The Role of Temperature and NOx in Ozone Trends in the Los Angeles Basin

Predicting plateau atmospheric ozone concentrations by a machine learning approach: A case study of a typical city on the southwestern plateau of China

Atmospheric ozone (O3) has been placed on the priority control pollutant list in China's 14th Five-Year Plan. Due to their unique meteorological conditions, plateau regions contain high concentrations of atmospheric O3. However, traditional experimental methods for determining O3 concentrations using automatic monitoring stations cannot predict O3 trends. In this study, two machine learning models (a nonlinear auto-regressive model with external inputs (NARX) and a temporal convolution network (TCN)) were developed to predict O3 concentrations in a plateau area in the Kunming region by considering the effects of meteorological parameters, air quality parameters, and volatile organic compounds (VOCs). The plateau O3 prediction accuracy of the machine learning models was found to be much higher than those of numerical models that served as a comparison. The O3 values predicted by the machine learning models closely matched the actual monitoring data. The temporal distribution of plateau O3 displayed a high all-day peak from February to May. A correlation analysis between O3 concentrations and feature parameters demonstrated that humidity is the feature with the highest absolute correlation (-0.72), and was negatively correlated with O3 concentrations during all test periods. VOCs and temperatures were also found to have high positive correlation coefficients with O3 during periods of significant O3 pollution. After negating the effects of meteorological parameters, the predicted O3 concentrations decreased significantly, whereas they increased in the absence of NOx. Although individual VOCs were found to greatly affect the O3 concentration, the total VOC (TVOC) concentration had a relatively small effect. The proposed machine learning model was demonstrated to predict plateau O3 concentrations and distinguish how different features affect O3 variations.

Temperature-dependent emissions dominate aerosol and ozone formation in Los Angeles

Despite declines in transportation emissions, urban North America and Europe still face unhealthy air pollution levels. This has challenged conventional understanding of the sources of their volatile organic compound (VOC) precursors. Using airborne flux measurements to map emissions of a wide range of VOCs, we demonstrate that biogenic terpenoid emissions contribute ~60% of emitted VOC OH reactivity, ozone, and secondary organic aerosol formation potential in summertime Los Angeles and that this contribution strongly increases with temperature. This implies that control of nitrogen oxides is key to reducing ozone formation in Los Angeles. We also show some anthropogenic VOC emissions increase with temperature, which is an effect not represented in current inventories. Air pollution mitigation efforts must consider that climate warming will strongly change emission amounts and composition.

Recent-year variations in O3 pollution with high-temperature suppression over central China

By analyzing environmental and meteorological monitoring data over recent years of 2015-2022, the Twain-Hu Basin (THB) in central China was identified as a regional O3 pollution center over China with the highest increasing trend at 1.10 %⸱yr-1 in interannual variations of O3 concentrations with deteriorating O3 pollution over recent years. We explored the spatiotemporal variations in O3 pollution in the THB with ozone suppression (OS) under high air temperature over metropolitan, small urban, and mountainous areas. The bipolarized interannual trends in interannual O3 variations in urban and mountainous areas over central China were characterized with the increasing and decreasing 90th percentiles of the daily maximum 8-h (MDA8-90) O3 concentrations respectively in polluted urban areas and clean mountainous areas over recent eight years. The changes of the near-surface O3 concentrations with air temperature exhibited the inflection points of OS from increasing to decreasing O3 at air temperature of 30.5 °C in mountainous areas, 32.5 °C in small urban areas, and 34.5 °C in metropolitan areas, and the intensity of OS was estimated in the ranking with mountainous areas (-2.30 μg⸱m-3⸱°C-1) > small urban areas (-1.96 μg⸱m-3⸱°C-1) > metropolitan areas (-1.54 μg⸱m-3⸱°C-1), indicating that the OS was more significant over the lower-O3 mountainous areas. This study has implications for understanding O3 pollution variations with the meteorological drivers.

A better representation of VOC chemistry in WRF-Chem and its impact on ozone over Los Angeles

The declining trend in vehicle emissions has underscored the growing significance of Volatile Organic Compound (VOC) emissions from Volatile Chemical Products (VCP). However, accurately representing VOC chemistry in simplified chemical mechanisms remains challenging due to its chemical complexity including speciation and reactivity. Previous studies have predominantly focused on VOCs from fossil fuel sources, leading to an underrepresentation of VOC chemistry from VCP sources. We developed an integrated chemical mechanism, RACM2B-VCP, that is compatible with WRF-Chem and is aimed to enhance the representation of VOC chemistry, particularly from VCP sources, within the present urban environment. Evaluation against the Air Quality System (AQS) network data demonstrates that our model configured with RACM2B-VCP reproduces both the magnitude and spatial variability of O3 as well as PM2.5 in Los Angeles. Furthermore, evaluation against comprehensive measurements of O3 and PM2.5 precursors from the Reevaluating the Chemistry of Air Pollutants in California (RECAP-CA) airborne campaign and the Southwest Urban NO x and VOC Experiment (SUNVEx) ground site and mobile laboratory campaign, confirm the model's accuracy in representing NOx and many VOCs and highlight remaining biases. Although there exists an underprediction in the total VOC reactivity of observed VOC species, our model with RACM2B-VCP exhibits good agreement for VOC markers emitted from different sectors, including biogenic, fossil fuel, and VCP sources. Through sensitivity analyses, we probe the contributions of VCP and fossil fuel emissions to total VOC reactivity and O3. Our results reveal that 52% of the VOC reactivity and 35% of the local enhancement of MDA8 O3 arise from anthropogenic VOC emissions in Los Angeles. Significantly, over 50% of this anthropogenic fraction of either VOC reactivity or O3 is attributed to VCP emissions. The RACM2B-VCP mechanism created, described, and evaluated in this work is ideally suited for accurately representing ozone for the right reasons in the present urban environment where mobile, biogenic, and VCP VOCs are all important contributors to ozone formation.

Applicability of evaluation metrics/schemes for human health burden attributable to regional ozone pollution: A case study in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), South China

This is a study to identify the applicable/preferable short- and long-term metrics/schemes to evaluate the premature mortality attributable to the ozone pollution in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), one of the most representative populous ozone pollution regions in China, by comprehensively accounting the uncertainty sources. The discrepancy between the observation and the CAQRA reanalysis datasets (2013-2019) was investigated in terms of the concentration variation pattern, which determines the exposure metric change. A set of domestic short-term C-R coefficients for the all-age population were integrated using the meta-analysis respectively corresponding to the metrics of MDA1, MDA8, and Daily average. The dataset-based deviations of the short-term attributable factors (AFs) and their corresponding premature mortalities were respectively about 16.9 ± 13.3 % and <5 % based on MDA8, much smaller than other two metrics; and the MDA8-based evaluation results were the most sensitive to the deteriorative ozone pollution, with the maximum upward trends of 0.095-0.129 %/year. Accordingly, MDA8 was recognized as the most applicable short-term metric. For the long-term exposure, the domestic summer metric SMDA8 could not exactly represent the peak-season ozone maximum level in the GBA, with the deviation from 6MMDA8 as much as 30 %. By considering the ability of metric to represent the peak-season ozone, the relatively smaller dataset-based discrepancies of AFs (6MMDA8-WHO2021: 23.3 ± 16.9 %, AMDA8-T2016: 20.7 ± 15.8 %) and the attributable premature mortalities (6MMDA8-WHO2021: 5 %, AMDA8-T2016: 8 %), and the higher sensitivity of the evaluation results to the deteriorative ozone pollution (6MMDA8-WHO2021: 0.13 %;year, p = 0.01; AMDA8-T2016: 0.15 %/year, p = 0.03), the schemes of 6MMDA8-WHO2021 and AMDA8-T2016 were recognized relatively more preferable for the adult (≥25-year) long-term evaluation. Based on the recognized metric/schemes, the central and the eastern PRE areas of higher NO2 level in the GBA were experiencing the highest health burdens from 2013 to 2019.

Limonene Enantiomeric Ratios from Anthropogenic and Biogenic Emission Sources

Emissions from volatile chemical products (VCPs) have been identified as contributors to air quality degradation in urban areas. Limonene can be a tracer compound for VCPs containing fragrances in densely populated regions, but limonene is also emitted from conifers that are planted in urban areas. This creates challenges for using limonene to estimate VCP emissions. In this study, the -/+ enantiomeric ratios of limonene from VCP and conifer emission sources were quantified to evaluate if this measurement could be used to aid in source apportionment and emission inventory development. Samples were analyzed using a gas chromatograph equipped with a chiral column and mass spectrometry. The results demonstrate that limonene exhibits distinct enantiomeric ratios when sourced from VCPs versus conifers. (+)-Limonene was dominant in VCP sources (>97%), which was not universally true for conifer sources. The results were compared to those of air samples collected outside at two locations and indoors. The levels of (-)-limonene in outdoor air in Irvine and Portland and in indoor air were 50%, 22%, and 4%, respectively. This suggests outdoor limonene had both VCP and plant emission sources while indoor air was dominated by VCP sources. This study demonstrates the potential utility of enantiomeric analysis for improving VCP emission estimates in urban areas.

Investigation of Ozone Formation Chemistry during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA)

Salt Lake City (SLC), UT, is an urban area where ozone (O3) concentrations frequently exceed health standards. This study uses an observationally constrained photochemical box model to investigate the drivers of O3 production during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA), which took place from August to September 2022 in SLC. During SAMOZA, a suite of volatile organic compounds (VOCs), oxides of nitrogen (NOx), and other parameters were measured at the Utah Technical Center, a high-NOx site in the urban core. We examined four high-O3 cases: 4 August and 3, 11, and 12 September, which were classified as a nonsmoky weekday, a weekend day with minimal smoke influence, a smoky weekend day, and a smoky weekday, respectively. The modeled O3 production on 4 August and 3 September was highly sensitive to VOCs and insensitive to NOx reductions of ≤50%. Box model results suggest that the directly emitted formaldehyde contributed to the rapid increase in morning O3 concentrations on 3 September. Model sensitivity tests for September 11-12 indicated that smoke-emitted VOCs, especially aldehydes, had a much larger impact on O3 production than NOx and/or anthropogenic VOCs. On 11 and 12 September, smoke-emitted VOCs enhanced model-predicted maximum daily 8 h average O3 concentrations by 21 and 13 parts per billion (ppb), respectively. Overall, our results suggest that regionwide VOC reductions of at least 30-50% or NOx reductions of at least 60% are needed to bring SLC into compliance with the national O3 standard of 70 ppb.

Improved Spatial Resolution in Modeling of Nitrogen Oxide Concentrations in the Los Angeles Basin

The extent to which emission control technologies and policies have reduced anthropogenic NOx emissions from motor vehicles is large but uncertain. We evaluate a fuel-based emission inventory for southern California during the June 2021 period, coinciding with the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign. A modified version of the Fuel-based Inventory of Vehicle Emissions (FIVE) is presented, incorporating 1.3 km resolution gridding and a new light-/medium-duty diesel vehicle category. NOx concentrations and weekday-weekend differences were predicted using the WRF-Chem model and evaluated using satellite and aircraft observations. Model performance was similar on weekdays and weekends, indicating appropriate day-of-week scaling of NOx emissions and a reasonable distribution of emissions by sector. Large observed weekend decreases in NOx are mainly due to changes in on-road vehicle emissions. The inventory presented in this study suggests that on-road vehicles were responsible for 55-72% of the NOx emissions in the South Coast Air Basin, compared to the corresponding fraction (43%) in the planning inventory from the South Coast Air Quality Management District. This fuel-based inventory suggests on-road NOx emissions that are 1.5 ± 0.4, 2.8 ± 0.6, and 1.3 ± 0.7 times the reference EMFAC model estimates for on-road gasoline, light- and medium-duty diesel, and heavy-duty diesel, respectively.

Location-Specific Control of Precursor Emissions to Mitigate Photochemical Air Pollution

The effects of precursor emission controls on air quality can vary greatly depending on where emission reductions occur. We use the adjoint of the Community Multiscale Air Quality (CMAQ) model to evaluate impacts of spatially targeted NO_x emission reductions on odd oxygen (O_x = O₃ + NO₂). The air quality responses studied here include one population-weighted regionwide and three city-level receptors in Central California. We map high-priority locations for NO_x control and their changes over decadal time scales. The desirability of NO_x-focused emission control programs has increased between 2000 and 2022. We find for present-day conditions that reducing NO_x emissions by 28% from targeted high-priority locations can achieve 60% of the air quality benefits of uniform NO_x reductions at all locations. High-priority source locations are found to differ for individual city-level versus regionwide receptors of interest. While high-impact emission hotspots for improving city-level metrics are found within the city itself or closely adjacent, the spatial pattern of emission hotspots for improving regionwide air quality is more complex and requires comprehensive consideration of upwind sources. Results of this study can help to inform strategic decision-making at local and regional levels about where to prioritize emission control efforts.

Policy-Related Gains in Urban Air Quality May Be Offset by Increased Emissions in a Warming Climate

Air quality policies have made substantial gains by reducing pollutant emissions from the transportation sector. In March 2020, New York City's activities were severely curtailed in response to the COVID-19 pandemic, resulting in 60-90% reductions in human activity. We continuously measured major volatile organic compounds (VOCs) during January-April 2020 and 2021 in Manhattan. Concentrations of many VOCs decreased significantly during the shutdown with variations in daily patterns reflective of human activity perturbations, resulting in a temporary ∼28% reduction in chemical reactivity. However, the limited effect of these dramatic measures was outweighed by larger increases in VOC-related reactivity during the anomalously warm spring 2021. This emphasizes the diminishing returns from transportation-focused policies alone and the risk of increased temperature-dependent emissions undermining policy-related gains in a warming climate.

Overview of ICARUS-A Curated, Open Access, Online Repository for Atmospheric Simulation Chamber Data

Atmospheric simulation chambers continue to be indispensable tools for research in the atmospheric sciences. Insights from chamber studies are integrated into atmospheric chemical transport models, which are used for science-informed policy decisions. However, a centralized data management and access infrastructure for their scientific products had not been available in the United States and many parts of the world. ICARUS (Integrated Chamber Atmospheric data Repository for Unified Science) is an open access, searchable, web-based infrastructure for storing, sharing, discovering, and utilizing atmospheric chamber data [https://icarus.ucdavis.edu]. ICARUS has two parts: a data intake portal and a search and discovery portal. Data in ICARUS are curated, uniform, interactive, indexed on popular search engines, mirrored by other repositories, version-tracked, vocabulary-controlled, and citable. ICARUS hosts both legacy data and new data in compliance with open access data mandates. Targeted data discovery is available based on key experimental parameters, including organic reactants and mixtures that are managed using the PubChem chemical database, oxidant information, nitrogen oxide (NOx) content, alkylperoxy radical (RO₂) fate, seed particle information, environmental conditions, and reaction categories. A discipline-specific repository such as ICARUS with high amounts of metadata works to support the evaluation and revision of atmospheric model mechanisms, intercomparison of data and models, and the development of new model frameworks that can have more predictive power in the current and future atmosphere. The open accessibility and interactive nature of ICARUS data may also be useful for teaching, data mining, and training machine learning models.

Driving Forces of Meteorology and Emission Changes on Surface Ozone in the Huaihe River Basin, China

Surface ozone (O3) pollution in China has become a serious environmental problem in recent years. In the present study, we targeted the HRB, a large region located in China's north-south border zone, to assess the driving forces of meteorology and emission changes on surface ozone. A Kolmogorov-Zurbenko (KZ) filter method was performed on the maximum daily average 8-h (MDA8) concentrations of ozone in the HRB during 2015-2020 to decompose the original time series. The findings demonstrated that the short-term (O3ST), seasonal (O3SN), and long-term components (O3LT) of MDA8 O3 variations accounted for 34.2%, 56.1%, and 2.9% of the total variance, respectively. O3SN has the greatest influence on the daily variation in MDA8 O3, followed by O3ST. In coastal cities, the influence of O3ST was enhanced. The influence of O3SN was stronger in the northwestern HRB. Air temperature is the prevailing variable that influences the photochemical formation of ozone. A clear phase lag (7-34 days) of the baseline component between MDA8 O3 and the atmospheric temperature was found in the HRB. Using multiple linear regression, the effect of temperature on ozone was removed. We estimated that the increase in ozone concentration in the HRB was mainly caused by the emission changes (79.4%), and the meteorological conditions made a small contribution (20.6%). This study suggests that reductions in volatile organic compounds (VOCs) will play an important role in further ozone pollution reduction in the HRB.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11270-023-06345-1.

Unexpected deterioration of O3 pollution in the South Coast Air Basin of California: The role of meteorology and emissions

Tropospheric ozone (O₃) pollution has long been a prominent environmental threat due to its adverse impacts on vulnerable populations and ecosystems. In recent years, an unexpected increase in O₃ levels over the South Coast Air Basin (SoCAB) of California has been observed despite reduced precursor emissions and the driving factors behind this abnormal condition remain unclear. In this work, we combine ambient measurements, satellite data, and air quality modeling to investigate O₃ and precursor emission trends and explore the impacts of meteorological variability and emission changes on O₃ over the SoCAB from 2012 to 2020. Changes in O₃ trends were characterized by declining O₃ in 2012-2015, and increasing O₃ afterwards with the most extreme O₃ exceedances in 2020. Basin-wide increases of MDA8 O₃ concentrations over warm season were depicted between 2012 and 2020, with the most significant enhancements (5-10 ppb) observed in San Bernardino County. Persistent heatwaves and weak ventilation on consecutive days were closely correlated with O₃ exceedances (r² above 0.6) over inland SoCAB. While decreasing trends in NO_x (-4.1%/yr) and VOC emissions (-1.8%/yr) inferred from emission inventory and satellites during 2012-2020 resulted in a slow transition for O₃ sensitivity from VOCs-limited to NO_x-limited, model simulations performed with fixed meteorology indicate that unfavorable meteorological conditions could largely offset regulation benefits, with meteorology anomaly-induced monthly O₃ changes reaching 20 ppb (May 2020) and the deterioration of O₃ pollution in 2016, 2017, and 2020 was largely attributed to unfavorable meteorological conditions. Nevertheless, anthropogenic emission changes may act as the dominant factor in governing O₃ variations across the SoCAB when net effects of meteorology are neutral (typically 2018). This work provides a comprehensive assessment of O₃ pollution and contributes valuable insights into understanding the long-term changes of O₃ and precursors in guiding future regulation efforts in the SoCAB.

Multiple Impacts of Aerosols on O3 Production Are Largely Compensated: A Case Study Shenzhen, China

Tropospheric ozone (O₃) is a harmful gas compound to humans and vegetation, and it also serves as a climate change forcer. O₃ is formed in the reactions of nitrogen oxides and volatile organic compounds (VOCs) with light. In this study, an O₃ pollution episode encountered in Shenzhen, South China in 2018 was investigated to illustrate the influence of aerosols on local O₃ production. We used a box model with comprehensive heterogeneous mechanisms and empirical prediction of photolysis rates to reproduce the O₃ episode. Results demonstrate that the aerosol light extinction and NO₂ heterogeneous reactions showed comparable influence but opposite signs on the O₃ production. Hence, the influence of aerosols from different processes is largely counteracted. Sensitivity tests suggest that O₃ production increases with further reduction in aerosols in this study, while the continued NO_x reduction finally shifts O₃ production to an NO_x-limited regime with respect to traditional O₃-NO_x-VOC sensitivity. Our results shed light on the role of NO_x reduction on O₃ production and highlight further mitigation in NO_x not only limiting the production of O₃ but also helping to ease particulate nitrate, as a path for cocontrol of O₃ and fine particle pollution.

Elucidating Contributions of Anthropogenic Volatile Organic Compounds and Particulate Matter to Ozone Trends over China

In China, emissions of ozone (O₃)-producing pollutants have been targeted for mitigation to reduce O₃ pollution. However, the observed O₃ decrease is slower than/opposite to expectations affecting the health of millions of people. For a better understanding of this failure and its connection with anthropogenic emissions, we quantify the summer O₃ trends that would have occurred had the weather stayed constant by applying a numerical tool that "de-weathers" observations across 31 urban regions (123 cities and 392 sites) over 8 years. O₃ trends are significant (p < 0.05) over 234 sites after de-weathering, contrary to the directly observed trends (only 39 significant due to high meteorology-induced variability). The de-weathered data allow categorizing cities in China into four different groups regarding O₃ mitigation, with group 1 exhibiting steady O₃ reductions, while group 4 showing significant (p < 0.05) O₃ increases. Analysis of the relationships between de-weathered odd oxygen and nitrogen oxides illustrates how the changes in NO_x, in anthropogenic volatile organic compounds (VOCs), and reductions in fine particulate matter (PM_2.5) affect the O₃ trends differently in these groups. While this analysis suggests that VOC reductions are the main driver of O₃ decreases in group 1, groups 3 and 4 are primarily affected by decreasing PM_2.5, which results in enhanced O₃ formation. Our analysis demonstrates both the importance of and possibility for isolating emission-driven changes from climate and weather for interpreting short-term air quality observations.

Two Decades of Changes in Summertime Ozone Production in California's South Coast Air Basin

Tropospheric ozone (O₃) continues to be a threat to human health and agricultural productivity. While O₃ control is challenging, tracking underlying formation mechanisms provides insights for regulatory directions. Here, we describe a comprehensive analysis of the effects of changing emissions on O₃ formation mechanisms with observational evidence. We present a new approach that provides a quantitative metric for the ozone production rate (OPR) and its sensitivity to precursor levels by interpreting two decades of in situ observations of the six criteria air pollutants(2001-2018). Applying to the South Coast Air Basin (SoCAB), California, we show that by 2016-2018, the basin was at the transition region between nitrogen oxide (NO_x)-limited and volatile organic compound (VOC)-limited chemical regimes. Assuming future weather conditions are similar to 2016-2018, we predict that NO_x-focused reduction is required to reduce the number of summer days the SoCAB is in violation of the National Ambient Air Quality Standard (70 ppbv) for O₃. Roughly, ∼40% (∼60%) NO_x reductions are required to reduce the OPR by ∼1.8 ppb/h (∼3.3 ppb/h). This change would reduce the number of violation days from 28 to 20% (10%) in a year, mostly in summertime. Concurrent VOC reductions which reduce the production rate of HO_x radicals would also be beneficial.

NOx and O3 Trends at U.S. Non-Attainment Areas for 1995-2020: Influence of COVID-19 Reductions and Wildland Fires on Policy-Relevant Concentrations

We analyzed NO2 and O3 data from 32 U.S. non-attainment areas (NAAs) for 1995-2020. Since 1995, all regions have shown steady reductions in NO2 and the weekend-weekday pattern indicates that the O3 production regime in most NAAs has transitioned to a NOx-limited regime, while a few NAAs remain NOx-saturated. In the eastern U.S., all NAAs have made steady progress toward meeting the current (70 ppb) O3 standard, but this is less true in midwestern and western NAAs, with most showing little improvement in peak O3 concentrations since about 2010. Due to COVID-19 restrictions, NO2 concentrations were substantially reduced in 2020. In the eastern NAAs, we see significant reductions in both NO2 and peak O3 concentrations. In the midwestern U.S., results were more variable, with both higher and lower O3 values in 2020. In the western U.S. (WUS), we see variable reductions in NO2 but substantial increases in O3 at most sites, due to the influence from huge wildland fires. The recent pattern over the past decade shows that the large amount of wildland fires has a strong influence on the policy-relevant O3 metric in the WUS, and this is making it more difficult for these regions to meet the O3 standard.

Responses of Photochemical Air Pollution in California's San Joaquin Valley to Spatially and Temporally Resolved Changes in Precursor Emissions

Ground-level ozone adversely affects human health and ecosystems. The effectiveness of control programs depends on which precursor(s) are controlled, by how much, and where and when emission reductions occur. We use the adjoint of the Community Multiscale Air Quality model to investigate odd oxygen (O_x ≡ O₃ + NO₂) sensitivities in California's San Joaquin Valley (SJV) to precursor emissions from local and upwind sources. Sensitivities are mapped and disaggregated by hour and day. Taken together, impacts of precursor emissions in the San Francisco Bay area and Sacramento Valley are similar in magnitude to impacts of local SJV emissions. Same-day emission sensitivities are mostly attributable to local sources, with the most influential anthropogenic emissions of VOCs (volatile organic compounds) and NO_x (nitrogen oxides) occurring in the morning (9-11 am) and early afternoon hours (1-3 pm), respectively. For the northernmost SJV receptor, the influence from Sacramento Valley emissions peaks 5-6 h later than Bay area emissions; this difference diminishes for SJV receptors located further downwind. Results show a shift toward more NO_x-sensitive conditions in the afternoon with all but the southernmost receptor shifting from VOC- to NO_x-sensitive conditions. We also evaluate opportunities to control pollution through shifts in precursor emission location and timing.

Resolving and Predicting Neighborhood Vulnerability to Urban Heat and Air Pollution: Insights From a Pilot Project of Community Science

Urban heat and air pollution, two environmental threats to urban residents, are studied via a community science project in Los Angeles, CA, USA. The data collected, for the first time, by community members, reveal the significance of both the large spatiotemporal variations of and the covariations between 2 m air temperature (2mT) and ozone (O₃) concentration within the (4 km) neighborhood scale. This neighborhood variation was not exhibited in either daily satellite observations or operational model predictions, which makes the assessment of community health risks a challenge. Overall, the 2mT is much better predicted than O₃ by the weather and research forecast model with atmospheric chemistry (WRF-Chem). For O₃, diurnal variation is better predicted by WRF-Chem than spatial variation (i.e., underestimated by 50%). However, both WRF-chem and the surface observation show the overall consistency in describing statistically significant covariations between O₃ and 2mT. In contrast, satellite-based land surface temperature at 1 km resolution is insufficient to capture air temperature variations at the neighborhood scale. Community engagement is augmented with interactive maps and apps that show the predictions in near real time and reveals the potential of green canopy to reduce air temperature and ozone; but different tree types and sizes may lead to different impacts on air temperature, which is not resolved by the WRF-Chem. These findings highlight the need for community science engagement to reveal otherwise impossible insights for models, observations, and real-time dissemination to understand, predict, and ultimately mitigate, urban neighborhood vulnerability to heat and air pollution.

Importance of ozone precursors information in modelling urban surface ozone variability using machine learning algorithm

Surface ozone (O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_3$$\end{document}3) is primarily formed through complex photo-chemical reactions in the atmosphere, which are non-linearly dependent on precursors. Even though, there have been many recent studies exploring the potential of machine learning (ML) in modeling surface ozone, the inclusion of limited available ozone precursors information has received little attention. The ML algorithm with in-situ NO information and meteorology explains 87% (R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{2}$$\end{document}2 = 0.87) of the ozone variability over Munich, a German metropolitan area, which is 15% higher than a ML algorithm that considers only meteorology. The ML algorithm trained for the urban measurement station in Munich can also explain the ozone variability of the other three stations in the same city, with R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{2}$$\end{document}2 = 0.88, 0.91, 0.63. While the same model robustly explains the ozone variability of two other German cities’ (Berlin and Hamburg) measurement stations, with R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{2}$$\end{document}2 ranges from 0.72 to 0.84, giving confidence to use the ML algorithm trained for one location to other locations with sparse ozone measurements. The inclusion of satellite O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_3$$\end{document}3 precursors information has little effect on the ML model’s performance.

Simulation of the Air Quality in Southern California, USA in July and October of the Year 2018

A numerical investigation of the air quality in Southern California, USA in the year 2018 is presented using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). In July, a heat wave occurred, and in October, Santa Ana conditions prevailed; these conditions and their impact on air quality are the scope of the present numerical study.The high spatial resolution in the simulation includes two nested domains of 1 km and 3 km, respectively. Local climate zones land use categories are combined with the complex urban model building effect parameterization coupled with the building energy model (BEP+BEM) and the detailed MOZCART-T1 chemical reaction mechanism, which is the MOZART-T1 mechanism for trace gases with GOCART aerosols. Thus, the model is suitable to compare simulation results to in situ and satellite measurements of O3, NO2, CH4, and CO. The meteorology is captured well by the model. Comparison of simulation results with observations shows a good agreement of NO2 and ozone, whereas CO mixing ratios are generally underestimated. This hints at missing emissions in the 2017 National Emissions Inventory (NEI) dataset. Both the heat wave and the Santa Ana winds increase the air pollution with gas-phase species in Los Angeles. In both cases, nighttime boundary layer heights are small, which causes emissions to reside near the ground. During Santa Ana winds, NOx removal on aerosols is reduced. Methane mixing ratios are modeled very well at most stations in Los Angeles, but predictions of low emissions near the University of California cause inaccuracies at that location. Modeled and observed PM2.5 agree well on low-pollution days, but high-pollution events are generally missed by the model. During the heat wave, both modeled and observed PM2.5 concentrations exceed the recommended NAAQS National Ambient Air Quality Standards value of 12.5 g/m3. The present modeling approach serves as a base for the study and prediction of special weather events and their impact on air pollution.

Human-health impacts of controlling secondary air pollution precursors

Exposure to ozone and fine particle (PM_2.5) air pollution results in premature death. These pollutants are predominantly secondary in nature and can form from nitrogen oxides (NO_X), sulfur oxides (SO_X), and volatile organic compounds (VOCs). Predicted health benefits for emission reduction scenarios often incompletely account for VOCs as precursors as well as the secondary organic aerosol (SOA) component of PM_2.5. Here, we show that anthropogenic VOC emission reductions are more than twice as effective as equivalent fractional reductions of SO_X or NO_X at reducing air pollution-associated cardiorespiratory mortality in the United States. A 25% reduction in anthropogenic VOC emissions from 2016 levels is predicted to avoid 13,000 premature deaths per year, and most (85%) of the VOC-reduction benefits result from reduced SOA with the remainder from ozone. While NO_X (-5.7 ± 0.2 % yr^-1) and SO_X (-12 ± 1 % yr^-1) emissions have declined precipitously across the U.S. since 2002, anthropogenic VOC emissions (-1.8 ± 0.3 % yr^-1) and concentrations of non-methane organic carbon (-2.4 ± 1.0 % yr^-1) have changed less. This work indicates preferentially controlling VOCs could yield significant benefits to human health.

Impact of OA on the Temperature Dependence of PM 2.5 in the Los Angeles Basin

Air quality policy in the Los Angeles megacity is a guidepost for other megacities. Over the last 2 decades, the policy has substantially reduced aerosol (OA) concentrations and the frequency of high aerosol events in the region. During this time, the emissions contributing to, and the temperature associated with, high aerosol events have changed. Early in the record, aerosol concentrations responded to a variety of different sources. We show that emission control has been effective with a strong decrease in temperature-independent sources. As a result, the response of aerosol to temperature has become a dominant feature of high aerosol events in the basin. The organic fraction of the aerosol (OA) increases with the temperature approaching 35% at 40 °C. We describe a simple conceptual model of aerosol in Los Angeles, illustrating how benzene, toluene, ethylbenzene, and xylenes (BTEX) and isoprene, along with molecules for which these are plausible surrogates such as monoterpenes, are sufficient to explain the observed temperature dependence of PM 2.5.