Characterizations of chemical oxidants in Mexico City: A regional chemical dynamical model (WRF-Chem) study

The formation of chemical oxidants, particularly ozone, in Mexico City were studied using a newly developed regional chemical/dynamical model (WRF-Chem). The magnitude and timing of simulated diurnal cycles of ozone (O₃), carbon monoxide (CO) and nitrogen oxides (NO_x)_, and the maximum and minimum O₃ concentrations are generally consistent with surface measurements. Our analysis shows that the strong diurnal cycle in O₃ is mainly attributable to photochemical variations, while diurnal cycles of CO and NO_x mainly result from variations of emissions and boundary layer height. In a sensitivity study, oxidation reactions of aromatic hydrocarbons (HCs) and alkenes yield highest peak O₃ production rates (20 and 18 ppbv h⁻¹, respectively). Alkene oxidations, which are generally faster, dominate in early morning. By late morning, alkene concentrations drop, and oxidations of aromatics dominate, with lesser contributions from alkanes and CO. The sensitivity of O₃ concentrations to NO_x and HC emissions was assessed. Our results show that daytime O₃ production is HC-limited in the Mexico City metropolitan area, so that increases in HC emissions increase O₃ chemical production, while increases in NO_x emissions decrease O₃ concentrations. However, increases in both NO_x and HC emissions yield even greater O₃ increases than increases in HCs alone. Uncertainties in HC emissions estimates give large uncertainties in calculated daytime O₃, while NO_x emissions uncertainties are less influential. However, NO_x emissions are important in controlling O₃ at night.     

Aircraft measurements of O3, NOx,CO, VOCs,and SO2 in the Yangtze River Delta region

In this study, air pollutants, including ozone (O₃), nitrogen oxides (NO_x = NO + NO₂), carbon monoxides (CO), sulfur dioxide (SO₂), and volatile organic compounds (VOCs) measured in the Yangtze River Delta (YRD) region during several air flights between September/30 and October/11 are analyzed. This measurement provides horizontal and vertical distributions of air pollutants in the YRD region. The analysis of the result shows that the measured O₃ concentrations range from 20 to 60 ppbv. These values are generally below the US national standard (84 ppbv), suggesting that at the present, the O₃ pollutions are modest in this region. The NO_x concentrations have strong spatial and temporal variations, ranging from 3 to 40 ppbv. The SO₂ concentrations also have large spatial and temporal variations, ranging from 1 to 35 ppbv. The high concentrations of CO are measured with small variations, ranging from 3 to 7 ppmv. The concentrations of VOCs are relatively low, with the total VOC concentrations of less than 6 ppbv. The relative small VOC concentrations and the relative large NO_x concentrations suggest that the O₃ chemical formation is under a strong VOC-limited regime in the YRD region. The measured O₃ and NO_x concentrations are strongly anti-correlated, indicating that enhancement in NO_x concentrations leads to decrease in O₃ concentrations. Moreover, the O₃ concentrations are more sensitive to NO_x concentrations in the rural region than in the city region. The ratios of Δ[O₃]/Δ[NO_x] are ?2.3 and ?0.25 in the rural and in the city region, respectively. In addition, the measured NO_x and SO₂ concentrations are strongly correlated, highlighting that the NO_x and SO₂ are probably originated from same emission sources. Because SO₂ emissions are significantly originated from coal burnings, the strong correlation between SO₂ and NO_x concentrations suggests that the NO_x emission sources are mostly from coal burned sources. As a result, the future automobile increases could lead to rapid enhancements in O₃ concentrations in the YRD region.     

Influence of nocturnal vertical stability on daytime chemistry: A one-dimensional model study

Nocturnal chemistry can play an important role in determining the initial morning conditions for daytime chemistry in urban areas. However, the impact on daytime O₃ levels is difficult to assess as the suppression of vertical trace gas transport leads to highly altitude dependent nocturnal chemistry, in particular with respect to the removal and conversion of nitrogen oxides (NO_x) and volatile organic compounds (VOC). One-dimensional (1-D) chemical transport model calculations for different nighttime vertical stabilities and different ozone formation regimes (i.e. NO_x- vs. VOC-sensitive) were performed assuming a 1000 m high daytime boundary layer and a growing nocturnal boundary layer reaching 200 m height at the end of the night. Exclusion of NO₃ chemistry from the model leads to daytime O₃ concentration changes from ?4% to +16% for different O₃ sensitivities. In all cases strong nocturnal vertical concentration profiles of NO_x, O₃, NO₃ and N₂O₅ and a dependence of these profiles on vertical stability were found at night. The nocturnal NO_x loss averaged over the lowest 1000 m changes by 9–24% for different vertical stabilities and ozone sensitivities. The impact of nocturnal vertical stability leads to 7–12% difference in O₃ concentration in the morning and ～0–2.5% in the afternoon.     

Analysis of ozone and VOCs measured in Shanghai: A case study

Shanghai Meteorological Administration has established a volatile organic compounds (VOCs) laboratory and an observational network for VOCs and ozone (O₃) measurements in the city of Shanghai. In this study, the measured VOCs and O₃ concentrations from 15 November (15-Nov) to 26 November (26-Nov) of 2005 in Shanghai show that there are strong day-to-day and diurnal variations. The measured O₃ and VOCs concentrations have very different characterizations between the two periods. During 15-Nov to 21-Nov (defined as the first period), VOCs and O₃ concentrations are lower than the values during 22-Nov to 28-Nov (defined as the second period). There is a strong diurnal variation of O₃ during the second period with maximum concentrations of 40–80 ppbv at noontime, and minimum concentrations at nighttime. By contrast, during the first period, the diurnal variation of O₃ is in an irregular pattern with maximum concentrations of only 20–30 ppbv. The VOC concentrations change rapidly from 30–50 ppbv during the first period to 80–100 ppbv during the second period. Two chemical models are applied to interpret the measurements. One model is a regional chemical/dynamical model (WRF-Chem) and another is a detailed chemical mechanism model (NCAR MM). Model analysis shows that the meteorological conditions are very different between the two periods, and are mainly responsible for the different chemical characterizations of O₃ and VOCs between the two periods. During the first period, meteorological conditions are characterized by cloudy sky and high-surface winds in Shanghai, resulting in a higher nighttime planetary boundary layer (PBL) and faster transport of air pollutants. By contrast, during the second period, the meteorological conditions are characterized by clear sky and weak surface winds, resulting in a lower nighttime PBL and slower transport of air pollutants. The chemical mechanism model calculation shows that different VOC species has very different contributions to the high-ozone concentrations during the second period. Alkane (40 ppbv) and aromatic (30 ppbv) are among the highest VOC concentrations observed in Shanghai. The analysis suggests that the aromatic is a main contributor for the O₃ chemical production in Shanghai, with approximately 79% of the O₃ being produced by aromatic. This analysis implies that future increase in VOC (especially aromatic) emissions could lead to significant increase in O₃ concentrations in Shanghai.     

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.     

Long-term observation of nitrate radicals in the continental boundary layer near Berlin

Long-term observations of the nitrate radical concentration and supporting parameters in the continental boundary layer at the rural site Lindenberg near Berlin, Germany, were performed using differential optical absorption spectroscopy (DOAS). Average nighttime NO₃ levels were 4.6 ppt, while NO₃ steady-state lifetimes (calculated from the NO₂–O₃ product and the NO₃ concentration) varied between 5 s and 615 s with an average of 92 s. The long-term observations offered the possibility to study the importance of NO₃ for the oxidation of VOCs (volatile organic compounds) and its contribution in the non-photochemical removal of NO_x from the atmosphere in different seasons. Analysis of the data showed, that NO₃ was depleted by both, reactions with VOCs and indirectly by loss of N₂O₅ on aerosol surfaces. A clear seasonal variation of the sink distribution was found. The VOC sink dominated during summer while indirect loss was of major importance during the winter months. The results are compared with former long-term campaigns of NO₃ in the marine boundary layer.     

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%.     

Multi-year hourly PM2.5 carbon measurements in New York: Diurnal,day of week and seasonal patterns

Multi-year hourly measurements of PM_2.5 elemental carbon (EC) and organic carbon (OC) from a site in the South Bronx, New York were used to examine diurnal, day of week and seasonal patterns. The hourly carbon measurements also provided temporally resolved information on sporadic EC spikes observed predominantly in winter. Furthermore, hourly EC and OC data were used to provide information on secondary organic aerosol formation. Average monthly EC concentrations ranged from 0.5 to 1.4 μg m^?3 with peak hourly values of several μg m^?3 typically observed from November to March. Mean EC concentrations were lower on weekends (approximately 27% lower on Saturday and 38% lower on Sunday) than on weekdays (Monday to Friday). The weekday/weekend difference was more pronounced during summer months and less noticeable during winter. Throughout the year EC exhibited a similar diurnal pattern to NO_x showing a pronounced peak during the morning commute period (7–10 AM EST). These patterns suggest that EC was impacted by local mobile emissions and in addition by emissions from space heating sources during winter months. Although EC was highly correlated with black carbon (BC) there was a pronounced seasonal BC/EC gradient with summer BC concentrations approximately a factor of 2 higher than EC. Average monthly OC concentrations ranged from 1.0 to 4.1 μg m^?3 with maximum hourly concentrations of 7–11 μg m^?3 predominantly in summer or winter months. OC concentrations generally correlated with PM_2.5 total mass and aerosol sulfate and with NO_x during winter months. OC showed no particular day of week pattern. The OC diurnal pattern was typically different than EC except in winter when OC tracked EC and NO_x indicating local primary emissions contributed significantly to OC during winter at the urban location. On average secondary organic aerosol was estimated to account for 40–50% of OC during winter and up to 63–73% during summer months.     

The sensitivity of the CHIMERE model to emissions reduction scenarios on air quality in Northern Italy

The sensitivity of the CHIMERE model to emission reduction scenarios on particulate matter PM2.5 and ozone (O₃) in Northern Italy is studied. The emissions of NO_x, PM2.5 SO₂, VOC or NH₃ were reduced by 50% for different source sectors for the Lombardy region, together with 5 additional scenarios to estimate the effect of local measures on improving the air quality for the Po valley area. Firstly, we evaluate the model performance by comparing calculated surface aerosol concentrations for the standard case (no emission reductions) with observations for January and June 2005. Calculated monthly mean PM10 concentrations are in general underestimated. For June, modelled PM10 concentrations slightly overestimate the measurements. Calculated monthly mean SO₄, NO₃^?, NH₄⁺ concentrations are in good agreement with the observations for January and June. Secondly, the model sensitivity of emission reduction scenarios on PM2.5 and O₃ calculated concentrations for the Po valley area is evaluated. The most effective scenarios to abate PM2.5 concentration are based on the SNAP2 (non-industrial combustion plants) and SNAP7 (road traffic) sectors, for which the NO_x and PM2.5 emissions are reduced by 50%. The number of days that the 2015 PM2.5 limit value of 25 μg m^?3 in Milan is exceeded by reducing primary PM2.5 and NO_x emissions for SNAP2 and 7 by 50%, does not change in January when compared to the standard case for the Milan area. It appears that 40% of the PM2.5 concentration in the greater Milan area is caused by the emissions surrounding the Lombardy region and from the model boundary conditions.This study also showed that a more effective pollutant reduction (emissions) per ton of pollutant reduced (concentrations) for the greater Milan area is obtained by reducing the primary PM2.5 emissions for SNAP7 by 50%. The most effective scenario on PM2.5 decrease for which precursor emissions are reduced is achieved by reducing SO₂ emissions by 50% for SNAP7.Our study showed that during summer time, the largest reductions in O₃ concentrations are achieved for SNAP7 emission reductions, when volatile organic compounds (VOCs) are reduced by 50%.     

Assessment of the Ozone-Nitrogen Oxide-Volatile Organic Compound Sensitivity of Mexico City through an Indicator-Based Approach: Measurements and Numerical Simulations Comparison

Abstract

The ozone (O₃) sensitivity to nitrogen oxides (NO_x, or nitric oxide [NO] + nitrogen dioxide [NO₂]) versus volatile organic compounds (VOCs) in the Mexico City metropolitan area (MCMA) is a current issue of scientific controversy. To shed light on this issue, we compared measurements of the indicator species O₃/NO_y (where NO_y represents the sum of NO + NO₂ + nitric acid [HNO₃] + peroxyacetyl nitrate [PAN] + others), NO_y, and the semiempirically derived O₃/NO_{z surrogate} (where NO_{z surrogate} is the derived surrogate NO_z, and NO_z represents NO_x reaction products, or NO_y – NO_x) with results of numerical predictions reproducing the transition regimes between NO_x and VOC sensitivities. Ambient air concentrations of O₃, NO_x, and NO_y were measured from April 14 to 25, 2004 in one downwind receptor site of photo-chemically aged air masses within Mexico City. MCMA-derived transition values for an episode day occurring during the same monitoring period were obtained through a series of photochemical simulations using the Multiscale Climate and Chemistry Model (MCCM). The comparison between the measured indicator species and the simulated spatial distribution of the indicators O₃/NO_y, O₃/NO_{z surrogate}, and NO_y in MCMA suggest that O₃ in this megacity is likely VOC-sensitive. This is in opposition to past studies that, on the basis of the observed morning VOC/NO_x ratios, have concluded that O₃ in Mexico City is NO_x-sensitive. Simulated MCMA-derived sensitive transition values for O₃/NO_y, hydrogen peroxide (H₂O₂)/HNO₃, and NO_y were found to be in agreement with threshold criteria proposed for other regions in North America and Europe, although the transition crossover for O₃/NO_z and O₃/HNO₃ was not consistent with values reported elsewhere. An additional empirical evaluation of weekend/weekday differences in average maximum O₃ concentrations and 6:00- to 9:00-a.m. NO_x and NO levels registered at the same site in April 2004 indirectly confirmed the above results. A preliminary conclusion is that additional reductions in NO_x emissions in MCMA might cause an increase in presently high O₃ levels.     

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.     

Modelling NOx from lightning and its impact on global chemical fields

We have used a three-dimensional off-line chemical transport model (CTM) to assess the impact of lightning emissions in the free troposphere both on NO_x itself and on other chemical species such as O₃ and OH. We have investigated these effects using two lightning emission scenarios. In the first, lightning emissions are coupled in space and time to the convective cloud top height calculated every 6 h by the CTM's moist convection scheme. In the second, lightning emissions are calculated as a constant, monthly mean field. The model's performance against observed profiles of NO_x and O₃ in the Atlantic and Pacific ocean improves significantly when lightning emissions are included. With the inclusion of these emissions, the CTM produces a significant increase in the NO_x concentrations in the upper troposphere, where the NO_x lifetime is long, and a smaller increase in the lower free troposphere, where the surface NO_x sources dominate. These changes cause a significant increase in the O₃ production in the upper troposphere and hence higher calculated O₃ there. The model indicates that lightning emissions cause local increases of over 50 parts per 10¹² by volume (pptv) in NO_x, 200 pptv in HNO₃ and 20 parts per 10⁹ by volume (ppbv) (>40%) in O₃. In addition, a smaller increase of O₃ in the lower troposphere occurs due to an increase in the downward transport of O₃. The O₃ change is accompanied by an increase in OH which is more pronounced in the upper troposphere with a corresponding reduction in CO. The method of emission employed in the model does not appear to have a significant effect globally. In the upper troposphere (above about 300 hPa) NO_x concentrations are generally lower with monthly mean emissions, because of the de-coupling of emissions from the model's convection scheme, which vents NO_x aloft more efficiently in the coupled scheme. Below the local convective outflow altitude, NO_x concentrations are larger when using the monthly mean emissions than when coupled to the convection scheme, because the more dilute emissions, and nighttime emissions, lead to a slower NO_x destruction rate. Only minor changes are predicted in the monthly average fields of O₃ if we emit lightning as a monthly constant field. However, the method of emission becomes important when we make a direct comparison of model results with time varying data. These differences should be taken into account when a direct comparison of O₃ with measurements collected at particular times and locations is attempted.     

Air quality measurements in urban green areas – a case study

The influence of traffic-induced pollutants (e.g. CO, NO, NO₂ and O₃) on the air quality of urban areas was investigated in the city of Essen, North Rhine-Westphalia (NRW), Germany. Twelve air hygiene profile measuring trips were made to analyse the trace gas distribution in the urban area with high spatial resolution and to compare the air hygiene situation of urban green areas with the overall situation of urban pollution. Seventeen measurements were made to determine the diurnal concentration courses within urban parks (summer conditions: 13 measurements, 530 30 min mean values, winter conditions: 4 measurements, 128 30 min mean values). The measurements were carried out during mainly calm wind and cloudless conditions between February 1995 and March 1996. It was possible to establish highly differentiated spatial concentration patterns within the urban area. These patterns were correlated with five general types of land use (motorway, main road, secondary road, residential area, green area) which were influenced to varying degrees by traffic emissions. Urban parks downwind from the main emission sources show the following typical temporal concentration courses: In summer rush-hour-dependent CO, NO and NO₂ maxima only occurred in the morning. A high NO₂/NO ratio was established during weather conditions with high global radiation intensities (K>800 W m⁻²), which may result in a high O₃ formation potential. Some of the values measured found in one of the parks investigated (Gruga Park, Essen, area: 0.7 km²), which were as high as 275 μg m⁻³ O₃ (30-min mean value) were significantly higher than the German air quality standard of 120 μg m⁻³ (30-min mean value, VDI Guideline 2310, 1996) which currently applies in Germany and about 20% above the maximum values measured on the same day by the network of the North Rhine–Westphalian State Environment Agency. In winter high CO and NO concentrations occur in the morning and during the afternoon rush-hour. The highest concentrations (CO=4.3 mg m⁻³, NO=368 μg m⁻³, 30-min mean values) coincide with the increase in the evening inversion. The maximum measured values for CO, NO and NO₂ do not, however, exceed the German air quality standards in winter and summer.     

Verification of a source-oriented externally mixed air quality model during a severe photochemical smog episode

The CIT/UCD three-dimensional source-oriented externally mixed air quality model is tested during a severe photochemical smog episode (Los Angeles, 7–9 September 1993) using two different chemical mechanisms that describe the formation of ozone and secondary reaction products. The first chemical mechanism is the secondary organic aerosol mechanism (SOAM) that is based on SAPRC90 with extensions to describe the formation of condensable organic products. The second chemical mechanism is the caltech atmospheric chemistry mechanism (CACM) that is based on SAPRC99 with more detailed treatment of organic oxidation products.The predicted ozone concentrations from the CIT/UCD/SOAM and the CIT/UCD/CACM models agree well with the observations made at most monitoring sites with a mean normalized error of approximately 0.4–0.5. Good agreement is generally found between the predicted and measured NO_x concentrations except during morning rush hours of 6–10 am when NO_x concentrations are under-predicted at most locations. Total VOC concentrations predicted by the two chemical mechanisms agree reasonably well with the observations at three of the four sites where measurements were made. Gas-phase concentrations of phenolic compounds and benzaldehyde predicted by the UCD/CIT/CACM model are higher than the measured concentrations whereas the predicted concentrations of other aromatic compounds approximately agree with the measured values.The fine airborne particulate matter mass concentrations (PM_2.5) predicted by the UCD/CIT/SOAM and UCD/CIT/CACM models are slightly greater than the observed values during evening hours and lower than observed values during morning rush hours. The evening over-predictions are driven by an excess of nitrate, ammonium ion and sulfate. The UCD/CIT/CACM model predicts higher nighttime concentrations of gaseous precursors leading to the formation of particulate nitrate than the UCD/CIT/SOAM model. Elemental carbon and total organic mass are under-predicted by both models during morning rush hour periods. When this latter finding is combined with the NO_x under-predictions that occur at the same time, it suggests a systematic bias in the diesel engine emissions inventory. The mass of particulate total organic carbon is under-predicted by both the UCD/CIT/SOAM and UCD/CIT/CACM models during afternoon hours. Elemental carbon concentrations generally agree with the observations at this time. Both the UCD/CIT/SOAM and UCD/CIT/CACM models predict low concentrations of secondary organic aerosol (SOA) (<3.5 μg m⁻³) indicating that both models could be missing SOA formation pathways. The representation of the aerosol as an internal mixture vs. a source-oriented external mixture did not significantly affect the predicted concentrations during the current study.     

Modeling an air pollution episode in northwestern United States: Identifying the effect of nitrogen oxide and volatile organic compound emission changes on air pollutants formation using direct sensitivity analysis

Air quality impacts of volatile organic compound (VOC) and nitrogen oxide (NO_x) emissions from major sources over the northwestern United States are simulated. The comprehensive nested modeling system comprises three models: Community Multiscale Air Quality (CMAQ), Weather Research and Forecasting (WRF), and Sparse Matrix Operator Kernel Emissions (SMOKE). In addition, the decoupled direct method in three dimensions (DDM-3D) is used to determine the sensitivities of pollutant concentrations to changes in precursor emissions during a severe smog episode in July of 2006. The average simulated 8-hr daily maximum O₃ concentration is 48.9 ppb, with 1-hr O₃ maxima up to 106 ppb (40 km southeast of Seattle). The average simulated PM_2.5 (particulate matter with an aerodynamic diameter <2.5 μm) concentration at the measurement sites is 9.06 μg m^?3, which is in good agreement with the observed concentration (8.06 μg m^?3). In urban areas (i.e., Seattle, Vancouver, etc.), the model predicts that, on average, a reduction of NO_x emissions is simulated to lead to an increase in average 8-hr daily maximum O₃ concentrations, and will be most prominent in Seattle (where the greatest sensitivity is??0.2 ppb per % change of mobile sources). On the other hand, decreasing NO_x emissions is simulated to decrease the 8-hr maximum O₃ concentrations in remote and forested areas. Decreased NO_x emissions are simulated to slightly increase PM_2.5 in major urban areas. In urban areas, a decrease in VOC emissions will result in a decrease of 8-hr maximum O₃ concentrations. The impact of decreased VOC emissions from biogenic, mobile, nonroad, and area sources on average 8-hr daily maximum O₃ concentrations is up to 0.05 ppb decrease per % of emission change, each. Decreased emissions of VOCs decrease average PM_2.5 concentrations in the entire modeling domain. In major cities, PM_2.5 concentrations are more sensitive to emissions of VOCs from biogenic sources than other sources of VOCs. These results can be used to interpret the effectiveness of VOC or NO_x controls over pollutant concentrations, especially for localities that may exceed National Ambient Air Quality Standards (NAAQS).

Implications: The effect of NO_x and VOC controls on ozone and PM_2.5 concentrations in the northwestern United States is examined using the decoupled direct method in three dimensions (DDM-3D) in a state-of-the-art three-dimensional chemical transport model (CMAQ). NO_x controls are predicted to increase PM_2.5 and ozone in major urban areas and decrease ozone in more remote and forested areas. VOC reductions are helpful in reducing ozone and PM_2.5 concentrations in urban areas. Biogenic VOC sources have the largest impact on O₃ and PM_2.5 concentrations.     

Chemical mass balance receptor model applied to ambient C2–C9 VOC concentration in Seoul,Korea: Effect of chemical reaction losses

A chemical mass balance (CMB) receptor model was used for estimating the diurnal contributions of VOC emission sources to the ambient C₂–C₉ VOC concentration in Seoul, Korea. For this purpose, the VOC concentrations were measured in the morning, the afternoon, and the evening. The samples were collected using a 2-h integrated SUMMA canister. The source profiles were developed for the CMB calculation in the Seoul area. To investigate the effect of the chemical reaction loss of VOCs on the CMB calculation, the modified model employing a decay factor and the standard model that considers no loss were compared. The modified model estimated that the vehicle exhaust (52%) was the largest leading source of VOCs in the Seoul atmosphere, followed by the use of solvents (26%), gasoline evaporation (15%), the use of liquefied petroleum gas (LPG) (5%), and the use of liquefied natural gas (LNG) (2%). Relative source contribution for vehicle exhaust showed a clear diurnal variation with a high in the morning and evening and a low in the afternoon, while the contribution of evaporative emissions (gasoline evaporation and solvent usage) showed a different diurnal pattern from that of the vehicle exhaust, exhibiting a high in the afternoon and evening and a low in the morning. It was found that the difference between the total source contribution (μg m⁻³) estimated from these two models was not statistically significant. However, when the paired-sample t-test is applied to the individual sources, a significant difference was found for the vehicle exhaust and the solvent use. In addition, the modified model brought forth a better performance with high R² and low χ² as compared to those obtained from the standard model in the CMB calculation. The vehicle exhaust and solvent use were estimated to be the largest and the second largest contributors to ambient benzene as well as ozone formation potential (OFP), respectively. Based on above results we believe that incorporating the reaction loss in the CMB calculations helps to better fit the source profile to the ambient VOC concentrations. However, the reaction loss does not significantly affect the estimation of source contributions.     

Biogenic VOC emissions from fresh leaf mulch and wood chips of Grevillea robusta (Australian Silky Oak)

The emissions of VOC from freshly cut and shredded Grevillea robusta (Australian Silky Oak) leaves and wood have been measured. The VOC emissions from fresh leaf mulch and wood chips lasted typically for 30 and 20 h respectively, and consisted primarily of ethanol, (E)-2-hexenal, (Z)-3-hexen-1-ol and acetaldehyde. The integrated emissions of the VOCs were 0.38±0.04 g kg⁻¹ from leaf mulch, and 0.022±0.003 g kg⁻¹ from wood chips. These emissions represent a source of VOCs in urban and rural air that has previously been unquantified and is currently unaccounted for. These VOCs from leaf mulch and wood chips will contribute to both urban photochemistry and secondary organic aerosol formation. Any CH₄ emissions from leaf mulch and wood chips were <1×10⁻¹¹ g g dry mass⁻¹ s⁻¹.     

Measurement and analysis of atmospheric concentrations of isoprene and its reaction products in central Texas

A field experiment was conducted in August 1998 to investigate the concentrations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level concentrations of isoprene ranged from 0.3 (lower limit of detection – LLD) to 10.2 ppbv in rural regions and from 0.3 to 6.0 ppbv in the Austin urban area. Rural ambient formaldehyde levels ranged from 0.4 ppbv (LLD) to 20.0 ppbv for 160 rural samples collected, while the observed range was smaller at Austin (0.4–3.4 ppbv) for a smaller set of samples (37 urban samples collected). Methacrolein levels did not vary as widely, with rural measurements from 0.1 ppbv (LLD) to 3.7 ppbv and urban concentrations varying between 0.2 and 5.7 ppbv. Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agreement with emission estimates based on local ground cover data. Ozone formation attributable to biogenic hydrocarbon oxidation was also calculated. The calculations indicated that if the ozone formation occurred at low VOC/NO_x ratios, up to 20 ppbv of ozone formed could be attributable to biogenic photooxidation. In contrast, if the biogenic hydrocarbon reaction products were formed under low NO_x conditions, ozone production attributable to biogenics oxidation would be as low as 1 ppbv. This variability in ozone formation potentials implies that biogenic emissions in rural areas will not lead to peak ozone levels in the absence of transport of NO_x from urban centers or large rural NO_x sources.     

Application of a Lagrangian particle model to assess the impact of harbour,industrial and urban activities on air quality in the Taranto area,Italy

This paper evaluates the relative impact on air quality of harbour emissions, with respect to other emission sources located in the same area. The impact assessment study was conducted in the city of Taranto, Italy. This area was considered as representative of a typical Mediterranean harbour region, where shipping, industries and urban activities co-exist at a short distance, producing an ideal case to study the interaction among these different sources. Chemical and meteorological field campaigns were carried out to provide data to this study. An emission inventory has been developed taking into account industrial sources, traffic, domestic heating, fugitive and harbour emissions. A 3D Lagrangian particle dispersion model (SPRAY) has then been applied to the study area using reconstructed meteorological fields calculated by the diagnostic meteorological model MINERVE. 3D short term hourly concentrations have been computed for both all and specific sources. Industrial activities are found to be the main contributor to SO₂. Industry and traffic emissions are mainly responsible for NO_x simulated concentrations. CO concentrations are found to be mainly related to traffic emissions, while primary PM₁₀ simulated concentrations tend to be linked to industrial and fugitive emissions. Contributions of harbour activities to the seasonal average concentrations of SO₂ and NO_x are predicted to be up to 5 and 30 μg m⁻³, respectively to be compared to a overall peak values of 60 μg m⁻³ for SO₂ and 70 μg m⁻³ for NO_x. At selected urban monitoring stations, SO₂ and NO_x average source contributions are predicted to be both of about 9% from harbour activities, while 87% and 41% respectively of total concentrations are predicted to be of industrial origin.