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Though drought is known to affect biogenic emissions of volatile organic compounds (BVOC), its effect on isoprene and formaldehyde (HCHO), a high yield product of isoprene, has not been investigated in East Asia where incidences of drought have increased in recent years. In this work, we analyzed the impact of drought on HCHO in the South Korea region during the summer period (June, July, and August) from 2005 to 2018 and found increased HCHO due to drought. The tropospheric HCHO column density retrieved by OMI increased by 8.02 % during extreme drought compared to the non-drought period, whereas no significant effect of drought on the NO 2 column was found. Regional variation of HCHO response to drought correlates significantly with the tree percentage of the region. This correlation indicates that the drought-led HCHO increases are most likely driven by the increase in isoprene emissions during drought. Indeed, model predicts isoprene emissions to be higher by 27.87 % during the extreme drought compared to the non-drought period in South Korea. From 2005 to 2018, the HCHO column has been increasing in South Korea by 0.16 × 10 15 molecules/cm 2 /year (1.56 % per year) during summer months, correlated with the increasing incidences of drought. HCHO increase is linked to higher ozone as most of South Korea is in the NOx-saturated or transitional regime., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

This study investigated the long-term variations in ambient levels of surface ozone, volatile organic compounds (VOCs), and nitrogen oxides (NO x ) within the Houston-Galveston-Brazoria (HGB) region. Analysis of ozone levels revealed an overall reduction in the maximum daily 8-h average ozone (MDA8 O 3 ) from 2000 to 2019 (April-October) with an average rate of ∼ -0.48 ppb/yr across HGB. With a few exceptions, the MDA8 O 3 reduction rates were more pronounced for the monitoring sites closer to the Houston Ship Channel (HSC). Meanwhile, ambient levels of NO x and most VOC species (across the three representative sites as Houston Bayland Park, Haden Road, and Lynchburg Ferry) decreased significantly within the same investigation period, reflecting the impact of emission reductions. The positive matrix factorization (PMF) model applied to the mentioned sites identified regional background ozone, petrochemical emissions, engine combustion, natural gas/fuel evaporation, and solvent/painting/rubber industries as the major sources of MDA8 O 3 . The regional background ozone was the predominant source, accounting for 59-70% of MDA8 O 3 across the three sites. Regarding the local anthropogenic emissions, natural gas/fuel evaporation was the largest contributor (19.5 ± 6.1%) to MDA8 O 3 at Houston Bayland Park, whereas petrochemical facilities (10.9 ± 4.9%) and solvent/painting/rubber industries (18.1 ± 9.5%) were the largest factor at Haden Road and Lynchburg Ferry, respectively. Notable reductions were found in the contributions of petrochemical emissions, engine combustion, and natural gas/fuel evaporation to MDA8 O 3 within 2000-2019, but an increasing trend was revealed in the role of solvent/painting/rubber industries on MDA8 O 3 most probably due to the enhanced demand for their products. Results of this study corroborated the success of emission control policies in limiting ozone precursors and provided useful details for prioritizing emission reduction policies to further reduce ozone pollution in the HGB., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Large day-to-day variability in O(3) and CO was observed at Chongming, a remote rural site east of Shanghai, in August 2010. High ozone periods (HOPs) that typically lasted for 3-5 days with daily maximum ozone exceeding 102 ppb were intermittent with low ozone periods (LOPs) with daily maximum ozone less than 20 ppb. The correlation analysis of ozone with meteorological factors suggests that the large variations of surface ozone are driven by meteorological conditions correlated with the changes in the location and intensity of the west Pacific subtropical high (WPSH) associated with the East Asian summer monsoon (EASM). When the center of WPSH with weaker intensity is to the southeast of Chongming site, the mixing ratios and variability of surface ozone are higher. When the center of WPSH with stronger intensity is to the northeast of Chongming site, the mixing ratios and variability of surface ozone are lower. Sensitivity simulations using the GEOS-Chem chemical transport model indicate that meteorological condition associated with WPSH is the primary factor controlling surface ozone at Chongming in August, while local anthropogenic emissions make significant contributions to surface ozone concentrations only during HOP.

The TRacking Aerosol Convection ExpeRiment – Air Quality (TRACER-AQ) campaign probed Houston air quality with a comprehensive suite of ground-based and airborne remote sensing measurements during the intensive operating period in September 2021. Two post-frontal high-ozone episodes (6–11 and 23–26 September) were recorded during the aforementioned period. In this study, we evaluated the simulation of the planetary boundary layer (PBL) height and the vertical ozone profile by a high-resolution (1.33 km) 3-D photochemical model, the Weather Research and Forecasting (WRF)-driven GEOS-Chem (WRF-GC). We evaluated the PBL heights with a ceilometer at the coastal site La Porte and the airborne High Spectral Resolution Lidar 2 (HSRL-2) flying over urban Houston and adjacent waters. Compared with the ceilometer at La Porte, the model captures the diurnal variations in the PBL heights with a very strong temporal correlation (R>0.7) and ±20 % biases. Compared with the airborne HSRL-2, the model exhibits a moderate to strong spatial correlation (R=0.26 –0.68), with ±20 % biases during the noon and afternoon hours during ozone episodes. For land–water differences in PBL heights, the water has shallower PBL heights compared to land. The model predicts larger land–water differences than the observations because the model consistently underestimates the PBL heights over land compared to water. We evaluated vertical ozone distributions by comparing the model against vertical measurements from the TROPospheric OZone lidar (TROPOZ), the HSRL-2, and ozonesondes, as well as surface measurements at La Porte from a model 49i ozone analyzer and one Continuous Ambient Monitoring Station (CAMS). The model underestimates free-tropospheric ozone (2–3 km aloft) by 9 %–22 % but overestimates near-ground ozone (<50 m aloft) by 6 %-39 % during the two ozone episodes. Boundary layer ozone (0.5–1 km aloft) is underestimated by 1 %–11 % during 8–11 September but overestimated by 0 %–7 % during 23–26 September. Based on these evaluations, we identified two model limitations, namely the single-layer PBL representation and the free-tropospheric ozone underestimation. These limitations have implications for the predictivity of ozone's vertical mixing and distribution in other models. [ABSTRACT FROM AUTHOR]

The Tracking Aerosol Convection Experiment Air Quality (TRACER-AQ) campaign probed Houston air quality with a comprehensive suite of ground-based and airborne remote sensing measurements during the intensive operating period in September 2021. Two post-frontal high-ozone episodes (September 6–11 and 23–26) were recorded during the said period. In this study, we evaluated the simulation of the planetary boundary layer (PBL) height and the vertical ozone profile by a high-resolution (1.33 km) 3-D photochemical model, Weather Research and Forecasting (WRF)-driven GEOS-Chem (WRF-GC). We contrasted the model performance between ozone-episode days and non-episode days. The model captures the diurnal variations of the PBL during ozone episodes (R = 0.72–0.77; normal mean bias (NMB) = 3 %–22 %) and non-episode days (R = 0.88; NMB = -21 %), compared with the ceilometer at La Porte. Land-water differences in PBL heights are captured better during non-episode days than episode days, compared with the airborne High Spectral Resolution Lidar-2 (HSRL-2). During ozone episodes, the simulated land-water differences are 50–60 m (morning), 320–520 m (noon), and 440–560 m (afternoon) in comparison with the observed values of 190 m, 130 m, and 260 m, respectively. During non-episode days, the simulated land-water differences are 140–220 m (morning) and 360–760 m (noon) in comparison with the observed values of 210 m and 420 m, respectively. For vertical ozone distributions, the model was evaluated against vertical profile measurements from the Tropospheric Ozone lidar (TROPOZ), the HSRL-2, and ozonesondes, as well as at the surface from a model 49i ozone analyzer and a site from the Continuous Ambient Monitoring Stations (CAMS) at La Porte. The model underestimates free tropospheric ozone (2–3 km aloft) by 9 %–22 % but overestimates near-ground ozone (< 50 m aloft) by 6 %–39 % during the two ozone episodes. Boundary layer ozone (0.5–1 km aloft) is underestimated by 1 %–11 % during September 8–11 but overestimated by 0 %–7 % during September 23–26. Based on these evaluations, we identified two model limitations: the single-layer PBL representation and free tropospheric ozone underestimation. These limitations have implications for the predictivity of ozone's vertical mixing and distribution in other models. [ABSTRACT FROM AUTHOR]

Coastal regions are susceptible to multiple complex dynamic and chemical mechanisms and emission sources that lead to frequently observed large tropospheric ozone variations. These large ozone variations occur on a mesoscale and have proven to be arduous to simulate using chemical transport models (CTMs). We present a clustering analysis of multi-dimensional measurements from ozone lidar in conjunction with both an offline GEOS-Chem chemical-transport model (CTM) simulation and the online GEOS-Chem simulation GEOS-CF, to investigate the vertical and temporal variability of coastal ozone during three recent air quality campaigns: 2017 Ozone Water-Land Environmental Transition Study (OWLETS)-1, 2018 OWLETS-2, and 2018 Long Island Sound Tropospheric Ozone Study (LISTOS). We developed and tested a clustering method that resulted in five ozone profile curtain clusters. The established five clusters all varied significantly in ozone magnitude vertically and temporally, which allowed us to characterize the coastal ozone behavior. The lidar clusters provided a simplified way to evaluate the two CTMs for their performance of diverse coastal ozone cases. An overall evaluation of the models reveals good agreement (R≈0.70) in the low-level altitude range (0 to 2000 m), with a low and unsystematic bias for GEOS-Chem and a high systemic positive bias for GEOS-CF. The mid-level (2000–4000 m) performances show a high systematic negative bias for GEOS-Chem and an overall low unsystematic bias for GEOS-CF and a generally weak agreement to the lidar observations (R=0.12 and 0.22, respectively). Evaluating cluster-by-cluster model performance reveals additional model insight that is overlooked in the overall model performance. Utilizing the full vertical and diurnal ozone distribution information specific to lidar measurements, this work provides new insights on model proficiency in complex coastal regions. [ABSTRACT FROM AUTHOR]

Drought is an extreme weather and climate event that has been shown to cause the worsening of ozone (O3) air pollution. Using 15‐year (2005–2019) surface O3 observations and weekly US Drought Monitor (USDM) indices, this study estimated that summertime US‐mean surface O3 increased by 1.47 ppb per USDM level. It is revealed that O3 responses to drought display a spatial east‐west variation: higher O3 enhancement in the Southeast (2.24 ppb/USDM), and no significant change or even a decrease in the west (e.g., −0.06 ppb/USDM in California). The diurnal changes of O3 with drought also show an opposite pattern between the Southeast and California. Formaldehyde (HCHO) and nitrogen dioxide (NO2) column, two satellite‐based O3 precursors proxies, show an increasing rate of 0.41 × 1015 molec/cm2/USDM and 0.03 × 1015 molec/cm2/USDM in the Southeast, respectively, while these rates are not statistically significant in California. We explained this spatial discrepancy from the perspective of O3 chemistry by applying a zero‐dimensional model at the sites with long‐term observations in California and Georgia. Isoprene concentrations decreased by ∼37% under exceptional drought in California causing a reduction of O3 production (PO3) by ∼23.7% during daytime. On the contrary, isoprene increased by ∼41% in Georgia inducing a consequent increase of PO3 by ∼33.4% which accounts for more than half of the O3 enhancement. This study reveals the key role of biogenic isoprene on ozone chemistry under drought conditions. Key Points: O3 responses to drought display an east‐west discrepancy in the contiguous US: higher enhancement in the southeast yet decrease in the westThe decrease of O3 in California is due to the up to 24% reduction of the O3 production rate (PO3) caused by the decrease of isopreneThe increase of isoprene in the southeast causes an increase of PO3 by up to 33%, accounting for more than half of the O3 enhancement [ABSTRACT FROM AUTHOR]

Prior work suggests drought exacerbates US air quality by increasing surface ozone concentrations. We analyze 2005–2015 tropospheric column concentrations of two trace gases that serve as proxies for surface ozone precursors retrieved from the OMI/Aura satellite: Nitrogen dioxide (ΩNO2; NOx proxy) and formaldehyde (ΩHCHO; VOC proxy). We find 3.5% and 7.7% summer drought enhancements (classified by SPEI) for ΩNO2 and ΩHCHO, respectively, corroborating signals previously extracted from ground‐level observations. When we subset by land cover type, the strongest ΩHCHO drought enhancement (10%) occurs in the woody savannas of the Southeast US. By isolating the influences of precipitation and temperature, we infer that enhanced biogenic VOC emissions in this region increase ΩHCHO independently with both high temperature and low precipitation during drought. The strongest ΩNO2 drought enhancement (6.0%) occurs over Midwest US croplands and grasslands, which we infer to reflect the sensitivity of soil NOx emissions to temperature. Plain Language Summary: Projected increases in drought severity and frequency for this century raise questions regarding possible impacts on air quality. Surface ozone, an air pollutant estimated to cause over 1 million annual premature deaths globally, forms when its precursor gases react in the sunlit atmosphere. These precursor gases depend on temperature and precipitation and thus can respond to drought. We analyze over a decade of satellite observations of two trace gases relevant to ozone formation and find that, on average, their concentrations increase during summer droughts in the Eastern US. While higher temperatures during droughts are usually associated with observed increases in trace gas concentrations, in the Southeast US we find increases associated with low precipitation independent of temperature. Satellite detection of these changes implies promise for application to other regions and more generally for improving mechanistic understanding of air quality responses to drought and other climate extremes. Key Points: Satellite retrievals of tropospheric NO2 and HCHO show drought enhancements of 3.5% and 7.7%, respectively, during Eastern US summersLow precipitation and high temperatures both independently drive HCHO drought enhancement (10%) in Southeast US woody savannasHigh temperatures drive NO2 drought enhancement (6.0%) in Midwest US croplands and grasslands [ABSTRACT FROM AUTHOR]

In coastal environments, sea breeze recirculation has been found to be an important mesoscale meteorological phenomenon that causes high ozone episodes, yet the identification of this small‐scale circulation pattern remains difficult. In this study, a new method was developed to automatically identify sea breeze recirculation in Houston, TX, by applying K‐Means clustering algorithm to surface winds measurements at near‐coast sites during the DISCOVER‐AQ (Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) field campaign period from August to October 2013. The key to the clustering algorithm is seven features derived from site‐based surface winds on each day, including zonal (U) and meridional (V) winds in the morning and afternoon, 24‐hr transport direction (θ) and the recirculation factor, which is the ratio of net transport distance (L) to wind run distance (S). For comparison, the same clustering was applied to San Antonio, TX, a noncoastal city yet within the synoptic‐scale distance from Houston. Four clusters were obtained for each region, including three synoptic patterns common to both regions and one mesoscale pattern that differs by region, classified as Stagnation and Sea Breeze Cluster for San Antonio and Houston, respectively. The clustering outputs were verified by wind profiler data in Houston. By linking the wind clusters with surface and aircraft ozone measurements, we revealed a clear connection between circulation patterns and daily ozone variability showing that maximum daily average 8‐hr (MDA8) ozone levels and spatial distributions differed by cluster type (e.g., the highest ozone found in the Stagnation/Sea Breeze Cluster and the lowest ozone in the Southerly Cluster). This automatable method of sea breeze identification we developed can be potentially applied to other coastal cities because it has low data requirement and no ad hoc location‐specific adjustments. Key Points: A new method was developed to automatically identify sea breeze recirculation in Houston, TX, by applying K‐Means clustering algorithmThis method has low data requirement and no ad hoc location‐specific adjustments and thus can be potentially applied to other coastal citiesResulting wind clusters in Houston can be linked with specific ozone levels and spatial distributions [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The diurnal cycle of surface ozone is directly influenced by the chemistry and meteorological processes which affect a region. This study uniquely employs a clustering methodology to examine the complete diurnal pattern of surface ozone for the Houston‐Galveston‐Brazoria region and links the identified patterns to meteorological regimes for June, July, and August of 3 years (2011, 2014, and 2015). Four features were implemented into the clustering algorithm: ozone rate of decrease at night, daily minimum before sunrise, rate of increase after sunrise, and an average of afternoon ozone. Four clusters were chosen, ranging from a mostly flat diurnal pattern with low mixing ratios (~20 ppbv) throughout the day (Cluster 1), to a more variable diurnal cycle with very high mixing ratios (>70 ppbv) in the afternoon (Cluster 4). The clusters are found to associate with distinctive circulation patterns and well‐known regional meteorological processes, such as the low‐level jet and Bermuda High. The uneven distribution of the clusters between the years helps elucidate ozone interannual variability due to meteorology: Cluster 4 had 0 days assigned from 2014 due to the greater influence of circulation patterns bringing clean air from the Gulf of Mexico. We show that the clustering method better characterizes ozone variability than the simplistic method of dividing peak ozone into quantiles. With the clustering analysis, we demonstrate that the ozone diurnal pattern holds more value than just peak ozone hours of the day in providing a clearer understanding of ozone variability and associated meteorological processes. Key Points: We developed a new method clustering diurnal ozone patterns that better characterizes ozone variability in Houston than peak ozone aloneClustering was heavily influenced by early morning ozone values and showed a strong relationship to peak afternoon ozoneUneven distributions of clusters between years elucidate interannual ozone variability and link to synoptic meteorological processes [ABSTRACT FROM AUTHOR]

Abstract: We use a global chemical transport model (GEOS-Chem) driven by the GISS GCM to investigate the effect on China's surface ozone from 2000 to 2050 global changes in climate and anthropogenic emissions as projected by the IPCC A1B scenario, with a focus on the different response between East and West China where present-day anthropogenic emissions, natural conditions, and ozone source attributions differ significantly. Over East China, climate change will increase both surface ozone and the possibility of high ozone episodes, implying a significant ‘climate change penalty’ that can be attributed mainly to increasing biogenic emissions of volatile organic compounds (VOCs). Over West China on the other hand, climate change will decrease mean surface ozone as a result of an increased ozone destruction rate in low-NO x regimes, assuming constant stratosphere–troposphere exchange (STE) of ozone. Chinese emissions change in 2050 will enlarge the East–West ozone difference in China, but emissions change from the rest of the world (excluding China) will decrease it. Driven by climate change and emissions change in combination, nation-mean surface ozone will increase, whereas East–West ozone contrast will decrease. In the future climate, the sensitivity of surface ozone to a given change in Chinese emissions will decrease over West China due to the accelerated ozone destruction rate and reduced transport from East China, but increase over East China as a result of the coupling effect between anthropogenic NO x and biogenic VOCs. The latter result suggests that the emission controls over East China need to be more aggressive in future climate. [Copyright &y& Elsevier]

The policy-relevant background (PRB) ozone is defined by the US Environmental Protection Agency (EPA) as the surface ozone concentration that would be present over the US in the absence of North American anthropogenic emissions. It is intended to provide a baseline for risk and exposure assessments used in setting the National Ambient Air Quality Standard (NAAQS). We present here three-year statistics (2006–2008) of PRB ozone over the US calculated using the GEOS-Chem global 3-D model of atmospheric composition with 1/2° × 2/3° horizontal resolution over North America and adjacent oceans (2° × 2.5° for the rest of the world). We also provide estimates of the US background (no anthropogenic US emissions) and natural background (no anthropogenic emissions worldwide and pre-industrial methane). Our work improves on previous GEOS-Chem PRB estimates through the use of higher model resolution, 3-year statistics, better representation of stratospheric influence, and updated emissions. PRB is particularly high in the intermountain West due to high elevation, arid terrain, and large-scale subsidence. We present for this region a detailed model evaluation showing that the model is successful in reproducing ozone exceedances up to 70 ppbv. However, the model cannot reproduce PRB-relevant exceptional events associated with wildfires or stratospheric intrusions. The mean PRB estimates for spring–summer are 27 ± 8 ppbv at low-altitude sites and 40 ± 7 ppbv at high-altitude sites. Differences between the PRB simulation and the natural simulation indicate a mean enhancement from intercontinental pollution and anthropogenic methane of 9 ppbv at low-altitude sites and 13 ppbv at high-altitude sites. The PRB is higher than average when ozone exceeds 60 ppbv, particularly in the intermountain West. Our PRB estimates are on average 4 ppbv higher than previous GEOS-Chem studies and we attribute this to higher lighting, increasing Asian emissions, and improved model resolution. Whereas previous studies found no occurrences of PRB exceeding 60 ppbv, we find here some occurrences in the intermountain West. The annual 4th-highest PRB values in the intermountain West are typically 50–60 ppbv, as compared to 35–45 ppbv in the East or on the West Coast. Such high PRB values in the intermountain West suggest that special consideration of this region may be needed if the ozone NAAQS is decreased to a value in the 60–70 ppbv range., Earth and Planetary Sciences, Engineering and Applied Sciences

Large intra-season differences in mixing ratios of CO and \(O_3\) were detected at Miyun, a rural site north of Beijing, in summer 2006. Despite an increase in mean daytime mixing ratio of CO from 500 ppbv in June to 700 ppbv in July, mean daytime \(O_3\) dropped from 67 ppbv in June to 50 ppbv in July and August. The observed changes in CO and \(O_3\) are attributed to the influence of the summer monsoonal circulation that develops over the North China Plain in July. Photochemical production of \(O_3\) is reduced as a consequence of increased cloudiness during July and August, as indicated by the strong negative correlation observed between \(O_3\) and satellite observations of cloud optical depth, with cloudiness having little effect on CO. The analysis suggests a strategy for emission controls that could be implemented in an economically efficient manner to minimize the frequency of high levels of \(O_3\) during summer in Beijing., Engineering and Applied Sciences

We investigated the impacts of cold fronts on area-wide peak O 3 and regional background O 3 mixing ratios on a daily scale over the Houston-Galveston-Brazoria (HGB) area of southeastern Texas during the O 3 seasons (April–October) of 2003–2016. Back trajectories showed that an 18h time lag existed between arrival of cold fronts in the HGB area and onset of a predominately northerly flow. Cold fronts showed increasing effects on both peak and background O 3 over the HGB area. Compared to no front days, average peak O 3 mixing ratios during the cold front 1st days, cold front 2+ days, and post frontal days increased 0.7, 5.9, and 9.0 ppbv, respectively while average background O 3 increased 2.9, 6.8, and 8.6 ppbv, respectively. The change in wind direction from southerly to northerly was the most important factor causing increasing O 3 levels. Wind direction shifts caused variation of other meteorological factors (i.e., wind speed, precipitation, temperature, cloud cover, and relative humidity) and tended to overshadow their effects on O 3 over the HGB area. On a long-term and large-scale view, cold fronts over the HGB area could be regarded as interruptions in the cleansing effects of predominantly marine southerly flow from the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

Cold fronts are mesoscale meteorological phenomena that play important roles in chemical transport processes. Effects of a single cold front episode on surface ozone may vary since meteorological factors may alter ozone in different (increasing or decreasing) directions. We studied cold front events and their effects on surface maximum daily 8-h average (MDA8) ozone over the Contiguous United States (CONUS) during 2003–2017. The average number of cold front days in the CONUS was 50.02 days per year per site with a slightly increasing trend (1.06 per year) during 2003–2017. The Rocky Mountains blocked most cold fronts for the Northwest and the Southwest. Percentages of 24 h back trajectories from north side in 12 sample cities show that cold front effects exist ±12 h to cold fronts' arrival at least. Compared to all days' average, MDA8 ozone differs from prefrontal days, cold front 1st days, cold front 2 + days, and post frontal days by 0.10, −0.94, −0.84, and −1.89 ppbv (−0.89 ppbv on average), respectively. By contrast, they overlapped 9.85%, 5.82%, 0.89%, and 5.04%, respectively (21.60% in total) with ozone exceedance days. Regional consistent long-term effects of cold fronts on surface MDA8 ozone have been found. Enhancements during prefrontal days can be seen in the Northeast and the Midwest during non-winter seasons. For cold front days and post frontal days, ozone differences in the Southwest, the Northwest, and the Northeast were negative in non-winter seasons then reversed to positive in winter. The areas along the coast of the Gulf of Mexico showed positive ozone difference in non-winter seasons and expanded to the inland area during summer then reversed to negative in winter. Synergy of cold fronts and other climatic and meteorological factors (e.g., prefrontal stagnation, southerly flow from the Gulf of Mexico, and winter stratospheric intrusions) can preliminarily explain these consistent long-term effects. • Cold front effects could exist ±12 h to cold fronts' arrival. • Cold fronts showed seasonal consistent effects on surface ozone in some regions. • The consistent effects reserved during winter in some regions. • Finding common features of cold fronts preliminarily explained consistent effects. [ABSTRACT FROM AUTHOR]