Summertime photochemical ozone formation in Santiago,Chile

The city of Santiago, Chile experiences frequent high pollution episodes and as a consequence very high ozone concentrations, which are associated with health problems including increasing daily mortality and hospital admissions for respiratory illnesses. The development of ozone abatement strategies requires the determination of the potential of each pollutant to produce ozone, taking into account known mechanisms and chemical kinetics in addition to ambient atmospheric conditions. In this study, the photochemical formation of ozone during a summer campaign carried out from March 8–20, 2005 has been investigated using an urban photochemical box model based on the Master Chemical Mechanism (MCMv3.1). The MCM box model has been constrained with 10 min averages of simultaneous measurements of HONO, HCHO, CO, NO, j(O¹D), j(NO₂), 31 volatile organic compounds (VOCs) and meteorological parameters. The O₃–NO_x–VOC sensitivities have been determined by simulating ozone formation at different VOC and NO_x concentrations. Ozone sensitivity analyses showed that photochemical ozone formation is VOC-limited under average summertime conditions in Santiago. The results of the model simulations have been compared with a set of potential empirical indicator relationships including H₂O₂/HNO₃, HCHO/NO_y and O₃/NO_z. The ozone forming potential of each measured VOC has been determined using the MCM box model. The impacts of the above study on possible summertime ozone control strategies in Santiago are discussed.     

The relation between ozone,NOx and hydrocarbons in urban and polluted rural environments

Research over the past ten years has created a more detailed and coherent view of the relation between O₃ and its major anthropogenic precursors, volatile organic compounds (VOC) and oxides of nitrogen (NO_x). This article presents a review of insights derived from photochemical models and field measurements. The ozone–precursor relationship can be understood in terms of a fundamental split into a NO_x-senstive and VOC-sensitive (or NO_x-saturated) chemical regimes. These regimes are associated with the chemistry of odd hydrogen radicals and appear in different forms in studies of urbanized regions, power plant plumes and the remote troposphere. Factors that affect the split into NO_x-sensitive and VOC-sensitive chemistry include: VOC/NO_x ratios, VOC reactivity, biogenic hydrocarbons, photochemical aging, and rates of meteorological dispersion. Analyses of ozone–NO_x–VOC sensitivity from 3D photochemical models show a consistent pattern, but predictions for the impact of reduced NO_x and VOC in indivdual locations are often very uncertain. This uncertainty can be identified by comparing predictions from different model scenarios that reflect uncertainties in meteorology, anthropogenic and biogenic emissions. Several observation-based approaches have been proposed that seek to evaluate ozone–NO_x–VOC sensitivity directly from ambient measurements (including ambient VOC, reactive nitrogen, and peroxides). Observation-based approaches have also been used to evaluate emission rates, ozone production efficiency, and removal rates of chemically active species. Use of these methods in combination with models can significantly reduce the uncertainty associated with model predictions.     

Long-term trends of primary and secondary pollutant concentrations in Switzerland and their response to emission controls and economic changes

A detrending technique is developed for short-term and yearly variations in order to identify long-term trends in primary and secondary pollutants. In this approach, seasonal and weekly variations are removed by using a mean year; the residual meteorological short-term variation is removed by using a multiple linear regression model. This methodology is employed to detrend ozone (O₃), NO_x, VOC and CO concentrations in Switzerland. We show that primary pollutants (NO_x,VOC and CO) at urban and sub-urban stations show a downward trend over the last decade which correlates well with the reductions in the estimated Swiss emissions. In spite of these large decreases achieved in precursor emissions, summer peak ozone concentrations do not show any statistically significant trend over the last decade. Application of this method to ozone concentrations measured at the Jungfraujoch (3580 m a.s.l.) also shows no trend over the last 10 years. Detrended summer ozone correlates well with European Union gross national product and industrial production growth rates. These results suggest that if substantial reductions in summer peak ozone in Switzerland are desired, emissions reduction strategies must be part of control program involving a much larger region.     

Spatial mapping of VOC and NOx-limitation of ozone formation in central California

Ambient aerometric data were used to predict whether ozone formation at specific times and locations in central California was limited by the availability of volatile organic compounds (VOC) or oxides of nitrogen (NO_x). The predictions were compared with differences between mean weekday and weekend peak ozone values. The comparison with weekend and weekday ozone levels provided a means for empirically investigating the effects of VOC and NO_x reductions on ozone formation, because the relative proportions and levels of ozone precursor species were significantly different on weekends than on weekdays. Weekend NO_x levels averaged 27 percent lower than weekday levels at the time of the peak ozone hour. Daytime weekend levels of VOC species were also consistently lower than weekday values throughout the region, though the differences between weekends and weekdays were not always statistically significant (p<0.05). Site-to-site differences between weekend and weekday mean peak hourly ozone were related to whether ozone formation was VOC- or NO_x-limited.     

Atmospheric oxidation capacity in the summer of Houston 2006: Comparison with summer measurements in other metropolitan studies

Both similarities and differences in summertime atmospheric photochemical oxidation appear in the comparison of four field studies: TEXAQS2000 (Houston, 2000), NYC2001 (New York City, 2001), MCMA2003 (Mexico City, 2003), and TRAMP2006 (Houston, 2006). The compared photochemical indicators are OH and HO₂ abundances, OH reactivity (the inverse of the OH lifetime), HO_x budget, OH chain length (ratio of OH cycling to OH loss), calculated ozone production, and ozone sensitivity. In terms of photochemical activity, Houston is much more like Mexico City than New York City. These relationships result from the ratio of volatile organic compounds (VOCs) to nitrogen oxides (NO_x), which are comparable in Houston and Mexico City, but much lower in New York City. Compared to New York City, Houston and Mexico City also have higher levels of OH and HO₂, longer OH chain lengths, a smaller contribution of reactions with NO_x to the OH reactivity, and NO_x-sensitivity for ozone production during the day. In all four studies, the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) are significant, if not dominant, HO_x sources. A problematic result in all four studies is the greater OH production than OH loss during morning rush hour, even though OH production and loss are expected to always be in balance because of the short OH lifetime. The cause of this discrepancy is not understood, but may be related to the under-predicted HO₂ in high NO_x conditions, which could have implications for ozone production. Three photochemical indicators show particularly high photochemical activity in Houston during the TRAMP2006 study: the long portion of the day for which ozone production was NO_x-sensitive, the calculated ozone production rate that was second only to Mexico City's, and the OH chain length that was twice that of any other location. These results on photochemical activity provide additional support for regulatory actions to reduce reactive VOCs in Houston in order to reduce ozone and other pollutants.     

Application of OMI observations to a space-based indicator of NOx and VOC controls on surface ozone formation

We investigated variations in the relative sensitivity of surface ozone formation in summer to precursor species concentrations of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) as inferred from the ratio of the tropospheric columns of formaldehyde to nitrogen dioxide (the “Ratio”) from the Aura Ozone Monitoring Instrument (OMI). Our modeling study suggests that ozone formation decreases with reductions in VOCs at Ratios <1 and NO_x at Ratios >2; both NO_x and VOC reductions may decrease ozone formation for Ratios between 1 and 2. Using this criteria, the OMI data indicate that ozone formation became: 1. more sensitive to NO_x over most of the United States from 2005 to 2007 because of the substantial decrease in NO_x emissions, primarily from stationary sources, and the concomitant decrease in the tropospheric column of NO₂, and 2. more sensitive to NO_x with increasing temperature, in part because emissions of highly reactive, biogenic isoprene increase with temperature, thus increasing the total VOC reactivity. In cities with relatively low isoprene emissions (e.g., Chicago), the data clearly indicate that ozone formation became more sensitive to NO_x from 2005 to 2007. In cities with relatively high isoprene emissions (e.g., Atlanta), we found that the increase in the Ratio due to decreasing NO_x emissions was not obvious as this signal was convolved with variations in the Ratio associated with the temperature dependence of isoprene emissions and, consequently, the formaldehyde concentration.     

Photochemical reactivities of common solvents: comparison between urban and regional domains

Solvents are one of the most abundant sources of anthropogenic VOCs in the atmosphere, and can comprise a large number of organic compounds having different impacts on the rate and amount of ozone formation. A three-dimensional photochemical air quality model has been used to study the relative impacts of eight solvents, acetone, ethane, ethanol, isobutane, m-xylene, tertiary butyl acetate (TBA), para-chlorobenzotrifluoride (PCBTF) and benzotrifluoride (BTF) in three very different domains: Los Angeles, an urban area with high ozone and NO_x levels; the Swiss Plateau, a more regional domain with much lower ozone and NO_x levels: and Mexico City, a very high VOC urban area with high ozone levels. The results show that there can be a wide range of VOC reactivities under variable environmental conditions. Variability also exists between metrics, which are used to quantify reactivity. In most cases, halogenated aromatics were the least reactive and isobutane and m-xylene the most. The results here, finding that normalized reactivities are less variable than the absolute reactivity, support the applicability of relative VOC reactivity scales for use in air quality management.     

Trends in on-road vehicle emissions and ambient air quality in Atlanta,Georgia, USA,from the late 1990s through 2009

On-road vehicle emissions of carbon monoxide (CO), nitrogen oxides (NO_x), and volatile organic compounds (VOCs) during 1995–2009 in the Atlanta Metropolitan Statistical Area were estimated using the Motor Vehicle Emission Simulator (MOVES) model and data from the National Emissions Inventories and the State of Georgia. Statistically significant downward trends (computed using the nonparametric Theil-Sen method) in annual on-road CO, NO_x, and VOC emissions of 6.1%, 3.3%, and 6.0% per year, respectively, are noted during the 1995–2009 period despite an increase in total vehicle distance traveled. The CO and NO_x emission trends are correlated with statistically significant downward trends in ambient air concentrations of CO and NO_x in Atlanta ranging from 8.0% to 11.8% per year and from 5.8% to 8.7% per year, respectively, during similar time periods. Weather-adjusted summertime ozone concentrations in Atlanta exhibited a statistically significant declining trend of 2.3% per year during 2001–2009. Although this trend coexists with the declining trends in on-road NO_x, VOC, and CO emissions, identifying the cause of the downward trend in ozone is complicated by reductions in multiple precursors from different source sectors.

Implications:Large reductions in on-road vehicle emissions of CO and NO_x in Atlanta from the late 1990s to 2009, despite an increase in total vehicle distance traveled, contributed to a significant improvement in air quality through decreases in ambient air concentrations of CO and NO_x during this time period. Emissions reductions in motor vehicles and other source sectors resulted in these improvements and the observed declining trend in ozone concentrations over the past decade. Although these historical trends cannot be extrapolated to the future because pollutant concentration contributions due to on-road vehicle emissions will likely become an increasingly smaller fraction of the atmospheric total, they provide an indication of the benefits of past control measures.     

The use of ambient data to corroborate analyses of ozone control strategies

Data from environmental-chamber studies and photochemical box-model simulations were used to evaluate and revise a method for developing a qualitative understanding of the sensitivity of ozone formation at a particular time and place to changes in concentrations of volatile organic compounds (VOC) and oxides of nitrogen (NO_x). The revised method requires measurements of ozone, NO, and either NO_x or NO_y. The sensitivities of the method to biases in measurements were evaluated. The method potentially can be used for qualitative assessment of VOC versus NO_x limitation, comparison with the predictions of grid-based photochemical air-quality models, and evaluation of trends over time in the relative effectiveness of VOC versus NO_x controls.     

Variability of indicator values for ozone production sensitivity: a model study in Switzerland and San Joaquin Valley (California)

The threshold values of indicator species and ratios delineating the transition between NO_x and VOC sensitivity of ozone formation are assumed to be universal by various investigators. However, our previous studies suggested that threshold values might vary according to the locations and conditions. In this study, threshold values derived from various model simulations at two different locations (the area of Switzerland by UAM Model and San Joaquin Valley of Central California by SAQM Model) are examined using a new approach for defining NO_x and VOC sensitive regimes. Possible definitions for the distinction of NO_x and VOC sensitive ozone production regimes are given. The dependence of the threshold values for indicators and indicator ratios such as NO_y, O₃/NO_z, HCHO/NO_y, and H₂O₂/HNO₃ on the definition of NO_x and VOC sensitivity is discussed. Then the variations of threshold values under low emission conditions and in two different days are examined in both areas to check whether the models respond consistently to changes in environmental conditions. In both cases, threshold values are shifted similarly when emissions are reduced. Changes in the wind fields and aging of the photochemical oxidants seem to cause the day-to-day variation of the threshold values. O₃/NO_z and HCHO/NO_y indicators are predicted to be unsatisfactory to separate the NO_x and VOC sensitive regimes. Although NO_y and H₂O₂/HNO₃ provide a good separation of the two regimes, threshold values are affected by changes in the environmental conditions studied in this work.     

Intercomparison of chemical mechanisms for air quality policy formulation and assessment under North American conditions

The intercomparison of seven chemical mechanisms for their suitability for air quality policy formulation and assessment is described. Box modeling techniques were employed using 44 sets of background environmental conditions covering North America to constrain the chemical development of the longer lived species. The selected mechanisms were modified to enable an unbiased assessment of the adequacy of the parameterizations of photochemical ozone production from volatile organic compound (VOC) oxidation in the presence of NO_x. Photochemical ozone production rates responded differently to 30% NO_x and VOC reductions with the different mechanisms, despite the striking similarities between the base-case ozone production rates. The 30% reductions in NO_x and VOCs also produced changes in OH. The responses in OH to 30% reductions in NO_x and VOCs appeared to be more sensitive to mechanism choice, compared with the responses in the photochemical ozone production rates. Although 30% NO_x reductions generally led to decreases in OH, 30% reductions in VOCs led to increases in OH, irrespective of mechanism choice and background environmental conditions. The different mechanisms therefore gave different OH responses to NO_x and VOC reductions and so would give different responses in terms of changes in the fate and behavior of air toxics, acidification and eutrophication, and fine particle formation compared with others, in response to ozone control strategies. Policymakers need to understand that there are likely to be inherent differences in the responses to ozone control strategies between different mechanisms, depending on background environmental conditions and the extents of NO_x and VOC reductions under consideration.

Implications: The purpose of this paper is to compare predicted ozone responses to NO_x and VOC reductions with seven chemical mechanisms under North American conditions. The good agreement found between the tested mechanisms should provide some support for their application in the air quality models used for policymaking.     

Projected ozone trends and changes in the ozone-precursor relationship in the South Coast Air Basin in response to varying reductions of precursor emissions

This study examined the effects of varying future reductions in emissions of oxides of nitrogen (NO_x) and volatile organic compounds (VOC) on the location and magnitude of peak ozone levels within California’s South Coast Air Basin (SoCAB or Basin). As ozone formation is currently VOC-limited in the Basin, model simulations with 2030 baseline emissions (?61% for NO_x and ?32% for VOC from 2008) predict 10–20% higher peak ozone levels (i.e., NO_x disbenefit) in the western and central SoCAB compared with the 2008 base simulation. With additional NO_x reductions of 50% beyond the 2030 baseline emissions (?81% from 2008), the predicted ozone levels are reduced by about 15% in the eastern SoCAB but remain comparable to 2008 levels in the western and central Basin. The Basin maximum ozone site shifts westward to more populated areas of the Basin and will result potentially in greater population-weighted exposure to ozone with even a relatively small shortfall in the required NO_x reductions unless accompanied by additional VOC reductions beyond 2030 baseline levels. Once committed to a NO_x-focused control strategy, NO_x reductions exceeding 90% from 2008 levels will be necessary to attain the ozone National Ambient Air Quality Standards (NAAQS). The findings from this study and other recent work that the current VOC emission estimates are underestimated by about 50% suggest that greater future VOC reductions will be necessary to reach the projected 2030 baseline emissions. Increasing the base year VOC emissions by a factor of 1.5 result in higher 2008 baseline ozone predictions, lower relative response factors, and about 20% lower projected design values. If correct, these findings have important implications for the total and optimum mix of VOC and NO_x emission reductions that will be required to attain the ozone NAAQS in the SoCAB.

Implications: Results of this study indicate that ozone levels in the western and central SoCAB would remain the same or increase with even a relatively small shortfall in the projected NO_x reductions under planned NO_x-focused controls. This possibility, therefore, warrants a rigorous analysis of the costs and effects of varying reductions of VOC and NO_x on the formation and combined health impacts of ozone and secondary particles. Given the nonlinearity of ozone formation, such analyses should include the implications of gradually increasing global background ozone concentrations and the Basin’s topography and meteorology on the practical limits of alternative emission control strategies.     

Air Pollution Control and Waste Management in a New World

Abstract

A research site for atmospheric chemistry and air pollution measurements was established at Pinnacle State Park in Addison, NY, in 1995. This paper presents an overview of the site characteristics and measurement program, as well as monthly average concentrations for many of the trace gas and aerosol pollutants over the full measurement period. Monthly averaged ozone concentrations range from values as low as 15 parts per billion (ppb) during cold-season months, to values approaching 50 ppb during some spring and summer months. Sulfur dioxide (SO₂), oxides of nitrogen (NO_x), and reactive odd nitrogen (NO_y) all show distinct seasonal variation, with summertime monthly averages as low as 1–3 ppb, and wintertime monthly averages from 6–12 ppb. The variation in carbon monoxide (CO) is much smaller, with minimums of approximately 150 ppb and maximums only rarely exceeding 250 ppb. Data for three hydrocarbon species propane, benzene, and isoprene—are presented. Propane and benzene show higher monthly averaged concentrations in the winter and lower values in the summer, with values ranging over a factor of 4–5. Isoprene, on the other hand has much higher values during the summer season, sometimes a factor of 10 or more greater than concentrations measured in the winter. Monthly averaged plots for fine particulate matter (PM_2.5) beginning in 1999 show a robust summer maximum and winter minimum, and roughly a factor of two difference between the two. An empirical measure of ozone production using the correlation of hour-averaged ozone and NO_y data illustrates relatively robust ozone production during some, but not all, summertime months over the time period. Also, an analysis of the frequency distribution of the hours of maximum ozone concentration shows a strong mid-afternoon peak, as expected, but also a prominent secondary maximum centered around midnight. The secondary peak is interpreted as ozone transported from ozone-producing areas to the west, including Buffalo, Cleveland, Pittsburgh, and the Ohio Valley. Finally, SO₂ concentrations as a function of wind direction clearly indicate maximum impacts when the winds are out of the south (Pittsburgh and Philadelphia), with a secondary peak when the winds are from the north-northeast, consistent with the locations of major SO₂ emission sources in the region.     

Photo-oxidant chemistry in the polluted boundary layer under changing UV-B radiation

UV-B radiation is a driving factor for the chemistry of the polluted boundary layer. It is involved in the formation of radicals and consequently influences the formation and concentration of photo-oxidants. The 3-D mesoscale photochemical Metphomod model was employed to study the effect of changes in UV-B radiation on the concentration of photo-oxidants in the boundary layer over the Swiss Plateau. The model chemistry is based on the RACM mechanism and a two-stream approximation of radiative transfer. A summer (July) and a late winter (February) episode were simulated. All simulations were replicated with relatively large changes in the prescribed total ozone. The results for an increase in UV-B radiation show increases in PAN, HNO₃, and ozone at noon in NO_x-rich areas and a decrease in NO_x. In NO_x-poor areas in summer the effect on ozone is weak and has a negative sign, the main effect being an increase in H₂O₂. The spatial variability of NO_x concentrations in the Swiss Plateau in the summer case is such that the effect of increased UV-B radiation on ozone is spatially variable. The effect on the ozone production rate in summer is strongest positive at the surface in the NO_x-rich regions in the morning and strongest negative at some altitude above ground in NO_x-poor regions in the early afternoon. In the winter episode, NO_x-rich conditions are found almost everywhere on the Swiss Plateau, the effect of increased UV-B radiation on the ozone production rate is positive all day long and is largest at 300 m above ground at noon. In this case, in contrast to the summer case, the increase in ozone is carried over to the next day. The model results for ozone are in good agreement with results from a case study and a time series analysis of surface ozone measurements. We estimate the effect of day-to-day changes in total ozone on surface ozone peaks to range from 4 to 6 ppb at most.     

Estimating changes in urban ozone concentrations due to life cycle emissions from hydrogen transportation systems

Hydrogen has been proposed as a low polluting alternative transportation fuel that could help improve urban air quality. This paper examines the potential impact of introducing a hydrogen-based transportation system on urban ambient ozone concentrations. This paper considers two scenarios, where significant numbers of new hydrogen vehicles are added to a constant number of gasoline vehicles. In our scenarios hydrogen fuel cell vehicles (HFCVs) are introduced in Sacramento, California at market penetrations of 9% and 20%. From a life cycle analysis (LCA) perspective, considering all the emissions involved in producing, transporting, and using hydrogen, this research compares three hypothetical natural gas to hydrogen pathways: (1) on-site hydrogen production; (2) central hydrogen production with pipeline delivery; and (3) central hydrogen production with liquid hydrogen truck delivery. Using a regression model, this research shows that the daily maximum temperature correlates well with atmospheric ozone formation. However, increases in initial VOC and NO_x concentrations do not necessarily increase the peak ozone concentration, and may even cause it to decrease. It is found that ozone formation is generally limited by NO_x in the summer and is mostly limited by VOC in the fall in Sacramento. Of the three hydrogen pathways, the truck delivery pathway contributes the most to ozone precursor emissions. Ozone precursor emissions from the truck pathway at 9% market penetration can cause additional 3-h average VOC (or NO_x) concentrations up to approximately 0.05% (or 1%) of current pollution levels, and at 20% market penetration up to approximately 0.1% (or 2%) of current pollution levels. However, all of the hydrogen pathways would result in very small (either negative or positive) changes in ozone air quality. In some cases they will result in worse ozone air quality (mostly in July, August, and September), and in some cases they will result in better ozone air quality (mostly in October). The truck pathway tends to cause a much wider fluctuation in degradation or improvement of ozone air quality: percentage changes in peak ozone concentrations are approximately −0.01% to 0.04% for the assumed 9% market penetration, and approximately −0.03% to 0.1% for the 20% market penetration. Moreover, the 20% on-site pathway occasionally results in a decrease of about −0.1% of baseline ozone pollution. Compared to the current ambient pollution level, all three hydrogen pathways are unlikely to cause a serious ozone problem for market penetration levels of HFCVs in the 9–20% range.     

Modeling the Effects of VOC/NOx Emissions on Ozone Synthesis in the Cascadia Airshed of the Pacific Northwest

ABSTRACT

A modeling system consisting of MM5, Calmet, and Calgrid was used to investigate the sensitivity of anthropogenic volatile organic compound (VOC) and oxides of nitrogen (NO_x) reductions on ozone formation within the Cascadia airshed of the Pacific Northwest. An ozone episode that occurred on July 11-14, 1996, was evaluated. During this event, high ozone levels were recorded at monitors downwind of Seattle, WA, and Portland, OR, with one monitor exceeding the 1 hr/120 ppb National Ambient Air Quality Standard (at 148 ppb), and six monitors above the proposed 8 hr/80 ppb standard (at 82-130 ppb). For this particular case, significant emissions reductions, between 25 and 75%, would be required to decrease peak ozone concentrations to desired levels. Reductions in VOC emissions alone, or a combination of reduced VOC and NO_x emissions, were generally found to be most effective; reducing NO_x emissions alone resulted in increased ozone in the Seattle area. When only VOC emissions were curtailed, ozone reductions occurred in the immediate vicinity of densely populated areas, while NO_x reductions resulted in more widespread ozone reductions.     

Differences between Weekday and Weekend Air Pollutant Levels in Atlanta; Baltimore; Chicago; Dallas–Fort Worth; Denver; Houston; New York; Phoenix; Washington,DC; and Surrounding Areas

Abstract

We evaluated day-of-week differences in mean concentrations of ozone (O₃) precursors (nitric oxide [NO], nitrogen oxides [NO_x], carbon moNO_xide [CO], and volatile organic compounds [VOCs]) at monitoring sites in 23 states comprising seven geographic focus areas over the period 1998– 2003. Data for VOC measurements were available for six metropolitan areas in five regions. We used Wednesdays to represent weekdays and Sundays to represent weekends; we also analyzed Saturdays. At many sites, NO, NO_x, and CO mean concentrations decreased at all individual hours from 6:00 a.m. to 3:00 p.m. on Sundays compared with corresponding Wednesday means. Statistically significant (p < 0.01) weekend decreases in ambient concentrations were observed for 92% of NO_x sites, 89% of CO sites, and 23% of VOC sites. Nine-hour (6:00 a.m. to 3:00 p.m.) mean concentrations of NO, NO_x, CO, and VOCs declined by 65, 49, 28, and 19%, respectively, from Wednesdays to Sundays (median site responses). Despite the large reductions in ambient NO_x and moderate reductions in ambient CO and VOC concentrations on weekends, ozone and particulate matter (PM) nitrate did not exhibit large changes from week-days to weekends. The median differences between Wednesday and Sunday mean ozone concentrations at all monitoring sites ranged from 3% higher on Sundays for peak 8-hr concentrations determined from all monitoring days to 3.8% lower on Sundays for peak 1-hr concentrations on extreme-ozone days. Eighty-three percent of the sites did not show statistically significant differences between Wednesday and weekend mean concentrations of peak ozone. Statistically significant weekend ozone decreases occurred at 6% of the sites and significant increases occurred at 11% of the sites. Average PM nitrate concentrations were 2.6% lower on Sundays than on Wednesdays. Statistically significant Sunday PM nitrate decreases occurred at one site and significant increases occurred at seven sites.     

Ozone uptake by citrus trees exposed to a range of ozone concentrations

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO_x). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s^?1 m^?2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.     

A diagnostic comparison of measured and model-predicted speciated VOC concentrations

This study compares speciated model-predicted concentrations (i.e., mixing ratios) of volatile organic compounds (VOCs) with measurements from the Photochemical Assessment Monitoring Stations (PAMS) network at sites within the northeastern US during June–August of 2006. Measurements of total non-methane organic compounds (NMOC), ozone (O₃), oxides of nitrogen (NO_x) and reactive nitrogen species (NO_y) are used for supporting analysis. The measured VOC species were grouped into the surrogate classes used by the Carbon Bond IV (CB4) chemical mechanism. It was found that the model typically over-predicted all the CB4 VOC species, except isoprene, which might be linked to overestimated emissions. Even with over-predictions in the CB4 VOC species, model performance for daily maximum O₃ was typically within ±15%. Analysis at an urban site in NY, where both NMOC and NO_x data were available, suggested that the reasonable ozone performance may be possibly due to compensating overestimated NO_x concentrations, thus modulating the NMOC/NO_x ratio to be in similar ranges as that of observations.     

Isoprene and its degradation products as strong ozone precursors in Insubria,Northern Italy

Frequent smog episodes occur during spring, summer, and autumn in Insubria, Northern Italy. On a test site in this area the atmospheric concentration of the photo-oxidants ozone and peroxyacetyl nitrate has been monitored over a year (2000) together with ozone precursors listed in the European Union Air Quality Directive 2002/3/EC, such as nitrous oxides (NO_X) and volatile organic compounds (VOC) including hydrocarbons and carbonyls. The results of this study revealed a strong impact of biogenic isoprene on the air quality.In winter isoprene was detected at the ppt level and correlated with anthropogenic VOC. However, during the growing season isoprene exhibited a distinct diurnal variation with maximum concentrations late in the afternoon reaching up 70 ppbC attributed to strong emissions from the abundant vegetation of broad-leaf deciduous trees in this area. A new HPLC-MS method was developed for the determination of isoprene's primary atmospheric oxidation products methacrolein as its 2,4-dinitrophenylhydrazone and methyl vinyl ketone as an unusual double derivative with 2,4-dinitrophenylhydrazine. Methacrolein and methyl vinyl ketone followed the same diurnal and annual trends as isoprene. The average monthly concentration of isoprene and these products ranged from around 10 ppbC in June, July and September to 20 ppbC in August, which constitutes 15–30% of C₃–C₉ VOCs. The contribution from isoprene photo-oxidation to the ambient air formaldehyde concentrations was also found to be high during this period ranging from 30% to 60% in May, June, July and August.From the atmospheric VOC and NO_X concentrations the local photochemical ozone formation was estimated by the incremental reactivity approach. The calculations showed that in summer isoprene's contribution to the local ozone formation was as high as 50–75%.