Increased night-time oxidation over China despite widespread decrease across the globe

Nitrogen oxides (NOx = NO + NO2) emitted from combustion and natural sources are reactive gases that regulate the composition of Earth’s atmosphere. Nocturnal oxidation driven by nitrate radicals is an important but poorly understood process in atmospheric chemistry, affecting the lifetimes of NOx and ozone and particulate pollution levels. Understanding the trends of nitrate radicals is important to formulating effective pollution mitigation strategies and understanding the influence of NOx on climate. Here we analyse publicly available monitoring data on NOx and ozone to assess production rates and trends of surface nitrate radicals from 2014 to 2021 across the globe. We show that nitrate radicals have undergone strong increases in China during 2014–2019 but exhibited modest decreases in the United States and the European Union. Accelerated night-time oxidation has shortened the lifetime of summer NOx in China by 30% during 2014–2019. This change will strongly affect ozone formation and has policy implications for the joint control of ozone and fine particulate pollution. Measurements show that night-time production of atmospheric nitrate radicals increased in China but decreased in the European Union and the United States from 2014 to 2019. This suggests the increasing contribution of night-time atmospheric oxidation in China to air pollution.

Nitrogen oxides (NO x = NO + NO 2 ) emitted from combustion and natural sources are reactive gases that regulate the composition of Earth's atmosphere. Nocturnal oxidation driven by nitrate radicals is an important but poorly understood process in atmospheric chemistry, affecting the lifetimes of NO x and ozone and particulate pollution levels. Understanding the trends of nitrate radicals is important to formulating effective pollution mitigation strategies and understanding the influence of NO x on climate.
Here we analyse publicly available monitoring data on NO x and ozone to assess production rates and trends of surface nitrate radicals from 2014 to 2021 across the globe. We show that nitrate radicals have undergone strong increases in China during 2014-2019 but exhibited modest decreases in the United States and the European Union. Accelerated night-time oxidation has shortened the lifetime of summer NO x in China by 30% during 2014-2019. This change will strongly affect ozone formation and has policy implications for the joint control of ozone and fine particulate pollution.
Nitrate radical (NO 3 ) is one of the major tropospheric oxidants and thus substantially impacts atmospheric chemical cycles important to air quality and climate 1,2 . NO 3 is primarily a night-time species that is formed by the reaction of nitrogen dioxide (NO 2 ) and ozone (O 3 ). It initiates the nocturnal oxidation of volatile organic compounds (VOCs), particularly olefins, and contributes to secondary organic aerosol (SOA) production [3][4][5] . For example, NO 3 oxidation accounts for 10-20% of global SOA on average and could be more important in polluted areas [6][7][8][9] . It further produces particulate inorganic nitrate via dinitrogen pentoxide (N 2 O 5 ) heterogeneous hydrolysis 10,11 . Night-time NO 3 chemistry influences next-day photochemistry by removing nitrogen oxides (NO x ) and VOCs, main precursors of O 3 , and through formation of nitryl chloride (ClNO 2 ), a photochemical Cl reservoir [12][13][14] . ClNO 2 acts as an important radical source and enhances the O 3 formation by up to 7.0 parts per billion by volume (ppbv) across the Northern Hemisphere 15 . NO 3 reactions thus act as a hub coupling the evolution of two critical Article https://doi.org/10.1038/s41561-022-01122-x We found that the increases in PNO 3 in China were dominated by an upward trend in O 3 rather than NO 2 (Fig. 1d). The reduction of nocturnal NO x emission led to a rapid increase of nocturnal O 3 (6.3%) in the four regions. In addition, we observed nocturnal surface temperature has an increasing tendency (0.06%) in this period that contributed to the rise in PNO 3 via k NO 2 +O 3 by 0.53%, showing that nocturnal chemical processes are modestly affected by global climate change 20 . The decrease in NO 2 offset the effect of O 3 increase to PNO 3 changes in three megacities (North China Plain, Yangtze River Delta and Pearl River Delta) and was significant in the North China Plain. NO 2 increased only in Sichuan Basin, which made a considerable contribution to PNO 3 . Similar distributions and trends also occurred in the cold season (Extended Data Figs. 3 and 4), suggesting that the hotspot of NO 3 chemistry persisted for the entire year in China.
We also calculated PNO 3 in late afternoon (16:00-19:00 lt), which is a proxy for PNO 3 in the residual layer to some extent 21 since late-afternoon surface measurements in a well-mixed boundary layer may in some cases be characteristic of the initial composition of the residual layer after dark. However, we acknowledge that this property of the afternoon boundary layer is not universal and depends on the nature of the site (for example, coastal versus continental) and season. Late-afternoon PNO 3 showed a similar distribution globally and an upward trend in China, highlighting that strong NO 3 oxidation occurs in both the nocturnal boundary layer and the residual layer (Supplementary Figs. 2 and 3).

Atmospheric impacts of PNO 3 change
Assuming the fate of produced NO 3 is to convert organic or inorganic nitrate, leading to removal of NO x , we show the intensive and accelerated night-time oxidation shortened the nocturnal surface NO x lifetime (τ NO 2 ) by ~30% from a median of 10.2 h in 2014 to 7.3 h in 2019 (Methods) in the warm season in China and shortened the residual-layer τ NO 2 from 4.1 h to 3.1 h, while it was stabilized at high level in other regions in this period (Extended Data Fig. 5 and Supplementary Table. 1). The nocturnal surface τ NO 2 in China is comparable to the daytime τ NO 2 in some urban regions 22,23 , while the residual-layer τ NO 2 is comparable to or faster than that of photochemical oxidation over a range of conditions. Reduction of the nocturnal τ NO 2 is important since the nocturnal contribution to oxidation of NO x varies from ~30% of the total in summer to as much as 90% in the winter [24][25][26][27] . The reduction in τ NO 2 changes the amount of NO x available for photochemistry at sunrise and the formation of night-time nitrate aerosols, demonstrating the growing importance of nocturnal oxidation processes in China. For example, an integrated 10 h PNO 3 of 1 ppbv hr −1 would be equivalent to a maximum of approximately 50 µg m −3 nitrate aerosol at unit yield. Since eastern China is characterized by intensive biogenic emissions 28 , anthropogenic monoterpene emission 29 and higher values of PNO 3 , more NO 3 is produced there, which accelerates the oxidation of VOC and has the potential to enhance organic aerosol pollution. For example, if a molecular weight of an oxidation product (monoterpene nitrate) were 250 g mol −1 with a yield of 30% (ref. 2 ), this would suggest 25 µg m −3 potential overnight (10 h) SOA formation if the limiting reagent were the VOC. In addition, the activation of chlorine atoms through the production of ClNO 2 via increased N 2 O 5 uptake promotes photochemical pollution 30,31 if the ClNO 2 yield does not change. Given that the much lower values of PNO 3 in both the European Union and the United States have been shown to exert significant impacts on the particulate nitrate and organic aerosol formation in those regions [3][4][5]32 , we infer that NO 3 chemistry may play a more critical role in atmospheric oxidation and aggravate both O 3 and PM 2.5 pollution in China.
Considering the short lifetime of NO 3 in the atmosphere, trends in the loss of NO 3 are also important but are probably separate from those in PNO 3 . Unlike the single dominant source of NO 3 , its loss is much more complicated, governed by the reaction of NO 3 with VOCs and the uptake of N 2 O 5 . In terms of the reaction between NO 3 and air pollutants (O 3 and particulate matter ≤ 2.5 µm in diameter (PM 2.5 )) of major concern. Despite its importance, less attention has been paid to night-time processes than to photochemical reactions. Specifically, the present-day increasing severe O 3 in China 16 and decreasing O 3 in the United States 17 may cause large-scale shifts in nocturnal NO 3 chemistry and its impact in these regions, but trends in the magnitude or rates of nocturnal oxidation processes have not been well assessed as yet.
Since NO 3 has a short lifetime, its impact is regulated by its formation process. Thus, we examine the nitrate radical production rate (PNO 3 ; equation (1)) as an indicator of NO 3 oxidation capacity 18 , here k NO 2 +O 3 is the reaction rate of NO 2 and O 3 . We use nocturnal PNO 3 (averaged over 20:00-06:00 local standard time (lt) at each site, the approximate darker half of the diel cycle; Supplementary Fig. 1 confirms the consistency of using a local standard time filter window with local solar zenith angles time window) and its trend to assess the evolution of night-time chemistry from a global perspective on the basis of a comprehensive surface observation dataset covering China, India, the European Union and the United States over 2014-2021 (Methods). We note that there are several regions not covered in this study (such as tropical regions and Southern Hemisphere) due to the gap of monitoring network, even though it is global in scope: (1) Figure 1a shows that the average nocturnal PNO 3 in the warm season (defined as April-September) during 2018-2019 in China was 1.07 ± 0.38 ppbv h −1 , higher than those in the United States, the European Union and India by 155%, 174% and 37%, respectively (Extended Data Table 1). Our result demonstrates that the most active night-time chemistry and strong nocturnal oxidation capacity occurs in China compared with other regions, an aspect of atmospheric oxidation that has not been recognized previously. The conventional view of night-time surface-level chemistry in polluted environments is that high NO x emissions strongly titrate O 3 and NO 3 at night, thus suppressing NO 3 chemistry in urban areas. Unexpectedly, the regions with high PNO 3 are concentrated in urban clusters in east China, with intensive NO x emission. The overall high surface nocturnal PNO 3 in China is determined by elevated nocturnal NO 2 and O 3 (Extended Data Fig. 1) 19 . In particular, the surface nocturnal NO 2 in China is about twice that of the United States and the European Union. Ground-level temperature contributes only slightly to the regional differences in PNO 3 through the temperature-dependence reaction rate term k NO 2 +O 3 (Extended Data Table 2) . This term in China is similar to that of the United States (3.1 × 10 −17 versus 3.0 × 10 −17 molecules cm −3 s −1 ), since both regions span similar latitudes, but is higher in India (lower latitude and higher temperatures) and lower in the European Union (higher latitude and lower temperatures). The PNO 3 trend in the warm season over 2014-2019 demonstrates a rapid growth of PNO 3 in China by 0.04 ppbv h −1 yr −1 (5.8%) on average, with 43.3% of sites showing positive trends with P < 0.1, while trends of PNO 3 in the European Union and the United States are much smaller and insignificant, with a larger fraction (47.9% in the European Union and 64.7% in the United States) of sites showing negative trends (Fig. 1c). In this Article, we estimate trends with monthly data in the warm season of 2014-2019 with the seasonal cycle and autocorrelation removed to derive robust trends even for a relatively short period 17 . The PNO 3 trends at the Chinese monitoring sites are overwhelmingly positive in all city clusters examined here, with large variabilities in the magnitudes of the trends (Extended Data Fig. 2 and Extended Data Table 3). By comparison, most sites in the European Union and the United States showed small increasing or decreasing trends. The diurnal pattern of PNO 3 is similar in the four regions and showed an overall change during the study period, rather than influenced by the curve shape change (Fig. 2).

Distributions and trends of PNO 3
Article https://doi.org/10.1038/s41561-022-01122-x VOCs, the major NO 3 reactants are biogenic terpenes at the global scale. Such biogenic emissions are influenced mainly by temperature and should not change much in terms of long-term trends during the six years analysed here. In the urban and city clusters scale, anthropogenic VOC emissions may be significant in the early period in areas such as Los Angeles and have been decreasing with the continuous implementation of emission reductions. Therefore, it can be assumed that the NO 3 reactivity contributed by anthropogenic VOC has decreased substantially in Los Angeles. In China, by contrast, VOC emissions showed a small increase in recent years (2010-2017) 33 ; thus, an increase in anthropogenic NO 3 reactivity can be expected. In terms of the N 2 O 5 uptake, the sharp decrease in aerosol loading in China during 2014-2019 34 led to a decreasing surface area for the reaction taking place. At the same time, the decrease in sulfate and the increase in nitrate fraction potentially further reduce the N 2 O 5 uptake rate. Long-term decreases in aerosol in the United States are well documented 35-37 but have not been analysed in terms of the influence on N 2 O 5 uptake. Above all, the NO 3 loss frequency may show some decline (considering the increased anthropogenic VOC enhanced little NO 3 reactivity) in China and be stable or decrease slightly in the European Union and the United States. This implies an increasing trend in NO 3 concentration in China.
This increasing trend was interrupted by the COVID-19 pandemic. Figure 1b shows that PNO 3 decreased by 10-40% in different regions during 2020-2021 compared with 2018-2019, which is attributed to the consistent global decline in concentrations of both nocturnal NO 2 and O 3 (Extended Data Fig. 1c,d). However, the dramatic emission change initiated by the pandemic does not appear to have been sustained 38 . Emissions and pollution conditions have largely rebounded to pre-pandemic levels in the United Sates and the European Union and will probably do so in China in the future 39 . Thus, atmospheric chemical cycles initiated by night-time chemistry are likely to return to pre-pandemic rates.

Air pollution control strategies from night-time chemical perspective
We further constructed a theoretical framework (Methods) to describe the dependence of PNO 3 on NO x emissions at different levels of VOCs (Fig. 3a). Under a fixed VOC emission scenario, a decrease in NO x from a high emission level weakens NO titration of O 3 and increases PNO 3 . We term this the NO x -saturated regime for night-time chemistry, analogous to the similarly named photochemical regime 40 Shijiazhuang) in China is comparable to that in 1990s Los Angeles, but with an overall increasing trend during 2014-2019 accompanied by NO x decrease (Fig. 4) This is because O 3 formation in many Chinese cities is under the VOC-limited regime, such that the reduction of NO x would increase O 3 (ref. 42 ). The long-term decreasing trend in PNO 3 in Los Angeles illustrates that the level of nocturnal O 3 can be stabilized while reducing NO x simultaneously, a result that may also apply in China. Combining the theoretical framework and the observed shift during the COVID-19 pandemic 38 , we propose that the night-time oxidation rate is closely affected by daytime O 3 pollution. Reduction in VOCs is therefore an effective mitigation strategy for both daytime O 3 pollution and night-time NO 3 -initiated chemical cycles in China and similar developing countries such as India 43 , unless a shift occurs to the NO x -limited regime.
The decreased NO x lifetime and increased night-time oxidation have the potential to significantly impact air quality. The recent trend in Chinese cities shows a counterintuitive increase of PNO 3 with NO x reduction, implying that night-time chemistry will become increasingly more important due to the continued future PNO 3 increase in China. This increase has the potential to further increase rates of important cycles influencing air quality such as nitrate aerosol and SOA production 2,44 and halogen cycling, which influences O 3 pollution 12,13 . These results demonstrate the challenge in mitigating photochemical and nitrate aerosol pollution related to nocturnal oxidation in the urban agglomeration of China 43,45,46 , and likewise in developing countries such as India 47 . Therefore, elucidating the mechanisms and developing cost-effective mitigation strategies for night-time oxidation changes will be critical for understanding the evolution of the tropospheric oxidation capacity [48][49][50][51] and its role in air quality and climate. Only a limited number of modelling studies quantitatively assess the response of O 3 and PM to night-time chemical processes, let alone to trends in the initiation of these processes. Explicit consideration of such trends in future modelling studies will be of substantial interest to assessment of air quality and climate responses to emissions changes.

Online content
Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41561-022-01122-x.

Observation and reanalysis data
The nationwide hourly observations of ground-level NO 2 and O 3 at more than 1,600 stations in Chinese cities for 2014-2021 were obtained from the China National Environmental Monitoring Center network. Concurrent hourly measurements in the United States (including Los Angeles for 1980-2021) and the European Union were obtained from the Environmental Protection Agency Air Quality System monitoring network and the European Environment Agency, respectively. Ground observations of NO 2 and O 3 monitored at the Central Pollution Control Board continuous stations in India are available from the Central Control Room for Air Quality Management for 2014-2021. Hourly air temperature at 2 m above the ground (T2M) was obtained from the NASA (National Aeronautics and Space Administration) Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) product 53 at a resolution of 0.5° × 0.625°. We sample MERRA-2 temperature data at the grid of a monitoring site to calculate the NO 3 production rate for individual sites.

Data quality control
The measurements of NO 2 and O 3 are converted to mixing ratio (ppbv) at local time. We apply data quality-control measures to exclude unreliable data outliers and select stations with continuous observations from 2014 to 2021 for trend analysis. Following previous studies 19,54,52 , we exclude data points (1) with values greater than 500 ppbv or less than 0 ppbv, (2) with an hourly standardized value ( z i = , where x i is the hourly data point, x is the monthly mean value and σ is the standard deviation calculated for each month) larger than 5, (3) showing extremely low variability in a specific day (the difference between the daily maximum and minimum is less than 2 ppbv), (4) with the same value for at least 4 of 5 consecutive hours or (5) showing unrealistic huge spikes in time series that meet the criteria of ref. 19 . These quality-control procedures removed 4.4%, 12.3%, 12.4% and 13.3% of the hourly records in China, the United States, the European Union and India, respectively. Some of the NO 2 monitoring network data used in this study are based on conversion to NO using a heated molybdenum catalyst, a method known to have positive interferences that increase with the ratio of oxidized reactive nitrogen to NO x (refs. 55,56 ). This effect makes the determined PNO 3 from this work an upper limit to the actual PNO 3 , except in networks that use photolytic converters or direct NO 2 monitors (for example, the US state of California) or in source regions where NO x is a large fraction of total reactive nitrogen.

The calculation of nocturnal NO x lifetime
The nocturnal NO x lifetime (τ NO x ) is calculated by equation (2). Here, we estimated the lower limit of nocturnal NO x lifetime assuming NO 3 loss is dominated by N 2 O 5 uptake; therefore, the loss of NO 3 is accompanied by an additional NO 2 loss:

Nocturnal nitrate production rate and trend analysis
Nocturnal nitrate production rate is the average of hourly NO 3 production rate for the 10 h period from 20:00 to 05:59 (the next day) lt. At least 75% ( 57 . We thus follow the strategy proposed by Cooper et al. 57 to calculate trends on the basis of monthly average anomalies. We first calculate monthly averages for the entire period 2014-2019 and then yield the monthly anomaly value as the difference between individual monthly mean and the 2014-2019 monthly mean values. The linear trends in NO 3 production rates are estimated using the following generalized least-squares method with autoregression 19 : where y t represents the monthly nocturnal NO 3 production rate anomaly in month t, t is the index of month during the study period (warm/ cold season for 2014-2019, that is, ranging from 1 to 36), b is the intercept, k denotes the linear trend coefficient, α and β are coefficients for a 6 month harmonic series of seasonal cycle (M ranges from 1 to 6) and AR t is to account for autocorrelation.

The set-up of framework for sensitivity test of PNO 3 and NO 2
An observationally constrained box model based on the Regional Atmospheric Chemical Mechanism version 2 (RACM2) 58 , with some modifications (such as the chloride chemistry was added 59 ) is applied in this study. A detailed description of the implementation of RACM2 can be found in a previous publication 59 . In this study, the model calculations are constrained to measurements of CO, SO 2 , C 2 −C 12 VOC as well as the measured photolysis frequencies, temperature and pressure, and water-vapour concentrations and aerosol surface-area density. NO x emission rate, retrieved by fitting the observed diurnal NO x and O 3 concentrations, was also constrained. The CH 4 and H 2 mixing ratios are assumed to be 1.9 ppmv and 550 ppbv, respectively. The model is operated in a time-dependent mode, in which constrained values are updated every 1 h. For all species that are produced in the model, an additional sink representing physical loss processes such as dry deposition is implemented at a rate equivalent to a lifetime of 24 h. The heterogeneous chemistries of N 2 O 5 and ClNO 2 were considered in the box model. The N 2 O 5 uptake coefficient was adopted by the parameterization scheme considering the ambient temperature and relative humidity 60 , with a fixed ClNO 2 yield of 0.5. The ClNO 2 uptake coefficient was adopted by an upper value of 5 × 10 −6 with a unit Cl 2 yield 61,62 . The dependence of PNO 3 on the NO x emission is calculated using the box model. The chemical mechanisms are the same as described in the preceding. Therefore, the model is prescribed to different NO x emission rates, which is more representative of the real-world conditions. We used the diurnal average observation data, obtained in Taizhou in east China during summer 2018 63 , as a base-case constraint with an additional 2 d spin-up. By adjusting the intensity of NO x emission along with the constrained VOC reactivity, the modelled NO 2 and O 3 are optimized to match the observations, and then we subsequently derived the NO x emission in the base case. By adjusting the factor of NO x emission and VOC reactivity, we can then enable the conceptual model to fit the observationally derived PNO 3 , O 3 and NO x for the average condition of China, the United States, the European Union and India during 2018-2019 and the case in Los Angeles in 1980 (Extended Data Fig. 6). We checked the influences of aerosol loading, solar radiation and surface temperature on the pattern of PNO 3 -NO x emission by sensitivity tests and confirmed that these factors did not change the pattern of the curve and relative positions of different cities but led to some squeeze and shift in the peak of the curve as well as its width.

Code availability
All figures in this article are produced by the IDL (Interactive Data Language version 8.3) and python, and the source codes can be obtained upon request to the corresponding authors.