comparison of the influence of O₃ versus light on increasing Hg^o emissions from dry Hg(II)-bearing substrates demonstrated that they have a similar amount of influence although O₃ appeared to be slightly more dominant. Experiments using water-saturated substrates showed that the presence of high-substrate moisture content minimizes reactions between atmospheric O₃ and substrate-bound Hg. Using conservative calculations developed in this paper, we conclude that because O₃ concentrations have roughly doubled in the last 100 years, this could have increased Hg^o emissions from terrestrial substrates by 65–72%.     

Synoptic weather patterns and their relationship to high ozone concentrations in the Taichung Basin

Frequent high ozone days (defined as daily maximum ozone concentration ⩾80 ppb) during recent years in the Taichung Basin have caused much concern. High ozone days occur mainly during autumn and spring. Statistically, there is no clear linear relationship between a single meteorological variable and ozone concentration. In this study, data from 1996–2000 has shown that high ozone concentrations occur during two types of synoptic weather patterns. The first type is a continental cyclone emanating from mainland China, the southern part of it swept towards Taiwan by easterly winds. The second pattern is a tropical depression moving northwards toward the region, the northern part of it affecting Taiwan via easterly winds. Both types cover Taiwan with easterly winds, which are blocked by the Central Mountain Ranges (altitude of 2000–3000 m). The ranges create lee cyclogenesis to the west, which is unfavorable for pollutant dispersion and leads to serious air pollution episodes.The statistical results of the synoptic weather patterns in relation to ozone concentrations are based on the 5 yr data (1996–2000). This was obtained from a network of air-pollution monitoring sites in the study area, while the vertical data come from two 3-day tethersonde experimental campaigns conducted during March and October 2000, measuring air pressure, air temperature, relative humidity, wind speed and direction, non-methane hydrocarbons, NO_x and O₃.     

Surface ozone background in the United States: Canadian and Mexican pollution influences

We use a global chemical transport model (GEOS-Chem) with 1° × 1° horizontal resolution to quantify the effects of anthropogenic emissions from Canada, Mexico, and outside North America on daily maximum 8-hour average ozone concentrations in US surface air. Simulations for summer 2001 indicate mean North American and US background concentrations of 26 ± 8 ppb and 30 ± 8 ppb, as obtained by eliminating anthropogenic emissions in North America vs. in the US only. The US background never exceeds 60 ppb in the model. The Canadian and Mexican pollution enhancement averages 3 ± 4 ppb in the US in summer but can be occasionally much higher in downwind regions of the northeast and southwest, peaking at 33 ppb in upstate New York (on a day with 75 ppb total ozone) and 18 ppb in southern California (on a day with 68 ppb total ozone). The model is successful in reproducing the observed variability of ozone in these regions, including the occurrence and magnitude of high-ozone episodes influenced by transboundary pollution. We find that exceedances of the 75 ppb US air quality standard in eastern Michigan, western New York, New Jersey, and southern California are often associated with Canadian and Mexican pollution enhancements in excess of 10 ppb. Sensitivity simulations with 2020 emission projections suggest that Canadian pollution influence in the Northeast US will become comparable in magnitude to that from domestic power plants.     

Ozone and increased nitrogen supply effects on the yield and nutritive quality of Trifolium subterraneum

The influence of ambient ozone (O₃) concentrations and nitrogen (N) fertilization, singly and in combination, on the growth and nutritive quality of Trifolium subterraneum was assessed. This is an important O₃-sensitive species of great pastoral value in Mediterranean areas. Plant material was enclosed in open-top chambers (OTCs). Three O₃ levels were established: Filtered air with O₃ concentrations below 15 ppb (CFA), non-filtered air with O₃ concentrations in the range of ambient levels (NFA), and non-filtered air supplemented with 40 ppb O₃ over ambient levels (NFA+). Similarly, three N levels were defined: 5, 15 and 30 kg ha⁻¹. The increase in O₃ exposure induced a reduction of the clover aerial green biomass and an increase of senescent biomass. Ozone effects were more adverse in the root system, inducing an impairment of the aerial/subterranean biomass ratio. Compared with the CFA treatment, nutritive quality of aerial biomass was 10 and 20% lower for NFA and NFA+ treatments, respectively, due to increased concentrations of acid detergent fiber, neutral detergent fiber and lignin. The latter effect appears to be related to senescence acceleration. The increment in N supplementation enhanced the increase of ADF concentrations in those plants simultaneously exposed to ambient and above-ambient O₃ concentrations, and reduced the incremental rate of foliar senescence induced by the pollutant.     

Impact of biogenic VOC emissions on a tropical cyclone-related ozone episode in the Pearl River Delta region,China

For quantitative estimate of biogenic volatile organic compound emissions (BVOCs) in South China and their impact on the regional atmospheric chemistry, a 3-day tropical cyclone-related ozone episode was modeled using chemical transport model CMAQ, which was driven by the mesoscale meteorological model MM5. Hourly biogenic emission inventories were constructed using the Sparse Matrix Operator Kernel Emissions (SMOKE) model. The simulation results show good agreement with observation data in air temperature, ozone and NO_x levels. The estimated biogenic emissions of isoprene, terpene, and other reactive VOCs (ORVOCs) during this tropical cyclone-related episode are 8500, 3400, and 11 300 ton day⁻¹, respectively. The ratio of isoprene to the total BVOCs was 36.4%. Two test runs were carried out with one incorporated biogenic emissions and the other without. The simulations show that Guangdong province, particularly the Pearl River Delta (PRD) region, was the area most reactive to biogenic emissions in South China. More ozone was produced in all layers under 1500 m when biogenic emissions were included in comparison to that without BVOCs. The net formation of ozone from 9:00 to 15:00 h was the highest near the surface and could reach 38 ppb, which include 4 ppb attributed to biogenic impact. The enhanced ozone due to biogenic emissions first appeared in the PRD region and slowly spread to a greater area in South China. Process analysis indicated that the surface ozone budget was dominated by the vertical transport and dry deposition. The horizontal transport and gas-phase chemical production were relatively small in the surface layer. Presumably, ozone was produced in upper layers within the atmospheric boundary layer and convected down to surface where it is destroyed. When BVOCs was included, apart from the enhancement of gas-phase chemical production of ozone, both the surface deposition and vertical transport were also augmented.     

Vegetation exposure to ozone over the continental United States: Assessment of exposure indices by the Eta-CMAQ air quality forecast model

The main use of air quality forecast (AQF) models is to predict ozone (O₃) exceedances of the primary O₃ standard for informing the public of potential health concerns. This study presents the first evaluation of the performance of the Eta-CMAQ air quality forecast model to predict a variety of widely used seasonal mean and cumulative O₃ exposure indices associated with vegetation using the U.S. AIRNow O₃ observations. These exposure indices include two concentration-based O₃ indices, M7 and M12 (the seasonal means of daytime 7-h and 12-h O₃ concentrations, respectively), and three cumulative exposure-based indices, SUM06 (the sum of all hourly O₃ concentrations ≥ 0.06 ppm), W126 (hourly concentrations weighed by a sigmoidal weighting function), and AOT40 (O₃ concentrations accumulated over a threshold of 40 ppb during daylight hours). During a three-month simulation (July–September 2005), the model over predicted the M7 and M12 values by 8–9 ppb, or a NMB value of 19% and a NME value of 21%. The model predicts a central belt of high O₃ extending from Southern California to Middle Atlantic where the seasonal means, M7 and M12 (the seasonal means of daytime 7-h and 12-h O₃ concentrations), are higher than 50 ppbv. In contrast, the model is less capable of reproducing the observed cumulative indices. For AOT40, SUM06 and W126, the NMB and NME values are two- to three-fold of that for M7, M12 or peak 8-h O₃ concentrations. The AOT40 values range from 2 to 33 ppm h by the model and from 1 to 40 ppm h by the monitors. There is a significantly higher AOT40 value experienced in the United States in comparison to Europe. The domain-wide mean SUM06 value is 14.4 ppm h, which is about 30% higher than W126, and 40% higher than AOT40 calculated from the same 3-month hourly O₃ data. This suggests that SUM06 and W126 represent a more stringent standard than AOT40 if either the SUM06 or the W126 was used as a secondary O₃ standard. Although CMAQ considerably over predicts SUM06 and W126 values at the low end, the model under predicts the extreme high exposure values (>50 ppm h). Most of these extreme high values are found at inland California sites. Based on our analysis, further improvement of the model is needed to better capture cumulative exposure indices.