Modeling weekday/weekend ozone differences in the Los Angeles region for 1997

Numerous studies of ambient ozone (O3) in the Los Angeles (LA) area have found both increases and decreases in elevated O3 levels on weekends, depending on location and year. Since the mid-1990s, average daily maximum O3 levels have been higher on weekends than on weekdays throughout most of the area. We used the Comprehensive Air-Quality Model with extensions to investigate causes of weekday/weekend O3 differences in the LA area for August 3-7, 1997, from the Southern California Ozone Study. Weekday/weekend emission changes were estimated, because explicit weekend inventories are not yet available from regulatory agencies. Changes to on-road motor vehicle (MV) emissions were derived from observed weekday/weekend traffic differences. The estimated changes in MV emissions of nitrogen oxides (NOx) were a 5% increase on Friday, a 27% decrease on Saturday, and a 37% decrease on Sunday, relative to Monday-Thursday levels. The corresponding changes in MV volatile organic carbon (VOC) emissions were an 8% increase on Friday, an 8% decrease on Saturday, and a 15% decrease on Sunday. Modeling these MV emissions changes explained the observed weekend O3 effect very well. Furthermore, changes to the mass of MV NOx emissions were the main contributor to O3 differences rather than changes to the timing of MV emissions. Ozone increases on weekends were caused by NOx emission decreases, because O3 formation is strongly VOC-limited throughout most of the LA area.     

Differences between weekday and weekend air pollutant levels in southern California

Ambient air quality data were analyzed to empirically evaluate the effects of reductions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) emissions on weekday and weekend levels of ozone (O3; 1991-1998) and particulate NO3- (1980-1999) in southern California. Despite significantly lower O3 precursor levels on weekends, 20 of 28 South Coast Air Basin (SoCAB) sites (28 of all 78 southern California sites) showed statistically significant higher mean O3 levels on Sundays than on weekdays (p < 0.01); 49 of the remaining 50 sites showed no significant differences between mean weekday and Sunday peak O3 levels. We also observed no statistically significant differences between mean weekday and weekend concentrations of particulate NO3- or nitric acid (HNO3, the precursor of particulate NO3-). Averaged over sites, the mean Sunday NOx and nonmethane hydrocarbon concentrations were 25-41% and 16-30% lower, respectively, than on weekdays. Site-to-site differences between weekend and weekday mean peak hourly O3 levels were related to whether O3 formation was limited by the availability of NOx. A thermodynamic equilibrium model predicts that particulate NO3- levels would decrease in response to a reduction of HNO3, and that particulate ammonium NO3- formation was not limited by the availability of ammonia. The similarity of mean weekday and weekend levels of NO3- therefore did not result from limitations on the formation of particulate NO3- from its precursor, HNO3.     

The spatial variation of the weekday/weekend differences in the Baltimore area

This study examined the spatial distribution of the weekday/weekend difference in Baltimore, MD, and vicinity. The difference in Baltimore was characterized by having approximately 39% more NOx, approximately 59% more CO, and approximately 27% more volatile organic compounds (VOCs) on the average weekday, but there was approximately 13% more O3 on the weekend day. Spatially, the difference was elongated in the northeast-southwest direction. It decreased from 13% more O3 in Baltimore to 6% at approximately 20 km west of Baltimore and to 4% at approximately 40 km south of Baltimore. It also appeared to decrease to the east, but the magnitude of the decrease could not be determined because of the lack of data east of the Chesapeake Bay. However, the difference increased to the north, reaching a value of almost 18% at a northern Delaware site.     

香港港口氮氧化物和臭氧的变化特征和周末效应

为了研究香港港口氮氧化物和臭氧（NO2、NOx和O3）的周末效应,本文收集香港港口15年的污染数据进行统计分析。首先分析污染物的日变化特征,结果发现工作日NO2和NOx浓度高于周六,周六浓度高于周日的情形。与此相反,工作日O3浓度低于周六,周六浓度低于周日。其次分析污染物年变化,结果表明近些年NO2和NOx呈降低趋势,O3呈增加趋势。还发现NO2、NOx和O3的周末效应减弱。然后使用线性回归分析光化学氧化剂（OX,O3+NO2）的局地和区域贡献,结果发现受港口作业周期性变化的影响,OX的局地贡献表现出工作日高于周六,周六高于周日的周末效应现象。最后分析了OX局地贡献的昼夜差异,结果显示白天OX局地贡献的周末效应明显强于晚上。     

Diurnal and weekday variations in the source contributions of ozone precursors in California's South Coast Air Basin

For at least 30 years, ozone (O3) levels on weekends in parts of California's South Coast (Los Angeles) Air Basin (SoCAB) have been as high as or higher than on weekdays, even though ambient levels of O3 precursors are lower on weekends than on weekdays. A field study was conducted in the Los Angeles area during fall 2000 to test whether proposed relationships between emission sources and ambient nonmethane hydrocarbon (NMHC) and oxides of nitrogen (NOx) levels can account for observed diurnal and day-of-week variations in the concentration and proportions of precursor pollutants that may affect the efficiency and rate of O3 formation. The contributions to ambient NMHC by motor vehicle exhaust and evaporative emissions, estimated using chemical mass balance (CMB) receptor modeling, ranged from 65 to 85% with minimal day-of-week variation. Ratios of ambient NOx associated with black carbon (BC) to NOx associated with carbon monoxide (CO) were approximately 1.25 +/- 0.22 during weekdays and 0.76 +/- 0.07 and 0.52 +/- 0.07 on Saturday and Sunday, respectively. These results demonstrate that lower NOx emissions from diesel exhaust can be a major factor causing lower NOx mixing ratios and higher NMHC/NOx ratios on weekends. Nonmobile sources showed no significant day-of-week variations in their contributions to NMHC. Greater amounts of gasoline emissions are carried over on Friday and Saturday evenings but are, at most, a minor factor contributing to higher NMHC/NOx ratios on weekend mornings.     

Weekday versus weekend activity patterns for ozone precursor emissions in California's South Coast Air Basin

Ambient O3 concentrations in California's South Coast Air Basin (SoCAB) can be as much as 55% higher on weekends than on weekdays under comparable meteorological conditions. This is paradoxical because emissions of O3 precursors (hydrocarbons, CO, and nitrogen oxides [NOx]) are lower on weekends. Day-of-week emissions activity data were collected and analyzed to investigate the hypothesized causes of the "weekend O3 effect." Emission activity data were collected for various mobile, area, and point sources throughout the SoCAB, including on-road vehicles, lawn and garden equipment, barbecues, fireplaces, solvent use, and point sources with continuous emission monitoring data. The results of this study indicate significant differences between weekday and weekend emission activity patterns and emissions. Their combined effect results in a 12-18% decrease in reactive organic gases (ROGs) and a 35-41% decrease in NOx emissions on Saturdays and Sundays, respectively, relative to weekdays in summer 2000. These changes in emissions result in an increase of more than 30% in the ROG/NOx ratio on weekends compared with weekdays, which, along with lower NOx emissions, leads to increased O3 production on weekends.     

Comparison of ozone temporal scales for large urban, small urban, and rural areas in Georgia

Ground-level ozone (O3) time series are characterized by the sum of several distinct temporal scales: long-term, seasonal, synoptic, diurnal (daily), and intraday variation. In this study, the authors use a Kolmorogov-Zurbenko filter to separate the 1981-2001 O3 time-series from many sites in and around Georgia into these various components. The authors compare the temporal components to examine differences between small and large metropolitan areas and between urban and rural areas. They then focus on the synoptic component to define a predominant transport region or airshed for each site.     

Day-of-Week behavior of atmospheric ozone in three U.S. cities

The weekly cycles of atmospheric ozone (O3) are of interest because they provide information about the response of O3 to changes in anthropogenic emissions from weekdays to weekends. The weekly behavior of O3 in Chicago, IL; Philadelphia, PA; and Atlanta, GA, is contrasted. In Chicago and Philadelphia, maximum 1-hr average O3 increases on weekends. In Atlanta, O3 builds up from Mondays to Fridays and declines during weekends. In all three areas, volatile organic compound (VOC)/nitrogen oxides (NOx) ratios are higher during weekends, resulting from greater than proportionate decreases in NOx relative to VOC emissions. The VOC/NOx ratios correlate with maximum 1-hr O3 concentrations in Chicago, a response consistent with a VOC-sensitive airshed. A weak correlation between O3 concentrations and VOC/NOx ratios in Philadelphia suggests the impact of transported O3, which is formed in upwind VOC-sensitive locations that may be hundreds of kilometers away. Ozone concentrations in Atlanta do not correlate with VOC/NOx ratios but with concentrations of NOx and total reactive nitrogen (NOy) carried over from the previous day. When data from 1986-1990 and 1995-1999 are compared, only small differences in the weekly behavior of O3 are observed in Chicago and Philadelphia. The day-of-week differences in O3 are amplified in the more recent period in Atlanta, a possible result of urban growth.     

臭氧氧化苯乙烯有机气体性能及机制

采用臭氧高级氧化处理高浓度苯乙烯有机废气,研究了进气苯乙烯浓度、臭氧浓度、停留时间、O₃/C₈H₈摩尔比对苯乙烯去除效率的影响。研究结果表明,臭氧氧化能有效净化苯乙烯有机废气,苯乙烯去除效率可达66.6%。适宜运行条件为：停留时间为3.6 s,O₃/C₈H₈摩尔比为0.46。采用GC-MS分析臭氧氧化苯乙烯出口气样,研究结果表明,苯甲醛（C₆H₅CHO）和苯甲酸（C₆H₅COOH）为臭氧氧化苯乙烯的中间产物。臭氧高级氧化苯乙烯机制为苯乙烯气体被臭氧氧化为苯甲醛和苯甲酸,然后继续臭氧氧化为最终产物二氧化碳和水。     

Neural network-based study for predicting ground-level ozone concentration in large urban areas, applied to the Sao Paulo metropolitan area

Events of high concentration of ground-level ozone constitute a matter of major concern in large urban areas in terms of air quality, and public health. In the Sao Paulo Metropolitan Area (SPMA), air quality data generated by a network of air quality measuring stations have been used in a number of studies correlating ozone formation with different variables. A study was carried out on the application of neural network models in the identification of typical sceneries leading to high ground-level ozone concentrations in the SPMA. The results were then applied in the selection of variables, and in the definition of neural network-based models for estimating ozone levels from meteorological variables. When combined with existing weather prediction tools, the models can be applied in the prediction of ozone levels in the SPMA     

An ozone climatology: relationship between meteorology and ozone in the Southeast USA

A statistical analysis of ozone (O₃) concentrations and meteorological parameters was performed to determine the relationship between meteorological changes and ambient O₃ concentrations in the Southeast United States. The correlation between average daily maximum O₃ concentration and various meteorological variables was analysed on a monthly basis from April through October during 1980-1994. The correlations were strongest during the summer months, particularly June, July, and August. Analysis of long term O₃ concentration trends indicates increasing trends during the 1980s and decreasing trends during the early 1990s.     

Trends in concentration of ground-level ozone and meteorological conditions during high ozone episodes in the Kao-Ping Airshed, Taiwan

This work analyzes the variations in daily maximum 1-hr ozone (O3) concentrations and the long-term trends in annual means of hourly ambient concentrations of O3, nitrogen oxides (nitrous oxide + nitrogen dioxide), and nonmethane hydrocarbons in the three administrative regions of Kao-Ping airshed in southern Taiwan over a recent 8-yr period. The annual or monthly means of all maxima, most 95th percentiles, and some 90th percentiles of the daily maximum 1-hr O3 concentrations exceed the daily limit of 120 parts per billion by volume in all three regions, namely, Kao-hsiung City, Kso-hsiung County, and P'ing-tung County. The monthly means of daily maximum 1-hr O3 concentrations exhibit distinct seasonal variations, with a bimodal form with the maxima in autumn and late winter to the middle of spring and a minimum in summer. The long-term variations in the annual means of hourly O3 concentrations in the three regions exhibit increasing trends. These increases in O3 are associated with the decline in ambient concentrations of nitrogen oxides and nonmethane hydrocarbons. High O3 episodes occur most often in autumn and most rarely in summer. The seasonal mean mixing heights in descending order follow the order of spring, summer, autumn, and winter. Meteorological parameters in autumn and winter indicate that the ground-level O3 tends to accumulate and trigger a high O3 episode on a warm day with sufficient sunlight and low wind in a high-pressure system, consistent with the low mixing heights in these two seasons.     

Prediction of ozone concentration in ambient air using multivariate methods

Multivariate statistical methods including pattern recognition (Principal Component Analysis--PCA) and modeling (Multiple Linear Regression--MLR, Partial Least Squares--PLS, as well as Principal Component Regression--PCR) methods were carried out to evaluate the state of ambient air in Miskolc (second largest city in Hungary). Samples were taken from near the ground at a place with an extremely heavy traffic. Although PCA is not able to determine the significance of variables, it can uncover their similarities and classify the cases. PCA revealed that it is worth to separate day and night data because different factors influence the ozone concentrations during all day. Ozone concentration was modeled by MLR and PCR with the same efficiency if the conditions of meteorological parameters were not changed (i.e. morning and afternoon). Without night data, PCR and PLS suggest that the main process is not a photochemical but a chemical one.     

Interaction between ozone and winter stress

In some countries, ozone (O3) is primarily a summer pollutant, but in much of Europe, elevated concentrations occur outside the growing season so perennials and over-wintering annuals may be subjected to the combined stresses of pollution, plus chilling, freezing, and winter desiccation. It is recognised that some air pollutants modify the response of plants to environmental stress, but little is known of interactions involving O3. This paper is part of a programme concerned with the effects of O3 on resistance to chilling, freezing, and winter desiccation. Pea (Pisum sativum L.) was used as a convenient model to confirm that O3 affects freezing resistance. The experiment also served as a further evaluation of the use of induced chlorophyll fluorescence kinetics to detect latent O3 injury. Two cultivars, 'Feltham First' and 'Conquest', were fumigated for 7 days, 7 h day(-1). Diffusive resistance and induced fluorescence were recorded daily during the period, then the plants were hardened at 4 degrees C day/2 degrees C night before exposure to 0, -2, -4, -6 and -8 degrees C. Ozone (0.075 ppm; 150 microg O3 m(-3)) caused stomatal closure in both cultivars, but the response was more rapid in 'Conquest'. There were also rapid effects on fluorescence kinetics, and it was concluded that FR, the rate of rise of induced fluorescence, is a useful parameter for indicating latent injury and for distinguishing between cultivars of different sensitivity. Exposure to O3 increased freezing injury and led to greater electrolyte leakage. The freezing resistance of 'Feltham First' was more affected than that of 'Conquest', probably because of the slower stomatal response to the pollutant leading to greater flux of O3 to the internal tissues. It is concluded that interactions involving pollutants and winter stress have implications for crop loss assessment. Perennials and over-wintering annuals should be exposed to the full range of environmental stresses.     

Evolution of the magnitude and spatial extent of the weekend ozone effect in California's South Coast Air Basin, 1981-2000

Since the mid-1970s, ozone (O3) levels in portions of California's South Coast Air Basin (SoCAB) on weekends have been as high as or higher than levels on weekdays, even though emissions of O3 precursors are lower on weekends. Analysis of the ambient data indicates that the intensity and spatial extent of the weekend O3 effect are correlated with-day-of-week variations in the extent of O3 inhibition caused by titration with nitric oxide (NO), reaction of hydroxyl radical (OH) with nitrogen dioxide (NO2), and rates of O3 accumulation. Lower NO mixing ratios and higher NO2/oxides of nitrogen (NOx) ratios on weekend mornings allow O3 to begin accumulating approximately an hour earlier on weekends. The weekday/weekend differences in the duration of O3 accumulation remained relatively constant from 1981 to 2000. In contrast, the rate of O3 accumulation decreased by one-third to one-half over the same period; the largest reductions occurred in the central basin on weekdays. Trends in mixing ratios of O3 precursors show a transition to lower volatile organic compound (VOC)/NOx ratios caused by greater reductions in VOC emissions. Reductions in VOC/NOx ratios were greater on weekdays, resulting in higher VOC/NOx ratios on weekends relative to weekdays. Trends in VOC/NOx ratios parallel the downward trend in peak O3 levels, a shift in the location of peak O3 from the central to the eastern portion of the basin, and an increase in the magnitude and spatial extent of the weekend O3 effect.     

Annual and seasonal trends of ambient ozone concentration and its impact on forest vegetation in Mercantour National Park (South-eastern France) over the 2000-2008 period

In the South-Eastern French Mediterranean region, high ozone concentrations were measured since many years and specific symptoms like chlorotic mottles were detected on Arolla pines. We presented results for the 2000-2008 period concerning the trend analysis for ambient ozone concentrations and related forest damages, with the Mann and seasonal Kendall tests. Ozone precursor’s emissions from Europe have been reduced over the last 20 years. Decreases in annual averages, median, 25th and 98th percentiles and maxima values were found. The seasonal trend analysis for the high-lying stations showed a decreasing trend for the warm season, when main ozone production is the photochemistry, and an increase for the cold period, caused by a reduced ozone titration. Statistics on Arolla Pine reveal strong correlations between mottling intensity and the high ozone concentrations. Finally, decreases for the ozone concentrations, and associated statistics, AOT40 values and for the mottling intensity on conifers needles were observed.     

Pesticides analysed in rainwater in Alsace region (Eastern France): Comparison between urban and rural sites

Current-used pesticides commonly applied in Alsace region (Eastern France) on diverse crops (maize, vineyard, vegetables, etc.) were analysed, together with Lindane, in rainwater between January 2002 and June 2003 simultaneously on two sites situated in a typical rural (Erstein, France) and urban area (Strasbourg, France).Rainwater samples were collected on a weekly basis by using two automatic wet only collectors associated with an open collector for the measurement of rainwater height.Pesticides were analysed by GC-MSMS and extracted from rainwater by SPME. Two runs were performed. The first one was performed by using a PDMS (100 μm) fibre for pesticides where direct injection into GC is possible (alachlor, atrazine, azinphos-ethyl, azinphos-methyl, captan, chlorfenvinphos, dichlorvos, diflufenican, α- and β-endosulfan, iprodione, lindane, metolachlor, mevinphos, parathion-methyl, phosalone, phosmet, tebuconazole, triadimefon and trifluralin). The second run was performed by using PDMS/DVB fibre and this run concerns pesticides where a preliminary derivatisation step with pentafluorobenzylbromide (PFBBr) is required for very low volatiles (bromoxynil,2,4-MCPA, MCPP and 2,4-D) or thermo labiles (chlorotoluron, diuron and isoproturon) pesticides.Results showed that the more concentrated pesticides detected were those used as herbicides in large quantities in Alsace region for maize crops (alachlor, metolachlor and atrazine). Maximum concentrations for these herbicides have been measured during intensive applications periods on maize crops following by rapid decrease immediately after use.For Alachlor, most important peaks have been observed between 21 and 28 April 2003 (3327 ng L⁻¹ at Erstein and 5590 ng L⁻¹ at Strasbourg). This is also the case for Metolachlor where most important peak was observed during the same week.Concentrations of pesticides measured out of application periods were very low for many pesticides and some others where never detected during this period. This is the case for diflufenican which was detected only during application. Two important peaks of concentrations were observed; a first one (101 ng L⁻¹) in Erstein in November 2002 (4–11 November) and a second one (762 ng L⁻¹) also in Erstein (28 April–15 May).The same behaviour can be seen for chlorfenvinphos and phosalone which have been detected, respectively, 2 and 4 times in Erstein and Strasbourg at high concentrations (28 April 2003–15 May 2003, 187 ng L⁻¹ of phosalone and 157 ng L⁻¹ of chlorfenvinphos in Erstein).MCPP, 2,4 MCPA and 2,4-D have been detected at high concentrations in rainwater but for the other pesticides very episodically and mainly during their use in agriculture. Maximal concentrations of MCPP and 2,4 MCPA have been measured in Erstein between 28 April and 15 May (904 and 746 ng L⁻¹, respectively).Comparison between rural and urban sites showed that concentrations in rural areas are generally higher except for pesticides commonly applied in urban areas like Diuron.No seasonal phenomenon was observed for Diuron. This herbicide has been detected in practically all of the rainwater samples in Strasbourg (40/41) with a maximum of 1025 ng L⁻¹ (16–23 September 2002) in 38 samples on 41 in Erstein with a maximum of 317 ng L⁻¹ (15–23 October 2002). The total concentration of Diuron measured between 4 March 2002 and 20 July 2003 is of 4721 ng L⁻¹ in Strasbourg and 5025 ng L⁻¹ in Erstein. This result shows that wet deposition of Diuron in urban and rural sites was equivalent and can be explained by the “urban use” of this molecule together with its potential persistence.     

Variability of ozone concentration in a montane environment,northern Italy

To evaluate the spatial variability of ozone concentrations, two studies were undertaken in the montane environment of Trentino region, northern Italy, in 2007. In the first study, a 225 km² area was considered. Here, a randomized design was used to evaluate the variability of ozone concentration at 1 and 225 km² scale. Measurements were carried out by passive samplers between May and June 2007. In a second study, the whole 6207 km² area of Trentino was considered. The area is covered by five grid cells of the European Monitoring and Evaluation Programme (EMEP). A systematic 15 × 15 km grid was used to allocate 15 passive samplers over the entire province, resulting into 1–4 samplers for each of the 5 EMEP grid cells (2500 km² each) overlapping the study area. Measurements were carried out between June and September 2007. Accuracy of passive samplers was checked by direct comparison with conventional ozone analysers. Significant differences (P = 0.034) were found in ozone concentration among 1 × 1 km quadrates within the 225 km² study area, while variability within the 1 × 1 km grid cells (coefficient of variation, CV′ = 0.12) slightly exceed the measurement error (CV′ = 0.08). At larger scales (225, 2500 and 6207 km²), ozone concentration shows much higher variability (CV′ from 0.18 to 0.28, with peak values at 0.40). Reported differences lead to very different AOT40 estimates even within the same EMEP grid cell. These findings suggest that 1 × 1 km resolution seems appropriate for ozone concentration modelling. On the other hand, significant sub-grid variation may exist at the resolution adopted by the EMEP model. Coupled with the likely variability of other important meteorological, soil and vegetation variables, our findings suggest that ozone risk assessment for vegetation based on large-scale modelled AOT40 and flux needs to be considered with great caution. The evidence reported in this paper asks for more detailed national-scale modelling, and the development of methods to incorporate local scale variations into large-scale models.     

Stochastic model to forecast ground-level ozone concentration at urban and rural areas

Stochastic models that estimate the ground-level ozone concentrations in air at an urban and rural sampling points in South-eastern Spain have been developed. Studies of temporal series of data, spectral analyses of temporal series and ARIMA models have been used. The ARIMA model (1,0,0) x (1,0,1)24 satisfactorily predicts hourly ozone concentrations in the urban area. The ARIMA (2,1,1) x (0,1,1)24 has been developed for the rural area. In both sampling points, predictions of hourly ozone concentrations agree reasonably well with measured values. However, the prediction of hourly ozone concentrations in the rural point appears to be better than that of the urban point. The performance of ARIMA models suggests that this kind of modelling can be suitable for ozone concentrations forecasting.