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Air quality simulations are performed to determine the impact of changes in future climate and emissions on regional air quality in the South Coast Air Basin (SoCAB) of California. The perturbation parameters considered in this study include (1) temperature, (2) absolute humidity, (3) biogenic VOC emissions due to temperature changes, and (4) boundary conditions. All parameters are first perturbed individually. In addition, the impact of simultaneously perturbing more than one parameter is analyzed. Air quality is simulated with meteorology representative of a summertime ozone pollution episode using both a baseline 2005 emissions inventory and a future emissions projection for the year 2023. Different locations within the modeling domain exhibit varying degrees of sensitivity to the perturbations considered. Afternoon domain wide average ozone concentrations are projected to increase by 13–18% as a result of changes in future climate and emissions. Afternoon increases at individual locations range from 10 to 36%. The change in afternoon particulate matter (PM) levels is a strong function of location in the basin, ranging from −7.1% to +4.7% when using 2005 emissions and −8.6% to +1.7% when using 2023 emissions. Afternoon secondary organic aerosol (SOA) concentrations for the entire domain are projected to decrease by over 15%, and the change in SOA levels is not a strong function of the emissions inventory utilized. Temperature increases play the dominant role in determining the overall impact on ozone, PM, and SOA concentrations in both the individual and combined perturbation scenarios. [ABSTRACT FROM AUTHOR]

Impact of emissions from ocean-going ships on ozone and particulate matter concentrations is quantified using UCI-CIT model for the South Coast Air Basin of California (SoCAB). The modeling domain encompasses Los Angeles and Long Beach ports and part of the Pacific Ocean that is traversed by ships to visit these ports. Impacts are assessed for a base year (2002) and a future year (2020) by analyzing results from simulations of a three-day summer episode. Contribution of ship emissions to peak 1-h and 8-h ozone concentrations is predicted to be up to 29 and 24ppb, respectively, for the year 2002. Similarly, particulate nitrate and sulfate concentrations increase up to 12.8 and 1.7μgm−3, respectively, in the basin when ship emissions are included. Maximum impacts are predicted to occur along the coasts of Ventura and Los Angeles and also at inland locations near Simi Valley. Future year simulations show substantial increase in impacts from ships due to expected growth in ship emissions. Ozone increases are as high as 59ppb for land-based locations when estimates of ship emissions for 2020 are included. Similarly, particulate nitrate and sulfate increase up to 14 and 2.5μgm−3. The results of this study show that control of ship emissions is important to mitigate air pollution. [Copyright &y& Elsevier]

This paper examines the errors introduced by operator splitting techniques in air quality models. Results are presented for different time steps used in the splitting schemes as well as for different ordering in which the operators are computed. Furthermore, a non-splitting technique is developed to analyze the performance of operator splitting techniques in air quality models. Convergence rates of operator splitting schemes are determined. Research indicates splitting techniques provides at most linear convergence. For fast-reacting species like N2O5, the convergence is not achieved when using splitting methods and time steps as small as 10 s. Symmetric and non-symmetric operator splitting does not provide significant difference in accuracy. Furthermore, operator splitting ordering with stiff operators computed last does not produce better results than with non-stiff operators computed last. The non-splitting method developed achieves convergence by reducing time steps, adapting time steps to insure convergence, and eliminating operator splitting. [Copyright &y& Elsevier]

Air quality models consider the formation and deposition of nitric acid (HNO[sub 3]) on surfaces to be an irreversible sink of atmospheric nitrogen oxides (NO[sub x]) and therefore an effective termination step in the ozone formation cycle. However, experimental evidence suggests that the reaction of gaseous nitric oxide with nitric acid on surfaces may convert HNO[sub 3] to photochemically active NO[sub x]. A first-order simulation of this surface-mediated renoxification process is performed using an air quality model of the South Coast Air Basin of California. Peak ozone concentrations are predicted closer to observed values in regions regularly underpredicted by base case models. In certain regions, ozone predictions are enhanced by as much as ∼ 30 ppb or ∼ 20% compared to the baseline simulation. These results suggest that renoxification processes may be a key to resolving long-standing shortcomings of air quality models, in addition to reconciling [HNO[sub 3]]/[NO[sub x]] ratios in remote regions. This study also illustrates that the surface terrain may play a more active chemical role than hitherto considered in air quality models. [ABSTRACT FROM AUTHOR]

This study assesses the potential impacts on emissions and air quality from the increased adoption of natural gas-fired distributed generation of electricity (DG), including displacement of power from central power generation, in the contiguous United States. The study includes four major tasks: (1) modeling of distributed generation market penetration; (2) modeling of central power generation systems; (3) modeling of spatially and temporally resolved emissions; and (4) photochemical grid modeling to evaluate the potential air quality impacts of increased DG penetration, which includes both power-only DG and combined heat and power (CHP) units, for 2030. Low and high DG penetration scenarios estimate the largest penetration of future DG units in three regions – New England, New York, and California. Projections of DG penetration in the contiguous United States estimate 6.3 GW and 24 GW of market adoption in 2030 for the low DG penetration and high DG penetration scenarios, respectively. High DG penetration (all of which is natural gas-fired) serves to offset 8 GW of new natural gas combined cycle (NGCC) units, and 19 GW of solar photovoltaic (PV) installations by 2030. In all scenarios, air quality in the central United States and the northwest remains unaffected as there is little to no DG penetration in those states. California and several states in the northeast are the most impacted by emissions from DG units. Peak increases in maximum daily 8-h average ozone concentrations exceed 5 ppb, which may impede attainment of ambient air quality standards. Overall, air quality impacts from DG vary greatly based on meteorological conditions, proximity to emissions sources, the number and type of DG installations, and the emissions factors used for DG units. [ABSTRACT FROM AUTHOR]

Highlights: [•] A statistical inference method is developed for natural gas burner emissions data. [•] The method is built to compensate for the typically small sample size. [•] The method integrates multiple measures of quantified goodness of fit. [•] The method provides a means to evaluate and report confidence of the result. [•] Estimates are developed for changes in emissions as functions of Wobbe Number. [ABSTRACT FROM AUTHOR]

Abstract: Three different mass-transfer expressions are employed within the Model of Aerosol, Gas, and Interfacial Chemistry (MAGIC) to study gas-phase molecular chlorine and bromine production from NaCl and NaBr aerosols, respectively. Simulations of chamber experiments are performed in which NaCl aerosols react with gas-phase ozone in the presence of UV light, in order to identify the importance of the Knudsen number and mass-transfer expression in systems with varying contributions from gas-phase, aqueous-phase, and interfacial chemistry. In the case of NaBr aerosols, simulations are performed of both dark and photolytic conditions. A range of Knudsen numbers spanning the continuum, transition and free-molecular regimes is studied. Particle size is varied over three orders of magnitude, and particle concentration is changed to keep either (a) total aerosol volume or (b) total aerosol surface area constant. When total aerosol volume is constant, the total amount of surface area available for interfacial reaction increases linearly with Knudsen number. Consequently peak gas-phase Cl2 and Br2 concentrations increase by two orders of magnitude from the continuum regime to the free-molecular regime. When total aerosol surface area is constant, total aerosol volume is inversely proportional to Knudsen number, with lesser volume being available at higher Knudsen numbers. Consequently Cl− depletion in the kinetic regime leads to most gas-phase Cl2 being produced in the transition regime. Gas-phase Br2 concentration trends are determined by aqueous-phase reaction mechanisms, leading to a monotonic decrease in production with Knudsen number. At all Knudsen numbers, more gas-phase bromine is produced in the photolytic case than in the dark case, the difference being significant in the transition regime. Results of this study suggest that halogen production is insensitive to the mass-transfer expression used in the simulations. [Copyright &y& Elsevier]

Abstract: Air quality models compute the transformation of species in the atmosphere undergoing chemical and physical changes. The numerical algorithms used to predict these transformations should obey mass conservation and positive definiteness properties. Among all physical phenomena, the chemical kinetics solver provides the greatest challenge to attain these two properties. In general, most chemical kinetics solvers are mass conservative but not positive definite. In this article, a new numerical algorithm for the computation of chemical kinetics is presented. The integrator is called Split Single Reaction Integrator (SSRI). It is both mass conservative and positive definite. It solves each chemical reaction exactly and uses operator splitting techniques (symmetric split) to combine them into the entire system. The method can be used within a host integrator to fix the negative concentrations while preserving the mass, or it can be used as a standalone integrator that guarantees positive definiteness and mass conservation. Numerical results show that the new integrator, used as a standalone integrator, is second order accurate and stable under large fixed time steps when other conventional integrators are unstable. [Copyright &y& Elsevier]

Photochemical mechanisms are a critical module of air quality models. In the past 20 years, several mechanisms have been developed to study the chemistry of the troposphere. This work compares several state-of-the-science photochemical mechanisms (including LCC, CBM-IV, RADM2, EMEP, RACM, SAPRC99, and CACM which have never been compared before in other studies). Concise descriptions of the chemical schemes are included. The main difference among existing mechanisms is the lumping technique used to classify organic compounds into surrogate groups. First, box model calculations are conducted to highlight the features that lead to differences in the modeled behaviors of photochemical pollutants and their precursors. Results indicate that most chemical schemes yield similar ozone concentrations. Nevertheless, there are significant discrepancies, mainly in predicted concentration of HNO3, HO2 and total PAN among model simulations. Finally, the sources of the discrepancy are identified. [Copyright &y& Elsevier]

Recent literature has shown that volatile organic compound (VOC) emission inventories for urban regions may be substantially underestimated. In particular, non-transportation sources including volatile chemical products (VCP) are increasing in relative importance due to both the current and historical focus on controlling transportation emissions. These findings have major implications for photo-chemical air quality modeling used to determine appropriate and effective regulatory controls to meet limits for primary and secondary pollutants. Using a regional air quality model, we quantify the changes in ozone and fine particulate matter (PM 2.5) simulated for updated VOC emissions reported in the recent literature relative to a baseline inventory for California. Results show that simulated maximum 8-hr ozone concentrations could increase by 17.4 ppb in summer and by 15.6 ppb in winter, and the 24-hr maximum PM 2.5 could increase by 7.8 μg/m3 in winter. Impacts reflect differences in the spatial location of VCP source emissions relative to those for transportation. However, compared to measurement data, model performance is not substantially improved by the adjustment of VOC emissions of current sources. In brief, augmented VOC emission inventories impact simulated concentrations of pollutants, but may not improve the performance of models used for the design of emission control policy without more refined representation of missing VCPs sources in the inventory. Image 1 • CMAQ sensitivities are quantified for potential underestimation of VOC emissions. • Underestimation of VOC emissions has substantial impact on air quality prediction. • Impacts on model performance are evaluated for enhanced VOC emissions. [ABSTRACT FROM AUTHOR]

Abstract In this study we analyze the impact of major drivers of future air quality, both separately and simultaneously, for the year 2035 in three major California air basins: the South Coast Air Basin (SoCAB), the San Francisco Bay Area (SFBA), and the San Joaquin Valley (SJV). A variety of scenarios are considered based on changes in climate-driven meteorological conditions and both biogenic and anthropogenic emissions. Anthropogenic emissions are based on (1) the California Air Resources Board (CARB) California Emissions Projection Analysis Model (CEPAM), (2) increases in electric sector emissions due to climate change, and (3) aggressive adoption of alternative energy technologies electrification of end-use technologies, and energy efficiency measures. Results indicate that climate-driven changes in meteorological conditions will significantly alter day-to-day variations in future ozone and PM 2.5 concentrations, likely increasing the frequency and severity of pollution periods in regions that already experience poor air quality and increasing health risks from pollutant exposure. Increases in biogenic and anthropogenic emissions due to climate change are important during the summer seasons, but have little effect on pollutant concentrations during the winter. Results also indicate that controlling anthropogenic emissions will play a critical role in mitigating climate-driven increases in both ozone and PM 2.5 concentrations in the most populated areas of California. In the absence of anthropogenic emissions controls, climate change will worsen ozone air quality throughout the state, increasing exceedances of ambient air quality standards. If planned reductions in anthropogenic emissions are implemented, ozone air quality throughout the less urban areas of the state may be improved in the year 2035, but regions such as the SoCAB and the east SFBA will likely continue to experience high ozone concentrations throughout the summer season. Climate change and anthropogenic emissions controls are both found to decrease wintertime PM 2.5 concentrations in the SJV, eliminating nearly all exceedances of PM 2.5 National Ambient Air Quality Standards (NAAQS) in the year 2035. However, reductions in anthropogenic emissions are unable to fully mitigate the impact of climate change on PM 2.5 concentrations in the SoCAB and east SFBA. Thus, while future air quality in the SJV is projected to be improved in the year 2035, air quality in the SoCAB and east SFBA will remain similar or marginally worsen compared to present day levels. Conversely, we find that aggressive adoption of alternative energy technologies including renewable resources, electrification of end-use technologies, and energy efficiency measures can offset the impacts of climate change. Overall, the two main drivers for air quality in 2035 are changes in meteorological conditions due to climate change and reductions in anthropogenic emissions. Highlights • Climate change and emission control are two main drivers for future air quality. • Climate-driven increase in emissions is more significant in summer than winter. • Climate changes would increase the frequency and severity of pollution episodes. • Restrict emission control is essential to mitigate the damage of climate change. • Highly populated areas are most affected by climate change increase health risk. [ABSTRACT FROM AUTHOR]

Reactions between ammonia (NH 3 ) and organic compounds containing carbonyl functional groups in aerosol particles can form organic products that are less basic than NH 3 and are thus unable to neutralize efficiently nitric and sulfuric acids. In this exploratory study, the University of California, Irvine - California Institute of Technology (UCI-CIT) model is used to investigate the potential air quality impacts of including of the chemical uptake of NH 3 by secondary organic aerosols (SOA) in a regional airshed model. A surface reaction of NH 3 with SOA is implemented into the model to determine the impact of this process on NH 3 and PM 2.5 concentrations in the South Coast Air Basin of California (SoCAB). Air quality simulations are conducted using uptake coefficients ranging from 10 −5 to 10 −2 to explore the sensitivity of changes in NH 3 and PM 2.5 concentrations to the magnitude of the uptake coefficient. Results indicate that the chemical uptake of NH 3 by SOA can potentially deplete gaseous NH 3 concentrations, causing indirect reductions in the amount of ammonium nitrate and ammonium sulfate in particulate matter. The magnitude of the impact on NH 3 and PM 2.5 concentrations exhibits a strong but non-linear dependence on the value of the uptake coefficient, with evidence for small but notable impacts on air quality even with the lowest assumed uptake coefficient of 10 −5 . [ABSTRACT FROM AUTHOR]

Reducing aviation emissions will be a major concern in the coming years, as the relative contribution of aviation to overall emissions is projected to increase in the future. The South Coast Air Basin of California (SoCAB) is an extreme nonattainment area with many airports located upwind of the most polluted regions in the basin. Techniques to reduce aviation emissions have been studied in the past, and strategies that can be implemented at airports include taxi-out times reduction, ground support equipment electrification and aviation biofuel implementation. These strategies have been analyzed only at the national scale, their effectiveness to improve air quality within the SoCAB given the local meteorology and chemical regimes is unclear. This work studies how the adoption of the techniques at commercial SoCAB airports affect ozone (O 3 ) and fine particulate matter (PM 2.5 ) concentrations. In addition, potential impacts on public exposure to PM 2.5 and O 3 resulting from changes in the concentration of these pollutants are estimated. In addition, the work calculates aviation emissions for each scenario and simulate the transport and atmospheric chemistry of the pollutants using the Community Multiscale Air Quality (CMAQ) model. The simultaneous application of all reduction strategies is projected to reduce the aviation-attributable population weighted ground-level PM 2.5 by 36% in summer and 32% in winter. On the other hand, O 3 increases by 16% in winter. Occurring mostly in densely populated areas, the decrease in ground-level PM 2.5 would have a positive health impact and help the region achieve attainment of national ambient air quality standards. [ABSTRACT FROM AUTHOR]

The introduction of fuel cell electric vehicles (FCEV) operating on hydrogen is a key strategy to mitigate pollutant emissions from the light duty vehicle (LDV) transportation sector in pursuit of air quality (AQ) improvements. Further, concomitant increases in renewable power generation could assist in achieving benefits via electrolysis-provided hydrogen as a vehicle fuel. However, it is unclear (1) reductions in emissions translate to changes in primary and secondary pollutant concentrations and (2) how effects compare to those from emissions in other transport sectors including heavy duty vehicles (HDV). This work assesses how the adoption of FCEVs in counties expected to support alternative LDV technologies affect atmospheric concentrations of ozone and fine particulate matter (PM 2.5 ) throughout California (CA) in the year 2055 relative to a gasoline vehicle baseline. Further, impacts of reducing HDV emissions are explored to facilitate comparison among technology classes. A base year emissions inventory is grown to 2055 representing a business-as-usual progression of economic sectors, including primarily petroleum fuel consumption by LDV and HDVs. Emissions are spatially and temporally resolved and used in simulations of atmospheric chemistry and transport to evaluate distributions of primary and secondary pollutants respective to baseline. Results indicate that light-duty FCEV Cases achieve significant reductions in ozone and PM 2.5 when LDV market shares reach 50–100% in early adoption counties, including areas distant from deployment sites. Reflecting a cleaner LDV baseline fleet in 2055, emissions from HDVs impact ozone and PM 2.5 at comparable or greater levels than light duty FCEVs. Additionally, the importance of emissions from petroleum fuel infrastructure (PFI) activity is demonstrated in impacts on ozone and PM 2.5 burdens, with large refinery complexes representing a key source of air pollution in 2055. Results presented provide insight into light duty FCEV deployment strategies that can achieve maximum reductions in ozone and PM 2.5 and will assist decision makers in developing effective transportation sector AQ mitigation strategies. [ABSTRACT FROM AUTHOR]

In this paper, the Spatially and Temporally Resolved Energy and Environment Tool (STREET) is used in conjunction with University of California Irvine – California Institute of Technology (UCI-CIT) atmospheric chemistry and transport model to assess the impact of deploying plug-in electric vehicles and integrating wind energy into the electricity grid on urban air quality. STREET is used to generate emissions profiles associated with transportation and power generation sectors for different future cases. These profiles are then used as inputs to UCI-CIT to assess the impact of each case on urban air quality. The results show an overall improvement in 8-h averaged ozone and 24-h averaged particulate matter concentrations in the South Coast Air Basin (SoCAB) with localized increases in some cases. The most significant reductions occur northeast of the region where baseline concentrations are highest (up to 6 ppb decrease in 8-h-averaged ozone and 6 μg/m 3 decrease in 24-h-averaged PM 2.5 ). The results also indicate that, without integration of wind energy into the electricity grid, the temporal vehicle charging profile has very little to no effect on urban air quality. With the addition of wind energy to the grid mix, improvement in air quality is observed while charging at off-peak hours compared to the business as usual scenario. [ABSTRACT FROM AUTHOR]

Abstract: The formation of gas-phase bromine from aqueous sodium bromide aerosols is investigated through a combination of chamber experiments and chemical kinetics modeling. Experiments show that Br2(g) is produced rapidly from deliquesced NaBr aerosols in the presence of OH radicals produced by ozone irradiated by UV light. The mechanisms responsible for the “bromine explosion” are examined using a comprehensive chemical kinetics Model of Aqueous, Gaseous, and Interfacial Chemistry (MAGIC). A sensitivity analysis on the model confirms that a complex mechanism involving gas-phase chemistry, aqueous-phase chemistry, and mass transfer is responsible for most of the observed bromine. However, the rate-limiting steps in the bromine explosion pathway vary, depending on the availability of ozone and bromide in the system. Interface reactions, an important source of bromine production under dark conditions, account for only a small fraction of total bromine under irradiation. Simulations performed with gaseous ozone and aerosol bromide concentrations typical of the marine boundary layer also show Br2(g) production, with BrO(g) and HOBr(g) as the dominant Br-containing products through this mechanism. Aerosol bromide is depleted after several hours of daylight, with photolysis of BrO(g) and HOBr(g) becoming major sources of Br atoms that continue generating Br2(g) after aerosol bromide is depleted. [Copyright &y& Elsevier]

Communities located adjacent to goods movement hubs (such as major ports) experience degraded air quality (AQ) because of emissions from on-road and off-road diesel equipment, including heavy-duty diesel trucks (HDDT), cargo, and materials handling equipment (CHE), ships, and rail technologies. In response, California is pursuing transitions to efficient and cleaner freight systems by introducing zero-emission technologies as the alternative to conventional technologies. Equipment and vehicles powered by hydrogen fuel cells represent a potential zero-emissions pathway for freight technologies at ports, including HDDT, CHE, ships, and rail applications, referred to collectively as fuel cell electric technologies (FCET). This work is the first to assess the AQ and human health impacts of deploying FCET to provide goods movement services at ports. Specific focus is given to southern California due to existing AQ challenges and the presence of significant activity from the San Pedro Bay Port Complex, which includes the Ports of Los Angeles and Long Beach. Sets of future vehicle and equipment cases are developed spanning a range of FCET penetrations and assessed to quantify how FCET provides improvements in primary and secondary pollutant concentrations and the value of corresponding public health benefits. If fuel cells are used in all technologies considered, the results show significant improvements in maximum 8-h ozone (−2.69 ppb to −5.09 ppb) and maximum 24-h PM 2.5 (−0.59 μg/m3 to −2.57 μg/m3) can be achieved with FCET deployment, and the valuation of health benefits is estimated to range from $3.21 to $7.11 million per day depending on the level of penetration reached in each technology category. Reducing emissions from ships and HDDT is found to attain the highest health savings. Heightening the importance of these benefits, socioeconomically disadvantaged communities are found to experience larger health savings from FCET deployment in contrast to the population as a whole. As a co-benefit, FCET deployment is shown to result in greenhouse gas (GHG) emission reductions which will further increase with the sourcing of hydrogen from renewable sources. These findings demonstrate the importance of addressing environmental quality associated with the goods movement sector in urban areas and validate support of zero-emission projects through incentives and other policy mechanisms. Furthermore , policies designed to support zero-emission strategies within HDDT and ships powered by renewable fuels represent a promising pathway for air quality and GHG co-benefits. [Display omitted] • Fuel cell electrification at a port attains air quality improvements. • Ocean going vessels and diesel trucks should be targeted first. • Health benefits are valuable and improve environmental justice. • GHG reductions are also sizeable if renewable hydrogen is used. [ABSTRACT FROM AUTHOR]

Abstract: A transition from gasoline internal combustion engine vehicles to hydrogen fuel cell electric vehicles (FCEVs) is likely to emerge as a major component of the strategy to meet future greenhouse gas reduction, air quality, fuel independence, and energy security goals. Advanced infrastructure planning can minimize the cost of hydrogen infrastructure while assuring that energy and environment benefits are achieved. This study presents a comprehensive advanced planning methodology for the deployment of hydrogen infrastructure, and applies the methodology to delineate fully built-out infrastructure strategies, assess the associated energy and environment impacts, facilitate the identification of an optimal infrastructure roll-out strategy, and identify the potential for renewable hydrogen feedstocks. The South Coast Air Basin of California, targeted by automobile manufacturers for the first regional commercial deployment of FCEVs, is the focus for the study. The following insights result from the application of the methodology: [•] Compared to current gasoline stations, only 11%–14% of the number of hydrogen fueling stations can provide comparable accessibility to drivers in a targeted region. [•] To meet reasonable capacity demand for hydrogen fueling, approximately 30% the number of hydrogen stations are required compared to current gasoline stations. [•] Replacing gasoline vehicles with hydrogen FCEVs has the potential to (1) reduce the emission of greenhouse gases by more than 80%, reduce energy requirements by 42%, and virtually eliminate petroleum consumption from the passenger vehicle sector, and (2) significantly reduce urban concentrations of ozone and PM2.5. [•] Existing sources of biomethane in the California South Coast Air Basin can provide up to 30% of the hydrogen fueling demand for a fully built-out hydrogen FCEV scenario. [•] A step-wise transition of judiciously located existing gasoline stations to dispense and accommodate the increasing demand for hydrogen addresses proactively key infrastructure deployment challenges including a viable business model, zoning, permitting, and public acceptance. [Copyright &y& Elsevier]

Abstract: This study assesses the air quality impacts of central power generation and compares them with the impacts of distributed generation (DG). The central power plant emissions factors used are from a newly installed combined cycle gas turbine system. Because location of power plants is a key parameter affecting air quality impacts, this study considers three potential locations for the installation of central power plants. Air quality impacts are evaluated for the South Coast Air Basin of California, in the year 2010, using a three-dimensional air quality model. Results are compared to air quality impacts from two potential DG scenarios to meet the same power demand as that of the central power plant case. Even though emissions from central generation are lower than emissions from the DG technology mix considered herein, central generation concentrates emissions in a small area, whereas DG spreads emissions throughout a larger cross-section of the air basin. As a result, air quality impacts from central generation are more significant than those from DG. The study also shows that assessment of air quality impacts from distributed and central generation should not only consider emissions levels, but also the spatial and temporal distribution of emissions and the air quality that results from atmospheric chemistry and transport – highly non-linear processes. Finally, analysis of population exposure to ozone and PM2.5 shows that central generation located in coastal areas upwind from populated areas would cause the highest population exposure and even though emissions from central generation are considerably lower than DG emissions spread throughout the basin, results show that central generation causes a higher pollutant exposure than DG. [Copyright &y& Elsevier]

Abstract: This paper presents a global sensitivity and uncertainty analysis of the bromine chemistry included in the Model of Aqueous, Gaseous and Interfacial Chemistry (MAGIC) in dark and photolytic conditions. Uncertainty ranges are established for input parameters (e.g. chemical rate constants, Henry''s law constants, etc.) and are used in conjunction with Latin hypercube sampling and multiple linear regression to conduct a sensitivity analysis that determines the correlation between each input parameter and model output. The contribution of each input parameter to the uncertainty in the model output is calculated by combining results of the sensitivity analysis with input parameters'' uncertainty ranges. Model runs are compared using the predicted concentrations of molecular bromine since Br2(g) has been shown in previous studies to be generated via an interface reaction between O3(g) and Br(surface) − during dark conditions [Hunt et al., 2004]. Formation of molecular bromine from the reaction of ozone with deliquesced NaBr aerosol: evidence for interface chemistry. Journal of Physical Chemistry A 108, 11559–11572]. This study also examines the influence of an interface reaction between OH(g) and Br(surface) − in the production of Br2(g) under photolytic conditions where OH(g) is present in significant concentrations. Results indicate that the interface reaction between O3(g) and Br(surface) − is significant and is most responsible for the uncertainty in MAGICs ability to calculate precisely Br2(g) under dark conditions. However, under photolytic conditions the majority of Br2(g) is produced from a complex mechanism involving gas-phase chemistry, aqueous-phase chemistry, and mass transport. [Copyright &y& Elsevier]

A sampling campaign was conducted in the city of Milan, Italy before and during a transportation strike in January 2004. This strike provided a unique opportunity to investigate the influence of public transportation on the air quality in a major metropolitan area. Twenty-four air samples were collected each day around the city on January 2nd, 7th and 9th. The samples were analyzed for methane, carbon monoxide, non-methane hydrocarbons (NMHCs), halocarbons and alkyl nitrates. Significant differences in the mixing ratios were observed among the three days of sampling, with January 2nd showing the lowest concentrations as a result of decreased activity in the city during the holiday season. January 9th showed the highest NMHC concentrations because of increased vehicular activity in the city due to a public transportation strike. This paper investigates the correlation between the increased number of vehicles and decreased air quality because of a reduction in public transportation. Computer simulations were able to reproduce measurements of ozone production during the January 2004 strike and a July 2005 strike. The measurements and simulations suggest that reduced VOC emissions due to the existence of public transportation lowers peak ozone by 11–33% during the summer months. [Copyright &y& Elsevier]

This paper presents a global sensitivity and uncertainty analyses of the chlorine chemistry included in the Model of Aqueous, Gaseous and Interfacial Chemistry (MAGIC). Uncertainty ranges are established for input parameters (e.g. chemical rate constants, Henry''s law constants, etc.) and are used in conjunction with Latin hypercube sampling and multiple linear regression to conduct a sensitivity analysis which determines the correlation between each input parameter and the model output. The contribution of each input parameter to the uncertainty in the model output is calculated by combining the results of the sensitivity analysis with the input parameters'' uncertainty ranges. The peak concentration of molecular chlorine, [Cl2(g)]peak, is used to compare model runs since MAGIC has demonstrated previously the importance of an interfacial reaction between OH(g) and Cl− (aq,surface) in the production of Cl2(g). Results indicate that the interface reaction rate is most strongly correlated with [Cl2(g)]peak and is most responsible for the uncertainty in MAGIC''s ability to calculate precisely [Cl2(g)]peak. In addition, the mass accommodation coefficient for OH and the aqueous-phase reaction rate for Cl2 +OH− →HOCl+Cl− and 2CO3 − +O2 +2H2O→2CO2·H2O+2O2 − also contribute significantly to the uncertainty in [Cl2(g)]peak. [Copyright &y& Elsevier]

High ozone mixing ratios are a serious concern of public health. While ozone concentrations are high on weekdays due to anthropogenic emissions, they are often higher on weekends. This phenomenon has been named the weekend effect. This study uses the University of California, Irvine-California Institute of Technology (UCI-CIT) air quality model to assess the weekend effect in the South Coast Air Basin (SoCAB) of California. The weekend effect is reproduced by the model using an emissions inventory that includes representative weekday and weekend emissions. Additionally, this study modifies the Caltech Atmospheric Chemistry Mechanism (CACM), used in the UCI-CIT model, by introducing new heterogeneous reactions involving nitrogen oxides and chlorine. Eight modeling scenarios that include the nitrogen oxide renoxification and heterogeneous/multiphase chlorine reactions are presented to quantify how these reactions impact the weekend effect. The renoxification reaction and chlorine chemistry are found to increase ozone levels during weekdays and weekend days. However, increases in weekdays are generally larger than these increases that occur in the weekend. As a result, renoxification and chlorine chemistry lead to a net decrease in the average weekend effect intensity. The influence of renoxification on the weekend effect depends on the reaction probability (P), and the impact on the weekend effect is significant for P larger than 0.1. The influence of chlorine chemistry on the weekend effect depends strongly on the sea-salt source function that activates the chlorine chemistry. An amplification factor of 10 for the sea-salt source function, which produces the best agreement with observed chlorine levels in the SoCAB, leads to a basin-wide overall decrease of 29% in the weekend effect intensity with respect to the base case. [Copyright &y& Elsevier]