Tropical tropospheric ozone observed in Thailand

The mixing ratios of surface ozone at two rural/remote sites in Thailand, Inthanon and Srinakarin, have been measured continuously for the first time. Almost identical seasonal variations of O₃ with dry season maximum and a wet season minimum with a large seasonal amplitude are observed at both sites during 1996–1998. At Inthanon, the monthly averaged O₃ mixing ratios range 9–55 ppb, with the annual average of 27 ppb. The ozone mixing ratios at Srinakarin are in the similar range, 9–45 ppb with annual average of 28 ppb. Based on trajectory analysis of O₃ data at Inthanon, the long-range transport of O₃ under Asian monsoon regime could primarily explain the low O₃ mixing ratios of 13 ppb in clean marine air mass from Indian Ocean during wet season but only partly explain the relatively low O₃ mixing ratios, 26 ppb or less, in continental air mass from northeast Asia either in wet or dry season. The highest O₃ mixing ratios are found in air masses transported within southeast Asia, averaged 46 ppb in dry season. The high O₃ mixing ratios during the dry season are suggested to be significantly due to the local/sub-regional scale O₃ production triggered by biomass burning in southeast Asia rather than long-range transport effect.     

An enhanced ozone forecasting model using air mass trajectory analysis

An enhanced ozone forecasting model using nonlinear regression and an air mass trajectory parameter has been developed and field tested. The model performed significantly better in predicting daily maximum 1-h ozone concentrations during a five-year model calibration period (1993–1997) than did a previously reported regression model. This was particularly true on the 28 “high ozone” days ([O₃]>120 ppb) during the period, for which the mean absolute error (MAE) improved from 21.7 to 12.1 ppb. On the 77 days meteorologically conducive to high ozone, the MAE improved from 12.2 to 9.1 ppb, and for all 580 calibration days the MAE improved from 9.5 to 8.35 ppb. The model was field-tested during the 1998 ozone season, and performed about as expected. Using actual meteorological data as input for the ozone predictions, the MAE for the season was 11.0 ppb. For the daily ozone forecasts, which used meteorological forecast data as input, the MAE was 13.4 ppb. The high ozone days were all anticipated by the ozone forecasters when the model was used for next day forecasts.     

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.     

Long term changes in nitrogen oxides and volatile organic compounds in Toronto and the challenges facing local ozone control

Ground level ozone represents a significant air quality concern in Toronto, Canada, where the national 65 ppb 8-h standard is repeatedly exceeded during the summer. Here we present an analysis of nitrogen dioxide (NO₂), ozone (O₃), and volatile organic compound (VOC) data from federal and provincial governmental monitoring sites from 2000 to 2007. We show that summertime VOC reactivity and ambient concentrations of NO₂ have decreased over this period of time by up to 40% across Toronto and the surrounding region. This has not resulted in significant summertime ozone reductions, and in some urban areas, it appears to be increasing. We discuss the competing effects of decreased ozone titration leading to an increase in O₃, and decreased local ozone production, both caused by significant decreases in NO_x concentrations. In addition, by using local meteorological data, we show that annual variability in summer ozone correlates strongly with maximum daily temperatures, and we explore the effect of atmospheric transport from the southwest which has a significant influence on early morning levels before local production begins. A mathematical model of instantaneous ozone production is presented which suggests that, given the observed decreases in NO_x and VOC reactivity, we would not expect a significant change in local ozone production under photochemically relevant conditions. These results are discussed in the context of Toronto's recent commitment to cutting local smog-causing pollutants by 20% by 2012.     

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.     

Ozone Transport

Elevated concentrations of ozone, often above the national ambient air quality standard for photochemical oxidants, have been measured in both urban and rural areas of Connecticut. One such episode took place on June 10, 1974. Ozone levels, after stabilizing at values slightly above the standard (i.e., 80 to 110 ppb; Connecticut generated ozone concentrations), rose sharply late in the afternoon reaching concentrations as high as 310 ppb (almost 4 times the standard) in Hartford. The trajectory of the air mass, which arrived in Hartford at the time of maximum O₃ occurence, had its origin in the metropolitan New York area during the early morning rush hour on the episode day. This illustrates that the advective transport of O₃ and O₃ precursors into Connecticut from New York are probably responsible for a significant portion (approximately two-thirds) of the elevated O₃ concentrations measured throughout Connecticut on days when winds are from the south-southwest direction. The fact that peak O₃ levels occur late in the afternoon, several hours after maximum sunlight intensity, reinforces the conclusion that excessive O₃ concentrations developed as O₃ and ozone precursors were generated in the vicinity of New York City and then drifted inland into Connecticut on the afternoon sea breeze.

It appears to be unrealistic to develop a hydrocarbon control strategy for Connecticut in order to meet the photochemical oxidant ambient air quality standard when O₃ and/or ozone precursors ad-vectively transported into the State cause oxidant levels to exceed the standard. The complete cessation of all anthropogenic hydrocarbon emissions in Connecticut would not necessarily assure that the standard would be attained here. The implication is that a regional (i.e., the eastern part of the United States) hydrocarbon control strategy is needed to reduce adequately ozone formation and transport so as to allow Connecticut to meet the current oxidant standard.     

Analysis of the effects of combustion emissions and Santa Ana winds on ambient ozone during the October 2007 southern California wildfires

Combustion emissions and strong Santa Ana winds had pronounced effects on patterns and levels of ambient ozone (O₃) in southern California during the extensive wildland fires of October 2007. These changes are described in detail for a rural receptor site, the Santa Margarita Ecological Reserve, located among large fires in San Diego and Orange counties. In addition, O₃ changes are also described for several other air quality monitoring sites in the general area of the fires. During the first phase of the fires, strong, dry and hot northeasterly Santa Ana winds brought into the area clean continental air masses, which resulted in minimal diurnal O₃ fluctuations and a 72-h average concentration of 36.8 ppb. During the second phase of the fires, without Santa Ana winds present and air filled with smoke, daytime O₃ concentrations steadily increased and reached 95.2 ppb while the lowest nighttime levels returned to ～0 ppb. During that period the 8-h daytime average O₃ concentration reached 78.3 ppb, which exceeded the federal standard of 75 ppb. After six days of fires, O₃ diurnal concentrations returned to pre-fire patterns and levels.     

Impacts of boundary layer mixing on pollutant vertical profiles in the lower troposphere: Implications to satellite remote sensing

Mixing in the planetary boundary layer (PBL) affects vertical distributions of air tracers in the lower troposphere. An accurate representation of PBL mixing is critical for chemical-transport models (CTMs) for applications sensitive to simulations of the vertical profiles of tracers. The full mixing assumption in the widely used global CTM GEOS-Chem has recently been supplemented with a non-local PBL scheme. This study analyzes the impact of the non-local scheme on model representation of PBL mixing, consequences for simulations of vertical profiles of air tracers and surface air pollution, and implications for model applications to the interpretation of data retrieved from satellite remote sensing. The non-local scheme significantly improves simulations of the vertical distributions for NO₂ and O₃, as evaluated using aircraft measurements in summer 2004. It also reduces model biases over the U.S. by more than 10 ppb for surface ozone concentrations at night and by 2–5 ppb for peak ozone in the afternoon, as evaluated using ground observations. The application to inverse modeling of anthropogenic NO_x emissions for East China using satellite retrievals of NO₂ from OMI and GOME-2 suggests that the full mixing assumption results in 3–14% differences in top–down emission budgets as compared to the non-local scheme. The top–down estimate combining the non-local scheme and the Lin et al. inverse modeling approach suggests a magnitude of 6.6 TgN yr^?1 for emissions of NO_x over East China in July 2008 and 8.0 TgN yr^?1 for January 2009, with the magnitude and seasonality in good agreement with bottom–up estimates.     

A comparison of model photochemical ozone formation potential with observed regional scale ozone formation during a photochemical episode over the United Kingdom in April 1987

A photochemical trajectory model has been employed to calculate the maximum potential for ozone generation in air parcels passing over the U.K. during a photochemical pollution episode in April 1987. In all, 11 trajectories have been studied and the model results compared against an objective analysis of the integrated ozone generation based on the observations reported from the U.K. Department of the Environment ground level O₃network. There is apparently good correlation between the observed O₃formation and the model O₃formation potential although the latter overestimates the former by a factor of about 2.8. The solar illumination conditions employed in the photochemical trajectory model may have caused this overestimation, since the model is formulated for O₃control strategy assessment and simulates the ‘worst case’ situation likely to give the maximum potential for secondary pollutant formation. In addition to the model results for O₃, a wide range of primary and secondary pollutant concentrations from the model were examined, together with the influence of precursor pollutant emissions.     

A study of the ozone formation by ensemble back trajectory-process analysis using the Eta–CMAQ forecast model over the northeastern U.S. during the 2004 ICARTT period

The integrated process rates (IPRs) estimated by the Eta–CMAQ model at grid cells along the trajectory of the air mass transport path were analyzed to quantitatively investigate the relative importance of physical and chemical processes for O₃ formation and evolution over the northeastern U.S. during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) period. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to determine the back trajectory of air masses reaching the northeast by linking a downwind receptor to upwind source areas. The process analysis is applied to a high O₃ episode occurring on July 22, 2004 at three selected sites in the northeastern U.S. The process analysis at the location of the site shows that during the daytime, the O₃ concentrations in the surface layer are mainly enhanced by the vertical diffusion of O₃-rich air from aloft, followed by horizontal advection (HADV) and chemical production (CHEM), whereas dry deposition (DDEP) and vertical advection (ZADV) mainly deplete O₃ concentrations at the sites of Valley Central (VC), NY and Castle Spring (CS), NH. By integrating the effects of each process over the depth of the daytime planetary boundary layer (PBL), it was found that at the VC site, CHEM and HADV contributed about 53% and 41%, respectively, to O₃ levels within the PBL. This confirms the significance of regional transport of O₃ from the industrialized areas into the Northeast. On the other hand, the process analysis results for O₃ formation in moving air masses indicate that on July 22, large chemical production of O₃ along the transport path over the polluted urban regions leads to significant increase in O₃ in the air mass reaching the VC site, whereas the low chemical production of O₃ along the transport path over the low emission regions leads to the low O₃ concentration at the site of Belleayre Mountain (BM), NY. The dramatic buildup of O₃ concentration from 50 ppb to 102 ppb in the air masses before reaching the VC site after 12:00 EST on 7/22 indicates the significant impact of pollution from the northeastern urban corridor at this site. On the basis of the results at the CS site, it was found that high NO_x emissions along the transport path led to large chemical production of O₃ in the air mass reaching the CS site on July 22. In contrast, the low chemical production of O₃ associated with low emission (relatively clean conditions) along the transport path over the northern portions of the domain is responsible for the low O₃ concentration at the CS site on July 26.     

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.     

The effect of variability in industrial emissions on ozone formation in Houston, Texas

Ambient observations have indicated that high concentrations of ozone observed in the Houston/Galveston area are associated with plumes of highly reactive hydrocarbons, mixed with NO_x, from industrial facilities. Ambient observations and industrial process data, such as mass flow rates for industrial flares, indicate that the VOCs associated with these industrial emissions can have significant temporal variability. To characterize the effect of this variability in emissions on ozone formation in Houston, data were collected on the temporal variability of industrial emissions or emission surrogates (e.g., mass flow rates to flares). The observed emissions variability was then used to construct regionwide emission inventories with variable industrial emissions, and the impacts of the variability on ozone formation were examined for two types of meteorological conditions, both of which lead to high ozone concentrations in Houston. The air quality simulations indicate that variability in industrial emissions has the potential to cause increases and decreases of 10–52 ppb (13–316%), or more, in ozone concentration. The largest of these differences are restricted to regions of 10–20 km², but the variability also has the potential to increase regionwide maxima in ozone concentrations by up to 12 ppb.     

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.     

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.     

Ground-level ozone in the Pearl River Delta region: Analysis of data from a recently established regional air quality monitoring network

The ambient air quality monitoring data of 2006 and 2007 from a recently established Pearl River Delta (PRD) regional air quality monitoring network are analyzed to investigate the characteristics of ground-level ozone in the region. Four sites covering urban, suburban, rural and coastal areas are selected as representatives for detailed analysis in this paper. The results show that there are distinct seasonal and diurnal cycles in ground-level ozone across the PRD region. Low ozone concentrations are generally observed in summer, while high O₃ levels are typically found in autumn. The O₃ diurnal variations in the urban areas are larger than those at the rural sites. The O₃ concentrations showed no statistically significant difference between weekend and weekdays in contrast to the findings in many other urban areas in the world. The average ozone concentrations are lower in urban areas compared to the sites outside urban centers. Back trajectories are used to show the major air-mass transport patterns and to examine the changes in ozone from the respective upwind sites to a site in the center of the PRD (Wanqingsha). The results show higher average ozone concentrations at the upwind sites in the continental and coastal air masses, but higher 1 h-max O₃ concentrations (by 8–16 ppbv) at the center PRD site under each of air-mass category, suggesting that the ozone pollution in the PRD region exhibits both regional and super-regional characteristics.     

Cumulative effects and threshold levels in air pollution mortality: Data analysis of nine large US cities using the NMMAPS dataset

We examined the existence of thresholds, cumulative effects and the homogeneity of five air pollutants on the relative risk of three mortality outcomes using data from nine major US cities using data from NMMAPS. Overall, PM₁₀ (usually 200-day accumulation) and ozone (3-day accumulation) were the two important predictors of outcome but their effect was not uniform across the nine cities. Many models exhibited thresholds (25–45 μm g/m³ for PM₁₀, and 10–45 ppb for O₃). Our preliminary exploratory analyses suggest that the use of a linear, no threshold, model for pollution studies is not consistent with the observed data. The heterogeneity in the risk estimates across the nine cities suggests combining the local risk estimates to obtain a national risk estimate may not be justifiable and the estimate is likely to be confounded.     

Elevated ozone phytotoxicity ameliorations in mung bean {Vigna radiata (L.) Wilczek} by foliar nebulization of silicic acid and ascorbic acid

The present work provides an insight into the development of biochemical adaptations in mung beans against ozone (O₃) toxicity. The study aims to explore the O₃ stress tolerance potential of mung bean genotypes under exogenous application of growth regulators. The seeds of twelve mung bean genotypes were grown in plastic pots under controlled conditions in the glasshouse. Six treatments, control (ambient ozone level 40–45 ppb), ambient O₃ with ascorbic acid, ambient ozone with silicic acid, elevated ozone (120 ppb), elevated O₃ with ascorbic acid (10 mM), and elevated ozone with silicic acid (0.1 mM) were applied. The O₃ fumigation was carried out using an O₃ generator. The results revealed that ascorbic acid and silicic acid application decreased the number of plants with foliar O₃ injury symptoms in different degrees, i.e., zero, first, second, third, and fourth degrees; whereas 0–4 degree symptoms represent, no symptoms, symptoms occupying?<?1/4, 1/4–1/2, 1/2–3/4, and?>?3/4 of the total foliage area, respectively. Application of ascorbic acid and silicic acid also prevented the plants from the negative effects of O₃ in terms of fresh as well as dry matter production, leaf chlorophyll, carotenoids, soluble proteins and ascorbic acid, proline, and malondialdehyde (MDA) contents. Overall, silicic acid application proved more effective in reducing the negative effects of O₃ on mung bean genotypes as compared to that of the ascorbic acid. Three mung bean genotypes (NM 20–21, NM-2006, and NM-2016) were identified to have a better adaptive mechanism for O₃ toxicity tolerance and may be good candidates for future variety development programs.

     

Influence of ship emissions on ozone concentrations around coastal areas during summer season

The influence of ship emissions on ozone (O₃) concentrations in a coastal area (CA) including Busan port, Korea was examined based on a numerical modeling approach during a high O₃ episode. The analysis was performed by two sets of simulation scenarios: (1) with ship emissions (e.g., TOTAL case) and (2) without ship emissions (e.g., BASE case). A process analysis (PA) (the integrated processes rate (IPR) and integrated reaction rate (IRR) analyses) was used to evaluate the relative contributions of individual physical and chemical processes in O₃ production in and around the CA (e.g., sites of Dong Sam (DS) and Dae Yeon (DY)). The model study suggested the possibility that pollutant gases emitted from the ships traversing Busan port can exert a direct impact on the O₃ concentration levels in the CA. Largest impacts of ship emissions on the O₃ concentrations were predicted at the coast (up to 15 ppb) and at inland locations (about 5 ppb) due to both the photochemical production of pollutant gases emitted from the ships and meteorological conditions. From the PA, the photochemical production of O₃ (P(O₃)) due to ship emissions in the CA was found to increase by a mean of 1.5 ppb h^?1 (especially by ≥10 ppb h^?1 at the DS site) during the day.     

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.