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A simple compartmental model is used to simulate the average (climatic) annual and seasonal distribution of ozone in the Northern Hemisphere midlatitude troposphere. The model (reminiscent of earlier examples) consists of a circular set of segments - 4 marine and 5 continental - which span the globe. Each segment consists of two well-mixed compartments, one representing the free troposphere overlying one representing the boundary layer. A minimal set of parameters with values taken from generally accepted and measured behavior is used to describe the ozone sources and sinks within the segments and inter-compartmental flow and mixing. The model accurately simulates measured seasonal cycles of ozone throughout the midlatitude troposphere. By virtue of relatively rapid circular zonal flow the model reproduces the observed nearly uniform free troposphere that behaves as a reservoir, responding to the combined boundary layer and stratospheric inputs. The distinctive seasonal cycles of the free troposphere and the marine boundary layer are well reproduced by the strong seasonal dependence of the loss mechanism initiated by ozone photolysis that yields O1D. The critical role of the marine boundary layer in the global ozone balance and the constraints that it places on the net continental production are clearly revealed. Sensitivity analysis identifies which of the basic set of process parameters most require better understanding. Minimalist models such as this, used in conjunction with detailed CTM model simulations, can help provide a comprehensive understanding of the climatic behavior and trends in tropospheric ozone.

Archived Ozone Design Values (ODVs) provide smoothed temporal records of maximum ozone concentrations impacting monitoring sites throughout the US. Utilizing time series of ODVs recorded at sites along the US West Coast, we separately estimate ODV contributions from US background ozone and from production driven by US anthropogenic precursor emissions. Sondes launched from Trinidad Head in northern California measure the vertical distribution of baseline ozone transported ashore from the Pacific; this profile is reflected in the increase of the US background ODV contribution with monitoring site elevation in both rural and urban areas. The ODVs that would result from US background ozone alone are small at coastal, sea level locations (average ~45 ppb), but increase with altitude; above 1 km US background ODVs can exceed 60 ppb. US background ozone contributions now constitute the majority of the maximum ODVs throughout the US west coast region, including the Los Angeles urban area, which records the country's highest ODVs. US anthropogenic emissions presently cause enhancements of 35 to 55 ppb to the maximum ODVs in the Los Angeles area; thus, local emission controls can further reduce ozone even though the background contribution is larger. In other US west coast urban areas ODV enhancements from US anthropogenic emissions are much smaller than the US background ODV contribution. The past decrease in US anthropogenic ODV enhancements from emission controls is larger than generally realized - a factor of more than 6 from 1980 to 2020, while US background ODV contributions varied to only a small extent over those four decades. Wildfire impacts on ODVs are significant in urban areas of the Pacific Northwest, but not over the vast northern US rural region. There is an indication that agricultural emissions of nitrogen oxides in California's Salinas Valley increase downwind maximum ODVs by 5-10 ppb.Implications: In 2020 the ozone design values (ODVs) resulting from transported background ozone alone are now larger than the ODV enhancements from US anthropogenic precursor emissions, even in the Los Angeles urban area, where the nation's highest ODVs are recorded. The US anthropogenic ODV enhancements have been reduced by more than a factor of 6 from 1980 to 2020. The maximum US background ODV contributions have varied somewhat, but in each of the US west coast urban areas it was 60 ppb or larger in 2000. These contributions are so large that reducing maximum urban ODVs to the 70 ppb required by the 2015 ozone NAAQS is very difficult. There remains relatively little room for further reducing ODVs through domestic emission controls alone. From this perspective, degraded US ozone air quality in the western US is primarily due to the US background ozone contribution, with the US anthropogenic enhancement making a significant, but smaller contribution. Notably, the US background ODV has slowly decreased (~1 ppb decade-1; Parrish, Derwent, and Faloona 2021) since the mid-2000s; cooperative, international emission control efforts aimed at continuing or even accelerating this background ozone decrease may be an effective approach to further ODV reductions, since the US background ODV is largely due to a hemisphere-wide, transported reservoir of ozone with contributions from all northern midlatitude continents. Given the major contribution of background ozone to observed ODVs, future reviews of the ozone NAAQS will be better informed if observational-based estimates of background ODV contributions are considered, in addition to model-derived estimates upon which past reviews have solely relied.

Quantification of the magnitude and long-term changes in ozone concentrations transported into the U.S. is important for effective air quality policy development. We synthesize multiple published trend analyses of western U.S. baseline ozone, and show that all results are consistent with an overall, non-linear change - a rapid increase (~5 ppb/decade) during the 1980s that slowed in the 1990s, maximized in the mid-2000s, and was followed by a slow decrease (~1 ppb/decade) thereafter. This non-linear change accounts for ~2/3 of the variance in 28 published linear trend analyses; we attribute the other 1/3 of the variance to unquantified autocorrelation in the analyzed data sets that result primarily from meteorologically driven interannual ozone variability. Recent systematic changes in baseline ozone on the U.S. West Coast have been relatively small - the standard deviation of the 2-year means over the 1990-2017 period is 1.5 ppb. International efforts to reduce anthropogenic precursor emissions from all northern mid-latitude sources could possibly reduce baseline ozone concentrations, thereby improving U.S. ozone air quality.Implications: Ozone is an air pollutant with significant human and ecological health impacts. Air masses transported into the western U.S. from over the Pacific Ocean carry ozone concentrations that are, on average, a large fraction of the U.S. health standard. The US EPA policy assessment conducted for the recent review of the ozone National Ambient Air Quality Standard (NAAQS) found that 2016 regional average MDA8 ozone concentrations in the western US maximized in summer at ~52 ppb and that ~40 ppb of that maximum was contributed by ozone of natural and transported anthropogenic contributions. Thus, quantifying these trans-boundary background ozone concentrations has been identified as an important issue for a complete understanding of US air quality. Published analyses of temporal trends of these transported ozone concentrations vary widely, from early reports of increases to more recent reports of decreases. We show that the long-term ozone changes are nonlinear, with substantial concentration increases (as large as ~5 ppb/decade) before the mid-2000s when a maximum is reached, followed by a small decrease of ~1 ppb/decade thereafter. Superimposed on the overall changes is significant interannual variability that makes accurate determination of systematic trends over decade-scale time periods uncertain. The end of the previously increasing trends, and the recent decrease in transported ozone concentrations, is a good news for U.S. air quality, as it eases the difficulty of achieving the ozone air quality standard.

Nighttime reactions of nitrogen oxides influence ozone, volatile organic compounds, and aerosol and are thus important to the understanding of regional air quality. Despite large emissions and rapid recent growth of nitrogen oxide concentrations, there are few studies of nighttime chemistry in China. Here we present measurements of nighttime nitrogen oxides, NO3 and N2O5, from a coastal mountaintop site in Hong Kong adjacent to the megacities of the Pearl River Delta region. This is the first study of nighttime chemistry from a site within the residual layer in China. Key findings include the following. First, highly concentrated urban NOx outflow from the Pearl River Delta region was sampled infrequently at night, with N2O5 mixing ratios up to 8 ppbv (1 min average) or 12 ppbv (1 s average) in nighttime aged air masses. Second, the average N2O5 uptake coefficient was determined from a best fit to the available steady state lifetime data as γ(N2O5) = 0.014 ± 0.007. Although this determination is uncertain due to the difficulty of separating N2O5 losses from those of NO3, this value is in the range of previous residual layer determinations of N2O5 uptake coefficients in polluted air in North America. Third, there was a significant contribution of biogenic hydrocarbons to NO3 loss inferred from canister samples taken during daytime. Finally, daytime N2O5 mixing ratios were in accord with their predicted photochemical steady state. Heterogeneous uptake of N2O5 in fog is determined to be an important production mechanism for soluble nitrate, even during daytime.

We utilize a simple, observational-based model to quantitatively estimate the US anthropogenic, background and wildfire contributions to the temporal and spatial distributions of maximum ozone concentrations throughout the southwestern US, including Texas and parts of California. The very different temporal variations of the separate contributions provide the basis for this analysis: over the past four decades the anthropogenic contribution has decreased at an approximately exponential rate by a factor of ~6.3, while the US background concentration rose significantly through the 1980s and 1990s, reached a maximum in the mid-2000s, and has since slowly decreased. We primarily analyze ozone design values (ODVs), an extreme value statistic of relatively rare maximum ozone concentrations upon which the US National Ambient Air Quality Standards (NAAQS) are based; ODV time series provide spatially and temporally resolved records of maximum ozone concentrations throughout the country. Recent contributions of US background ozone to ODVs (primarily due to transported baseline ozone) are 64 to 70 ppb over most of the southwestern US, and wildfires (also generally considered a background contribution) add further enhancements of 2 to 6 ppb in southwestern US urban areas. US anthropogenic emissions from urban and industrial sectors now produce only relatively modest enhancements to ODVs (less than ~6 ppb in 2020) outside of the three largest urban areas considered (Dallas, Houston and Los Angeles), where the 2020 enhancements were in the 17 to 30 ppb range. As a consequence, US background ozone concentrations now dominate over US anthropogenic contributions in the western US, including the Los Angeles urban basin, where the largest US ozone concentrations are observed. This finding has several implications: 1) A pronounced shift in the spatial distribution of maximum US ozone concentrations has occurred; once ubiquitous nearly nationwide, ODVs of 75 ppb or greater have nearly disappeared in the eastern US, but are still frequent in the southwestern US. 2) By 2021, the trend of maximum ODVs in two major eastern urban areas (i.e., New York City and Atlanta) had decreased to the point that they were smaller than those in smaller southwestern US urban areas, and nearly as small as ODVs recorded at isolated rural southwestern US sites. 3) Together, the US background plus wildfire contributions approach or exceed the US NAAQS for ozone of 70 ppb (implemented in 2015) and 75 ppb (implemented in 2008); consequently, in the southwestern US NAAQS achievement has been precluded. 4) Alternate emission control approaches may provide more effective approaches to air quality improvement; since background ozone makes the dominant contribution to even the highest observed concentrations, an international effort to reduce northern midlatitude baseline ozone concentrations could be pursued, or a standard based on the anthropogenic increment above the regionally-varying US background ozone concentration could be considered to provide a regionally uniform challenge of standard achievement. 5) The predominant contribution of US background ozone across the southwestern US presents a profound challenge for air quality modelling, since a manifold of stratospheric and tropospheric processes occurring on small spatial scales, but over hemisphere-wide distances, must be treated in detail to predict present and future background contributions to daily maximum ozone concentrations at local scales. [ABSTRACT FROM AUTHOR]

During the CalNex (California Research at the Nexus of Air Quality and Climate Change) field study in May-June 2010, measurements of volatile organic compounds (VOCs) were performed in the Los Angeles (LA) basin onboard a NOAA research aircraft and at a ground site located in Pasadena. A weekday-weekend effect in ozone, caused by lower NOx emissions due to reduced diesel truck traffic in the weekends, has been previously observed in Los Angeles and other cities. Measurements in the Caldecott tunnel show that emission ratios of VOCs do not vary with the day of the week, but measurements during CalNex2010 show a VOC weekday-weekend effect through faster photochemical processing at lower ambient NOx mixing ratios. Ambient VOC enhancement ratios of long-lived species such as benzene are the same between weekdays and weekends, whereas enhancement ratios of short-lived species, such as trimethyl benzene, are up to a factor of three lower on weekends. Based upon the observed differences in VOC enhancement ratios to CO, we determine that photochemical processing was on average 65%-75% faster on weekends during CalNex2010, which indicates that ambient OH radical concentrations were larger by this factor causing the observed change in VOC composition. A box model calculation based on the Master Chemical Mechanism was used to verify the increase in photochemical processing in the weekends. Key PointsSpatial and temporal photochemical processing in Los AngelesVOC weekday-weekend effectFaster photochemistry in weekends ©2013. American Geophysical Union. All Rights Reserved.

Detailed airborne, surface, and subsurface chemical measurements, primarily obtained in May and June 2010, are used to quantify initial hydrocarbon compositions along different transport pathways (i.e., in deep subsurface plumes, in the initial surface slick, and in the atmosphere) during the Deepwater Horizon oil spill. Atmospheric measurements are consistent with a limited area of surfacing oil, with implications for leaked hydrocarbon mass transport and oil drop size distributions. The chemical data further suggest relatively little variation in leaking hydrocarbon composition over time. Although readily soluble hydrocarbons made up ∼25% of the leaking mixture by mass, subsurface chemical data show these compounds made up ∼69% of the deep plume mass; only ∼31% of the deep plume mass was initially transported in the form of trapped oil droplets. Mass flows along individual transport pathways are also derived from atmospheric and subsurface chemical data. Subsurface hydrocarbon composition, dissolved oxygen, and dispersant data are used to assess release of hydrocarbons from the leaking well. We use the chemical measurements to estimate that (7.8 ± 1.9) × 10(6) kg of hydrocarbons leaked on June 10, 2010, directly accounting for roughly three-quarters of the total leaked mass on that day. The average environmental release rate of (10.1 ± 2.0) × 10(6) kg/d derived using atmospheric and subsurface chemical data agrees within uncertainties with the official average leak rate of (10.2 ± 1.0) × 10(6) kg/d derived using physical and optical methods.

Airborne measurements of volatile organic compounds (VOCs) were performed during CalNex 2010 (California Research at the Nexus of Air Quality and Climate Change) in the Los Angeles (LA) basin in May-June 2010 and during ITCT2k2 (Intercontinental Transport and Chemical Transformation) in May 2002. While CO2 enhancements in the basin were similar between the two years, the δ CO/DCO2 ratio had decreased by about a factor of two. The δ VOC/δCO; emission ratios stayed relatively constant between the two years. This indicates that, relative to CO2, VOCs in the LA basin also decreased by about a factor of two since 2002. These data are compared with the results from various previous field campaigns dating back as early as 1960 and from the extensive air quality monitoring system in the LA basin going back to 1980. The results show that the mixing ratios of VOCs and CO have decreased by almost two orders of magnitude during the past five decades at an average annual rate of about 7.5%. Exceptions to this trend are the small alkanes ethane and propane, which have decreased slower due to the use and production of natural gas. A comparison with trends in London, UK shows that, due to stricter regulations at the time, VOC mixing ratios in LA decreased earlier than in London, albeit at a slower rate, such that typical mixing ratios in both cities in 2008 were at about the same level. © 2012. American Geophysical Union. All Rights Reserved.

Methane (CH4) and carbon monoxide (CO) mixing ratios were measured at an air quality monitoring station near the Mt. Wilson (MW) Observatory in southern California starting in the spring of 2007. Diurnal variation and mixing ratio correlation (R2 = 0.81) were observed. The correlation results observed agree with previous aircraft measurements collected over the greater Los Angeles (LA) metropolitan area. The consistent agreement between CH4 and CO indicates these gases are well-mixed before reaching the sampling site and the emission source contributions of both compounds are reasonably constant. Since CH4 and CO are considered non-reactive on the time scale of dispersion within the LA urban area and their emission sources are likely to be similarly distributed (e.g., associated with human activities) they are subject to similar scales of atmospheric transport and dilution. This behavior allows the relationship of CH4 and CO to be applied for estimation of CH4 emissions using well-documented CO emissions. Applying this relationship a "top-down" CH4 inventory was calculated for LA County based on the measurements observed at MW and compared with the California Air Resources Board (CARB) "bottom-up" CH4 emissions inventory based on the Intergovernmental Panel on Climate Change recommended methodologies. The "top-down" CH4 emissions inventory is approximately one-third greater than CARB's "bottom-up" inventory for LA County. Considering the uncertainties in both methodologies, the different CH4 emissions inventory approaches are in good agreement, although some under and/or uninventoried CH4 sources may exist.

Elevated tropospheric ozone concentrations driven by anthropogenic precursor emissions are an environmental hazard scientifically similar to the depletion of the stratospheric ozone layer and global climate change; however, the tropospheric ozone issue lacks the generally accepted, international assessment efforts that have greatly informed our understanding of the other two. Here, we briefly review those successful science-into-policy approaches and outline the elements required to conduct a similar process for tropospheric ozone. Particular emphasis is placed on the need to establish a conceptual model to fully understand the underpinning science, useful policy metrics, and motivating international policy forums for regulating anthropogenic ozone production over the hemispheric and global scales, thereby expanding beyond the traditional regional, air basin approach that has dominated air quality regulatory philosophy to date. [ABSTRACT FROM AUTHOR]

A non-linear change in baseline ozone concentrations at northern midlatitudes has been quantified over preceding decades. During the past few years several studies, using linear trend analyses, report relatively small trends over selected time periods - results inconsistent with the earlier developed picture. We show that reported COVID-19 related ozone changes in the background troposphere based on the linear analysis are significantly larger than those derived considering recent longterm decreases in background ozone, which the linear trend analyses do not quantify. We further point out that the extensive loss of lower stratospheric ozone in the unprecedented 2020 springtime Arctic stratospheric ozone depletion event likely reduced the natural source to the troposphere rendering the background anomalously low that year. Consideration of these two issues indicates that the COVID-19 restrictions had a much smaller impact on background tropospheric ozone in 2020 than previously reported. A consensus understanding of baseline ozone changes and their causes is important for formulating policies to improve ozone air quality; cooperative, international emission control efforts aimed at continuing or even accelerating the ongoing decrease in in hemisphere-wide background ozone concentrations may be the most effective approach to further reducing urban and rural ozone in the more developed northern midlatitude countries, as well as improving ozone air quality in all countries within these latitudes. Analysis of baseline ozone measurements over several years following the COVID-19 impact is expected to provide a firm basis for resolving the inconsistencies between the two views of long-term northern midlatitude ozone changes and better quantifying the COVID-19 impact. [ABSTRACT FROM AUTHOR]

High-resolution dropwindsonde and in-flight measurements collected by a research aircraft during the Severe Clear-Air Turbulence Colliding with Aircraft Traffic (SCATCAT) experiment and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Spectral, wavelet, and structure function analyses performed with the 25-Hz in situ data are used to investigate the relationship between gravity waves and turbulence. Mesoscale dynamics are analyzed with the 20-km hydrostatic Rapid Update Cycle (RUC) model and a nested 1-km simulation with the nonhydrostatic Clark-Hall (CH) cloud-scale model. Turbulence occurred in association with a wide spectrum of upward propagating gravity waves above the jet core. Inertia-gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold. Turbulent kinetic energy fields forecast by the RUC and CH models displayed a strongly banded appearance associated with these mesoscale gravity waves (horizontal wavelengths of ~120-216 km). Smaller-scale gravity wave packets (horizontal wavelengths of 1-20 kin) within the mesoscale wave field perturbed the background wind shear and stability, promoting the development of bands of reduced Richardson number conducive to the generation of turbulence. The wavelet analysis revealed that brief episodes of high turbulent energy were closely associated with gravity wave occurrences. Structure function analysis provided evidence that turbulence was most strongly forced at a horizontal scale of 700 m. Fluctuations in ozone measured by the aircraft correlated highly with potential temperature fluctuations and the occurrence of turbulent patches at altitudes just above the jet core, but not at higher flight levels, even though the ozone fluctuations were much larger aloft. These results suggest the existence of remnant 'fossil turbulence' from earlier events at higher levels, and that ozone cannot be used as a substitute for more direct measures of turbulence. The findings here do suggest that automated turbulence forecasting algorithms should include some reliable measure of gravity wave activity.

In the past decade, ozone (O3) pollution has become a severe environmental problem in major cities in China. Here, based on available observational records, we investigated the long-term trend of ozone pollution in China during 2014–2020. Ozone concentrations were higher in urban areas than in non-urban areas. During these seven years, the highest ozone concentrations primarily occurred in summer in northern China, and in autumn or spring in southern China. Although ozone precursors, including nitrogen oxides (NOx) and carbon monoxide (CO), continuously decreased throughout the seven years, four ozone metrics that were used to characterize ozone exposure levels increased from 2014 to 2017 and reached a plateau after 2017. The long-term trend of ozone concentrations differed across seasons; especially from 2019 to 2020 when ozone concentrations decreased in summer and increased in winter. To analyze the causes of this observed trend, a photochemical box model was used to investigate the change in ozone sensitivity regime in two representative cities – Beijing and Shanghai. Our model simulations suggest that the summertime ozone sensitivity regime in urban areas of China has changed from a VOC-limited regime to a transition regime during 2014–2020; by 2020, the urban photochemistry is in a transition regime in summer but in a VOC-limited regime in winter. This study helps to understand the distinct trends of ozone in China and provides insights into efficient future ozone control strategies in different regions and seasons. [ABSTRACT FROM AUTHOR]