Urban Airshed Model Research Articles

Sub-Grid Scale Plume Modeling

Multi-pollutant chemical transport models (CTMs) are being routinely used to predict the impacts of emission controls on the concentrations and deposition of primary and secondary pollutants. While these models have a fairly comprehensive treatment of the governing atmospheric processes, they are unable to correctly represent processes that occur at very fine scales, such as the near-source transport and chemistry of emissions from elevated point sources, because of their relatively coarse horizontal resolution. Several different approaches have been used to address this limitation, such as using fine grids, adaptive grids, hybrid modeling, or an embedded sub-grid scale plume model, i.e., plume-in-grid (PinG) modeling. In this paper, we first discuss the relative merits of these various approaches used to resolve sub-grid scale effects in grid models, and then focus on PinG modeling which has been very effective in addressing the problems listed above. We start with a history and review of PinG modeling from its initial applications for ozone modeling in the Urban Airshed Model (UAM) in the early 1980s using a relatively simple plume model, to more sophisticated and state-of-the-science plume models, that include a full treatment of gas-phase, aerosol, and cloud chemistry, embedded in contemporary models such as CMAQ, CAMx, and WRF-Chem. We present examples of some typical results from PinG modeling for a variety of applications, discuss the implications of PinG on model predictions of source attribution, and discuss possible future developments and applications for PinG modeling.

Lumping, testing, tuning: The invention of an artificial chemistry in atmospheric transport modeling

Since the late 1950s computer simulation has been used to investigate the transport of pollutants in the atmosphere. About 20 years later also the chemical transformation of atmospheric pollutants was included in computer models of photochemical smog formation. Due to limited knowledge of atmospheric chemistry and due to limited computer capacity, chemical processes in the atmosphere were modeled with the help of simplified chemical models. In these models chemical substances are lumped together forming artificial virtual compounds with virtual characteristics. The paper aims at studying the practices developed in chemical model building and the creation of confidence in these models. Core of the paper will be the analysis of the Urban Airshed Model (UAM) for the Los Angeles region, a pioneering development in the early 1970s. The construction of the UAM involved the “lumping” of chemical processes and extensive testing and tuning. These practices led to a consistent model representation, in which diverse pieces of information fitted and were mutually stabilized. The pragmatic achievement of consistency created confidence, even though empirical tests of the models remained ambiguous and problematic.

The Molecular Mechanism of Tropospheric Nitrous Acid Production on Mineral Dust Surfaces

Nitrous acid, HONO, was first detected spectroscopically in the troposphere in 1979. Its photolysis results in a significant source of hydroxyl radicals that go on to initiate the oxidation of many pollutants and are a key intermediate in the formation of photochemical smog. Uncertainties remain regarding the processes that produce HONO, their magnitude and their mechanisms. Urban nitrous acid concentrations maximise during the night due to its formation via the heterogeneous hydrolysis of NO2 (2NO2+H2O!HONO+HNO3). The mechanism of this important reaction is unclear and as a consequence reported rate constants vary by several orders of magnitude. Earlier laboratory studies aimed at elucidating the underlying chemistry were confronted by experimental difficulties due to the slow nature of the reaction and the inability to detect HONO and the surface species involved in its formation under conditions pertinent to the troposphere. This has led to experiments being carried out at NO2 concentrations much higher (up to ~10 times) than those encountered in the troposphere. Accordingly, the surface species and processes observed under such laboratory conditions may not be relevant to the problem at hand. Much hinges on the position of the 2NO2(g)=N2O4(g) equilibrium. Current mechanistic proposals invoke N2O4 as the reaction initiating gaseous species; we argue that this cannot be correct and report laboratory measurements carried out under carefully controlled conditions which demonstrate that NO2(g) is the crucial reactant. This has significant implications for the inclusion of this reaction in urban air shed models and explains the large variability in reported kinetic parameters. 8] Using realistic partial pressures, temperatures and surface coverages, we studied the interaction of NO2 and water on an authentic mineral dust sample obtained from the Gobi desert. This major source, along with other arid areas, contributes ~1500 Tgyear 1 globally which corresponds to ~50% by mass of all aerosol, both natural and anthropogenic, emitted into the troposphere. The catalytically active surface provided by this material is responsible for elevated levels of tropospheric HONO that occur during dust storms. Experiments were conducted in an ultra high vacuum chamber, which allowed precise exposure of the dust sample to NO2 and H2O. In particular the NO2 pressure never exceeded 1>10 8 mbar, important for the reasons explained above. In situ X-ray photoelectron spectroscopy showed that the surface of the sample consisted principally of SiO2 and Al2O3, as is the case for most mineral samples as well as many urban surfaces such as window glass, road surfaces and cement. After exposing a sample of Gobi dust to 1 L (1 Langmuir=10 6 Torr.s) of H2O followed by an equal dose of NO2 at 170 K, temperature programmed reaction/desorption (TPR/TPD) data were acquired by heating the sample at ~8 Ks and detecting desorbing species using electron impact mass spectrometry. (The gas exposures used were such that the amount of adsorbate was equivalent to a monolayer or less—see Supporting Information). Representative results are shown in Figure 1: the width of the desorption features reflects the multi-component nature of the polycrystalline sample and unavoidable temperature gradients due to its poor thermal conductivity.

Effects of anthropogenic emission sources on maximum ozone concentrations over Greece

The impacts of anthropogenic emissions from the maritime transport and from the countries neighboring Greece on the maximum ground-level ozone concentrations over Greece were studied. Ozone concentrations were simulated across a part of the Eastern Mediterranean during a summer period using a modeling system that consisted of the photochemical Urban Airshed Model known as UAM-V coupled with the meteorological mesoscale model known as MM5. Simulated, maximum hourly ozone concentrations were higher over continental and maritime areas of Greece influenced by the emissions of the large urban agglomerations. When air quality simulations were performed assuming zero maritime transport emissions, the greater part of Greece experienced reductions in maximum ozone levels. Over the continental part of the country, the largest ozone reductions occurred along the coastline of southern and western Greece and were approximately 20 ppb. Over maritime areas, the decreases in maximum ozone concentrations were more pronounced. However, in the regions influenced by high amounts of nitrogen oxides emitted from the sea transport activities, the O 3 concentrations were increased. The impact of anthropogenic emissions from all continental areas within the modeling domain, other than Greece, on the maximum ozone concentrations over Greece was small. Considering that the emission inventory for Greece was more detailed, while the official emission data for some of the countries in the modeling domain were generally poor, the assessed impact might have been underestimated.

Mining and modeling for a metropolitan Atlanta ozone pollution decision-making framework

In this paper, we present a Decision-Making Framework (DMF) for reducing ozone pollution in the metropolitan Atlanta region. High ground-level concentrations of ozone continue to be a serious problem in several US cities, and Atlanta is one of the most serious of these cases. In contrast to the “trial and error” approach utilized by state government decision-makers, our DMF searches for dynamic and focused control strategies that require a lower total reduction of emissions than current control strategies. Our DMF utilizes a rigorous stochastic dynamic programming formulation and includes an Atmospheric Chemistry Module to represent how ozone concentrations change over time. This paper focuses on the procedures within the Atmospheric Chemistry Module. Using the US EPA's Urban Airshed Model for Atlanta, we use mining and metamodeling tools to develop a computationally efficient representation of the relevant ozone air chemistry. The proposed approach is able to effectively model changes in ozone concentrations over a 24-hour period.

Influence of dense surface meteorological data assimilation on the prediction accuracy of ozone pollution in the southeastern coastal area of the Korean Peninsula

In order to clarify the influence of surface meteorological data assimilation with various resolutions on the photochemical ozone concentration in the southeastern Korean Peninsula, several numerical experiments were conducted. The meteorological and photochemical reaction models used in this study are the fifth-generation mesoscale model (MM5) and the three-dimensional photochemical urban airshed model (UAM-V), respectively. Dense meteorological data make it easier to obtain accurate estimates and surface characteristics than coarse-resolution data. As a result, the estimated temperature obtained from high resolution surface data assimilation in the Busan and Ulsan metropolitan areas is higher than that obtained from coarse resolution surface data assimilation. These high temperatures resulted in strong winds from the sea and a significant dispersion of ozone. In analyses involving an index of agreement (IOA) and root mean square deviation (RMSD), the temperature and wind speed estimated with dense surface data assimilation agreed well with those obtained from observations. However, the influence of dense surface data assimilation tends to be stronger in the flat Ulsan metropolitan area than in the mountainous Busan metropolitan area. This is caused by the heterogeneity of the surface characteristics, including the topography. If the surface parameters induced by regional circulation, such as the topography and land use, are complex and heterogeneous, the efficiency of observational data on data assimilation has to be verified before air pollution is assessed.

Air Quality Modeling in the South Coast Air Basin of California: What Do the Numbers Really Mean?

This study evaluates air quality model sensitivity to input and to model components. Simulations are performed using the California Institute of Technology (CIT) airshed model. Results show the impacts on ozone (O3) concentration in the South Coast Air Basin (SCAB) of California because of changes in: (1) input data, including meteorological conditions (temperature, UV radiation, mixing height, and wind speed), boundary conditions, and initial conditions (ICs); and (2) model components, including advection solver and chemical mechanism. O3 concentrations are strongly affected by meteorological conditions and, in particular, by temperature. ICs also affect O3 concentrations, especially in the first 2 days of simulation. On the other hand, boundary conditions do not significantly affect the absolute peak O3 concentration, although they do affect concentrations near the inflow boundaries. Moreover, predicted O3 concentrations are impacted considerably by the chemical mechanism. In addition, dispersion of pollutants is affected by the advection routine used to calculate its transport. Comparison among CIT, California Photochemical Grid Model (CALGRID), and Urban Airshed Model air quality models suggests that differences in O3 predictions are mainly caused by the different chemical mechanisms used. Additionally, advection solvers contribute to the differences observed among model predictions. Uncertainty in predicted peak O3 concentration suggests that air quality evaluation should not be based solely on this single value but also on trends predicted by air quality models using a number of chemical mechanisms and with an advection solver that is mass conservative.

Measurement and modelling of air pollution and atmospheric chemistry in the U.K. West Midlands conurbation: Overview of the PUMA Consortium project

The PUMA (Pollution of the Urban Midlands Atmosphere) Consortium project involved intensive measurement campaigns in the Summer of 1999 and Winter of 1999/2000, respectively, in which a wide variety of air pollutants were measured in the UK West Midlands conurbation including detailed speciation of VOCs and major component analysis of aerosol. Measurements of the OH and HO 2 free radicals by the FAGE technique demonstrated that winter concentrations of OH were approximately half of those measured during the summer despite a factor of 15 reduction in production through the photolysis of ozone. Detailed box modelling of the fast reaction chemistry revealed the decomposition of Criegee intermediates formed from ozone–alkene reactions to be responsible for the majority of the formation of hydroxyl in both the summer and winter campaigns, in contrast to earlier rural measurements in which ozone photolysis was predominant. The main sinks for hydroxyl are reactions with NO 2, alkenes and oxygenates. Concentrations of the more stable hydrocarbons were found to be relatively invariant across the conurbation, but the impacts of photochemistry were evident through analyses of formaldehyde which showed the majority to be photochemical in origin as opposed to emitted from road traffic. Measurements on the upwind and downwind boundaries of the conurbation revealed substantial enhancements in NO x as a result of emissions within the conurbation, especially during westerly winds which carried relatively clean air. Using calcium as a tracer for crustal particles, it proved possible to reconstruct aerosol mass from the major chemical components with a fairly high degree of success. The organic to elemental carbon ratios showed a far greater influence of photochemistry in summer than winter, presumably resulting mainly from the greater availability of biogenic precursors during the summer campaign. Two urban airshed models were developed and applied to the conurbation, one Eulerian, the other Lagrangian. Both were able to give a good simulation of concentrations of both primary and secondary pollutants at urban background locations.

Model simulation of meteorology and air quality during the summer PUMA intensive measurement campaign in the UK West Midlands conurbation

The Regional Atmospheric Modeling System (RAMS) and Urban Airshed Model (UAM IV) have been implemented for prediction of air pollutant concentrations within the West Midlands conurbation of the United Kingdom. The modelling results for wind speed, direction and temperature are in reasonable agreement with observations for two stations, one in a rural area and the other in an urban area. Predictions of surface temperature are generally good for both stations, but the results suggest that the quality of temperature prediction is sensitive to whether cloud cover is reproduced reliably by the model. Wind direction is captured very well by the model, while wind speed is generally overestimated. The air pollution climate of the UK West Midlands is very different to those for which the UAM model was primarily developed, and the methods used to overcome these limitations are described. The model shows a tendency towards under-prediction of primary pollutant (NO x and CO) concentrations, but with suitable attention to boundary conditions and vertical profiles gives fairly good predictions of ozone concentrations. Hourly updating of chemical concentration boundary conditions yields the best results, with input of vertical profiles desirable. The model seriously underpredicts NO 2 / NO ratios within the urban area and this appears to relate to inadequate production of peroxy radicals. Overall, the chemical reactivity predicted by the model appears to fall well below that occurring in the atmosphere.

A GIS framework for Traffic Emission Information System

A coefficient-based vehicular exhaust emission model is being developed jointly with the variable-grid urban airshed model (UAM-V) of Systems Applications International (SAI), and integrated within a GIS (ArcInfo) framework. When coupled with a road network, traffic features and meteorological as well as geographical databases, this framework produces a Traffic Emission Information System (TEIS). With this TEIS, the emission coefficients of CO, NOx and VOC for Sydney in the calendar year 2000 are derived; and the emission rates of the pollutants are then estimated using sample traffic profile data for the Sydney metropolitan area. Finally, ongoing improvements to the system are presented.

The BOND project: Biogenic aerosols and air quality in Athens and Marseille greater areas

The role of Secondary Biogenic Organic Aerosol in aerosol budget is examined using the Atmospheric Dispersion of Pollutants over Complex Terrain–Urban Airshed Model–Aerosols (ADREA‐I/UAM‐AERO) modeling system in two representative Mediterranean areas. The areas have been selected, because of their elevated biogenic emission levels and the sufficient degree of meteorological and land use diversity characterizing the locations. Comparison of the model results with and without biogenic emissions reveals the significant role biogenic emissions play in modulating ozone and aerosol concentrations. Biogenic emissions are predicted to affect the concentrations of organic aerosol constituents through the reactions of terpenes with O3, OH and NO3. The ozonolysis of terpenes is predicted to cause an increase in OH radical concentrations that ranges from 10% to 78% for Athens, and from 20% to 95% for Marseilles, depending on the location, compared to the predictions without biogenic emissions. The reactions of this extra hydroxyl radical with SO2 and NOx have as final products increased concentrations of sulfates and nitrates in the particulate phase. As a result, biogenic emissions are predicted to affect the concentrations not only of organic aerosols, but those of inorganic aerosols as well. Thus biogenic emissions should be taken into consideration when models for the prediction and enforcement of abatement strategies of atmospheric pollution are applied.

Model study with UAM-V in the Milan area (I) during PIPAPO: simulations with changed emissions compared to ground and airborne measurements

Abstract The three-dimensional photochemical Urban Airshed Model with variable grid (UAM-V) is used to investigate the temporal and spatial dynamics of the photooxidant production in the highly polluted Milan area (Italy). A simulation of the 13 May 1998 event is presented, when peak ozone levels of 190 ppb were measured in the urban plume. A large field campaign provided data of numerous chemical species and meteorological quantities from ground based and airborne platforms. The highest measured ozone mixing ratios occurred in the urban plume, where a local peroxide decrease was found simultaneously. The model base case is able to reproduce the ozone concentration but more peroxide is formed in the centre of the plume than in the adjacent areas. This can be interpreted as a tendency of the model towards a stronger NOx sensitivity compared to observation. Modification of the emission inventory, which is based on observed discrepancies between inventory and measurements, lead to a better agreement with respect to peroxide formation and NOx removal in the urban plume. It seems that the different rearrangement of the emissions in urban and non-urban areas are important for the improved response of the model. Ozone levels did not differ much between the base case simulation and those with a modified emission inventory, while peroxide formation and NOx concentrations in the urban plume did.

Modelling of Air Quality with CAMx: A Case Study in Switzerland

The three-dimensional Eulerian model CAMx (Comprehensive Air QualityModel with Extensions) was applied for the first time to simulate bothgaseous and particulate photochemical air pollution in Switzerland during July 28–30, 1993. The meteorological input data were prepared using the Systems Applications International Mesoscale Model (SAIMM). The CAMx model results were compared with the measurements carried out at ground level andfrom airborne measuring platforms within the frame of the Swiss POLLUMET research programme. In general, the CAMx performance for gaseous species wasfound to be better than that of the previously used Urban Airshed Model (UAM)and the Variable Grid Urban Airshed Model (UAM-V). The most significant improvement for the gaseous species is in the prediction of HNO3 concentrations, due to the inclusion of aerosol chemistry. Aerosol species such as NO3 -, NH4 +, SO4 2-, and secondary organic aerosols were calculated in one particle size range (0.04–2.5 μm) and compared with a few measurements available. Although July 29 was reasonably well simulated, overestimated wind speeds by SAIMM for July 30 caused a too fast transport of pollutants. Similarly to measurements, significant spatial correlation of the secondarily formed aerosols with ozone and formaldehyde is found in the afternoon.

Benzene, toluene, ozone, NO 2 and SO 2 measurements in an urban street canyon in Thessaloniki, Greece

Benzene, toluene, sulphur dioxide, ozone and nitrogen dioxide were measured at a mean level of 13.5 m above ground in a narrow, four-lane street canyon (height 30 m, width 20 m) in Thessaloniki, Greece during the period January–July 1997 by means of a commercial differential optical absorption spectrometer (OPSIS DOAS). Primary pollutant levels were found to be 2.5–4.4 times higher during the cold part of the year than during the warm part of the year, the winter/summer ratio increasing with the reaction rate constant with OH for each of the measured species. Ozone, on the other hand, exhibited a winter/summer ratio of 0.36. NO 2 originates from both primary and secondary sources; its winter/summer concentration ratio of 1.4 lies, therefore, between those of primary pollutants and ozone. Pollution levels were influenced considerably by wind speed, while for the street canyon under study wind direction did not influence pollutant levels considerably. While primary pollution was found to decrease with increasing wind speed, ozone increased. Benzene mean levels during the study period were around 6 ppb and hence much higher than the EU annual limit value of 5 μg m −3 (1.44 ppb at STP). Toluene mean levels were around 14 ppb and hence also several times above the WHO recommendation of 2 ppb for 24 h. The apportionment of traffic emissions in four time zones used in most inventories in urban airshed models was tested using benzene and toluene measurements at low (<1 m s −1) wind speeds. The agreement between model emissions and calculated emissions apportionment into the four time zones was good, except for Zone D (23:00–1:59), where model inventory emissions were somewhat too low.

A model to estimate mixing height and its effects on ozone modeling

A mixing height estimation model (PMH) whose output can be used in air quality models, especially the Urban Airshed Model (UAM-IV), is developed. The evaluation of the performance of the PMH shows that the predictions match the observations very well. The average geometric mean of the predicted to observed hourly mixing height ratios is equal to 0.99. The outputs of the PMH along with those of the mixing height model incorporated in the UAM-IV package (RAMMETX) are used in the UAM-IV to simulate hourly average concentrations of tropospheric ozone (O 3) in the Indianapolis metropolitan statistical area (MSA). A comparative study performed to assess the effects of mixing height shows that the UAM-IV performs better when the mixing height fields are from the PMH. For example, the episodic mean of the average normalized bias and the average normalized absolute bias of predicted to observed O 3 concentrations are 1.2% and 18.6%, respectively, when the mixing height fields are from the PMH. They are 6.6% and 20.1%, respectively, when the mixing height fields are from RAMMETX.

Evaluation of a photochemical grid model using estimates of concentration probability density functions

A method for evaluating photochemical grid models based on the estimation of the probability density function (PDF) of the variations in ozone concentrations is described and demonstrated. It is assumed that the ozone concentration PDF is determined from 100 Monte Carlo uncertainty runs based on uncertainties in photochemical grid model input variables. The exercise involves the Urban Airshed Model with Variable grid (UAM-V) as applied to the July 1995 Ozone Transport Assessment Group episode in the eastern US. The focus is on the distribution of model residuals (observed concentration minus model-predicted ensemble mean concentration) for 66 ozone monitors on the OTAG domain. It is concluded that the distribution of observed model residuals is within the 95% range of expected uncertainty, suggesting that the model is performing adequately.

Evaluation of a Data Assimilation Technique for a Mesoscale Meteorological Model Used for Air Quality Modeling

Abstract An observational data assimilation (ODA) technique was evaluated based on both its direct effect on meteorological model fields and its indirect effect on the results of two air quality models that input these meteorological fields: a Lagrangian particle model (LPM) and a photochemical model, the variable-grid version of the Urban Airshed Model (UAM-V). The purpose was to investigate the model performance improvements that are derived from using field-study observations with an ODA technique. The ODA technique, based upon Newtonian relaxation, was incorporated into the Colorado State University Regional Atmospheric Modeling System (RAMS). The technique was applied with rawinsonde, profiler, and sodar observations of winds, temperature, and moisture from an intensive field campaign during 3–7 August 1990 over the San Joaquin Valley in California. The RAMS meteorological fields, produced with and without the use of the ODA technique, and the results from the two air quality models using these two f...

Compilation and evaluation of a Paso del Norte emission inventory

Emission inventories of ozone precursors are routinely used as input to comprehensive photochemical air quality models. Photochemical model performance and the development of effective control strategies rely on the accuracy and representativeness of an underlying emission inventory. This paper describes the tasks undertaken to compile and evaluate an ozone precursor emission inventory for the El Paso/Ciudad Juárez/Southern Doña Ana region. Point, area and mobile source emission data were obtained from local government agencies and were spatially and temporally allocated to a gridded domain using region-specific demographic and land-cover information. The inventory was then processed using the US Environmental Protection Agency (EPA) recommended Emissions Preprocessor System 2.0 (UAM-EPS 2.0) which generates emissions files compatible with the Urban Airshed Model (UAM). A top–down evaluation of the emission inventory was performed to examine how well the inventory represented ambient pollutant compositions. The top–down evaluation methodology employed in this study compares emission inventory ratios of non-methane hydrocarbon (NMHC)/nitrogen oxide (NO x ) and carbon monoxide (CO)/NO x ratios to corresponding ambient ratios. Detailed NMHC species comparisons were made in order to investigate the relative composition of individual hydrocarbon species in the emission inventory and in the ambient data. The emission inventory compiled during this effort has since been used to model ozone in the Paso del Norte airshed (Emery et al., CAMx modeling of ozone and carbon monoxide in the Paso del Norte airshed. In: Proc of Ninety-Third Annual Meeting of Air & Waste Management Association, 18–22 June 2000, Air & Waste Management Association, Pittsburgh, PA, 2000).

An Operational Evaluation of Two Regional–Scale Ozone Air Quality Modeling Systems over the Eastern United States

In this paper, the performance of two commonly used regional-scale Eulerian photochemical modeling systems, namely, RAMS/UAM-V and MM5/SAQM, from the regulatory or operational perspective, is examined. While the Urban Airshed Model with Variable Grid (UAM-V) is driven with the meteorological fields derived from the Regional Atmospheric Model System (RAMS), the San Joaquin Valley Air Quality Model (SAQM) used the meteorological fields derived from the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). The model's performance in reproducing the observed ozone air quality over the eastern United States is evaluated for three typical high-ozone episodic events that occurred during 16–20 June, 12–16 July, and 30 July–2 August of 1995. The prevailing meteorological conditions associated with these three episodes are characterized by a slow eastward-moving high pressure system, westerly and southwesterly low-level jets, stable boundary layers, and the Appalach...

Uncertainties in Episodic Ozone Modeling Stemming from Uncertainties in the Meteorological Fields

This paper examines the uncertainty associated with photochemical modeling using the Variable-Grid Urban Airshed Model (UAM-V) with two different prognostic meteorological models. The meteorological fields for ozone episodes that occurred during 17‐20 June, 12‐15 July, and 30 July‐2 August in the summer of 1995 were derived from two meteorological models, the Regional Atmospheric Modeling System (RAMS) and the Fifth-Generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model (MM5). The simulated ozone concentrations from the two photochemical modeling systems, namely, RAMS/ UAM-V and MM5/UAM-V, are compared with each other and with ozone observations from several monitoring sites in the eastern United States. The overall results indicate that neither modeling system performs significantly better than the other in reproducing the observed ozone concentrations. The results reveal that there is a significant variability, about 20% at the 95% level of confidence, in the modeled 1-h ozone concentration maxima from one modeling system to the other for a given episode. The model-to-model variability in the simulated ozone levels is for most part attributable to the unsystematic type of errors. The directionality for emission controls (i.e., NOx versus VOC sensitivity) is also evaluated with UAM-V using hypothetical emission reductions. The results reveal that not only the improvement in ozone but also the VOC-sensitive and NOx-sensitive regimes are influenced by the differences in the meteorological fields. Both modeling systems indicate that a large portion of the eastern United States is NOx limited, but there are model-to-model and episode-to-episode differences at individual grid cells regarding the efficacy of emission reductions.