Nocturnal O3 Research Articles

Cause of nocturnal surface ozone enhancement in the North China Plain

The North China Plain (NCP), known for its dense population, extensive urbanization, and developed industry and agriculture, faces one of the foremost ozone (O3) pollution issues nationwide and even globally. Currently, most studies focus on daytime peak O3 levels, with insufficient understanding of the increase in nighttime O3 concentrations. Based on data from 204 national atmospheric composition monitoring sites in the NCP from 2015 to 2023, we investigated the characteristics of nocturnal surface O3 enhancement (NSOE) events and explored potential formation mechanisms. The mean annual frequencies of single-site and regional NSOE event in the NCP between 2015 and 2023 are 42 % and 21 %, respectively. The daytime peak O3 concentrations before and after NSOE events exceeded those during the corresponding periods of non-NSOE events by 84 ± 19 and 32 ± 15 μg/m3, respectively. The overall effect of the NSOE events was to decelerate the rate of decline in nighttime O3 concentrations and resulted in a reduction of NO2 and CO concentrations from 22:00 onwards. Low level jet (LLJ) and vertical mixing were the main factors affecting NSOE events in the NCP. The proportion of NSOE events affected by LLJ in four representative cities ranged from 57.6 % to 79.5 %. Furthermore, the high concentration of O3 in the residual layer before the NSOE event and the reduction of atmospheric stability during the NSOE event favored downward mixing of upper layer O3. The primary weather systems influencing the four most severe regional NSOE events were LLJ, typhoon, and cold fronts. The first two events were dominated by vertical mixing of O3, while the latter two events were mainly affected by horizontal transport. Our findings provide the first overview of NSOE events in the NCP from characteristics to mechanisms, emphasizing the necessity for future detailed studies based on nocturnal vertical O3 observations.

Impacts of different vehicle emissions on ozone levels in Beijing: Insights into source contributions and formation processes

Beijing, with the highest number of motor vehicles in China, significantly contributes to O3 pollution through substantial NOx and VOC emissions in the on-road transportation sector. Understanding the unique impact of emissions from different vehicle types on O3 levels is crucial for developing targeted strategies for O3 pollution. This study applied the Community Multiscale Air Quality Modeling System (CMAQ) to comprehensively investigate the impacts of emissions from different vehicle types on O3 levels in various regions of Beijing and to provide valuable insights into source contributions and formation processes. The results revealed that various vehicle types exhibited different spatial-temporal emission patterns, with medium-heavy duty trucks (HDT) and mini-light passenger vehicles (LDPV) identified as the primary contributors to NOx (36.1 %) and VOC (57.6 %) emissions. Using the Integrated Source Apportionment Method (ISAM) coupled in CMAQ, we found the total vehicle emissions contributed to over 20 % of daily maximum 8–h average O3 (MDA8 O3) concentration, ranked as the second largest contributor after regional transport. Contributions to O3 formation from LDPV and medium-large passenger vehicles (MDPV) were 2.6–4.0 and 4.2–6.8 ppb and mainly concentrated in urban areas, while the contributions from mini-light duty trucks (LDT) and HDT were 3.5–4.8 and 3.7–6.2 ppb and mainly concentrated in suburban areas. Through scenario analysis that removed emissions from specific types of vehicles, we found removing LDPV emissions led to decreases in daytime O3 concentration by 0.3–3.8 ppb. In contrast, removing MDPV emissions led to notable O3 increases by 4.0–11.8 ppb at rush hours. Removing LDT and HDT emissions resulted in 0.6–8.0 ppb increases in nocturnal O3 concentrations while 0.8–2.0 ppb decreases during the afternoon. This research highlights the necessity of tailoring control strategies for different vehicle types to effectively reduce O3 levels in Beijing and provides useful information for decision-makers to formulate effective measures of vehicle management in the future.

Spatiotemporal Distribution Characteristics of Ground-level-ozone and Its Relationship with Meteorological Conditions in a Representative City in the Bohai Rim from 2017 to 2022

In recent years, ground-level-ozone（O3） pollution in urban areas in the Bohai Rim has attracted wide attention. Based on the analysis of the spatiotemporal distribution characteristics of O3 concentration in Dongying, a representative city in the Bohai Rim from 2017 to 2022, the effects of meteorological factors and sea-land breeze circulation on O3 concentration were evaluated. The results showed that： ① From 2017 to 2022, the annual assessment value of O3 concentration in Dongying showed a fluctuating upward trend, and the pollution days with O3 as the primary pollutant increased. O3 pollution mainly occurred in spring, summer, and autumn, with the most severe O3 pollution episodes typically occurring in May and June, and the duration of O3 pollution season tended to be longer. The monthly variation in the daily maximum 8-h average ozone （MDA8 O3） presented a bimodal distribution, with significant increases in the 5th and 25th percentiles, and the spatial distribution was "high in the north and south, low in the middle." In addition, the nocturnal O3 concentration in recent years in Dongying also showed a significant increase trend. ② Meteorological factors greatly influenced O3 concentration in Dongying. When the temperature was greater than 30℃, the relative humidity was less than 50%, and the wind direction was south-southwest or east-northeast, a high O3 value was more likely to occur. Meteorological factors contributed 30% of the MDA8 O3 variation in Dongying during the study period. In the case of moderate and severe O3 pollution, the contribution of meteorological factors to the change in MDA8 O3 could be as high as 40%. ③ To some extent, sea-land breeze contributed to the occurrence of MDA8 O3 exceeding the secondary standard limit value of the National Ambient Air Quality Standard. In the afternoon, the hourly concentration of O3 during the sea-land breeze days was approximately 20 μg·m-3 higher than that during the non-sea-land breeze days. On the days of moderate and severe O3 pollution, the O3 concentration during the sea-land breeze days from 10：00 to 16：00 was higher than that during non-sea-land breeze days, and the O3 concentration was also at a high level from 20：00 to 23：00 on sea-land breeze days. In the O3 pollution season, the sea-land breeze could significantly affect the O3 level in coastal cities, which could bring significant challenges for O3 pollution prevention and control in this region. In the future, cities in the Bohai Rim need to further strengthen regional joint prevention and control of O3 pollution and increase emission reduction efforts of nitrogen oxides and volatile organic compounds. This strategy could effectively lower pollutant concentrations within the land breeze air mass, consequently reducing the impact of the sea breeze air mass on air quality in cities in the Bohai Rim.

Investigation into the nocturnal ozone in a typical industrial city in North China Plain, China

Ozone (O3) concentrations usually peak at midday by photochemical reactions and gradually decline after sunset due to chemical destruction and dry deposition. However, an increase in the frequency of elevated nocturnal ozone enhancement (NOE) and high nocturnal ozone value (HNOV) has been frequently observed in urban areas of eastern China, but the reasons are not well understood. In this study, taking a typical industrial city, ZiBo as a case study, we analyzed the trends, characteristics, and causes of the NOE and HNOV events in historical years by combining observations and model simulations. During the warm season (April–September) of 2017–2023, HNOV events are accompanied by low humidity, high temperature, large friction velocity, and a high boundary layer (52 days in total), whereas NOE events coincide with increases in humidity, wind speed, friction velocity, and boundary layer height (141 days in total). During the HNOV and NOE events, the nighttime average concentrations of Ox were 77 ± 7 and 12 ± 6 μg m−3 higher than the non-nocturnal O3 period, indicating enhanced atmospheric oxidizing capacity during nighttime. The modeling results indicate that both the HNOV and NOE events were mainly driven by vertical mixing and regional transport. We selected a typical period with high O3 pollution and frequent NOE and HNOV events to conduct the modeling study. Three typical nocturnal O3 events are identified: Case I was mainly driven by horizontal transport; while in the two subsequent cases, the vertical transport contribution was 80 μg m−3 h−1 (20:00 LT on June 21, 2021) and 35 μg m−3 h−1 (02:00 LT on June 26, 2021), respectively. Our study reveals that the O3 pollution in industrial cities has been extending to nighttime, primarily attributed to vertical mixing and horizontal transport within the boundary layer. This highlights the critical role of implementing regional joint control action to reduce primary emissions and eliminate residual ozone.

Investigating the causes and reduction approaches of nocturnal ozone increase events over Tai'an in the North China Plain

China's O3 pollution events continue to be frequent, and accurately understanding the sources and sinks of O3 pollution is crucial. Previous research predominantly focused on controlling O3 pollution through daytime photochemical production, overlooking the significant impact of nocturnal high O3 values on the subsequent days' O3 concentrations. Understanding the causes of nocturnal O3 pollution and implementing effective control measures are essential for O3 pollution alleviation. In light of this, we utilized clustering analysis, combined with meteorological and chemical factors to comprehend the nocturnal variation characteristics and mechanisms of nocturnal O3 increase (NOI) events in Tai'an from July 2021 to June 2022. The results qualitatively identified that three clusters have distinct influencing factors: horizontal transport predominantly drives NOIflucts, meanwhile, NOIdecline events, mainly occurring in summer, experienced more intense photochemical reactions. For NOIstable events, primarily observed in spring, there was greater downward transport of high-altitude O3 and more intense titration reactions. Further, using interpretable machine learning models, we quantitatively assessed the contributions of several factors to nocturnal O3. The results reveal that nocturnal titration is the predominant factor, followed by the accumulation of daytime photochemical generation. Additionally, we explored approaches for reducing daytime nitrogen dioxide (NO2) and formaldehyde (HCHO), quantifying their impact on nocturnal O3 concentrations. The results indicate that reducing emissions of HCHO and NO2 can effectively lower O3 levels in NOIdecline and focusing on reducing HCHO emissions proves to be more beneficial in NOIstable events, with NOIflucts events falling in between these two clusters. Targeted implementation of diverse reduction approaches and proportions in different NOI clusters can more effectively mitigate the occurrence of nocturnal O3 pollution, thereby influencing the O3 levels of the following day.

Statistical analysis of the relationships between boundary layer meteorological processes and nocturnal O3 increases in Beijing from 2018 to 2022

In the absence of photochemical reactions, nocturnal ozone (O3) variation was found to be caused by boundary layer meteorological processes. In addition to low-level jet (LLJ) and convective storm (CS) events, cold front and foehn processes also cause nocturnal O3 increases (NOIs). The subsidence warming effect (Subsi.) caused by weak cold front and foehn processes cause the nocturnal surface temperature and O3 to increase. Statistical analysis revealed that NOI days accounted for 39.4 ± 6.4% of the total number of days per year in Beijing, and the average occurrence frequency decreased by 1.7%/year over a recent five-year period (2018–2022). The LLJ process is the primary factor causing NOIs, and the average annual contributions of the LLJ, CS and Subsi. processes to NOIs were approximately 32.2 ± 10.5%, 11.7 ± 2.3% and 8.0 ± 3.0%, respectively. From a seasonal point of view, the most NOI events occurred in summer, and the least occurred in autumn. LLJs contributed the most to the NOIs in each season, and the CS contribution was almost equal to that of LLJs in summer. LLJs that occurred near 00:00 LT and 04:00 LT had a greater probability of causing NOI events. Most of the nocturnal CS events occurred during 22:00 LT-02:00 LT (next day), and the contribution of CSs to NOIs fluctuated around 10.0%. More than half (68.0%) of the Subsi. events occurred during 23:00 LT-03:00 LT (next day). Subsi. events occurred during this period contributed more to NOIs than other periods. Meteorological processes more easily caused NOI events in the urban, Daxing and Tongzhou districts than other areas. The average surface O3 concentration during the early morning hours (04:00–06:00 LT) on NOI days was 8.8 ± 3.2 μg/m3 higher than that on non-NOI days, suggesting that NOIs can elevate the base O3 concentration during the following day.

A Comparative Investigation of the Characteristics of Nocturnal Ozone Enhancement Events and Their Effects on Ground-Level Ozone and PM2.5 in the Central City of the Yellow River Delta, China, in 2022 and 2023

In recent years, nocturnal ozone enhancement (NOE) events have emerged as a prominent research focus in the field of the atmospheric environment. By using statistical analysis methods, we conducted a comparative investigation of nocturnal ozone concentrations and NOE events in Dongying, the central city of the Yellow River Delta, China, in 2022 and 2023, and further explored the effects of NOE events on O3 and PM2.5 on the same night and the subsequent day. The results showed that from 2022 to 2023, in Dongying, the annual average nocturnal ozone concentrations increased from 51 μg/m3 to 59 μg/m3, and the frequency of NOE events was higher in the spring, summer, and autumn, and lower in the winter. The NOE events not only exhibited promoting effects on nocturnal O3 and Ox, and on the daily maximum 8 h average concentration of O3 (MDA8-O3) on the same day (comparatively noticeable in summer and autumn), but also demonstrated a clear impact on nocturnal PM2.5 and PM2.5-bounded NO3− and SO42− (especially in winter). Additionally, the NOE events also led to higher concentrations of O3 and Ox, as well as higher MDA8-O3 levels during the subsequent day, with more observable impacts in the summer. The results could strengthen our understanding about NOE events and provide a scientific basis for the collaborative control of PM2.5 and O3 in urban areas in the Yellow River Delta in China.

Comment on “Transport of substantial stratospheric ozone to the surface by a dying typhoon and shallow convection” by Chen et al. (2022)

Abstract. Chen et al. (2022) analyzed the event of rapid nocturnal O3 enhancement (NOE) observed on 31 July 2021 at the surface level in the North China Plain and proposed transport of substantial stratosphere ozone to the surface by Typhoon In-fa followed by downdraft of shallow convection as the mechanism of the NOE event. The analysis seems to be valid from the viewpoint of atmospheric physics. This comment revisits the NOE phenomenon on the basis of the China National Environmental Monitoring Center (CNEMC) network data used in Chen et al. (2022), together with the CNEMC data from Zibo (ZB) and O3, NOx, PAN (peroxyacetic nitric anhydride), and VOC (volatile organic compound) data from the Zibo supersite operated by the China Research Academy of Environmental Sciences (CRAES). We found (a) Ox (O3 + NO2) levels during the NOE period approaching those of O3 during 14:00–17:00 LT, (b) levels of PAN and the relationship between O3 and PAN consistent with dominance of chemical and physical processes within the boundary layer, and (c) estimated photochemical ages of air mass shorter than 1 d and showing no drastic increases during the NOE. We argue that the NOE was not caused by typhoon-induced stratospheric intrusion but originated from fresh photochemical production in the lower troposphere. Our argument is well supported by the analysis of atmospheric transport as well as ground-based remote sensing data.

Nocturnal ozone enhancement in Shandong Province, China, in 2020–2022: Spatiotemporal distribution and formation mechanisms

Nighttime ozone enhancement (NOE) can increase the oxidation capacity of the atmosphere by stimulating nitrate radical formation and subsequently facilitating the formation of secondary pollutants, thereby affecting air quality in the following days. Previous studies have demonstrated that when nocturnal ozone (O3) concentrations exceed 80 μg/m3, it leads to water loss and reduction of plant yields. In this study, the characteristics and mechanisms of NOE over Shandong Province as well as its 16 cities were analyzed based on observed hourly O3 concentrations from 2020 to 2022. The analysis results show that NOE predominantly occurred in the periods of 0:00–3:00 (41 %). The annual mean frequency of NOE events was ~64 days/year, approximately 4–7 days per month. The average concentration of nocturnal O3 peak (NOP) was ~72.6 μg/m3. Notably, high NOP was observed in the period from April to September with the maximum in June. Coastal cities experienced more NOE events. Typical NOE events characterized by high NOP concentrations in the coastal cities of QingDao, WeiHai and YanTai in June 2021 were selected for detailed analysis with a regional chemical transport model. The results showed that high levels of O3 in eastern coastal cities during NOE events primarily originate from horizontal transport over the sea, followed by vertical transport. During the daytime, O3 and its precursors are transported to the Yellow Sea by westerly winds, leading to the accumulation of O3 near the sea and coastline. Consequently, under the influence of prevailing winds, the movement of O3 pollution belts from the sea to land causes rapid increases in near-surface O3 levels. Meanwhile, vertical transport can also contribute to NOE in coastal areas. The high-level O3 in the upper atmosphere generally originates from long-distance transport and turbulent transport of O3 produced near the ground during the daytime. At night, the absence of chemicals that consume O3 in the upper air and descending air flow carries O3 to the near-surface. The impacts of other O3-depletion processes (such as dry deposition) on NOE are less pronounced than those of transport processes.

Nocturnal downward transport of NO3 radical from the residual layer to a surface site by the mountain breeze in Seoul, South Korea

Using the long-path differential optical absorption spectroscopy (LP-DOAS) technique, the nocturnal mixing ratios of NO3 radicals were measured at an urban site in Seoul, South Korea, from May 13 to June 12, 2022. The primary objectives of the study were to estimate the NO3 radical concentration in the nighttime boundary layer and to examine the variations in their generation and loss over time, including an evaluation of their sources and sinks. The observed mean NO3 mixing ratios and lifetimes were 16 pptv and 73 s, respectively. NO3 concentrations and lifetimes were closely associated with their production, involving O3 and NO2, as well as the loss processes regulated by NO, VOCs, and aerosol surface area. Applying the steady-state assumption for NO3 and N2O5, the mean N2O5 uptake coefficient on the aerosol surface for the entire period was 0.031. When foggy nights were excluded from the analysis, the uptake coefficient decreased to 0.025. These values are within the higher range of the results reported in other studies. High relative humidity, which averaged 75% at night throughout the collection period, favored high N2O5 uptake coefficients. NO3 concentrations generally peaked shortly after sunset and gradually declined until sunrise. However, we frequently observed distinct increases in NO3 throughout the early morning hours, especially during clear nights, when local circulation with the westerly mountain breeze prevailed at the research site. The mountain breeze transported residual air downward, resulting in 110% and 38% higher NO3 and O3 levels at the sampling locations, respectively, compared to normal conditions. This additional source of nocturnal radicals and O3 has consequences for increased secondary aerosols and O3 production, particularly at urban surface levels where they are readily depleted in the early morning hours. To this end, a more thorough examination is required to measure N2O5 and a larger spectrum of VOCs species, as these parameters are crucial for nighttime radical budgets, secondary aerosols, and O3 generation in cities like Seoul.

From lowland plains to the Altiplano: The impacts of regional transport of wildfire smoke on the air quality of Bolivian cities

Wildfires emerged as a global concern in recent decades, because they deteriorate the air quality in urban centres situated downwind, impact human health, ecosystems and regional climate. In this study, we assessed the air quality of Bolivia's three largest metropolitan areas during the period 2017–2019 by analysing in situ criteria air pollutant data (PM10, O3 and NOx) coupled with satellite (aerosol optical depth AOD and total CO column), fire foci data, and airmass backward trajectories. The Bolivian lowland plains were highly impacted by smoke transported from the Amazon and Cerrado biomes. Due to its proximity to the fire foci, Santa Cruz was the most affected city, where PM10, CO and AOD increased between 1.3- and 3.9-fold in September (maximum fire activity) compared to April (lowest fire activity). In Cochabamba and La Paz-El Alto, the impacts of wildfires on AOD and CO were lower (15–16% increase). A case study of regional smoke in September 2019 using Cochabamba as an example revealed that: i) airmass trajectories passed over regions with high fire radiative power density in southern Bolivia, Paraguay and northern Argentina; ii) the advection of wildfire smoke caused unusual episodes of enhanced nocturnal O3 concentrations (20–45 μg/m3) over a four-day pollution event. The results indicated that local sources (e.g., motorised traffic, brick kilns and dust resuspension) largely contributed to deteriorate the air quality, particularly in the dry months, surpassing the World Health Organization recommendations for PM10, in La Paz-El Alto and Cochabamba. We highlight the need of international cooperation among South American countries to combat fire practices in the region, given the transboundary nature of the biomass burning plumes. Moreover, it is crucial to implement local measures aimed at tackling local air pollution sources, especially in a changing climate in which drier conditions can favour the onset and propagation of wildfires and accumulation of air pollutants.

Transport mechanisms of nocturnal surface ozone over Riyadh, Kingdom of Saudi Arabia

This study investigated ozone (O3) transport mechanisms and their impact on observed nocturnal surface O3 (NSO) enhancements over Riyadh, Kingdom of Saudi Arabia (KSA). We used O3 measurements for the summer of 2012 to examine the NSO enhancements across the city. We found that out of the 88 days for which observations were available, NSO enhancements could be observed on 38 days. The average difference in the NSO concentration between NSO-enhanced days and nonenhanced days was ∼15 ppb, which was statistically significant (p < 0.05). Our analysis of the regional surface emission sources, potential vorticity (PV), and back trajectories using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated that out of 38 days, the NSO enhancements on 18 days (47%), 14 days (37%), 4 days (11%), and 2 days (5%) are due to local transport, long-range transport from highly polluted regions and from stratospheric intrusion regions located at a considerable distance from Riyadh, and a combination of long-range transport from stratospheric intrusion and highly polluted regions, respectively. We then investigated the relevance of each transport mechanism for the increased NSO concentrations during five protracted episodes, each marked by three consecutive days of NSO enhancement. Our results demonstrated that the local transport, long-range transport of O3-rich air masses from highly polluted regions to the east and northeast of the KSA, and local descent over Riyadh increased the average NSO concentration by up to 43%. Moreover, we found that long-range transport from stratospheric intrusion regions north of the KSA increased the average NSO concentration by up to 46%. The combination of long-range transport and stratospheric intrusion increased the average NSO concentration by ∼180%. These increases were usually associated with strong northwest regional winds flowing over highly polluted and major stratospheric intrusion regions.

Comprehensively exploring the characteristics and meteorological causes of ozone pollution events in Beijing during 2013–2020

Ozone (O3) pollution deteriorated in recent years in Beijing; however, the evolution characteristics and formation mechanism of O3 pollution events (OPEs) lack comprehensive exploration from a long-term perspective. Based on 35-site (Beijing) and multi-city (North China) O3 and meteorological monitoring and reanalysis data, totally 61 OPEs during 2013–2020 were counted in Beijing to reveal their characteristics, the relationship with regional pollution, and meteorological causes. Mild, moderate, and heavy pollution in all OPEs (203 ± 28 μg/m3 for averaged MDA8 O3) accounted for 68.4%, 27.8%, and 3.8% of 291 days, respectively. 78.7% of OPEs were concentrated in May–July. The averaged MDA8 O3 concentration, total days and durations of OPEs increased notably from 2013 to 2020, with a rate of 1.6 μg/m3/yr, 2.7 d/yr, and 0.2 d/yr, respectively. Overall, 90% (53%) of OPEs in Beijing co-occurred with regional pollution covering Beijing-Tianjin-Hebei (North China) under the predominant southerly airflow. Meteorological causes of different levels and spatial distribution of OPEs were explored through multi-scale weather processes. The increasing proportion of southwesterly-westerly synoptic circulation (49%–80%) and mountain-valley breeze (51%–70%), along with enhanced temperature, UV radiation, southerly wind, and temperature inversion and decreased relative humidity, collectively contributed to elevated levels of OPEs. The aforementioned meteorological condition facilitated O3 photochemical reactions, the transport of pollutants, and hindered diffusion. Three spatial types (urban, northern and southern suburban pollution) of OPEs were determined by the intensity and direction of southerly wind. Additionally, daytime O3 pollution elevated the mean peak O3 of nocturnal O3 enhancement events (NOEs) by 31 μg/m3 (41%) mainly induced by convective storms and low-level jets. Our comprehensive investigations of OPEs provide important scientific basis for the precise prediction and control of O3 pollution in Beijing.

The Aggravation of Summertime Nocturnal Ozone Pollution in China and Its Potential Impact on the Trend of Nitrate Aerosols

AbstractTropospheric ozone (O3) concentration continued to increase over past years, thereby becoming a growing environmental concern in China. Most studies have focused on the analysis of daily maximum 8‐hr O3 concentration, while there is still a dearth of investigations of nocturnal O3. Here, by analyzing the data of 1,313 sites from the China National Environmental Monitoring Center, we show a remarkable increase in the nocturnal O3 concentration during summertime of 2015–2019 in most regions of China, revealing an aggravation of nocturnal O3 pollution. Combining with a GeoDetector model and statistical analysis, we clarify that the aggravation of nocturnal O3 pollution is mainly caused by reduction in both ambient nitrogen dioxide (NO2) concentration and wet scavenging in recent years. We further reveal that the increasing O3 may have enhanced the nocturnal particulate nitrate () formation through N2O5 heterogeneous hydrolysis, and thereby driving the variation and long‐term trend of nocturnal .

Drivers of nocturnal interactions between ground-level ozone and nitrogen dioxide

&lt;p&gt;This study analyses nighttime (7 p.m. to 6 a.m.) light pollution effects on ground-level ozone production in urban and sub-urban sites in Malaysia. In the absence of solar radiation, no photochemical reaction will occur during nighttime, resulting in zero readings of ground-level ozone (O3). O3 and nitrogen dioxide (NO2) data collected in 2020 were analysed to assess time series and diurnal variability at each site and between sites. The highest nighttime mean O3 concentration is Minden (60 ppb); meanwhile, for nighttime mean NO2 concentrations, the highest is Klang (43 ppb). The results show that sub-urban experienced higher O3 variations compared to urban areas. The monthly mean nighttime O3 and NO2 in urban and suburban areas display a gradual increase in O3 and NO2 variations from March to April, followed by a decreasing trend in the mid of the year. These variations in monthly air pollutants are related to the MCO, CMCO, and RMCO in Malaysia during 2020. Putrajaya (sub-urban site) was the darkest site (average lux: 20) for the whole dataset. In contrast, Minden (sub-urban site) was recorded as the brightest site with maximum light pollution (average lux: 70). The relationship between O3 and NO2 shows a negative correlation during nighttime for urban and suburban sites. Light pollution can reach levels that might affect nocturnal O3 and NO2 concentrations; therefore, the long-term variability of light pollution is essential for air pollution studies.&lt;/p&gt;&#x0D;

An observed nocturnal ozone transport event in the Sichuan Basin, Southwestern China

The ozone (O3) pollution in China drew lots of attention in recent years, and the Sichuan Basin (SCB) was one of the regions confronting worsening O3 pollution problem. Many previous studies have shown that regional transport is an important contributor to O3 pollution. However, very few features of the O3 profile during transport have been reported, especially in the border regions between different administrative divisions. In this study, we conducted tethered balloon soundings in SCB during the summer of 2020 and captured a nocturnal O3 transport event during the campaign. Vertically, the O3 transport occurred in the bottom of the residual layer, between 200 and 500 m above ground level. Horizontally, the transport pathway was directed from southeast to northwest based on the analysis of the wind field and air mass trajectories. The effect of transport in the residual layer on the surface O3 concentration was related to the spatial distribution of O3. For cities with high O3 concentrations in the upwind region, the transport process would bring clean air masses and abate pollution. For downwind lightly polluted cities, the transport process would slow down the decreasing or even increase the surface O3 concentration during the night. We provided observational facts on the profile features of a transboundary O3 transport event between two provincial administrative divisions, which implicated the importance of joint prevention and control measures. However, the sounding parameters were limited and the quantitative analysis was preliminary, more integrated, and thorough studies of this topic were called for in the future.

Quantitative impacts of vertical transport on the long-term trend of nocturnal ozone increase over the Pearl River Delta region during 2006–2019

Abstract. The Pearl River Delta (PRD) region in southern China has been subject to severe ozone (O3) pollution during daytime and anomalous nocturnal O3 increase (NOI) during nighttime. In this study, the spatiotemporal variation of NOI events in the PRD region from 2006 to 2019 is comprehensively analysed, and the role of vertical transport in the occurrence of NOI events is quantified based on observed surface and vertical O3 and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) dataset. The results show that the average annual frequency of NOI events in the whole PRD region during the 14 year period is estimated to be 53 ± 16 d yr−1, with an average of 58 ± 11 µg m−3 for the nocturnal O3 peak (NOP) concentration. Low-level jets (LLJs) are the main meteorological processes triggering NOI events, explaining on average 61 % of NOI events. Annual NOI events exhibit an upward trend before 2011 (4.70 d yr−1) and a downward trend thereafter (−0.72 d yr−1), which is consistent with the annual variation of LLJs (r=0.88, p&lt;0.01). Although the contribution of convective storms (Conv) to NOI events is relatively small with an average value of 11 %, Conv-induced NOI events steadily increased at a rate of 0.26 d yr−1 during this 14 year period due to the impact of urbanisation. Seasonally, a relatively higher frequency of NOI events is observed in spring and autumn, which is consistent with the seasonal pattern of LLJs and maximum daily 8 h average (MDA8) O3. Spatially, NOI events are frequent in the eastern PRD, which agrees well with the spatial distribution of the frequency of LLJs and partially overlaps with the distribution of MDA8 O3 concentration, suggesting that vertical transport plays a more important role in NOI events than daytime O3 concentration. The Weather Research and Forecasting (WRF) model coupled with the Community Multiscale Air Quality (CMAQ) model and the observed vertical O3 profiles are further applied to illustrate the mechanisms of NOI formation caused by LLJs and Conv. The results confirm that both LLJs and Conv trigger NOI events by inducing downdrafts with the difference being that LLJs induce downdrafts by wind shear, while Conv by compensating downdrafts. Through observational and modelling analysis, this study presents the long-term (2006–2019) trends of NOI events in the PRD region and quantifies the contribution of meteorological processes for the first time, emphasising the importance of vertical transport, as well as daytime O3 concentration for the occurrence of NOI events.

Understanding the nocturnal ozone increase in Nanjing, China: Insights from observations and numerical simulations

Surface ozone (O3) is mainly photochemically formed by nitrogen oxides (NOX) and volatile organic compounds (VOCs), and therefore O3 usually has a distinct diurnal variation with high concentrations in the afternoon and low values at night. However, eight nocturnal O3 increase (NOI) events were identified in Nanjing in June 2021. To understand the mechanism of NOI events, we selected two events (June 6–7, and 24–25) for observational data analysis. The Community Multiscale Air Quality (CMAQ) model was employed for the process analysis (PA) and regional transport of O3. By analyzing the O3 observation data and meteorological factors, we found that there were clear southeastward O3 transport paths. The O3 peak clearly moved from the upwind to the downwind cities in both events. Model simulations showed that when nocturnal O3 enhancement occurred, horizontal transport resulted in a negative to positive net O3 production rate. O3 continued to get accumulated in Nanjing. Nocturnal O3 in the first event was dominated by long-range transport, with the top two contributing cities being Huzhou (5.6 %) and Jiaxing (4.7 %). NOI during the second event was from the nearby upwind cities. The top three contributing cities were Shanghai (18.3 %), Wuxi (9.1 %), and Suzhou (8.8 %). We conclude that the June NOI events in Nanjing were mainly driven by the horizontal transport of southeasterly winds. This study provides scientific support for O3 prevention and control in Nanjing in the summer.

Seasonal variation in nitryl chloride and its relation to gas-phase precursors during the JULIAC campaign in Germany

Abstract. Ambient measurements of nitryl chloride (ClNO2) were performed at a rural site in Germany, covering three periods in winter, summer, and autumn 2019, as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed to understand the photochemical processes in air masses typical of midwestern Europe. Measurements were conducted at 50 m aboveground, which was mainly located in the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO2 is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N2O5) on chloride (Cl−) that contains aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO2 mixing ratio was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N2O5 necessary for ClNO2 formation comes from the reaction of nitrate radicals (NO3) with nitrogen dioxide (NO2), where NO3 itself is formed by a reaction of NO2 with ozone (O3). Measured concentrations of ClNO2, NO2, and O3 were used to quantify ClNO2 production efficiencies, i.e., the yield of ClNO2 formation per NO3 radical formed, and a box model was used to examine the idealized dependence of ClNO2 on the observed nocturnal O3 and NO2 concentrations. Results indicate that ClNO2 production efficiency was most sensitive to the availability of NO2 rather than that of O3 and increased with decreasing temperature. The average ClNO2 production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO2 production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO2 chemistry in rural environments in midwestern Europe.

Effects of Horizontal Transport and Vertical Mixing on Nocturnal Ozone Pollution in the Pearl River Delta

Based on the meteorological, ozone (O3), and vertical observation data of 2020, this study sought to evaluate the daily variation in O3, particularly the characteristics of nocturnal ozone pollution. We also discuss the effect of local and mesoscale horizontal transport and vertical mixing on the formation of nocturnal O3 pollution. Distinct seasonal characteristics of the daily O3 variation in Shenzhen were identified. In particular, significant nocturnal peaks were found to regularly occur in the winter and spring (November–December and January–April). The monthly average of daily variation had a clear bimodal distribution. During the period, O3 pollution frequently occurred at night, with the maximum hourly O3 concentration reaching 203.5 μg/m3. Nocturnal O3 pollution was closely associated with horizontal transport and vertical mixing. During the study period, the O3 maximum values were recorded on 68 nights, primarily between 23:00 and 03:00, with occasional observation of two peaks. The impact of horizontal transport and vertical mixing on the nocturnal secondary O3 maximum values was elaborated in two case studies, where vertical mixing was mainly associated with low-level jets, with strong wind shear enhancing turbulent mixing and transporting O3 from the upper layers to the surface.