scholarly journals Characterizing sources of high surface ozone events in the southwestern U.S. with intensive field measurements and two global models

2019 ◽  
Author(s):  
Li Zhang ◽  
Meiyun Lin ◽  
Andrew O. Langford ◽  
Larry W. Horowitz ◽  
Christoph J. Senff ◽  
...  
Keyword(s):  
Large Scale ◽  
Las Vegas ◽  
Surface Ozone ◽  
Field Measurements ◽  
Ambient Air ◽  
Daily Maximum ◽  
Global Models ◽  
High Background ◽  
Stratospheric Intrusions ◽  
Surface O3

Abstract. The detection and attribution of high background ozone (O3) events in the southwestern U.S. is challenging but relevant to the effective implementation of the lowered National Ambient Air Quality Standard (NAAQS; 70 ppbv). Here we leverage intensive field measurements from the Fires, Asian, and Stratospheric TransportLas Vegas Ozone Study (FAST-LVOS) in MayJune 2017, alongside high-resolution simulations with two global models (GFDL-AM4 and GEOS-Chem), to pinpoint the sources of O3 during high-O3 events. We show stratospheric influence on four out of the ten events with daily maximum 8-hour average (MDA8) surface O3 above 65 ppbv in the greater Las Vegas region. While O3 produced from regional anthropogenic emissions dominates pollution in the Las Vegas Valley, stratospheric intrusions can mix with regional pollution to push surface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics of deep stratospheric intrusions consistent with ozonesondes, lidar profiles, and co-located measurements of O3, CO, and water vapor at Angel Peak, whereas GEOS-Chem has difficulty simulating the observed features and underestimates observed O3 by ~ 20 ppbv at the surface. The two models also differ substantially during a wildfire event, with GEOS-Chem estimating ~ 15 ppbv greater O3, in better agreement with lidar observations. At the surface, the two models bracket the observed MDA8 O3 values during the wildfire event. Both models capture the large-scale transport of Asian pollution, but neither resolves some fine-scale pollution plumes, as evidenced from aerosol backscatter, aircraft, and satellite measurements. U.S. background O3 estimates from the two models differ by 5 ppbv on average and up to 15 ppbv episodically. Our multi-model approach tied closely to observational analysis yields process insights, suggesting that elevated background O3 may pose challenges to achieving a potentially lower NAAQS level (e.g., 65 ppbv) in the southwestern U.S.

scholarly journals Characterizing sources of high surface ozone events in the southwestern US with intensive field measurements and two global models

2020 ◽  
Vol 20 (17) ◽  
pp. 10379-10400
Author(s):  
Li Zhang ◽  
Meiyun Lin ◽  
Andrew O. Langford ◽  
Larry W. Horowitz ◽  
Christoph J. Senff ◽  
...  
Keyword(s):  
Las Vegas ◽  
Stratospheric Ozone ◽  
Surface Ozone ◽  
Field Measurements ◽  
Ambient Air ◽  
Daily Maximum ◽  
Global Models ◽  
High Background ◽  
Stratospheric Intrusions ◽  
Surface O3

Abstract. The detection and attribution of high background ozone (O3) events in the southwestern US is challenging but relevant to the effective implementation of the lowered National Ambient Air Quality Standard (NAAQS; 70 ppbv). Here we leverage intensive field measurements from the Fires, Asian, and Stratospheric Transport−Las Vegas Ozone Study (FAST-LVOS) in May–June 2017, alongside high-resolution simulations with two global models (GFDL-AM4 and GEOS-Chem), to study the sources of O3 during high-O3 events. We show possible stratospheric influence on 4 out of the 10 events with daily maximum 8 h average (MDA8) surface O3 above 65 ppbv in the greater Las Vegas region. While O3 produced from regional anthropogenic emissions dominates pollution events in the Las Vegas Valley, stratospheric intrusions can mix with regional pollution to push surface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics of deep stratospheric intrusions consistent with ozonesondes, lidar profiles, and co-located measurements of O3, CO, and water vapor at Angel Peak, whereas GEOS-Chem has difficulty simulating the observed features and underestimates observed O3 by ∼20 ppbv at the surface. On days when observed MDA8 O3 exceeds 65 ppbv and the AM4 stratospheric ozone tracer shows 20–40 ppbv enhancements, GEOS-Chem simulates ∼15 ppbv lower US background O3 than GFDL-AM4. The two models also differ substantially during a wildfire event, with GEOS-Chem estimating ∼15 ppbv greater O3, in better agreement with lidar observations. At the surface, the two models bracket the observed MDA8 O3 values during the wildfire event. Both models capture the large-scale transport of Asian pollution, but neither resolves some fine-scale pollution plumes, as evidenced by aerosol backscatter, aircraft, and satellite measurements. US background O3 estimates from the two models differ by 5 ppbv on average (greater in GFDL-AM4) and up to 15 ppbv episodically. Uncertainties remain in the quantitative attribution of each event. Nevertheless, our multi-model approach tied closely to observational analysis yields some process insights, suggesting that elevated background O3 may pose challenges to achieving a potentially lower NAAQS level (e.g., 65 ppbv) in the southwestern US.

scholarly journals Why does surface ozone peak before a typhoon landing in southeast China?

2015 ◽  
Vol 15 (17) ◽  
pp. 24623-24642 ◽  
Author(s):  
Y. C. Jiang ◽  
T. L. Zhao ◽  
J. Liu ◽  
X. D. Xu ◽  
C. H. Tan ◽  
...  
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Surface Ozone ◽  
High Surface ◽  
Ambient Air ◽  
Ambient Air Quality ◽  
Substantial Impact ◽  
Upper Troposphere ◽  
Horizontal Transport ◽  
Surface O3

Abstract. A high O3 episode with the large increases in surface ozone by 21–42 ppbv and the nocturnal surface O3 levels exceeding 70 ppbv was observed in the region between Xiamen and Quanzhou over the southeastern coast of China during 12–14 June 2014, before Typhoon Hagibis landing. Variations in the surface O3, NO2, CO and meteorology during Typhoon Hagibis event clearly suggest a substantial impact of the peripheral downdrafts in the large scale typhoon circulation on such an O3 episode with excluding the contributions of photochemical production and the horizontal transport. The influence of vertical O3 transport from the upper troposphere and lower stratosphere (UTLS) region on high surface O3 levels is further confirmed by a negative correlation between surface O3 and CO concentrations as well as dry surface air observed during the O3 episode. This study provides observational evidence of typhoon-driven intrusion of O3 from the UTLS region to surface air, revealing a significant effect of such a process of stratosphere–troposphere exchange (STE) of O3 on tropospheric O3 and ambient air quality.

scholarly journals Why does surface ozone peak before a typhoon landing in southeast China?

2015 ◽  
Vol 15 (23) ◽  
pp. 13331-13338 ◽  
Author(s):  
Y. C. Jiang ◽  
T. L. Zhao ◽  
J. Liu ◽  
X. D. Xu ◽  
C. H. Tan ◽  
...  
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Surface Ozone ◽  
High Surface ◽  
Ambient Air ◽  
Ambient Air Quality ◽  
Substantial Impact ◽  
Upper Troposphere ◽  
Horizontal Transport ◽  
Surface O3

Abstract. A high O3 episode with the large increases in surface ozone by 21–42 ppbv and the nocturnal surface O3 levels exceeding 70 ppbv was observed in the region between Xiamen and Quanzhou over the southeastern coast of China during 12–14 June 2014, before the Typhoon Hagibis landing. Variations in the surface O3, NO2, CO and meteorology during the Typhoon Hagibis event clearly suggest a substantial impact of the peripheral downdrafts in the large-scale typhoon circulation on such an O3 episode excluding the contributions of photochemical production and the horizontal transport. The influence of vertical O3 transport from the upper troposphere and lower stratosphere (UTLS) region on high surface O3 levels is further confirmed by a negative correlation between surface O3 and CO concentrations as well as dry surface air observed during the O3 episode. This study provides observational evidence of typhoon-driven intrusion of O3 from the UTLS region to surface air, revealing a significant effect of such a process of stratosphere–troposphere exchange (STE) of O3 on tropospheric O3 and ambient air quality.

scholarly journals Ozone and NO<sub>x</sub> chemistry in the eastern US: evaluation of CMAQ/CB05 with satellite (OMI) data

2015 ◽  
Vol 15 (4) ◽  
pp. 4427-4461 ◽  
Author(s):  
T. P. Canty ◽  
L. Hembeck ◽  
T. P. Vinciguerra ◽  
D. C. Anderson ◽  
D. L. Goldberg ◽  
...  
Keyword(s):  
Air Quality ◽  
Surface Ozone ◽  
Quality Standards ◽  
Ambient Air ◽  
Daily Maximum ◽  
Air Quality Model ◽  
Ozone Monitoring Instrument ◽  
Air Quality Standards ◽  
The Us ◽  
Us Northeast

Abstract. Regulatory air quality models, such as the Community Multiscale Air Quality model (CMAQ), are used by federal and state agencies to guide policy decisions that determine how to best achieve adherence with National Ambient Air Quality Standards for surface ozone. We use observations of ozone and its important precursor NO2 to test the representation of the photochemistry and emission of ozone precursors within CMAQ. Observations of tropospheric column NO2 from the Ozone Monitoring Instrument (OMI), retrieved by two independent groups, show that the model overestimates urban NO2 and underestimates rural NO2 under all conditions examined for July and August 2011 in the US Northeast. The overestimate of the urban to rural ratio of tropospheric column NO2 for this baseline run of CMAQ (CB05 mechanism, mobile NOx emissions from the National Emissions Inventory; isoprene emissions from MEGAN v2.04) suggests this model may under estimate the importance of interstate transport of NOx. This CMAQ simulation leads to a considerable overestimate of the 2 month average of 8 h daily maximum surface ozone in the US Northeast, as well as an overestimate of 8 h ozone at AQS sites during days when the state of Maryland experienced NAAQS exceedances. We have implemented three changes within CMAQ motivated by OMI NO2 as well as aircraft observations obtained in July 2011 during the NASA DISCOVER-AQ campaign: (a) the modeled lifetime of organic nitrates within CB05 has been reduced by a factor of 10, (b) emissions of NOx from mobile sources has been reduced by a factor of 2, and (c) isoprene emissions have been reduced by using MEGAN v2.10 rather than v2.04. Compared to the baseline simulation, the CMAQ run using all three of these changes leads to a considerably better simulation of the ratio of urban to rural column NO2, better agreement with the 2 month average of daily 8 h maximum ozone in the US Northeast, fewer number of false positives of an ozone exceedance throughout the domain, as well as an unbiased simulation of surface ozone at ground based AQS sites in Maryland that experienced an ozone exceedance during July and August 2007. These modifications to CMAQ may provide a framework for use in studies focused on achieving future adherence to specific air quality standards for surface ozone by reducing emission of NOx from various anthropogenic sectors.

scholarly journals Spatio-temporal of surface ozone (O3) variations at urban and suburban sites in Sarawak region of Malaysia

2021 ◽  
Vol 880 (1) ◽  
pp. 012004
Author(s):  
H Mahidin ◽  
M T Latif ◽  
A Hamdan ◽  
J Salleh ◽  
D Dominick ◽  
...  
Keyword(s):  
Air Quality ◽  
Surface Ozone ◽  
Ambient Air ◽  
Quality Standard ◽  
Transportation Systems ◽  
Pollutant Emissions ◽  
Daily Maximum ◽  
Spatio Temporal ◽  
Ambient Air Quality Standard ◽  
The Relationship

Abstract Sarawak Region of Malaysia is currently experiencing a high demand for capital needs such as transformation forest to plantations, economic development, and improving transportation systems. Those land cover changes will increase primary pollutant emissions and trigger surface O3 formation. Surface O3 is a secondary pollutant and a significant greenhouse gas contributing to climate change and declining air quality. In this study, variations in surface O3 concentrations at urban and suburban sites in Sarawak were explored using the Malaysian Department of Environment data spanning a two-year cycle (2018-2019). The primary aim of this study is to ascertain the variation of surface O3 concentrations reported at four monitoring stations in Sarawak, namely Kuching (SQ1) (Urban), Sibu (SQ2) (Suburban), Bintulu (SQ3) (Suburban), and Miri (SQ4) (Suburban). The study also analysed the relationship between O3 distribution and nitrogen oxides (NO and NO2). The findings showed that O3 concentrations observed in the region during the study period were lower than the maximum permissible value of 100 ppbv suggested by the Malaysian Ambient Air Quality Standard (2020). SQ4 (Miri) at suburban sites recorded the highest average surface O3 concentrations with an hourly average and daily maximum O3 concentration of 15.7 and 89.5 ppbv, respectively. Temperatures, UV exposure, and wind speed all impact the concentration of surface O3 in Sarawak. In all stations, concentrations of O3 were inversely linked with NO, NO2, and relative humidity (RH). This research will assist the relevant agency in forecast, monitor, and mitigate the level of O3 in the ambient environment, especially in the Sarawak Region.

scholarly journals Detection of deep stratospheric intrusions by cosmogenic 35S

2016 ◽  
Vol 113 (40) ◽  
pp. 11131-11136 ◽  
Author(s):  
Mang Lin ◽  
Lin Su ◽  
Robina Shaheen ◽  
Jimmy C. H. Fung ◽  
Mark H. Thiemens
Keyword(s):  
Air Quality ◽  
Southern California ◽  
Ground Level ◽  
Ambient Air ◽  
Quality Standard ◽  
Quantitative Detection ◽  
Daily Maximum ◽  
Model Calculations ◽  
Mixing Ratios ◽  
Stratospheric Intrusions

The extent to which stratospheric intrusions on synoptic scales influence the tropospheric ozone (O3) levels remains poorly understood, because quantitative detection of stratospheric air has been challenging. Cosmogenic 35S mainly produced in the stratosphere has the potential to identify stratospheric air masses at ground level, but this approach has not yet been unambiguously shown. Here, we report unusually high 35S concentrations (7,390 atoms m−3; ∼16 times greater than annual average) in fine sulfate aerosols (aerodynamic diameter less than 0.95 µm) collected at a coastal site in southern California on May 3, 2014, when ground-level O3 mixing ratios at air quality monitoring stations across southern California (43 of 85) exceeded the recently revised US National Ambient Air Quality Standard (daily maximum 8-h average: 70 parts per billion by volume). The stratospheric origin of the significantly enhanced 35S level is supported by in situ measurements of air pollutants and meteorological variables, satellite observations, meteorological analysis, and box model calculations. The deep stratospheric intrusion event was driven by the coupling between midlatitude cyclones and Santa Ana winds, and it was responsible for the regional O3 pollution episode. These results provide direct field-based evidence that 35S is an additional sensitive and unambiguous tracer in detecting stratospheric air in the boundary layer and offer the potential for resolving the stratospheric influences on the tropospheric O3 level.

scholarly journals Two-scale multi-model ensemble: is a hybrid ensemble of opportunity telling us more?

2018 ◽  
Vol 18 (12) ◽  
pp. 8727-8744 ◽  
Author(s):  
Stefano Galmarini ◽  
Ioannis Kioutsioukis ◽  
Efisio Solazzo ◽  
Ummugulsum Alyuz ◽  
Alessandra Balzarini ◽  
...  
Keyword(s):  
Time Series ◽  
Air Quality ◽  
Surface Ozone ◽  
Regional Scale ◽  
Probability Of Detection ◽  
Daily Maximum ◽  
Regional Models ◽  
Global Models ◽  
Ensemble Performance ◽  
Scale Models

Abstract. In this study we introduce a hybrid ensemble consisting of air quality models operating at both the global and regional scale. The work is motivated by the fact that these different types of models treat specific portions of the atmospheric spectrum with different levels of detail, and it is hypothesized that their combination can generate an ensemble that performs better than mono-scale ensembles. A detailed analysis of the hybrid ensemble is carried out in the attempt to investigate this hypothesis and determine the real benefit it produces compared to ensembles constructed from only global-scale or only regional-scale models. The study utilizes 13 regional and 7 global models participating in the Hemispheric Transport of Air Pollutants phase 2 (HTAP2)–Air Quality Model Evaluation International Initiative phase 3 (AQMEII3) activity and focuses on surface ozone concentrations over Europe for the year 2010. Observations from 405 monitoring rural stations are used for the evaluation of the ensemble performance. The analysis first compares the modelled and measured power spectra of all models and then assesses the properties of the mono-scale ensembles, particularly their level of redundancy, in order to inform the process of constructing the hybrid ensemble. This study has been conducted in the attempt to identify that the improvements obtained by the hybrid ensemble relative to the mono-scale ensembles can be attributed to its hybrid nature. The improvements are visible in a slight increase of the diversity (4 % for the hourly time series, 10 % for the daily maximum time series) and a smaller improvement of the accuracy compared to diversity. Root mean square error (RMSE) improved by 13–16 % compared to G and by 2–3 % compared to R. Probability of detection (POD) and false-alarm rate (FAR) show a remarkable improvement, with a steep increase in the largest POD values and smallest values of FAR across the concentration ranges. The results show that the optimal set is constructed from an equal number of global and regional models at only 15 % of the stations. This implies that for the majority of the cases the regional-scale set of models governs the ensemble. However given the high degree of redundancy that characterizes the regional-scale models, no further improvement could be expected in the ensemble performance by adding yet more regional models to it. Therefore the improvement obtained with the hybrid set can confidently be attributed to the different nature of the global models. The study strongly reaffirms the importance of an in-depth inspection of any ensemble of opportunity in order to extract the maximum amount of information and to have full control over the data used in the construction of the ensemble.

scholarly journals Sensitivity analysis of the surface ozone and fine particulate matter to meteorological parameters in China

2020 ◽  
Vol 20 (21) ◽  
pp. 13455-13466
Author(s):  
Zhihao Shi ◽  
Lin Huang ◽  
Jingyi Li ◽  
Qi Ying ◽  
Hongliang Zhang ◽  
...  
Keyword(s):  
Particulate Matter ◽  
Air Quality ◽  
Meteorological Parameters ◽  
Surface Ozone ◽  
Fine Particulate Matter ◽  
Great Influence ◽  
Meteorological Conditions ◽  
Daily Maximum ◽  
Fine Particulate ◽  
Surface O3

Abstract. Meteorological conditions play important roles in the formation of ozone (O3) and fine particulate matter (PM2.5). China has been suffering from serious regional air pollution problems, characterized by high concentrations of surface O3 and PM2.5. In this study, the Community Multiscale Air Quality (CMAQ) model was used to quantify the sensitivity of surface O3 and PM2.5 to key meteorological parameters in different regions of China. Six meteorological parameters were perturbed to create different meteorological conditions, including temperature (T), wind speed (WS), absolute humidity (AH), planetary boundary layer height (PBLH), cloud liquid water content (CLW) and precipitation (PCP). Air quality simulations under the perturbed meteorological conditions were conducted in China in January and July of 2013. The changes in O3 and PM2.5 concentrations due to individual meteorological parameters were then quantified. T has a great influence on the daily maximum 8 h average O3 (O3-8 h) concentrations, which leads to O3-8 h increases by 1.7 in January in Chongqing and 1.1 ppb K−1 in July in Beijing. WS, AH, and PBLH have a smaller but notable influence on O3-8 h with maximum change rates of 0.3 ppb %−1, −0.15 ppb %−1, and 0.14 ppb %−1, respectively. T, WS, AH, and PBLH have important effects on PM2.5 formation of both in January and July. In general, PM2.5 sensitivities are negative to T, WS, and PBLH and positive to AH in most regions of China. The sensitivities in January are much larger than in July. PM2.5 sensitivity to T, WS, PBLH, and AH in January can be up to −5 µg m−3 K−1, −3 µg m−3 %−1, −1 µg m−3 %−1, and +0.6 µg m−3 %−1, respectively, and in July it can be up to −2 µg m−3 K−1, −0.4 µg m−3 %−1, −0.14 µg m−3 %−1, and +0.3 µg m−3 %−1, respectively. Other meteorological factors (CLW and PCP) have negligible effects on O3-8 h (less than 0.01 ppb %−1) and PM2.5 (less than 0.01 µg m−3 %−1). The results suggest that surface O3 and PM2.5 concentrations can change significantly due to changes in meteorological parameters, and it is necessary to consider these effects when developing emission control strategies in different regions of China.

scholarly journals Integrating O<sub>3</sub> influences on terrestrial processes: photosynthetic and stomatal response data available for regional and global modeling

2013 ◽  
Vol 10 (4) ◽  
pp. 6973-7012 ◽  
Author(s):  
D. Lombardozzi ◽  
J. P. Sparks ◽  
G. Bonan
Keyword(s):  
Stomatal Conductance ◽  
Radiative Forcing ◽  
Large Scale ◽  
Surface Ozone ◽  
Plant Interactions ◽  
Global Models ◽  
Response Data ◽  
Integral Role ◽  
Strong Motivation ◽  
Using Data

Abstract. Plants have a strong influence on climate by controlling the transfer of carbon dioxide and water between the biosphere and atmosphere during the processes of photosynthesis and transpiration. Chronic exposure to surface ozone (O3) differentially affects photosynthesis and transpiration because it damages stomatal conductance, the common link that controls both processes, in addition to the leaf biochemistry that only affects photosynthesis. Because of the integral role of O3 in altering plant interactions with the atmosphere, there is a strong motivation to incorporate the influence of O3 into regional and global models. However, there are currently no analyses documenting both photosynthesis and stomatal conductance responses to O3 exposure through time using a standardized O3 parameter that can be easily incorporated into models. Therefore, models often rely on photosynthesis data derived from the responses of one or a few plant species that exhibit strong negative correlations with O3 exposure to drive both rates of photosynthesis and transpiration, neglecting potential divergence between the two fluxes. Using data from the peer-reviewed literature, we have compiled photosynthetic and stomatal responses to chronic O3 exposure for all plant types with data available in the peer-reviewed literature as a standardized function of cumulative uptake of O3 (CUO), which integrates O3 flux into leaves through time. These data suggest that stomatal conductance decreases ~ 11% after chronic O3 exposure, while photosynthesis independently decreases ~ 21%. Despite the overall decrease in both variables, high variance masked any correlations between the decline in photosynthesis or stomatal conductance with increases in CUO. Though correlations with CUO are not easily generalized, existing correlations demonstrate that photosynthesis tends to be weakly but negatively correlated with CUO while stomatal conductance is more often positively correlated with CUO. Results suggest that large-scale models using data with strong negative correlations that only affect photosynthesis need to reconsider the generality of their response. Data from this analysis are now available to the scientific community and can be incorporated into global models to improve estimates of photosynthesis, global land carbon sinks, hydrology, and indirect radiative forcing that are influenced by chronic O3 exposure.

scholarly journals Ozone and NO<sub><i>x</i></sub> chemistry in the eastern US: evaluation of CMAQ/CB05 with satellite (OMI) data

2015 ◽  
Vol 15 (19) ◽  
pp. 10965-10982 ◽  
Author(s):  
T. P. Canty ◽  
L. Hembeck ◽  
T. P. Vinciguerra ◽  
D. C. Anderson ◽  
D. L. Goldberg ◽  
...  
Keyword(s):  
Air Quality ◽  
Rural Areas ◽  
Surface Ozone ◽  
Quality Standards ◽  
Ambient Air ◽  
Daily Maximum ◽  
Air Quality Model ◽  
Air Quality Standards ◽  
The Us ◽  
Us Northeast

Abstract. Regulatory air quality models, such as the Community Multiscale Air Quality model (CMAQ), are used by federal and state agencies to guide policy decisions that determine how to best achieve adherence with National Ambient Air Quality Standards for surface ozone. We use observations of ozone and its important precursor NO2 to test the representation of the photochemistry and emission of ozone precursors within CMAQ. Observations of tropospheric column NO2 from the Ozone Monitoring Instrument (OMI), retrieved by two independent groups, show that the model overestimates urban NO2 and underestimates rural NO2 under all conditions examined for July and August 2011 in the US Northeast. The overestimate of the urban to rural ratio of tropospheric column NO2 for this baseline run of CMAQ (CB05 mechanism, mobile NOx emissions from the National Emissions Inventory; isoprene emissions from MEGAN v2.04) suggests this model may underestimate the importance of interstate transport of NOx. This CMAQ simulation leads to a considerable overestimate of the 2-month average of 8 h daily maximum surface ozone in the US Northeast, as well as an overestimate of 8 h ozone at AQS sites during days when the state of Maryland experienced NAAQS exceedances. We have implemented three changes within CMAQ motivated by OMI NO2 as well as aircraft observations obtained in July 2011 during the NASA DISCOVER-AQ campaign: (a) the modeled lifetime of organic nitrates within CB05 has been reduced by a factor of 10, (b) emissions of NOx from mobile sources has been reduced by a factor of 2, and (c) isoprene emissions have been reduced by using MEGAN v2.10 rather than v2.04. Compared to the baseline simulation, the CMAQ run using all three of these changes leads to considerably better simulation of column NO2 in both urban and rural areas, better agreement with the 2-month average of daily 8 h maximum ozone in the US Northeast, fewer number of false positives of an ozone exceedance throughout the domain, as well as an unbiased simulation of surface ozone at ground-based AQS sites in Maryland that experienced an ozone exceedance during July and August 2007. These modifications to CMAQ may provide a framework for use in studies focused on achieving future adherence to specific air quality standards for surface ozone by reducing emission of NOx from various anthropogenic sectors.

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