Introduction

Compelling evidence has suggested that high ozone (O3) concentrations in the surface layer can be harmful to human health and vegetation growth1. As known, tropospheric O3 originates from two sources: photochemical production within the troposphere and dynamical injection from the stratosphere. Though the injected stratospheric O3 is estimated to account for only 5-10% of the tropospheric O3 sources2, case studies of stratospheric intrusions (SI) to the troposphere have documented how SI occurred and impacted tropospheric and even surface O33,4,5. Events of deep and fast stratospheric intrusions to the surface (SITS) can lead to high-O3 episodes, inducing severe O3 pollution6,7,8,9. In the recent decade, China has confronted a severe O3 pollution problem, even after the implementation of strict policies on emission reductions of O3 precursors. The causes for this environmental issue have been investigated from the perspectives of chemical responses to the changes in both emissions and meteorology10,11,12. Yet, the contribution of natural stratospheric O3 inputs to surface O3 pollution is often neglected. Verstraeten et al.13 found a substantial positive trend in stratospheric contributions to tropospheric O3 over China for 2005-2010 based on numerical simulations. It is still highly uncertain about the influence of SITS on the surface O3 over a long period. Such long-term variation of stratospheric influence on surface O3 is not only an issue for China, but also for elsewhere in the world3.

Though SITS events are transient and limited in local areas, their impact on surface O3 can be substantial during SITS periods over the affected areas, regarding the absolute O3 enhancement and fractional contribution to overall surface O3. For example, Chen et al.14 showed that a SITS event during the COVID-19 lockdown period in 2020 enhanced surface O3 concentrations in Beijing, China, by 8 ppbv, which contributed to 23% of overall surface O3. These extra stratospheric O3 inputs, plus the background O3, can exacerbate air pollution beyond the recommended O3 threshold. The SITS-induced O3 exceedances depend not only on the absolute SITS O3 inputs, but also on surface background O3, which is modulated by the interplay of multiple chemical and physical processes, varying at different time scales, and sensitive to O3 precursor emissions and environmental conditions. Therefore, to what extent SITS events are harmful to human health and crop yield is largely uncertain, not only in their absolute magnitudes, but also in relative contributions to surface O3. To our knowledge, this issue has not been resolved on a national scale for the worsening ground-level O3 pollution in China to date.

Assessing the impact of SITS on surface O3 for large areas over long periods requires a good understanding of three key SITS metrics: frequency, duration, and intensity. These three metrics are essential to evaluate the O3 exposure associated with SITS concerning the health effect. Despite the knowledge obtained from SITS case studies, it is challenging to obtain an overview of SITS events in terms of these three metrics on a national, continent, or global scale, due to the difficulty of explicitly resolving them. For stratospheric air to reach the surface, it has to be transported downward across the first barrier, the tropopause, through deep SI, and then quickly descends and crosses the second barrier, the planetary boundary layer (PBL)15,16,17. Insufficient consideration of multi-scale atmospheric dynamical processes, including large-scale tropopause folding and small-scale PBL mixing, can easily lead to biased representation of the frequency, duration, and intensity of SITS events. Chemical transport models are often applied to evaluate stratospheric O3 transported into the troposphere using tagged stratospheric tracers18,19, but the estimated stratospheric influences are heavily dependent on tracer definitions and resolvable dynamical processes20,21. Especially, the abundant and complicated chemical sink compounds of O3 in the PBL would result in enhanced uncertainty of the intruded stratospheric O3 if they are not accurately described in models7,16. The instantaneous and local nature of SITS events3 requires comprehensive observations with a high spatial and temporal resolution in order to assess their impacts on surface O3. The routine weekly ozonesonde observations can directly monitor vertical O3 variation, they are not able to capture SITS events that persist only for several hours4,8,9. Running multi-instrumental campaigns with targeted stratospheric tracers, such as O3 and cosmogenic radionuclide, can provide reliable indicators of SITS5,22,23. Performed in limited regions and periods, however, these campaigns are spatially and temporally too scant to provide a national-scale estimation of stratospheric influences on surface O3 for long periods. Therefore, how can we overcome our insufficient ability to achieve a nation-wide assessment of SITS events regarding their frequency, duration, and intensity?

Stratospheric air is characterized by high O3, low carbon monoxide (CO), and low relative humidity (RH)3,4,24. Intrusions of stratospheric air downward to the surface level produce sharp upward (downward) spikes in the surface-measured O3 (CO), as documented in many case studies6,7,8,9. Such stratospheric signals can provide a clear indication of SI that have reached the surface level with the aid of dense surface observations. China has built a nationwide network consisting of more than 1600 surface stations that are capable of providing hourly observations of O3, CO, NO2 and SO2 concentrations25,26. This comprehensive dataset is both spatially and temporally dense, so it is possible that the data from this network could capture the signals of SITS events over large areas much more effectively than those mentioned traditional data, and hence provide an opportunity to investigate the stratospheric contribution to surface O3 and its long-term trends across the nation.

Here, we take advantage of the dense surface observations and develop a SITS detection method using O3 and CO as stratospheric indicators and their distinct spikes during SITS as constraints (see “Methods” section), based on our previous case studies9,14. This method bypasses the need for detailed knowledge about the multi-scale dynamical processes and complicated chemical sinks for the descending stratospheric air. The general features of SITS from our detection method are in line with those derived from multi-instrumental observations in published literatures (see ”Methods” section). This gives us confidence to resolve the spatial and temporal variations of SITS events in China. Basing on large samples of SITS events detected locally at individual stations across the nation in 8 years (27,616 events in total), here we aim to address the following scientific questions: (1) what are the characteristics of SITS events in China over 2015-2022 in terms of frequency, duration, and intensity? (2) to what extent SITS-induced O3 impacts surface O3 variations and contributes to the occurrences of surface O3 pollution? and (3) what is the trend of SITS-induced O3 in China over 2015-2022?

Results

Spatial distributions and seasonal variations of SITS in China

A total of 27,616 SITS events were screened out during 2015-2022 using surface observations from 1500 stations with continuous 8-year measurements in China (see “Methods” section). Figure 1 shows the spatial distributions of the annual frequency, annual total duration, and surface O3 enhancements of the SITS averaged over 2015-2022. The hot spots for SITS frequency and duration are located in high-elevation regions in the southeast rim of the Tibetan Plateau in southwest China, Tianshan Mountains in northwest China and Changbai Mountains in northeast China (Fig. 1a, b). The annual SITS frequency averaged is 8-12 occurrences per year over these regions, with an average of the total duration of 120-150 h per year, i.e., ~1.7% of the entire year. The highest SITS frequency is 16 and 14 occurrences per year in cities Panzhihua (3130 m above sea level) and Chuxiong (2530 m above sea level), both of which are situated closely to the Tibetan Plateau. Correspondingly, the total duration of SITS there reaches 345 hours and 318 hours per year, respectively, accounting for 3.94% and 3.63% of the time in a year. In the plain areas of eastern China, however, SITS events are relatively sparse and rare with a mean frequency of 2–3 occurrences per year and a total duration of 30-60 hours annually.

Fig. 1: Spatial distributions of the three key metrics of stratospheric intrusions to the surface (SITS) events in China.
figure 1

a The annual frequency (unit: occurrence per year), (b) the annual total duration (unit: hours per year) and (c) the surface O3 enhancements (unit: ppbv) of SITS events averaged over 2015-2022. Mt. Changbaishan, Tianshan and Hehuan are indicated in (a), and locations of Panzhihua and Chuxiong are marked by the magenta triangles. SCB in (c) refers to the Sichuan Basin. Source data are provided as a Source Data file.

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Figure 1c shows the SITS-induced O3 enhancements above their corresponding reference baselines averaged over the SITS duration (see “Methods” section for the baseline definition). Unlike the spatial variations in frequency and duration, the surface O3 enhancements are larger in plain than in high-elevation areas. In central and eastern China, average surface O3 enhancements are 15–25 ppbv during the SITS, while in western China, where elevations are high, only 7–15 ppbv. The SITS-induced O3 enhancements varying with elevation are more evident in the Sichuan basin (SCB). The reasons for such O3 enhancements varying with elevation can be subject to further studies. Here we suggest that for those SITS events descending to plain regions, the air parcels originated in the stratosphere have to travel a deeper vertical extension experiencing more chance to be diluted by tropospheric air. Hence, only those strong SITS events are more likely to reach the surface over plain regions. Trickl et al.27 suggested that the intensity of O3 enhancements during the SITS partially depend on how high the intrusions start in the stratosphere. Possibly, SITS events over plain regions in eastern China may initiate at higher altitudes within the stratosphere.

The frequency of the SITS exhibits distinct seasonality with a maximum in early spring, a secondary maximum in autumn and a minimum in summer (Fig. 2a). Such seasonality is in agreement with multiple studies of SI climatology in the midlatitudes of the Northern Hemisphere28,29,30,31,32. In particular, the number of detected SITS events in China shows a pronounced peak in March and a minimum in August. In terms of the duration of the SITS events, responding to the rapid mixing processes with tropospheric air and abundant chemical sinks, SITS-induced O3 can elevate surface O3 concentrations above the baseline for 18 hours on average (Fig. 2b). In general, SITS occurring in spring and autumn persist longer than in winter and summer. Besides showing the highest SITS frequency, March also exhibits the longest duration of SITS events, yielding elongated stratospheric impacts on surface O3. Note that the duration here does not include the time before which stratospheric O3 is chemically destroyed in the troposphere, instead, it refers to the period when the stratospheric air parcel reaches the surface and retains its rich-O3 and poor-CO properties that are distinguishable from tropospheric air.

Fig. 2: Monthly variations in the three key metrics of stratospheric intrusions to the surface (SITS) events in China.
figure 2

a Frequency of SITS events scaled to the maximum value in March. b Duration of STIS events in each month (unit: hours). c O3 enhancements of SITS events relative to the corresponding surface O3 reference baselines (unit: ppbv). The horizontal blue dashed lines in b and c represent the corresponding annual means. d Fractions of SITS associated with different intensity categories, i.e., weak (O3 enhancements<15 ppbv, green bar), moderate (15 ppbv<O3 enhancements<40 ppbv, blue bar), and strong (O3 enhancements >40 ppbv, red bar). The four seasons are separated by red dashed lines for winter (December-February), spring (March-May), summer (June-August), and autumn (September-November). Source data are provided as a Source Data file.

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The SITS-induced O3 enhancements averaged over the SITS duration in each month are compared in Fig. 2c. Generally, the stratospheric O3 enhancements are stronger in warmer months, with an average of 18.1 ppbv above the baseline value from March to October, and weaker during colder months, with an average of 12.2 ppbv above the baseline. Furthermore, Fig. 2d illustrates the distinct seasonal cycles of the SITS events with three levels of intensity measured by O3 enhancements27. The SITS events with O3 enhancements exceeding the surface O3 baseline by less than 15 ppbv (weak SITS) exhibit a maximum in cold months, while those with an exceedance over 40 ppbv (strong SITS) appear frequently in warm months, especially in summer. The distinct seasonality of SITS with different intensities is in line with Trickl et al.27, who attributed the maximum of stratospheric O3 intensity in summer to the air parcels’ higher origins in the stratosphere than in the other seasons. Due to an elevated tropopause in summer33, the summertime SITS events originating at higher altitudes would have high O3 concentrations because the altitudes are closer to the stratospheric O3 reservoir ~20–25 km.

Substantial stratospheric contribution to surface ozone pollution episodes

Based on a complete depiction of the three key SITS metrics, we estimate the magnitude of injected stratospheric O3 at ground level by integrating the hourly excess of O3 concentrations relative to their reference baselines28 (Fig. 3a, unit: ppbv*hour, see “Methods” section). Averaged over 2015-2022, the monthly stratospheric O3 inputs to the surface layer in China show a peak in March, a second peak in October and a minimum in December and January. Governed by the three key metrics of SITS, it is clear that stratospheric influences maximize in early spring due to the high frequency and duration and moderate intrusion intensity. The combination of high intrusion intensity and moderate frequency and duration of SITS in autumn leads to a second peak of stratospheric influences. The intrusion frequency is relatively high in winter (Fig. 2a); however, the short duration and weak magnitudes of the SITS (Fig. 2b, c) result in the least stratospheric inputs to surface O3 in winter. During the short periods of the SITS (referred to as the SITS duration), these additional stratospheric O3 inputs substantially enhance the surface O3 concentrations, consisting of 30-45% of surface O3 over SITS-affected areas (Fig. 3b–e). The ratio of stratospheric O3 to overall surface O3 can reach 58% in March and October, calling for extra consideration of the stratospheric influence on O3 budgets in the 2 months.

Fig. 3: The monthly sum of stratospheric O3 inputs (\({{{{{{\boldsymbol{O}}}}}}}_{{{{{{\bf{3}}}}}}}^{{{{{{\boldsymbol{strat}}}}}}}\)) and the ratios of \({{{{{{\boldsymbol{O}}}}}}}_{{{{{{\bf{3}}}}}}}^{{{{{{\boldsymbol{strat}}}}}}}\) to the sum of surface O3 concentrations during stratospheric intrusions to the surface (SITS) events (\({{{{{{\boldsymbol{Ratio}}}}}}}_{{{{{{\boldsymbol{SITS}}}}}}}\)).
figure 3

a The monthly sum of \({O}_{3}^{{strat}}\) (unit: ppbv*hour; see Eq. (5) in “Methods” section) in China averaged over 2015-2022. The horizontal blue dashed line represents its annual mean. b–e Spatial distributions of \({{Ratio}}_{{SITS}}\) during the periods of the SITS events (unit: %; see Eq. (4) in “Methods” section) in December, March, August and October. The red numbers in the lower right corner are the mean \({{Ratio}}_{{SITS}}\) and the mean duration of SITS events in the corresponding months on a national scale. Source data are provided as a Source Data file.

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The natural stratosphere-to-troposphere (STT) processes have direct influences on the chemical compositions in the troposphere34. Figure 4 presents a statistical overview of the instantaneous impacts of SITS on surface gaseous compounds, from 12 h before the start of the SITS to 20 hours after, averaged over all the detected SITS events. The synchronously sharp enhancement of O3 and reduction of CO, the two indicators of SITS used in our detection method, are obvious at the moment when SITS events start. Compared with the baseline values representing the non-SITS conditions, surface O3 is generally enhanced by 20 ppbv in the initial hours when stratospheric O3 reaches the surface and still retains its stratospheric properties largely, leading to a positive O3 anomaly of 60–70%. Right after SITS occurrences, the 90th percentile of surface O3 concentrations is directly enhanced by up to 40 ppbv above their normal values, which can affect adversely human health and ecosystems.

Fig. 4: Composite analysis of hourly O3, CO, NO2, SO2 and total oxidants (Ox = O3 + NO2) concentrations during all detected stratospheric intrusions to the surface (SITS) events in China over 2015-2022.
figure 4

The zero hour represents the start hour of all SITS events. A period of 33 h is applied starting 12 h before the SITS start hour and ending 20 h after the start hour. O3 concentrations of each SITS events (SITS O3, red solid line, unit: ppbv) are aligned into the 33-h period and averaged in each hour. The red arrows measure the 10th and 90th percentile of SITS O3 concentrations. The same procedure is applied to the hourly O3 reference baselines (Non_SITS O3, green solid line), CO (SITS CO, light blue solid line), NO2 (SITS NO2, deep blue solid line), SO2 (SITS SO2, orange solid line), and Ox (SITS Ox, black solid line). Source data are provided as a Source Data file.

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The recommended O3 threshold by the World Health Organization (WHO) is 50 ppbv35, which is measured as a daily maximum 8-h average (MDA8), while the threshold is 70 ppbv in USA7, and 80 ppbv in China12. When MDA8 O3 is larger than the threshold, O3 exceedances occur. Figure 5 shows the fraction of SITS events with O3 exceedances (based on the three O3 thresholds above) to the total number of SITS events in each month, averaged over all the stations and all the years. Here for each standard, if there is at least one O3 exceedance during a SITS event, we regard the event as a SITS-induced O3 exceedance. Referring to the WHO standard (Fig. 5a), over 70% of SITS events are associated with O3 exceedances from the middle of spring to summer, with an annual mean ratio of 41.1%. Similarly, the inputs of stratospheric O3 can lead to high O3 exposure to more harmful-level concentrations, even above the US and Chinese O3 standards (Fig. 5b, c). In spring, O3 in the lowermost stratosphere builds up and frequent tropopause folding events take place36,37,38. In addition, the seasonal variation in the surface background O3 shows an overall maximum in spring and summer in China39 (Fig. 5d). Under such conditions, air pollution is greatly exacerbated by SITS events from April to September, raising great health concerns especially during the warm months.

Fig. 5: The fraction of stratospheric intrusions to the surface (SITS) events with O3 exceedance to the total SITS events (unit: %) under different air quality standards and mean surface O3 concentrations in each month over 2015–2022.
figure 5

The recommended O3 standard by the World Health Organization (WHO) is 50 ppbv (a) measured as 8-h maximum moving average within a day (MDA8), while it is 70 ppbv (b) and 80 ppbv (c) in the USA and China, respectively. In each detected SITS event, the MDA8 O3 is calculated and compared against the standards above to determine the occurrence of O3 exceedance. d Mean surface O3 concentrations (unit: ppbv) averaged over all surface stations in each month. The monthly variations in the fraction and surface O3 concentrations are the mean over 2015–2022, while the rightmost column represents the annual mean. Source data are provided as a Source Data file.

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For CO variations during SITS periods as shown in Fig. 4, the composite analysis indicates a reduction of 30-35% compared with their values before SITS occurrences. Other gas pollutants that are not used for identifying SITS events here, such as NO2 and SO2, also exhibit a prominent decline during the SITS hours. We also examined the responses of atmospheric oxidation capacity (AOC) to SITS using total oxidant (Ox=O3 + NO2) as an indicator. Despite a reduction of NO2 when SITS events occur, the injected stratospheric O3 compensates for the loss of NO2 and instantly increases the AOC by 20-35%. As a consequence, the enhanced AOC would help stimulate the formation of particulate nitrate and secondary organic aerosols40. Stratospheric air injected into the surface can substantially alter the tropospheric air compositions, reaction sensitivity regimes between O3-NOx-VOC and tropospheric oxidative states34,41,42. Even though SITS events are transient, the complicated perturbation from the stratosphere induces changes in the balanced tropospheric air and can amplify the initial effects of SI18,43,44,45. Therefore, SITS should be seriously considered in tropospheric chemistry and air pollution control46.

Declining influences of SITS on surface ozone variations in 2015-2022

The quasi-decadal observations enable us to examine the long-term variation of stratospheric influence on surface O3. Supplementary Fig. 1a shows variations in the monthly stratospheric O3 inputs to the surface (in the unit of ppbv*hour, see “Methods” section, Eq. (5)) in China over 2015-2022 and their ratio to the overall surface O3 during SITS periods in the affected areas (see “Methods” section, Eq. (4)). The ratio ranges between 25-50%, and shows substantial interannual variations. Over 2015-2022, the magnitude of direct stratospheric influence on surface O3 across China appears declining. Figure 6a further shows the time series of deseasonalized monthly accumulated SITS-induced O3 over 2015-2022. A declining trend significantly at a 95% level is observed at a rate of -6.7 ppbv*hour per year, which is ~1–2% per year. An independent indicator of the stratospheric influence, the occurrence of very-high-O3 concentrations during SITS (e.g., 80–100 ppbv; Supplementary Fig. 2), also exhibits a decreasing trend, supporting the results of Fig. 6a.

Fig. 6: Deseasonalized monthly stratospheric O3 inputs and their ratios to surface O3 concentrations averaged at all study sites in China over 2015-2022.
figure 6

The gray shaded areas represent the time series of deseasonalized monthly means of (a) stratospheric O3 inputs (see Eq. (5) in “Methods” section) and (b) their ratios to the surface O3 concentrations (see Eq. (6) in “Methods” section). The red lines represent the linear trends for stratospheric O3 inputs and stratospheric O3 ratios, respectively, and the red numbers in the upper right corner are the trends that are statistically significant above the 95% and 99% confidence level, respectively. Source data are provided as a Source Data file.

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If scaled to longer timescales of a month or year (see Eq. (6) in “Methods” section), the ratio of stratospheric inputs to surface O3 is much lower (Fig. 6b and Supplementary Fig. 1b). On a annual basis, 1.6-2.2% of surface O3 is attributable to the stratospheric inputs in the affected areas. The stratospheric O3 ratios at the surface exhibit a maximum in early spring (2.7%) and autumn (2.2%), but a minimum in the summer (1.3%). Low fractions of stratospheric O3 inputs to surface O3 over annual or longer periods have been documented. For example, Cristofanelli et al.47 estimated that deep SI contributed 2% of surface O3 on the southern slope of the Himalayan region, and Lin et al.48 found that the SI contributed 1.3% of surface O3 in Mt. Hehuan of Taiwan.

Over 2015–2022, the ratio of SITS-induced O3 to overall surface O3 concentrations also declined significantly at -0.074% per year (Fig. 6b). The model simulations of Verstraeten et al.13 suggested increasing stratospheric contributions to the tropospheric O3 increases in China over 2005–2010. Yet, based on the analysis of surface observations, this study suggests a minor and declining stratospheric influence on the surface O3 increase in China over 2015–2022, at least for the direct and deep SI events. The causes for the declining trend of stratospheric influences may include the weakened O3 abundance in lower stratosphere49,50 (Supplementary Fig. 3a) where STT events take place mostly, and the reduced strong SITS occurrences (Supplementary Fig. 2). Regarding the tropospheric environments where stratospheric O3 intruded into, the capping stable layer (thermal inversion) tends to descent and intensify since 2010 in China (Supplementary Fig. 3b), hindering the formation of deep PBL for downward transport of stratospheric air to the surface15,16. The stratospheric O3 injected to the troposphere averaged over China, deriving from the Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST)51 data, also indicates a declining tendency over 2015-2022 (Supplementary Fig. 4). Supplementary Fig. 5 presents the estimated amounts of stratospheric O3 in the surface layer from the MERRA-2 GMI global atmospheric chemistry model52, which also shows a decreasing trend of stratospheric contributions to surface O3.

Discussion

Estimates of the stratospheric influences on surface O3 are necessary for making effective mitigation policies, since these inputs of natural stratospheric O3 can substantially enhance the risk of O3 pollution episodes and partially determine the floor value for air quality managements. In a short term, SITS-induced O3 has non-negligible significance for transient high-O3 episodes, given its large fractions of surface O3 budget (30-45%) and high risks of O3 exposure to harmful-level concentrations during SITS periods in affected areas. While the absolute stratospheric influences are highest in March and October, special attention to O3 pollution control should be paid in spring and summer when extra SITS-induced O3 inputs, plus the high background O3, promote possibility to exacerbate O3 pollution beyond the WHO and national standards. In 10% of the SITS cases, surface O3 can be elevated by over 40 ppbv, setting alarms for possible severe O3 pollution in affected areas. On the other hand, SITS could synchronously reduce concentrations of other air pollutants, including CO, NO2, and SO2. Such stratospheric perturbation can also substantially enhance the oxidation capacity of tropospheric air. On the annual basis, detectable O3 with stratospheric origins consists of 1.6–2.2% of surface O3 in China, implying that O3 pollution mitigation over long terms in China should mainly focus on surface O3 variations through photochemical reactions under the influence of meteorology and anthropogenic emissions. In this study, we conservatively estimate the stratospheric influences by only including the dynamically injected stratospheric O3 associated with deep, direct, and fast SITS events. The influences of aged stratospheric air injected into the troposphere from the stratosphere are spared, since such influences could not be easily identified from surface measurements. A combination of satellite remote sensing technology and deep machine learning methods in future work can help solve these issues. The chemically-induced O3 production due to stratospheric perturbation may also contribute to surface O3 variations in the presence of nonlinear chemical reactions during SITS53,54. The above factors can amplify the influences of the stratosphere especially in transient surface O3 pollution events, and hence enhance the impact of O3 on human health and crop yield.

Methods

Screening SITS based on comprehensive surface observations

Taking advantage of surface gaseous pollutant measurements, e.g., O3, CO, NO2, and SO2, with a high spatial and temporal resolution, here we develop a methodology of detecting SITS events over large areas and for long periods, based on and further refined from our previous studies9,14. This method is effective in detecting deep, direct, and fast SI retaining stratospheric properties, such as “O3-rich and CO-poor”3,4,15. In this study, we focus on such SITS events, while aged stratospheric air that has reached the surface but lost its stratospheric properties is not considered. Relying on the characteristics of stratospheric air reaching the surface (richer O3 and poorer CO relative to tropospheric air), we identify a SITS event based on hourly concurrent O3 and CO measurements at the surface based on the following points.

  1. 1.

    Distinct upward and downward spikes of O3 and CO indicators, respectively. The hourly measurements of stratospheric indicators (O3 and CO) are screened to filter out their distinct spikes, e.g., the sharp increase in O3 and decrease in CO simultaneously, a unique indication for air with recent stratospheric origins. Hourly variations of O3 and CO concentrations throughout the year are calculated site by site and year by year. The 95th percentile of the O3 rising rate and 5th percentile of the CO decline rate in each year are chosen to identify those sudden and sharp spikes when stratospheric air initially reaches the surface55. The synchronous appearance of extreme O3 increase and CO decrease could help isolate the sudden surface O3 change due to SI from that due to O3 transport or photochemical processes. As shown in Supplementary Fig. 6, the two parameters simultaneously determine the start timing of a SITS event (SITS_start).

  2. 2.

    The large departures of O3 and CO from their normal values. The intruded stratospheric air contains higher O3 than that in the troposphere; therefore, surface O3 with additional inputs in SITS events is supposed to increase from its normal values. To minimize the blurring of photochemically produced tropospheric O3, we consider that the O3 concentrations at the SITS_start hour should exceed the seasonal mean value during noontime (\({\bar{{O}_{3}}}^{{noon}}\), 1st O3 criterion) when photochemical reactions are active. Simultaneously, the CO concentrations during the SITS should decline below their seasonal mean value (CO criterion). These criteria could also help remove the occasions that could be falsely identified when O3 is transported downward from the residual layer, an O3-rich “reservoir” containing photochemically produced O3 in the preceding day56,57. Due to the mixing with tropospheric air and chemical sinks of O3, the properties of the intruded stratospheric air subside over the time58. When the O3 concentrations fall back to their seasonal mean values (\({\bar{{O}_{3}}}^{{season}}\), 2nd O3 criterion) or the CO concentrations rebound over the CO criterion, stratospheric air is not distinguishable from the tropospheric air and hence the SITS events end (SITS_end; referred to the case illustrated in Supplementary Fig. 6).

Provided with the start and end timing of SITS events, we estimate the amounts of injected stratospheric O3 reaching the surface by integrating the excess of O3 concentrations above their reference baselines (the seasonal means at the corresponding hour) during the SITS events (see details in the next section). At a given station for a period, such as a month, the number of SITS occurrences, the length of time between SITS_start and SITS_end averaged over all SITS events in the period, and the averaged excess of O3 concentrations above the baselines are regarded as the frequency, duration, and intensity of the SITS at that station for that period.

Similar to the definition of a chemical tropopause59, we rely on the variations in atmospheric chemical constituents O3 and CO, rather than some dynamic indicators, to define the frequency, duration, and intensity of SITS events. These definitions are referred throughout this manuscript. Both O3 abundance in the lower stratosphere and frequency of deep stratosphere-to-troposphere processes primarily determine the injected amounts of stratospheric O3 into the troposphere60. When intruded into the troposphere, stratospheric O3 can be strongly mixed with tropospheric air and be chemically destroyed, responding to the complicated dynamical and chemical processes in the troposphere, especially in the PBL. Therefore, assessing the stratospheric contribution to surface O3 depends on not only the detailed information of SITS (e.g., their frequencies and magnitudes), but also the varying tropospheric environments that control the fate of injected stratospheric O3 (SITS duration).

Although stratospheric air is also characterized with low RH, RH is not selected as an indicator in our detect algorithm. This is because RH is inversely related to temperature. Low RH may also appear when air parcels descend from higher altitudes to the lower troposphere experiencing adiabatic warming. The air parcels can also experience various atmospheric moisture conditions on their way to the surface, so RH of the air parcels is less conservative than O3 and CO3,8. In addition, concurrent RH measurements are usually unavailable in air quality monitoring stations in China.

We have developed this SITS methodology with a goal of being objective, robust, and accurate, i.e., reducing the commission and omission errors as much as possible. We have inclined to be conservative and set the detecting criteria rather strictly. For example, we assure that SITS would enhance surface O3 concentrations, i.e., as long as surface O3 concentrations are not above the background value, SITS stops. In this way, the detected SITS events are highly likely to be the cases, while some weak SITS events may be omitted.

Estimation of contributions of SITS to surface O3

The contributions of SITS to surface O3 are estimated event by event and station by station. The hourly mean surface O3 concentrations (\({\bar{O}}_{3}^{h}\); where h = 1, 2, 3,…24) are calculated by averaging O3 observations at each of the 24 hours in each season based on station-level observations in each year. The \({\bar{O}}_{3}^{h}\) values are taken as reference baselines to measure the departure of O3 concentrations from their baselines during SITS periods. Provided with the start and end timing of SITS events, we integrate the excess of O3 above the reference baselines during SITS periods (unit: ppbv*hour), and take it as the amount of injected stratospheric O3 (\({O}_{3}^{{strat}}\)) in each SITS event14,28,31,48:

$${O}_{3}^{{strat}}={{\int }}_{{SITS}{{\_}}{start}}^{{{SITS}{{\_}}{end}}} ({O}_{3}^{h}-\,{\bar{O}}_{3}^{h}){dt}$$
(1)

where \({O}_{3}^{h}\) denotes the in situ hourly O3 observations at hour h, and the \({\bar{O}}_{3}^{h}\) represents the baseline O3 concentrations at the same hour. The differences between \({O}_{3}^{h}\) and \({\bar{O}}_{3}^{h}\) are summed over the SITS period with a temporal resolution of 1 hour (i.e., dt = 1 hour).

The sum of O3 concentrations during each SITS event and its corresponding month (unit: ppbv*hour) are calculated following Eqs. (2) and (3), respectively:

$${{O}_{3}}_{{SITS}}^{{sum}}={{\int }}_{{SITS}{{\_}}{start}}^{{{SITS}{{\_}}{end}}}{O}_{3}^{h}{dt}$$
(2)
$${{O}_{3}}_{{month}}^{{sum}}={{\int }}_{{month}{{\_}}{start}}^{{{month}{{\_}}{end}}} {O}_{3}^{h}{dt}$$
(3)

The ratio of stratospheric O3 (\({O}_{3}^{{strat}}\)) to the sum of O3 concentrations during each SITS event (\({{Ratio}}_{{SITS}}\)) is given by Eq. (4):

$$R{{atio}}_{{SITS}}=\frac{{O}_{3}^{{strat}}}{{{O}_{3}}_{{SITS}}^{{sum}}} * 100\%$$
(4)

The sum of stratospheric O3 inputs during all SITS events in a month (\({{O}_{3}}_{{month}}^{{strat}}\), unit: ppbv*hour) is calculated by Eq. (5), given the number of SITS events in the month being N:

$${{O}_{3}}_{{month}}^{{strat}}={\sum }_{i=1}^{N}{O}_{3,i}^{{strat}}$$
(5)

Finally, the ratio of stratospheric O3 to the sum of O3 concentrations in the corresponding month (\({{Ratio}}_{{month}}\)) is given by Eq. (6):

$$R{{atio}}_{{month}}=\frac{{{O}_{3}}_{{month}}^{{strat}}}{{{O}_{3}}_{{month}}^{{sum}}} * 100\%$$
(6)

The time series (2015–2022) of SITS-induced O3 and its ratio to overall surface O3 concentrations during SITS periods and the entire month are showed in Supplementary Fig. 1.

Validations of the SITS detection algorithm

As SITS appears as rare events in local areas, it is important to verify the reliability of our detection algorithm in order to address the impact of deep SI on surface O3. We previously published a detailed analysis of two SITS events that occurred in China, which were selected from the large samples of SITS events9,14. The stratospheric origins and transport pathways of the two SITS cases were revealed by means of surface air pollutant observations, vertical profiles of RH and O3, PV evolution, and backward trajectory simulations. The general characteristics of SITS detected with our algorithm, such as their seasonality and contribution to surface O3, are in good agreement with the observed deep SI climatology by Stohl et al.24, Elbern et al.28 and Cristofanelli et al.31. The detected frequency of SITS is further compared with published observational studies. For example, based on multiple stratospheric tracers including RH, CO, and cosmogenic radionuclide 7Be, Lin et al.48 identified 14 SI days during a 13-month campaign in a high-elevation station (3380 m asl) located in Mt. Hehuan of Taiwan (Fig. 1a). The annual frequencies of SITS in Panzhihua (16 per year) and Chuxiong (14 per year) detected in the present study agree reasonably with those at Mt. Hehuan which is with a similar latitudes and altitudes (Fig. 1a). Using a combination of stratospheric tracers including RH, potential vorticity (PV), 7Be and the tropopause height anomaly, Cristofanelli et al.31 reported that there were 33 days (average 5.5 days per year) of deep and direct SI which were characterized by distinct stratospheric properties at Mt. Cimone (2165 m asl) in Italy over 1998-2003. We apply the SITS detection algorithm to the O3 and CO measurements collected in Mt. Cimone during 2013–2016, and find a total of 26 direct SI events (average 6.5 days per year). The results from our detection algorithm are in line with these SI studies shown above and indicate the feasibility of using sudden and sharp spikes of O3 and CO to identify SI reaching the surface.

The origins of SITS events are investigated with backward trajectory simulations of 10 days over selected cities (Supplementary Fig. 7; see details of the backward trajectory simulations in the following section). We select Panzhihua as an example where the highest SITS frequency is detected. The cities Beijing and Fuzhou are also selected to examine SITS occurred in northern and southern China, respectively. The trajectory analysis indicates that the majority of air parcels at the surface during the detected SITS events originated in the upper troposphere and lower stratosphere (UTLS; above 400 hPa), i.e., 96% in Panzhihua, and 100% in Fuzhou and Beijing. As another piece of evidence, the ensemble RH profiles during the 33 SITS events over Beijing show substantial dryness characterized by RH < 30% in the PBL and near the ground level15, suggesting the dry stratospheric air has descended into the surface. In addition to these selected cities, we further analyze the ensemble of backward trajectories associated with the detected 27,616 SITS events (Supplementary Fig. 8). We evenly divide every trajectory from the beginning to the end into three travel stages. The height, PV, and O3 concentrations in each stage are extracted from MERRA-2 reanalysis data. The air parcels of SITS initially reside in 300-200 hPa, where PV values exceed 2 PVU (an iso-surface representing the dynamical tropopause) and O3 concentrations are larger than 250 ppbv61,62, showing prominent stratospheric origins.

Backward trajectory simulations and MERRA-2 reanalysis data

Backward trajectories are simulated to check the origins of airmass of detected SITS events (Supplementary Fig. 7 and Supplementary Fig. 8) using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. HYSPLIT is developed by National Oceanic Atmospheric Administration’s (NOAA)63 (https://www.arl.noaa.gov/hysplit). The 10-day backward trajectories are driven by the meteorological data from the Global Forecast System (GFS) with a resolution of 0.25°. The PV values and O3 concentrations along the trajectory are extracted from the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) reanalysis data. The MERRA-264 reanalysis data have a spatial resolution of 0.5° latitude × 0.625° longitude with 72 model levels (https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2; DOI: 10.5067/WWQSXQ8IVFW8).

Surface observational data and stratospheric O3 tracer data

The present study is mainly based on analysis of hourly ground-based measurements of O3, CO, SO2, and NO2 concentrations at more than 1,600 stations in Chinese cities. For O3 and CO, they are measured with a CO analyzer (Thermo Fisher Model 48i) and an O3 analyzer (Thermo Fisher Model 49i). The detection limit (precision) for Model 48i and Model 49i are 0.04 ppmv (±0.1 ppmv) and 0.5 ppbv (±1 ppbv), respectively. The data are from the public website of the Chinese Ministry of Ecology and Environment (MEE) (https://english.mee.gov.cn/).

To explore the variations in stratospheric O3 during the study period (Supplementary Fig. 3a), the stratospheric O3 profile observations are acquired from the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) dataset65 (https://csl.noaa.gov/groups/csl8/swoosh). SWOOSH provides a merged record of stratospheric O3 on the basis of a number of limb sounding and solar occultation satellites from 1984 to the present.

To investigate the thermal inversion variations over China during 2015-2022 (Supplementary Fig. 3b), radiosonde observations in China are processed, which are available from https://www.ncei.noaa.gov/products/weather-balloon/ integrated-global-radiosonde-archive.

The Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST)49,66 is a 3-dimensional O3 dataset derived from ozonesondes at over 100 stations using a trajectory-based mapping methodology with the HYSPLIT model. The thermal tropopause height is determined for each O3 profile, and the stratospheric O3 distribution is mapped for the O3 with stratospheric origination along the trajectory paths. All O3 values along the trajectory paths are binned into grids of 5° × 5° × 1 km (latitude, longitude, and altitude) from sea level to 26 km in each month. TOST has been validated against independent ozonesondes and widely used in global O3 climatology studies67. In the present study, we further extend the TOST data by conducting 10-day forward trajectories simulations over 2015-2021 (Supplementary Fig. 4).

Simulations from the MERRA-2 GMI global chemical transport model are analyzed for long-term variations in stratospheric O3 inputs to the surface-layer during the period (Supplementary Fig. 5). The MERRA-2 GMI (Global Modeling Initiative’s) model with stratosphere-troposphere chemical mechanisms is driven by MERRA-2 meteorology including winds, temperature and pressure52 (https://acd-ext.gsfc.nasa.gov/Projects/GEOSCCM/MERRA2GMI/). This model is run at a MERRA-2 native horizontal resolution of ∼50 km with 72 vertical levels. The model applies a stratospheric O3 tracer to diagnose the stratospheric O3 influence on the troposphere.