Odd Oxygen Research Articles

Evaluating the impact of control measures on sulfate-nitrate-ammonium aerosol variations and their formation mechanism in northern China during 2022 Winter Olympic Games

Sulfate, nitrate, and ammonium (SNA) significantly contribute to fine particulate matter (PM2.5). To ensure good air quality during 2022 Winter Olympic Games (WOG), stringent control measures were implemented in northern China. In this study, comprehensive atmospheric observations were conducted from 2022 January to March in four representative cities to investigate the changes in SNA and their formation mechanism in response to these control measures. The SNA concentrations obviously decreased (29.45%–42.83%) during the control periods in the four cities, but their proportion of PM2.5 remained relatively stable (>50%). SNA was the key factor driving the decrease and increase in the PM2.5 concentration during WOG, with nitrate making the most prominent contribution. The formation of SNA was largely dominated by aqueous heterogeneous chemistry instead of gas-phase oxidation. However, the aerosol liquid water content (ALWC) decreased while odd oxygen (OX) increased during the control periods. Such a variation influenced the role of aqueous heterogeneous and gas-phase reactions in SNA formation, with the former's dominant role being replaced by the latter. After the control periods, the aqueous heterogeneous reactions were enhanced again due to the increased ALWC. Among the cities in the study region, SNA formation became more intense from north to south with increased air temperature and relative humidity (RH). Moreover, ISORROPIA-II thermodynamic modelling revealed notable impacts of sulfate, ammonium, and nonvolatile cations (NVCs) on variations in particle acidity throughout WOG. These results provide a more comprehensive understanding of SNA responses to pollution control on a regional scale and insight into future conventional control.

Differences in aerosol chemistry at a regional background site in Hong Kong before and during the COVID-19 pandemic

Restrictions on human-related activities implemented in Hong Kong to curb the spread of the coronavirus disease 2019 (COVID-19) pandemic provided an opportunity to investigate the anthropogenic impact on organic aerosols (OA) composition. In this study, we conducted a comparative analysis of online measurements of non-refractory submicron particulate matters (NR-PM1) at a regional background site in Hong Kong, covering the periods before the COVID-19 control (November 2018) and during the COVID-19 control (October to November 2020), to investigate changes in OA sources and formation mechanisms. Among the measured NR-PM1 components, organics were the most dominant species with an average percentage of 51.0 ± 0.5 %, exceeding pre-control levels of 44.0 ± 0.7 %. Moreover, 88 % of the organics were attributed to oxygenated OA (OOA). Diurnal variations of all bulk components in NR-PM1 consistently showed afternoon peaks, indicating photochemical processes during COVID-19 control. Similar to the pre-restriction period, the positive matrix factorization (PMF) model showed that OOA was composed of three factors, including two less-oxidized oxygenated factors (LO-OOA1 and LO-OOA2) and one more-oxidized oxygenated factor (MO-OOA). The contribution of the LO-OOA2 factor remained small and stable during both sampling campaigns, which might imply background levels of OOA at this site. The formation of the two predominant components of organics (e.g., LO-OOA1 and MO-OOA) was further discussed. Compared with before control, observational evidence showed that the levels of MO-OOA exceeded LO-OOA1 during the control period, and the average concentration of odd oxygen (Ox = ozone + nitrogen dioxide) increased by 53 % during the COVID-19 control. Besides, the results showed that both LO-OOA1 and MO-OOA exhibited similar diurnal variations to Ox, and their concentrations generally enhanced with increasing Ox levels. This suggested that the formation of OOA was closely related to the photochemical oxidation processes when anthropogenic emissions were reduced. By correlating LO-OOA1 and MO-OOA with speciated OA markers, we found that the formation of LO-OOA1 remained associated with anthropogenic sources, while biogenic emissions contributed to the formation of MO-OOA during the COVID-19 control. Our findings highlight the interplay between emissions, atmospheric conditions, and aerosol composition, providing valuable insights to guide strategic decisions for future air quality improvement.

Decreasing Production and Potential Urban Explosion of Nighttime Nitrate Radicals amid Emission Reduction Efforts.

Nighttime oxidation by nitrate (NO3) radicals has important ramifications on nocturnal aerosol formation and hence the climate and human health. Nitrate radicals are produced by the reaction of NO2 and O3. Despite large decreases in anthropogenic emissions of nitrogen oxides (NOx = NO + NO2), a previous study found significant increases in NO3 production (PNO3) from 2014 to 2019 in China, in contrast to decreasing trends in the U.S. and Europe. Using the summer observations from 2014 to 2022, we analyze the interannual variability of nocturnal PNO3 using a systematic framework, in which PNO3 is diagnosed as a function of odd oxygen (Ox = O3 + NO2) and the NO2/O3 ratio. We did not find an increase of PNO3 from 2014 to 2022 in China due to a continuous decrease in the NO2/O3 ratio, although PNO3 is modulated by the variation in Ox. Using in situ observations obtained in Beijing in 2007, we demonstrate the potential for an upsurge resembling an "explosion" in urban nighttime NO3 radicals amid emission reduction efforts.

Long-term airborne measurements of pollutants over the United Kingdom to support air quality model development and evaluation

Abstract. The ability of regional air quality models to skilfully represent pollutant distributions throughout the atmospheric column is important to enabling their skilful prediction at the surface. This provides a requirement for model evaluation at elevated altitudes, though observation datasets available for this purpose are limited. This is particularly true of those offering sampling over extended time periods. To address this requirement and support evaluation of regional air quality models such as the UK Met Offices Air Quality in the Unified Model (AQUM), a long-term, quality-assured dataset of the three-dimensional distribution of key pollutants was collected over the southern United Kingdom from July 2019 to April 2022. Measurements were collected using the Met Office Atmospheric Survey Aircraft (MOASA), a Cessna 421 instrumented for this project to measure gaseous nitrogen dioxide, ozone, sulfur dioxide and fine-mode (PM2.5) aerosol. This paper introduces the MOASA measurement platform, flight strategies and instrumentation and is not intended to be an in-depth diagnostic analysis but rather a comprehensive technical reference for future users of these data. The MOASA air quality dataset includes 63 flight sorties (totalling over 150 h of sampling), the data from which are openly available for use. To illustrate potential uses of these upper-air observations for regional-scale model evaluation, example case studies are presented, which include analyses of the spatial scales of measured pollutant variability, a comparison of airborne to ground-based observations over Greater London and initial work to evaluate performance of the AQUM regional air quality model. These case studies show that, for observations of relative humidity, nitrogen dioxide and particle counts, natural pollutant variability is well observed by the aircraft, whereas SO2 variability is limited by instrument precision. Good agreement is seen between observations aloft and those on the ground, particularly for PM2.5. Analysis of odd oxygen suggests titration of ozone is a dominant chemical process throughout the column for the data analysed, although a slight enhancement of ozone aloft is seen. Finally, a preliminary evaluation of AQUM performance for two case studies suggests a large positive model bias for ozone aloft, coincident with a negative model bias for NO2 aloft. In one case, there is evidence that an underprediction in the modelled boundary layer height contributes to the observed biases at elevated altitudes.

Atmospheric Oxidation Capacity and Its Impact on the Secondary Inorganic Components of PM2.5 in Recent Years in Beijing: Enlightenment for PM2.5 Pollution Control in the Future

In recent years, the concentrations of PM2.5 in urban ambient air in China have been declining; however, the strong atmospheric oxidation capacity (AOC) represents challenges to the further reduction of PM2.5 concentration and the continuous improvement of ambient air quality in China in the future, since the overall AOC is still at a high level. For this paper, based on ground observation data recorded in Beijing from 2016 to 2019, the variation in AOC was characterized according to the concentration of odd oxygen (OX = O3 + NO2). The concentrations of the primary and secondary components of PM2.5 were analyzed using empirical formulas, the correlation between AOC and the concentrations of secondary PM2.5 and the secondary inorganic components (SO42−, NO3−, NH4+, and SNA) in Beijing were explored, the impact of atmospheric photochemical reaction activity on the generation of atmospheric secondary particles was evaluated, and the impact of atmospheric oxidation variations on PM2.5 concentrations and SNA in Beijing was investigated. The results revealed that OX concentrations reached their peak in 2016 and reached their lowest point in 2019. The OX concentrations followed a descending seasonal trend of summer, spring, autumn, and winter, along with a spatial descending trend from urban observation stations to suburban stations and background stations. The degree of photochemical activity and the magnitude of the AOC have a large influence on the production of atmospheric secondary particles. When the photochemical reaction was more active and the AOC was stronger, the mass concentrations of the secondary generated PM2.5 fraction were higher and accounted for a higher proportion of the total PM2.5 mass concentrations. In the PM2.5 fraction, SNA accounted for 50.7% to 94.4% of the total mass concentrations of water-soluble inorganic ions in the field observations. Higher concentrations of the atmospheric oxidant OX in ambient air corresponded to a higher sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR), suggesting that the increase in AOC could promote the increase of PM2.5 concentration. Based on a relationship analysis of SOR, NOR, and OX, it was inferred that the relationship between OX and SOR and the relationship between OX and NOR were both nonlinear. Therefore, when establishing PM2.5 control strategies in Beijing in the future, the impact of the AOC on PM2.5 generation should be fully considered, and favorable measures should be taken to properly regulate the AOC, which would be more effective when carrying out further control measures regarding PM2.5 pollution.

Effect of Global Warming on the Atmospheric Lifetime of Odd Oxygen

Effect of Global Warming on the Atmospheric Lifetime of Odd Oxygen

The Effect of Global Warming on the Atmospheric Lifetime of Odd Oxygen

Data on the effect of global warming on the atmospheric lifetime of odd oxygen, Ox. The results obtained characterize the effect at latitude 50°C. in the altitude range of 15–55 km in January–June 2000–2100s, the data for calculations were obtained using the interactive radiation-chemical two-dimensional SOCRATES model, with which the total rate of Ox death in the Ox, HOx, NOx, ClOx and BrOx catalytic cycles, as well as the concentration of Ox equal to the sum of the concentrations of O3, O(3P) and O(1D) for the above conditions, which are necessary for calculating the lifetime of Ox, were preliminarily calculated. Scenarios of the Intergovernmental Panel on Climate Change (IPCC) were used as initial conditions for calculations using the SOCRATES model RCP 4.5 and RCP 6.0 for the above conditions.

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 NOx emission reductions on odd oxygen (Ox = 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 NOx control and their changes over decadal time scales. The desirability of NOx-focused emission control programs has increased between 2000 and 2022. We find for present-day conditions that reducing NOx emissions by 28% from targeted high-priority locations can achieve 60% of the air quality benefits of uniform NOx 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.

Formation pathways of organic aerosols over a tropical coastal atmosphere

The characteristics of non-refractory submicron aerosols (NR-PM1), focusing on organic aerosols, are examined over a tropical coastal location, Thumba, in southern peninsular India. The measurements were carried out using an aerosol chemical speciation monitor during 2017–2021. The results indicated organic aerosols as the major component (67–85%) in the NR-PM1 mass loading during all seasons. Positive matrix factorization (PMF) analysis on the OA mass spectra yielded two factors resolving Hydrocarbon-like organic aerosols (HOA) and Oxygenated organic aerosols (OOA), with the major contribution to OA coming from OOA (mass fraction >50%) during all the seasons. Examination of the diurnal variations revealed the daytime enhancement in OOA and sulfate mass fractions, indicating photochemical production overwhelming the dilution at the surface due to local atmospheric boundary layer expansion. The OOA mass concentration showed a good correlation (R > 0.5) with odd oxygen (O3+NO2) and with an increase in oxygen-to-carbon ratio (ΔO:C = 0.11 to 0.35 with Δodd oxygen = 60 μg m−3 for different seasons) (i.e., increased oxidation degree of OA) signifying the dominance of photochemistry and subsequent enhanced atmospheric oxidative capacity in OOA formation. Interestingly, the aerosol liquid water content (ALWC) doubled during the nighttime (>40 μg m−3) compared to the daytime (<20 μg m−3) in different seasons, indicating the combined impact of varying submicron aerosol composition and ambient relative humidity. Since highly humid (relative humidity >85%) conditions prevail over this tropical coastal station, especially during the nighttime, the role of aqueous-phase formation of OA is also examined. It was found that less oxidized/less-hygroscopic OA were prevalent during the nighttime, resulting in lower aqueous-phase OA processing. During all the seasons, the hygroscopicity of organics (кorganics) reached as high as > 0.22 and was mainly driven by photochemical processing rather than aqueous-phase chemistry. These results highlight the importance of the OA formation pathways in affecting their hygroscopic properties over the tropical atmosphere.

Identifying the O3 chemical regime inferred from the weekly pattern of atmospheric O3, CO, NOx, and PM10: Five-year observations at a center urban site in Shanghai, China

Ozone pollution is still considered a severe environmental problem in China despite the fact that great efforts have been devoted to monitoring and alleviating its impact by the Chinese government including the establishment of numerous observational networks. One of the issues most relevant to the design of emission reduction policies is to distinguish the O3 chemical regime. Here a method of quantifying the fraction of the radical loss versus NOx chemistry was applied to identify the O3 chemical regime inferred from the weekly pattern of atmospheric O3, CO, NOx, and PM10, which were monitored by Ministry of Ecology and Environment of China (MEEC). During spring and autumn, O3 and the total odd oxygen (Ox, Ox = O3 + NO2) weekend afternoon concentrations are both higher than the weekday values during 2015–2019 except in 2016, while CO and NOx weekend morning concentrations were generally both smaller than weekday values except 2017. Results from the calculated values of fraction of the radical loss by NOx chemistry relative to total radical loss (Ln/Q) suggested a volatile organic compound (VOC)-limited regime at this site in the spring of 2015–2019, as expected from the decreasing trend in NOx concentration and essentially constant CO after 2017. With respect to autumn, a shift from a transition regime during 2015–2017 to a VOC-limited regime in 2018 was found, which rapidly took place to a NOx-limited regime in 2019. No significant differences were detected in the Ln/Q values under different assumptions on photolysis frequencies both in spring and autumn mostly from 2015 to 2019, giving the same conclusion of determining the O3 sensitivity regime. This study develops a new method in determining the O3 sensitivity regime in the typical season in China and provides insight into efficient O3 control strategies in different seasons.

Effects of regional air pollutants on respiratory diseases in the basin metropolitan area of central Taiwan

This study divided a basin metropolitan area with high air pollution into three subareas, namely urban, suburban, and rural, on the basis of population density for a systematic analysis of the effects of local air pollutants on respiratory diseases. A panel data regression model was used to estimate the annual incidence growth rates (AIGRs) of the four respiratory diseases, namely lung cancer, chronic obstructive pulmonary disease, asthma, and pneumonia, resulting from exposure to fine particulate matter (PM2.5, diameter of 2.5 μm or less), odd oxygen (ODO), or nonmethane hydrocarbon (NMHC). The results indicate that the prevailing wind direction is not a major factor determining the distribution of air pollutants. The spatial distributions of ODO and NMHC differed from that of PM2.5. Three air pollutants contributed to positive AIGRs of the four diseases in the study area, but PM2.5 which had a negative AIGR for asthma in the rural subarea. The pollutants with the strongest effects on AIGR, in descending order, were NMHC, PM2.5, and ODO. The effect of ambient NMHC was significant and nonnegligible, especially in the urban subarea. A dimensionless potential AIGR (PAIGR) formula was established to quantitatively compare the effects of different air pollutants on the four respiratory diseases. The results indicate that ambient NMHC had the strongest effect on the incidences of the respiratory diseases, followed by that of ambient PM2.5. The effect of ambient NMHC was significant and nonnegligible, especially in the urban subarea. The PAIGR ratio ranges of PM2.5 to ODO and NMHC to ODO for the four diseases in urban subsarea were from 3 to 19 and from 289 to 920, respectively. This study also applied multivariate regression to assess the association among 5 aspects, namely air quality, point source, line source, area source, and socioeconomic status, and the incidences of the four respiratory diseases. The results indicate that the model has favorable fit and can thus reflect the associations of the 15 factors of 5 aspects with the four respiratory diseases in each subarea.

A Closure Study of Secondary Organic Aerosol Estimation at an Urban Site of Yangtze River Delta, China

Secondary organic aerosols (SOA) are crucial components of ambient particulate matters. However, their composition and formation mechanisms remain uncertain. To investigate the SOA formation and evaluate various SOA estimation approaches, a comprehensive measurement was conducted at an urban site, Changzhou, in Yangtze River Delta (YRD) region. 98 kinds of volatile organic compounds (VOCs) were measured by an online gas chromatography-mass spectrometer/flame ionization detector (GC-MS/FID). Non-refractory submicron particulate matters (NR-PM1) were measured by an Aerodyne Aerosol Chemical Speciation Monitor (ACSM). Both bottom-up approaches, i.e., VOCs oxidation yield method, and top-down approaches, i.e., elemental carbon (EC) tracer method and ACSM, combined with positive matrix factorization (PMF) method, were utilized to estimate SOA. ACSM-PMF method estimated the highest SOA concentration, followed by EC tracer method. SOA from VOCs oxidation yield method accounted for 43.2 ± 41.9% of that from EC tracer method, suggesting the existence of missing SOA precursors, e.g., semivolatile organic compounds. The influencing factors of SOA formation were investigated and a good correlation of SOA with odd oxygen rather than aerosol liquid water content was found, suggesting the importance of photochemical formation of SOA.

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 NOx, in anthropogenic volatile organic compounds (VOCs), and reductions in fine particulate matter (PM2.5) 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 PM2.5, 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.

Secondary organic aerosol formation at an urban background site on the coastline of South China: Precursors and aging processes

Understanding the formation mechanisms of secondary organic aerosols (SOA) is an arduous task in atmospheric chemistry. In November 2018, a sampling campaign was conducted at an urban background site in Hong Kong for characterization of secondary air pollution. A high-resolution time-of-flight aerosol mass spectrometer was used to monitor the compositions of non-refractory submicron particulate matters (NR-PM1), and multiple online instruments provided us with comprehensive auxiliary data. Organic aerosol (OA) constituted the largest fraction (43.8%) of NR-PM1, and 86.5% of the organics was contributed by the oxygenated OA (OOA, secondary components). Formation mechanisms of a dominant and more variable component of the less-oxidized OOA (labelled as LO-OOA1 in this study) and the more-oxidized OOA (MO-OOA) were explored. Based on the multilinear regression with molecular markers of OA (e.g., hydroxybenzonic acids and 2,3-dihydroxy-4-oxopentanoic acid), we presumed that anthropogenic organic compounds, especially aromatics, were the most likely precursors of LO-OOA1. MO-OOA correlated well with odd oxygen (Ox), and its concentration responded positively to the increase of liquid water content (LWC) in NR-PM1, indicating that the formation of MO-OOA involved photochemical oxidation and aqueous processes. It exhibited the best correlation with malic acid which can be formed through the oxidation of various precursors. Moreover, it was plausible that LO-OOA1 was further oxidized to MO-OOA through aqueous processes, as indicated by the consistent diurnal variations of MO-OOA to LO-OOA1 ratio and LWC. This study highlights the important roles of anthropogenic emissions and aqueous processes in SOA formation in coastal areas downwind of cities.

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 (Ox ≡ 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 NOx (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 NOx-sensitive conditions in the afternoon with all but the southernmost receptor shifting from VOC- to NOx-sensitive conditions. We also evaluate opportunities to control pollution through shifts in precursor emission location and timing.

Enhanced secondary organic aerosol formation during dust episodes by photochemical reactions in the winter in Wuhan

To investigate the effect of frequently occurring mineral dust on the formation of secondary organic aerosol (SOA), 106 volatile organic compounds (VOCs), trace gas pollutants and chemical components of PM2.5 were measured continuously in January 2021 in Wuhan, Central China. The observation period was divided into two stages that included a haze period and a following dust period, based on the ratio of PM2.5 and PM10 concentrations. The average ratio of secondary organic carbon (SOC) to elemental carbon (EC) was 1.98 during the dust period, which was higher than that during the haze period (0.69). The contribution of SOA to PM2.5 also increased from 2.75% to 8.64%. The analysis of the relationships between the SOA and relative humidity (RH) and the odd oxygen (e.g., OX = O3 + NO2) levels suggested that photochemical reactions played a more important role in the enhancement of SOA production during the dust period than the aqueous-phase reactions. The heterogeneous photochemical production of OH radicals in the presence of metal oxides during the dust period was believed to be enhanced. Meanwhile, the ratios of trans-2-butene to cis-2-butene and m-/p-xylene to ethylbenzene (X/E) dropped significantly, confirming that stronger photochemical reactions occurred and SOA precursors formed efficiently. These results verified the laboratory findings that metal oxides in mineral dust could catalyse the oxidation of VOCs and induce higher SOA production.

Decisive role of ozone formation control in winter PM2.5 mitigation in Shenzhen, China

During the COVID-19 lockdown, atmospheric PM2.5 in the Pearl River Delta (PRD) showed the highest reduction in China, but the reasons, being a critical question for future air quality policy design, are not yet clear. In this study, we analyzed the relationships among gaseous precursors, secondary aerosols and atmospheric oxidation capacity in Shenzhen, a megacity in the PRD, during the lockdown period in 2020 and the same period in 2021. The comprehensive observational datasets showed large lockdown declines in all primary and secondary pollutants (including O3). We found that, however, the daytime concentrations of secondary aerosols during the lockdown period and normal period were rather similar when the corresponding odd oxygen (Ox≡O3+NO2, an indicator of photochemical processing avoiding the titration effect of O3 by freshly emitted NO) were at similar levels. Therefore, reduced Ox, rather than the large reduction in precursors, was a direct driver to achieve the decline in secondary aerosols. Moreover, Ox was also found to determine the spatial distribution of intercity PM2.5 levels in winter PRD. Thus, an effective strategy for winter PM2.5 mitigation should emphasize on control of winter O3 formation in the PRD and other regions with similar conditions.

The roles of aqueous-phase chemistry and photochemical oxidation in oxygenated organic aerosols formation

The formation mechanism and evolution process of secondary organic aerosol (SOA) remain poorly understood due to measurement uncertainties and chemical complexities. Here, we present the characterization of non-refractory fine particles (NR-PM2.5) analyzed by means of a time-of-flight ACSM (ToF-ACSM) at a rural site in the North China Plain during winter. Our results show that air quality in rural areas was heavily polluted by primary organic aerosols (POA), which accounted for 83% of total organic aerosol (OA). The oxygenated organic aerosols (OOA) were mainly generated from local emissions. As pieces of evidence, firstly, less-oxidized OOA (LO-OOA) had good correlations with primary species like biomass burning OA (BBOA) (R2= 0.54), EC (R2= 0.46), and chloride (R2= 0.46), implying LO-OOA was formed with the POA emission processes; secondly, there was a significant increase in OOA concentration after sunrise, indicating a lot of local generation during the daytime; thirdly, the potential source region of OOA was restricted in that of POA, implying that OOAs were converted from where primary emission exists. The formation processes of LO-OOA and more-oxidized OOA (MO-OOA) are different. Aqueous-phase chemistry had a dominant effect on the formation of LO-OOA due to the better correlation between aerosol liquid water content (ALWC) with LO-OOA (R2= 0.54) than MO-OOA (R2= 0.15). In comparison, both aqueous-phase and photochemical processes acted on the MO-OOA formation, when odd oxygen (Ox = O3+NO2) was low (Ox < 35 ppb), MO-OOA concentration increased with the increment of ALWC. However, at high Ox level (Ox > 35 ppb), photochemical oxidation played an essential role in MO-OOA formation.

Seasonal characteristics of PM1 in Seoul, Korea, measured using HR-ToF-Aerosol Mass Spectrometer in 2018

In Korea, the characteristics of aerosols vary according to the four distinct seasons (spring, summer, fall, and winter); the seasonal variations are influenced by different air mass inflows of each season. Therefore, research is required to analyze the composition and characteristics of aerosols in different seasons. In this study, high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and positive matrix factorization (PMF) analyses were conducted in 2018 to examine the seasonal characteristics of aerosols with focus on organic aerosol in Seoul metropolitan area of South Korea. The results indicated that the seasonal non-refractory submicron particle (NR-PM1) concentration in Seoul metropolitan area ranged from 6.3 to 41.0 μg/m3, which was lower than those in other metropolitan areas of Northeast Asia and slightly higher than those in suburban and rural areas of South Asia and Europe. The organic aerosol concentration, which was the highest among PM1 components, ranged 3.5–14.24 μg/m3. In addition, a large increase in nitrate content was observed in high-concentration episodes. In all seasons, the O:C ratio and oxidation state were higher than those in previous studies, and it was estimated that the oxidation state was higher than those in other countries due to the influx of aged aerosols by long-range transport. In summer, the odd oxygen (Ox) concentration tends to be proportional to temperature and inversely proportional to humidity, indicating that the formation of fresh organic aerosol (OA) by active photochemical reaction is the main process of organic aerosol generation. In spring, fall, and winter, the Van Krevelen graph showed a slope of −0.68 ~ –0.99, indicating that carboxylic acid production was an important process for OA production. In addition, the causes of the high concentration episodes observed in spring and winter were estimated through the oxidation state of organic aerosols, PMF factor, and backward trajectory analyses.

Critical Role of Simultaneous Reduction of Atmospheric Odd Oxygen for Winter Haze Mitigation.

The lockdown due to COVID-19 created a rare opportunity to examine the nonlinear responses of secondary aerosols, which are formed through atmospheric oxidation of gaseous precursors, to intensive precursor emission reductions. Based on unique observational data sets from six supersites in eastern China during 2019-2021, we found that the lockdown caused considerable decreases (32-61%) in different secondary aerosol components in the study region because of similar-degree precursor reductions. However, due to insufficient combustion-related volatile organic compound (VOC) reduction, odd oxygen (Ox = O3 + NO2) concentration, an indicator of the extent of photochemical processing, showed little change and did not promote more decreases in secondary aerosols. We also found that the Chinese provinces and international cities that experienced reduced Ox during the lockdown usually gained a greater simultaneous PM2.5 decrease than other provinces and cities with an increased Ox. Therefore, we argue that strict VOC control in winter, which has been largely ignored so far, is critical in future policies to mitigate winter haze more efficiently by reducing Ox simultaneously.