Nighttime NO3 Research Articles

Aromatic Nitration Enhances Absorption of Biomass Burning Brown Carbon in an Oxidizing Urban Environment.

Brown carbon (BrC) from biomass burning constitutes a significant portion of light-absorbing components in the atmosphere. Although the aging of BrC surrogates from biomass burning has been studied in many laboratory settings, BrC aging behavior in real-world urban environments is not well understood. In this study, through a combination of online dynamic monitoring and offline molecular characterization, the ambient optical aging of BrC was linked to its dynamic changes in molecular composition. Enhanced light absorption by BrC was consistently observed during the periods dominated by oxygenated biomass burning organic aerosol (BBOA), in contrast to periods dominated by primary emissions or secondary formation in aqueous-phase. This enhancement was linked to the formation of nitrogen-containing compounds during the ambient aging of BBOA. Detailed molecular characterization, alongside analysis of environmental parameters, revealed that an increased atmospheric oxidizing capacity, marked by elevated levels of ozone and nighttime NO3 radicals, facilitated the formation of nitrated aromatic BrC chromophores. These chromophores were primarily responsible for the enhanced light absorption during the ambient aging of BBOA. This study elucidates the nitration processes that enhance BrC light absorption for ambient BBOA, and highlights the crucial role of meteorological conditions. Furthermore, our findings shed light on the chemical and optical aging processes of biomass burning BrC in ambient air, offering insights into its environmental behavior and effects.

Impact of Heatwaves and Declining NOx on Nocturnal Monoterpene Oxidation in the Urban Southeastern United States.

Nighttime oxidation of monoterpenes (MT) via the nitrate radical (NO3) and ozone (O3) contributes to the formation of secondary organic aerosol (SOA). This study uses observations in Atlanta, Georgia from 2011-2022 to quantify trends in nighttime production of NO3 (PNO3) and O3 concentrations and compare to model outputs from the EPA's Air QUAlity TimE Series Project (EQUATES). We present urban-suburban gradients in nighttime NO3 and O3 concentrations and quantify their fractional importance (F) for MT oxidation. Both observations and EQUATES show a decline in PNO3, with modeled PNO3 declining faster than observations. Despite decreasing PNO3, we find that NO3 continues to dominate nocturnal boundary layer (NBL) MT oxidation (FNO3 = 60%) in 2017, 2021, and 2022, which is consistent with EQUATES (FNO3 = 80%) from 2013-2019. This contrasts an anticipated decline in FNO3 based on prior observations in the nighttime residual layer, where O3 is the dominant oxidant. Using two case studies of heatwaves in summer 2022, we show that extreme heat events can increase NO3 concentrations and FNO3, leading to short MT lifetimes (<1 h) and high gas-phase organic nitrate production. Regardless of the presence of heatwaves, our findings suggest sustained organic nitrate aerosol formation in the urban SE US under declining NOx emissions, and highlight the need for improved representation of extreme heat events in chemistry-transport models and additional observations along urban to rural gradients.

Seasonal variation in nighttime NO radiative cooling as observed by TIMED/SABER in lower thermosphere during solar maximum and solar minimum

Both composition and temperature play a crucial role in determining the NO radiative cooling in lower thermosphere as observed by TIMED/SABER. In this work, we present a detailed investigation of seasonal variation in thermospheric NO radiative cooling. We have carried forward the investigation of Li et al. (2018) regarding the variations in local nighttime peak NO radiative cooling and its altitude during solar maximum and solar minimum conditions. By analyzing latitudinal changes over quiet times for each month in year 2018, it is evident that both the investigated parameters exhibit summer-winter variability. The qualitative contribution of different species (i.e., NO, and O), and temperatures in determining the vertical profile of NO radiative cooling for different latitudes is investigated by utilizing the NRLMSISE-00 estimated parameters, and SNOE observed NO density. The temperature, NO density, and compositional variations due to asymmetrical solar heating and associated summer-to-winter circulation in both the hemispheres during solar minimum conditions seem to be the dominating factor in controlling the NO radiative cooling during different seasons. The altitudes at which maximum cooling by NO occurs exhibits an inverse correlation with the amount of radiative cooling. The region of enhanced NO densities (polar and summer hemispheric low-mid latitude regions) have larger NO radiative cooling with lower peak altitudes in comparison to other regions (equatorial to winter hemispheric low-mid latitude regions), where NO radiative cooling is low with higher peak altitude values.

Characteristics and source apportionment of water-soluble organic nitrogen (WSON) in PM2.5 in Hong Kong: With focus on amines, urea, and nitroaromatic compounds

Water-soluble organic nitrogen (WSON) is ubiquitous in fine particulate matter (PM2.5) and poses health and environmental risks. However, there is limited knowledge regarding its comprehensive speciation and source-specific contributions. Here, we conducted chemical characterization and source apportionment of WSON in 65 PM2.5 samples collected in Hong Kong during a 1-yr period. Using various mass-spectrometry-based techniques, we quantified 22 nitrogen-containing organic compounds (NOCs), including 17 nitroaromatics (NACs), four amines, and urea. The most abundant amine and NACs were dimethylamine and 4-nitrocatechol, respectively. Two secondary (i.e., secondary formation and secondary nitrate) and five primary sources (i.e., sea salt, fugitive dust, marine vessels, vehicle exhaust, and biomass burning) of WSON and these three categories of NOCs were identified. Throughout the year, secondary sources dominated WSON formation (69.0%), while primary emissions had significant contributions to NACs (77.1%), amines (75.9%), and urea (83.7%). Fugitive dust was the leading source of amines and urea, while biomass burning was the main source of NACs. Our multi-linear regression analysis revealed the significant role of sulfate, NO3, nitrate, liquid water content, and particle pH on WSON formation, highlighting the importance of nighttime NO3 processing and heterogeneous and aqueous-phase formation of NOCs in the Hong Kong atmosphere.

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.

Nighttime NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi

Abstract. Atmospheric pollution in urban regions is highly influenced by oxidants due to their important role in the formation of secondary organic aerosol (SOA) and smog. These include the nitrate radical (NO3), which is typically considered a nighttime oxidant, and the chlorine radical (Cl), an extremely potent oxidant that can be released in the morning in chloride-rich environments as a result of nocturnal build-up of nitryl chloride (ClNO2). Chloride makes up a higher percentage of particulate matter in Delhi than has been observed anywhere else in the world, which results in Cl having an unusually strong influence in this city. Here, we present observations and model results revealing that atmospheric chemistry in Delhi exhibits an unusual diel cycle that is controlled by high concentrations of NO during the night. As a result of this, the formation of both NO3 and dinitrogen pentoxide (N2O5), a precursor of ClNO2 and thus Cl, are suppressed at night and increase to unusually high levels during the day. Our results indicate that a substantial reduction in nighttime NO has the potential to increase both nocturnal oxidation via NO3 and the production of Cl during the day.

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;

Extreme Concentrations of Nitric Oxide Control Daytime Oxidation and Quench Nocturnal Oxidation Chemistry in Delhi during Highly Polluted Episodes.

Delhi, India, suffers from periods of very poor air quality, but little is known about the chemical production of secondary pollutants in this highly polluted environment. During the postmonsoon period in 2018, extremely high nighttime concentrations of NOx (NO and NO2) and volatile organic compounds (VOCs) were observed, with median NOx mixing ratios of ∼200 ppbV (maximum of ∼700 ppbV). A detailed chemical box model constrained to a comprehensive suite of speciated VOC and NOx measurements revealed very low nighttime concentrations of oxidants, NO3, O3, and OH, driven by high nighttime NO concentrations. This results in an atypical NO3 diel profile, not previously reported in other highly polluted urban environments, significantly perturbing nighttime radical oxidation chemistry. Low concentrations of oxidants and high nocturnal primary emissions coupled with a shallow boundary layer led to enhanced early morning photo-oxidation chemistry. This results in a temporal shift in peak O3 concentrations when compared to the premonsoon period (12:00 and 15:00 local time, respectively). This shift will likely have important implications on local air quality, and effective urban air quality management should consider the impacts of nighttime emission sources during the postmonsoon period.

Contrasting Influence of Nitrogen Oxides on the Cloud Condensation Nuclei Activity of Monoterpene‐Derived Secondary Organic Aerosol in Daytime and Nighttime Oxidation

AbstractAnthropogenic nitrogen oxides may influence the cloud condensation nuclei (CCN) activity of biogenic secondary organic aerosols (SOA) in both daytime photooxidation and nighttime NO3 oxidation, which has significant implications for the climatic impact of SOA. We investigated the influence of NOx on the CCN activity of monoterpene‐derived SOA in OH oxidation and in NO3 oxidation. In OH oxidation, NOx had little influence on the hygroscopic parameter κ of organic aerosol (κOrg), which was attributed to the minor fraction of organic nitrates (ON) in SOA (&lt;24%), resulted from the low branching ratio of RO2 + NO to form ON. In contrast, in NO3 oxidation κOrg was much reduced compared to OH/O3 oxidation due to a dominant fraction of ON. We report κ of MT‐derived ON formed in photo‐oxidation and NO3 oxidation (0.029–0.052) for the first time to our knowledge, which may be used to improve model simulations of CCN concentrations.

Secondary organic aerosol formation from atmospheric reactions of anisole and associated health effects

Anisole (methoxybenzene) represents an important marker compound of lignin pyrolysis and a starting material for many chemical products. In this study, secondary organic aerosols (SOA) formed by anisole via various atmospheric processes, including homogeneous photooxidation with varying levels of OH• and NOx and subsequent heterogeneous NO3• dark reactions, were investigated. The yields of anisole SOA, particle-bound organoperoxides, particle-induced oxidative potential (OP), and cytotoxicity were characterized in view of the atmospheric fate of the anisole precursor. Anisole SOA yields ranged between 0.12 and 0.35, depending on the reaction pathways and aging degrees. Chemical analysis of the SOA suggests that cleavage of the benzene ring is the main reaction channel in the photooxidation of anisole to produce low-volatility, highly oxygenated small molecules.Fresh anisole SOA from OH• photooxidation are more light-absorbing and have higher OP and organoperoxide content. The high correlation between SOA OP and organoperoxide content decreases exponentially with the degree of OH• aging. However, the contribution of organoperoxides to OP is minor (<4%), suggesting that other, non-peroxide oxidizers play a central role in anisole SOA OP. The particle-induced OP and particulate organoperoxides yield both reach a maximum value after ∼2 days’ of photooxidation, implicating the potential long impact of anisole during atmospheric transport. NOx-involved photooxidation and nighttime NO3• reactions facilitate organic nitrate formation and enhance particle light absorption. High NOx levels suppress anisole SOA formation and organoperoxides yield in photooxidation, with decreased aerosol OP and cellular oxidative stress. In contrast, nighttime aging significantly increases the SOA toxicity and reactive oxygen species (ROS) generation in lung cells. These dynamic properties and the toxicity of anisole SOA advocate consideration of the complicated and consecutive aging processes in depicting the fate of VOCs and assessing the related effects in the atmosphere.

The Levels and Sources of Nitrous Acid (HONO) in Winter of Beijing and Sanmenxia

AbstractHONO concentrations in Beijing (BJ) and Sanmenxia (SMX) were simultaneously measured in winter with a duration of 1 month, and the sources and sinks of HONO in the two cities were comparably analyzed. BJ and SMX had different pollution characteristics. Direct vehicle emission made a contribution to observed HONO of about 28% in BJ, whereas it contributed to HONO only about 12% in SMX. Additionally, direct emission from coal combustion in SMX also made a significant contribution to atmospheric HONO, which could achieve to be 13%. In BJ, nighttime NO2 conversion on the aerosol and ground surfaces contributed to HONO sources of 15% and 27%, respectively. In SMX, nighttime NO2 conversion on the aerosol and ground surfaces contributed to HONO sources of 40% and 30%, respectively. Daytime NO2 heterogeneous conversion (including photo‐enhanced conversion) on the aerosol and ground surfaces was important in SMX. Daytime NO3− photolysis contributed 9% HONO in SMX and 4% in BJ. The primary OH production rates from photolysis of daytime HONO in both BJ and SMX were three orders of magnitude faster than those from photolysis of O3, revealing the dominant role of HONO in local atmospheric chemistry of the two cities in winter.

Oxidation pathways and emission sources of atmospheric particulate nitrate in Seoul: based on &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N and Δ&amp;lt;sup&amp;gt;17&amp;lt;/sup&amp;gt;O measurements

Abstract. PM2.5 haze pollution driven by secondary inorganic NO3- has been a great concern in East Asia. It is, therefore, imperative to identify its sources and oxidation processes, for which nitrogen and oxygen stable isotopes are powerful tracers. Here, we determined the δ15N (NO3-) and Δ17O (NO3-) of PM2.5 in Seoul during the summer of 2018 and the winter of 2018–2019 and estimated quantitatively the relative contribution of oxidation pathways for particulate NO3- and investigated major NOx emission sources. In the range of PM2.5 mass concentration from 7.5 µg m−3 (summer) to 139.0 µg m−3 (winter), the mean δ15N was −0.7 ‰ ± 3.3 ‰ and 3.8 ‰ ± 3.7 ‰, and the mean Δ17O was 23.2 ‰ ± 2.2 ‰ and 27.7 ‰ ± 2.2 ‰ in the summer and winter, respectively. While OH oxidation was the dominant pathway for NO3- during the summer (87 %), nighttime formation via N2O5 and NO3 was relatively more important (38 %) during the winter, when aerosol liquid water content (ALWC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Δ17O was coupled with the lowest δ15N and highest NOR during the record-breaking winter PM2.5 episodes, revealing the critical role of photochemical oxidation process in severe winter haze development. For NOx sources, atmospheric δ15N (NOx) estimated from measured δ15N (NO3-) considering isotope fractionation effects indicates vehicle emissions as the most important emission source of NOx in Seoul. The contribution from biogenic soil and coal combustion was slightly increased in summer and winter, respectively. Our results built on a multiple-isotope approach provide the first explicit evidence for NO3- formation processes and major NOx emission sources in the Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.

Nocturnal atmospheric chemistry of NO3 and N2O5 over Changzhou in the Yangtze River Delta in China

Comprehensive observations of the nocturnal atmospheric oxidation of NO3 and N2O5 were conducted at a suburban site in Changzhou in the YRD using cavity ring-down spectroscopy (CRDS) from 27 May to 24 June, 2019. High concentrations of NO3 precursors were observed, and the nocturnal production rate of NO3 was determined to be 1.7 ± 1.2 ppbv/hr. However, the nighttime NO3 and N2O5 concentrations were relatively low, with maximum values of 17.7 and 304.7 pptv, respectively, illustrating the rapid loss of NO3 and N2O5. It was found that NO3 dominated the nighttime atmospheric oxidation, accounting for 50.7%, while O3 and OH only contributed 34.1% and 15.2%, respectively. For the reactions of NO3 with volatile organic compounds (VOCs), styrene was found to account for 60.3%, highlighting its dominant role in the NO3 reactivity. In general, the contributions of the reactions between NO3 and VOCs and the N2O5 uptake to NO3 loss were found to be about 39.5% and 60.5%, respectively, indicating that N2O5 uptake also played an important role in the loss of NO3 and N2O5, especially under the high humidity conditions in China. The formation of nitrate at night mainly originated from N2O5 uptake, and the maximum production rate of NO3− reached 6.5 ppbv/hr. The average NOx consumption rate via NO3 and N2O5 chemistry was found to be 0.4 ppbv/h, accounting for 47.9% of the total NOx removal. The predominant roles of NO3 and N2O5 in nitrate formation and NOx removal in the YRD region was highlighted in this study.

Clustering Analysis on Drivers of O3 Diurnal Pattern and Interactions with Nighttime NO3 and HONO

The long-path differential optical absorption spectroscopy (LP-DOAS) technique was deployed in Shanghai to continuously monitor ozone (O3), formaldehyde (HCHO), nitrogen dioxide (NO2), nitrous acid (HONO), and nitrate radical (NO3) mixing ratios from September 2019 to August 2020. Through a clustering method, four typical clusters of the O3 diurnal pattern were identified: high during both the daytime and nighttime (cluster 1), high during the nighttime but low during the daytime (cluster 2), low during both the daytime and nighttime (cluster 3), and low during the nighttime but high during the daytime (cluster 4). The drivers of O3 variation for the four clusters were investigated for the day- and nighttime. Ambient NO caused the O3 gap after midnight between clusters 1 and 2 and clusters 3 and 4. During the daytime, vigorous O3 generation (clusters 1 and 4) was found to accompany higher temperature, lower humidity, lower wind speed, and higher radiation. Moreover, O3 concentration correlated with HCHO for all clusters except for the low O3 cluster 3, while O3 correlated with HCHO/NOx, but anti-correlated with NOx for all clusters. The lower boundary layer height before midnight hindered O3 diffusion and accordingly determined the final O3 accumulation over the daily cycle for clusters 1 and 4. The interactions between the O3 diel profile and other atmospheric reactive components established that higher HONO before sunrise significantly promoted daytime O3 generation, while higher daytime O3 led to a higher nighttime NO3 level. This paper summarizes the interplays between day- and nighttime oxidants and oxidation products, particularly the cause and effect for daytime O3 generation from the perspective of nighttime atmospheric components.

Ozone chemistry and dynamics at a tropical coastal site impacted by the COVID-19 lockdown

The nationwide lockdown in India to curb the spread of Coronavirus disease 2019 (COVID-19) led to colossal reduction in anthropogenic emissions. Here, we investigated the impact of lockdown on surface ozone (O3) and nitrogen dioxide (NO2) over a tropical coastal station – Thumba, Thiruvananthapuram (8.5°N, 76.9°E). Daytime as well as night-time NO2 showed reduction by 0.8 (40%) and 2.3 (35%) ppbv, respectively during the lockdown period of 25–30 March 2020 as compared with the same period of previous 3 years. Unlike many urban locations, daytime surface O3 is found to be dramatically reduced by 15 ppbv (36%) with O3 production rate being lower by a factor of 3 during the lockdown. Interestingly, a feature of O3-hump during the onset of land breeze typically observed during 1997–1998 has reappeared with magnitude of 5–10 ppbv. A photochemical box model, capturing this feature, revealed that significant O3 sustained till onset of land breeze over the land due to weaker titration with NOx during lockdown. It is suggested that the transport of this O3 rich air with onset of land breeze led to the observed hump. Our measurements unravel a remarkable impact of the COVID-19 lockdown on the chemistry and dynamics of O3 over this tropical coastal environment.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12040-021-01666-3.

Switching to electric vehicles can lead to significant reductions of PM2.5 and NO2 across China

Switching to electric vehicles can lead to significant reductions of PM2.5 and NO2 across China

Seasonality of nitrous acid near an industry zone in the Yangtze River Delta region of China: Formation mechanisms and contribution to the atmospheric oxidation capacity

HONO along with other trace gases and actinic flux were continuously measured in selected months from December 2015 to November 2016 near a major industrial zone of the Yangtze River Delta (YRD) region of China. The seasonalities of HONO and its formation mechanisms were explored. A box model was developed to assess the role of HONO chemistry in the atmospheric oxidative capacity in the study area. The model was constrained by in-situ observations to minimize simulation uncertainties. Our measurement results demonstrate that HONO mean concentration and diurnal profile varied noticeably between different seasons with the maximum (1.32 ± 0.92 ppbv) observed in winter, coincided with high mass loading of fine particles (PM2.5 = 114 ± 59.6 μg m−3). The averaged nighttime NO2 to HONO conversion ratios (hr−1) were (1.83 ± 0.73)% in spring, (2.91 ± 1.06)% in summer, (0.72 ± 0.44)% in autumn, and (0.84 ± 0.45)% in winter. The homogeneous reaction of OH + NO was responsible for a large portion of HONO production throughout the year but was relatively weaker compared to other mechanisms in winter. Heterogeneous reactions on ground surfaces were much stronger than those on aerosol surfaces, dominating nighttime HONO production, except in summer. Primary HONO emissions were only responsible for a small portion of the HONO budget. Particulate nitrate photolysis was a significant source of HONO and it showed very little variation in different seasons. OH budget analysis shows that O3 photolysis was a major contributor to the OH production in the study area while HONO photolysis provided an extra boost to the photochemical reactivity in the early morning, particularly during summer. However, HONO chemistry played a more important role in autumn and winter by dominating OH production throughout the day. Accordingly, effective control of NOx emissions dominated by industries can significantly reduce HONO production and thus weaken the atmospheric oxidative capacity in the study area.

Chemistry of Atmospheric Fine Particles During the COVID-19 Pandemic in a Megacity of Eastern China.

Air pollution in megacities represents one of the greatest environmental challenges. Our observed results show that the dramatic NOx decrease (77%) led to significant O3 increases (a factor of 2) during the COVID‐19 lockdown in megacity Hangzhou, China. Model simulations further demonstrate large increases of daytime OH and HO2 radicals and nighttime NO3 radical, which can promote the gas‐phase reaction and nocturnal multiphase chemistry. Therefore, enhanced NO3 − and SO4 2− formation was observed during the COVID‐19 lockdown because of the enhanced oxidizing capacity. The PM2.5 decrease was only partially offset by enhanced aerosol formation with its reduction reaching 50%. In particular, NO3 − decreased largely by 68%. PM2.5 chemical analysis reveals that vehicular emissions mainly contributed to PM2.5 under normal conditions in Hangzhou. Whereas, stationary sources dominated the residual PM2.5 during the COVID‐19 lockdown. This study provides evidence that large reductions in vehicular emissions can effectively mitigate air pollution in megacities.

Assessment of Nighttime Ground Level Ozone Concentration in Klang During Wet and Dry Month

Ground level ozone (O3) is the most significant secondary air pollutants in Malaysia, and this air pollutant exhibited different variations during daytime and nighttime due to differences in photochemistry. This utilizing seven variables (O3, NO2, NO, SO2, PM10, temperature, and relative humidity) secondary data acquired from the Air Pollution Division, Department of Environment Malaysia. The nighttime data (7 p.m. – 6 p.m.) on March and December 2015 were used to represent the dry and wet months, respectively. Box and whisker plots were used to show the variation of nighttime O3, NO2, NO, SO2, PM10, temperature, and relative humidity during the dry and wet months. Results suggested that there are variations among the selected variables between dry and wet month with temperature, O3, NO2, and PM10 showed higher value during dry month compared to wet month. Meanwhile, relative humidity, NO, and SO2 showed the opposite result.

Fluctuations in nighttime ground-level ozone concentrations during haze events in Malaysia

This study focused on O3 variations and the titration effects of NOx during nighttime at urban, industrial, sub-urban and background sites. Nighttime O3 concentration variations and the presence of high particles with an aerodynamic diameter of less than 10 μm (PM10) were examined because haze disturbs the photochemical reactions of O3. Hourly data on O3, NO2, NO and PM10 concentrations provided by the Air Quality Division of the Department of Environment were divided into two groups of daytime and nighttime and analysed. The maximum O3 concentrations during daytime were generally observed during noon. At nighttime, the concentration of O3 decreased, indicating that destruction activities occurred mainly via titration. The retention of O3 during daytime caused the nighttime O3 during haze events to be higher than that during normal days. Apparent fluctuations in nighttime O3 concentrations were observed in the urban site (20 ± 13 ppb) during haze events. The NO2/NO ratio in the urban site during haze was higher than that on normal days; amongst the sites, the urban one had the highest value (6.6). Results indicated that during haze, the reactions between NO and O3 were enhanced at nighttime, leading to low nighttime NO concentrations. The low nighttime NO concentrations led to low nighttime NO titration rates, which enabled O3 to persist in ambient air. Nighttime O3 was not completely absent due to anthropogenic sources. This condition accelerated NO titration to NO2, thus promoting O3 production even during haze.