Optimizing a twin-chamber system for direct ozone production rate measurement

High Ozone Production Rate (OPR) leads to O3 pollution episodes and adverse human health outcomes. OPR observation (Obs-OPR) and OPR modelling (Mod-OPR) have been obtained from observed and modelled peroxy radicals and nitrogen oxides. However, discrepancies between them remind of an imperfect understanding of O3 photochemistry. Direct measurement of OPR (Mea-OPR) by a twin-chamber system emerges. Herein, we optimized Mea-OPR design, i.e., minimizing the chamber surface area to volume ratio (S/V) to 9.8 m-1 from 18 m-1 and the dark uptake coefficient of O3 to 9.9 × 10-9 from 7.1 × 10-8 in the literature. In addition, control experiments further revealed and quantified a photo-enhanced O3 uptake, and therefore recommended an essential correction of Mea-OPR. We finally characterized a measurement uncertainty of ±38% and a detection limit of 3.2 ppbv h-1 (3SD), which suggested that Mea-OPR would be sensitive enough to measure OPR in urban or suburban environments. Further application of this system in urban Beijing during the Beijing 2022 Olympic Winter Games recorded a noontime OPR of 7.3 (±3.3, 1SD) ppbv h-1. These observational results added up to our confidence in future field application of Mea-OPR, to facilitate pollution control policy evaluation and to shed light on O3 photochemistry puzzle.

Peroxy radical chemistry during ozone photochemical pollution season at a suburban site in the boundary of Jiangsu-Anhui-Shandong-Henan region, China

Ambient peroxy radical (RO2⁎ = HO2 + RO2) concentrations were measured at a suburban site in a major prefecture-level city (Huaibei) in the boundary of Jiangsu-Anhui-Shandong-Henan region, which is the connecting belt of air pollution in the Beijing-Tianjin-Hebei region and the Yangtze River Delta. Measurements were carried out during the period of September to October 2021 to elucidate the formation mechanism of O3 pollution. The observed maximum concentration of peroxy radicals was 73.8 pptv. A zero-dimensional box model (Framework for 0-Dimensional Atmospheric Modeling, F0AM) based on Master Chemical Mechanism (MCM3.3.1) was used to predict radical concentrations for comparison with observations. The model reproduced the daily variation of peroxy radicals well, but discrepancies still appear in the morning hours. As in previous field campaigns, systematic discrepancies between modelled and measured RO2⁎ concentrations are observed in the morning for NO mixing ratios higher than 1 ppbv. Between 6:00 and 9:00 am, the model significantly underpredicts RO2⁎ by a mean factor of 7.2. This underprediction can be explained by a missing RO2⁎ source of 1.2 ppbv h-1 which originated from the photochemical conversion of an alkene-like chemical species. From the model results it shows that the main sources of ROx (= OH + HO2 + RO2) are the photolysis of oxygenated volatile organic compounds (OVOCs, 33 %), O3 and HONO (25 %), and HCHO (24 %). And the major sinks of ROx transitioned from a predominant reaction of radicals with NOx in the morning to a predominant peroxy self- and cross-reaction in the late afternoon. The introduction of an alkene-like species increased RO2 radical concentration and resulted in 14 % increase in net daily integrated ozone production, indicating the possible significance of the mechanism of alkene-like species oxidation to peroxy radicals. This study provides important information for subsequent ozone pollution control policies in Jiangsu-Anhui-Shandong-Henan region.

Direct evidence of local photochemical production driven ozone episode in Beijing: A case study

We present a comprehensive field campaign conducted in Beijing, September 2016, to elucidate the photochemical smog pollution, i.e. Ozone (O₃). The observed daily maximum hydroxyl radical (OH) and hydroperoxy radical (HO₂) concentrations were up to 1 × 10⁷ cm^-3 and 6 × 10⁸ cm^-3, respectively, indicating the active photochemistry in autumn Beijing. Photolysis of nitrous acid (HONO) and O₃ contributed 1-2 ppbv h^-1 to OH primary production during daytime. OH termination were dominated by the reaction with nitric oxide (NO) and nitrogen dioxide (NO₂), which were in general larger than primary production rates, indicating other primary radical sources maybe important. The measurement of radicals facilitates the direct determination of local ozone production rate P (O_x) (O_x = O₃ + NO₂). The integrated P(O_x) reached 75 ppbv in afternoon (for 4 h) when planetary boundary layer was well developed. At the same time period, the observed total oxidant concentrations O_x, increased significantly by 70 ppbv. In addition, the O_x measurement showed compact increase in 12 stations both temporally and spatially in Beijing, indicating that active photochemical production happened homogenously throughout the city. The back-trajectory analysis showed that Beijing was isolated from the other cities during the episode, which further proved that the fast ozone pollution was contributed by local photochemical production rather than regional advection.

Season-wise analyses of VOCs, hydroxyl radicals and ozone formation chemistry over north-west India reveal isoprene and acetaldehyde as the most potent ozone precursors throughout the year

The north-west Indo-Gangetic Plain is the agricultural cereal-basket of India owing to its prolific wheat and rice production. Surface ozone pollution is of growing concern over it, yet no detailed year-round in-situ measurements of its most reactive precursors, particularly the volatile organic compounds (VOCs) are available from this region. Here, using the first year-long continuous measurements of 23 major VOCs, ozone, NO_x, CO and their atmospheric oxidation products from a regionally representative site in north-west India, we evaluated speciated OH reactivities (OHR), ozone formation potential (OFP) and ozone production regimes (OPR) across all seasons. The average seasonal OHR ranged from 14 s^-1 (winter) to 21.5 s^-1 (summer). We provide the first estimate of OH radical mixing ratios varying between 0.06 and 0.37 ppt in different seasons for the peak daytime hours in this region. Recycling via HO₂+NO was the most important pathway contributing to >85% of the OH production throughout the year. Contrary to satellite derived proxies and chemical transport models which predict NO_x sensitive OPR, we show it to be strongly sensitive to both VOCs and NO_x (>90% days in a year). Remarkably for densely populated regions, isoprene and acetaldehyde collectively accounted for ~30-50% of the total OFP in all seasons. Biogenic emissions of isoprene (reaching 12.9 mg/m²/h) and high acetaldehyde from anthropogenic and photochemical sources were observed for all seasons. Monitoring and control of isoprene and acetaldehyde are therefore urgently required for efforts focused on mitigating surface ozone pollution in this demographically important region of the world.

Variability and Time of Day Dependence of Ozone Photochemistry in Western Wildfire Plumes

Understanding the efficiency and variability of photochemical ozone (O₃) production from western wildfire plumes is important to accurately estimate their influence on North American air quality. A set of photochemical measurements were made from the NOAA Twin Otter research aircraft as a part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) experiment. We use a zero-dimensional (0-D) box model to investigate the chemistry driving O₃ production in modeled plumes. Modeled afternoon plumes reached a maximum O₃ mixing ratio of 140 ± 50 ppbv (average ± standard deviation) within 20 ± 10 min of emission compared to 76 ± 12 ppbv in 60 ± 30 min in evening plumes. Afternoon and evening maximum O₃ isopleths indicate that plumes were near their peak in NO_x efficiency. A radical budget describes the NO_x volatile - organic compound (VOC) sensitivities of these plumes. Afternoon plumes displayed a rapid transition from VOC-sensitive to NO_x-sensitive chemistry, driven by HO_x (=OH + HO₂) production from photolysis of nitrous acid (HONO) (48 ± 20% of primary HO_x) and formaldehyde (HCHO) (26 ± 9%) emitted directly from the fire. Evening plumes exhibit a slower transition from peak NO_x efficiency to VOC-sensitive O₃ production caused by a reduction in photolysis rates and fire emissions. HO_x production in evening plumes is controlled by HONO photolysis (53 ± 7%), HCHO photolysis (18 ± 9%), and alkene ozonolysis (17 ± 9%).

Coupled Air Quality and Boundary-Layer Meteorology in Western U.S. Basins during Winter: Design and Rationale for a Comprehensive Study

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical-meteorological interactions that drive high pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in Western U.S. basins. Approximately 120 people participated, representing 50 institutions and 5 countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary-layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological-chemical linkages outlined here, nor to validate complex processes within coupled atmosphere-chemistry models.

Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H2O2 and Particulate Sulfate in the Winter North China Plain

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H₂O₂ formation through HO₂ recombination. Surprisingly, H₂O₂ mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H₂O₂, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H₂O₂ mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H₂O₂ formation rates of about 0.2 ppbv h^-1 during the initial irradiation periods are consistent with the H₂O₂ rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H₂O₂ formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM_2.5 ≥ 75 μg m^-3 agree with the observed H₂O₂ concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO₂ reaction with OH and HSO₃^- oxidation by H₂O₂. Overall, under high pollution, the H₂O₂-caused sulfate formation rate is above 250 ng m^-3 h^-1, contributing to the sulfate formation by more than 70%.

Elucidating the effect of HONO on O3 pollution by a case study in southwest China

Photolysis of nitrous acid (HONO) is one of the major sources for atmospheric hydroxyl radicals (OH), playing significant role in initiating tropospheric photochemical reactions for ozone (O₃) production. However, scarce field investigations were conducted to elucidate this effect. In this study, a field campaign was conducted at a suburban site in southwest China. The whole observation was classified into three periods based on O₃ levels and data coverage: the serious O₃ pollution period (Aug 13-18 as P1), the O₃ pollution period (Aug 22-28 as P2) and the clean period (Sep 3-12 as P3), with average O₃ peak values of 96 ppb, 82 ppb and 44 ppb, respectively. There was no significant difference of the levels of O₃ precursors (VOCs and NOx) between P1 and P2, and thus the evident elevation of OH peak values in P1 was suspected to be the most possible explanation for the higher O₃ peak values. Considering the larger contribution of HONO photolysis to HO_X primary production than photolysis of HCHO, O₃ and ozonolysis of Alkenes, sensitivity tests of HONO reduction on O₃ production rate in P1 are conducted by a 0-dimension model. Reduced HONO concentration effectively slows the O₃ production in the morning, and such effect correlates with the calculated production rate of OH radicals from HONO photolysis. Higher HONO level supplying for OH radical initiation in the early morning might be the main reason for the higher O₃ peak values in P1, which explained the correlation (R² = 0.51) between average O₃ value during daytime (10:00-19:00 LT) and average HONO value during early morning (00:00-05:00 LT). For nighttime accumulation, a suitable range of relative humidity that favored NO₂ conversion within P1 was assumed to be the reason for the higher HONO concentration in the following early morning which promoted O₃ peak values.

Introductory lecture: air quality in megacities

Urbanization is an ongoing global phenomenon as more and more people are moving from rural to urban areas for better employment opportunities and a higher standard of living, leading to the growth of megacities, broadly defined as urban agglomeration with more than 10 million inhabitants. Intense activities in megacities induce high levels of air pollutants in the atmosphere that harm human health, cause regional haze and acid deposition, damage crops, influence air quality in regions far from the megacity sources, and contribute to climate change. Since the Great London Smog and the first recognized episode of Los Angeles photochemical smog seventy years ago, substantial progress has been made in improving the scientific understanding of air pollution and in developing emissions reduction technologies. However, much remains to be understood about the complex processes of atmospheric oxidation mechanisms; the formation and evolution of secondary particles, especially those containing organic species; and the influence of emerging emissions sources and changing climate on air quality and health. While air quality has substantially improved in megacities in developed regions and some in the developing regions, many still suffer from severe air pollution. Strong regional and international collaboration in data collection and assessment will be beneficial in strengthening the capacity. This article provides an overview of the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management in a few megacities, and the impacts on health and climate. The challenges and opportunities facing megacities due to lockdown during the COVID-19 pandemic is also discussed.

Changes in ozone photochemical regime in Fresno, California from 1994 to 2018 deduced from changes in the weekend effect

Significant progress has been made in reducing emissions of air pollutants in the San Joaquin Valley in California. Nevertheless, from May to October, the valley still experiences numerous exceedances of the ozone health standard. As the standards are tightened, it is becoming harder to design policies to attain them. To better understand historical emissions reductions in the context of necessary future control efforts, we analyze 25 years of hourly measurements of ozone and nitrogen oxides concentrations for the hottest one third of days in Fresno using multiple linear regression analysis. We then analyze the changing dynamics of the weekend effect over the years in order to evaluate the growing importance of day-to-day carryover on ozone concentrations. A simplified model of the day-of-week pattern of ozone concentrations is used to explore the impact of same-day and previous-day concentrations. In addition to ozone, O_x (O₃ + NO₂) is used to distinguish reductions of atmospheric oxidants from short-duration exchanges between O₃ and NO₂. The analysis shows that there has been a significant increase in the importance of day-to-day carryover on ozone levels, and that consequently the ozone weekend effect in Fresno has changed over the last 25 years. In the 1990s, lower NO_x on the weekend led to increased ozone on Saturdays and Sundays but levels of O_x remained constant. In the 2010s, lower weekend NO_x led to reduced ozone on Saturdays, Sundays and Mondays showing that reductions in primary pollutants are sufficient to yield immediate decreases in secondary pollutants. Overall, the photochemical regime in the atmosphere has evolved such that carryover and regional pollution will be increasingly important in determining local ozone concentrations. Policies will therefore need to pay greater attention to regional emissions as local reductions may not be sufficient to meet the health standard.

Explicit diagnosis of the local ozone production rate and the ozone-NOx-VOC sensitivities

In the troposphere, ozone is a harmful gas compound to both human health and vegetation. Ozone is produced from the reaction of NO_x (NO + NO₂) and VOCs (volatile organic compounds) with light. Due to the highly nonlinear relationships between ozone and its precursors, proper ozone mitigation relies on the knowledge of chemical mechanisms. In this study, an observation-based method is used to simulate ozone formation and elucidate its controlling factors for a rural site on the North China Plain. The instantaneous ozone production rate is calculated utilizing a box model using the dataset obtained from the Wangdu campaign. First, the model was operated in a time-dependent mode to calculate the ozone production rate at each time stamp. The calculated ozone formation rate showed a diurnal average maximum value of 17 ppbv/h (1-h diurnal averaged). The contribution of individual peroxy radicals to ozone production was analyzed. In addition, the functional dependence of calculated P(O₃) reveals that ozone production was in a NO_x-limited regime during the campaign. Furthermore, the missing peroxy radical source will further extend NO_x-limited conditions to earlier in the day, making NO_x limitation dominate more of a day than the current chemical model predicts. Finally, a multiple scenarios mode, also known as EKMA (empirical kinetic modeling approach), was used to simulate the response of P(O₃) to the imaginary change in precursor concentrations. We found that ozone production was in the NO_x-limited region. However, the use of NO₂ measured by the molybdenum converter and/or the absence of a peroxy radical source in the current chemical model could over-emphasize the VOC-limited effect on ozone production.

Agriculture is a major source of NO x pollution in California

Nitrogen oxides (NO _x = NO + NO₂) are a primary component of air pollution-a leading cause of premature death in humans and biodiversity declines worldwide. Although regulatory policies in California have successfully limited transportation sources of NO _x pollution, several of the United States' worst-air quality districts remain in rural regions of the state. Site-based findings suggest that NO _x emissions from California's agricultural soils could contribute to air quality issues; however, a statewide estimate is hitherto lacking. We show that agricultural soils are a dominant source of NO _x pollution in California, with especially high soil NO _x emissions from the state's Central Valley region. We base our conclusion on two independent approaches: (i) a bottom-up spatial model of soil NO _x emissions and (ii) top-down airborne observations of atmospheric NO _x concentrations over the San Joaquin Valley. These approaches point to a large, overlooked NO _x source from cropland soil, which is estimated to increase the NO _x budget by 20 to 51%. These estimates are consistent with previous studies of point-scale measurements of NO _x emissions from the soil. Our results highlight opportunities to limit NO _x emissions from agriculture by investing in management practices that will bring co-benefits to the economy, ecosystems, and human health in rural areas of California.

Source apportionment of VOCs and their impacts on surface ozone in an industry city of Baoji, Northwestern China

Level of surface ozone (O₃) has been increasing continuously in China in recent years, while its contributors and formation pathways are less understood. In this study, distributions of volatile organic compounds (VOCs) and the roles on O₃ pollution have been investigated in a typical industrial city of Baoji in Northwestern China by means of monitoring of their concentrations and other trace gases_. The air samples have been collected at three sites according to urban function area. Concentration of VOCs in Weibin site, which near to industrial zone, was higher than most of other cities in China, and the ambient VOCs were dominated by aromatics and alkenes. The temporal variations of VOCs and O₃ coincided with the surface wind, implying that the formation of O₃ was impacted by both exports of plumes upwind and local photochemical reactions. Result of source apportionment indicated that industrial emission, vehicular exhaust, and solvent evaporation were three major pollution origins. Alkenes and aromatics contributed to the largest fractions of photochemical reactivity, suggesting the strong influences from industrial and traffic sectors. The study presents the characteristic VOCs and other factors in the contribution of O₃ formation in China.

Urban eddy covariance measurements reveal significant missing NOx emissions in Central Europe

Nitrogen oxide (NO_x) pollution is emerging as a primary environmental concern across Europe. While some large European metropolitan areas are already in breach of EU safety limits for NO₂, this phenomenon does not seem to be only restricted to large industrialized areas anymore. Many smaller scale populated agglomerations including their surrounding rural areas are seeing frequent NO₂ concentration violations. The question of a quantitative understanding of different NO_x emission sources is therefore of immanent relevance for climate and air chemistry models as well as air pollution management and health. Here we report simultaneous eddy covariance flux measurements of NO_x, CO₂, CO and non methane volatile organic compound tracers in a city that might be considered representative for Central Europe and the greater Alpine region. Our data show that NO_x fluxes are largely at variance with modelled emission projections, suggesting an appreciable underestimation of the traffic related atmospheric NO_x input in Europe, comparable to the weekend-weekday effect, which locally changes ozone production rates by 40%.