Severe level of photochemical oxidants (Ox) over the western coast of Japan during autumn after typhoon passing

Photochemical oxidants (Ox; mainly O3) are a concern in East Asia. Because of the prevailing westerly wind in the midlatitudes, O3 concentration generally shows a high in spring over Kyushu Island, western Japan, and Ox warnings have been issued in spring. However, the record from 2000 to 2021 of Ox warning days in Kyushu Island contains one warning case in autumn 2020. Interestingly, a typhoon had passed the day before this Ox warning. To relate these events, a modelling simulation was conducted and it showed the transboundary O3 transport from the Asian continent to the western coast of Japan due to the strong wind field determined by the location of Typhoon Haishen (2020). The sensitivity simulations for changing Chinese anthropogenic sources suggested that both nitrogen oxides (NOx) and volatile organic compound (VOC) emission regulations in China could decrease high O3 over the downwind region of Japan. Furthermore, VOC emission regulation in China led to an overall O3 decrease in East Asia, whereas NOx emission regulation in China had complex effects of decreasing (increasing) O3 during the daytime (nighttime) over China. The association between air quality and meteorology related to typhoons should be considered along with global warming in the future.


Syuichi Itahashi
Photochemical oxidants (O x ; mainly O 3 ) are a concern in East Asia.Because of the prevailing westerly wind in the midlatitudes, O 3 concentration generally shows a high in spring over Kyushu Island, western Japan, and O x warnings have been issued in spring.However, the record from 2000 to 2021 of O x warning days in Kyushu Island contains one warning case in autumn 2020.Interestingly, a typhoon had passed the day before this O x warning.To relate these events, a modelling simulation was conducted and it showed the transboundary O 3 transport from the Asian continent to the western coast of Japan due to the strong wind field determined by the location of Typhoon Haishen (2020).The sensitivity simulations for changing Chinese anthropogenic sources suggested that both nitrogen oxides (NO x ) and volatile organic compound (VOC) emission regulations in China could decrease high O 3 over the downwind region of Japan.Furthermore, VOC emission regulation in China led to an overall O 3 decrease in East Asia, whereas NO x emission regulation in China had complex effects of decreasing (increasing) O 3 during the daytime (nighttime) over China.The association between air quality and meteorology related to typhoons should be considered along with global warming in the future.
Photochemical oxidants (O x ), which mainly consist of tropospheric ozone (O 3 ), are related to chemical reactions involving nitrogen oxides (NO x ) and volatile organic compounds (VOCs) 1 .O x causes urban smog, poses major risks to human health and the natural environment, and is an important greenhouse gas 2 .The Japanese environmental quality standard (EQS) for O x was established in 1973, and hourly O x values should not exceed 0.06 ppm (118 μg/m 3 ) 3 .In addition, the Air Pollution Control Law (Paragraph 1, Article 23) stipulates that when hourly O x concentration exceeds 0.12 ppm and the status is expected to continue due to weather conditions, a warning should be issued to prevent damage to human health or the living environment.The EQS has not been satisfied at most monitoring stations in Japan, and this is a continuing issue that should be solved 4,5 .In general, high O x concentrations and subsequent O x warnings have generally occurred in spring over western Japan and in summer toward eastern Japan (e.g., Osaka, Nagoya, and Tokyo).This seasonal variation of O x in Japan is related to the transboundary O x transport from the Asian continent during spring and local photochemical production in Japan during summer [6][7][8][9][10][11] .A recent report on Japanese local photochemical production found a decreasing trend in the 3-year average of the annual 99 th percentile of the daily maximum 8-h concentration (a new index for evaluating O x ), especially over the Kanto region (e.g., Tokyo) 12 , and this result suggested that transboundary O x pollution has an important effect over Japan.The worsening of O x pollution in East Asia has been a concern recently [13][14][15] ; thus, the effect of transboundary transport on O x behaviour is the focus of the present work.
Over Kyushu Island, western Japan, high O x concentrations and related warnings are generally issued during spring.The summary of the number of O x warning days from 2000 to 2021 over Kyushu Island is presented in Fig. 1.The O x warning days have mostly been observed in May, and sometimes in April and June.In Nagasaki Prefecture, located on western Kyushu Island, an O x warning was issued for the first time in May 2006.However, there was one warning day in September 2020 in Nagasaki Prefecture (Fig. 1).This warning was issued on Goto Island (westernmost Nagasaki Prefecture, Fig. 1) on 8 September, 2020 at 15:00 local time when an O x concentration of 123 ppb was observed, the maximum concentration was recorded as 126 ppb at 16:00 local time, and the warning was canceled at 19:00 local time.In the present study, this severe O x concentration event was analysed because this is the first case of a warning issued in autumn in Nagasaki Prefecture.

Results
First, the meteorological conditions in September 2020 were analysed.Typhoons Maysak (2020) and Haishen (2020) occurred in this period 16 .Typhoon Maysak (2020) formed on 27 August, 2020 over the eastern Philippines and passed over western Kyushu Island on 2-3 September, and Typhoon Haishen (2020) formed on 31 August, 2020 over the Northwest Pacific and passed over western Kyushu Island on 6-7 September (Supplementary Fig. S1).The O x warning on Goto Island in September 2020 was issued the day after Typhoon Haishen (2020) passed.Typhoons are usually associated with hazards, such as strong winds, storm surges, and rainfall, that can cause considerable damage and disruption.Although such a strong wind field could simply result in good ventilation and reduce air pollutant concentrations, several studies have reported that typhoons degraded air quality in terms of PM 2.5 17,18 , O x 19,20 , and deposition 21,22 .Because the O x warning issued in September 2020 occurred in a remote area of Goto Island, westernmost Nagasaki Prefecture, the horizontal distribution over East Asia should also be examined.Thus, a numerical modelling simulation covering the whole of East Asia (Supplementary Fig. S2) was applied to analyse this severe O x event further (see "Methods" section for details of the modelling).
The surface pressure anomaly, which is a unique characteristic of typhoons, was investigated and evaluated with the typhoon best-track data 16 .The typhoon tracks (Supplementary Fig. S1) and the simulated surface pressure anomaly are shown in Fig. 2. In this comparison, the anomaly was calculated by the difference between the surface pressure during each typhoon and the 1-month average (from 15 August, 2020 to 14 September, 2020; see "Methods" section).The typhoon best-track data and the simulated lower anomaly associated with Typhoons Maysak (2020) and Haishen (2020) agreed well.The comparison shown in Fig. 2 indicated that the general features of typhoon movement were captured well in the present modelling simulations.
The meteorological parameters were measured by the Automated Meteorological Data Acquisition System (AMeDAS) at the Fukue observatory, which is on Goto Island (Fig. 1), and the corresponding simulated results are shown in Fig. 3.The pressure (Fig. 3a) was simulated well, and the decrease in pressure was larger in the period when Typhoon Haishen (2020) passed compared with that when Typhoon Maysak (2020) passed.The precipitation (Fig. 3b) was generally underestimated but the timing was well captured.The wind speed (Fig. 3c) and direction (Fig. 3d) were also well captured, although the wind speed tended to be overestimated.This overestimation was mainly due to the insufficient wind reduction over the land.In addition to the case at Fukue in Fig. 3, another validation at Nomozaki, southern Nagasaki Prefecture is presented in Supplementary Fig. S3.The Nomozaki site observed a record-breaking maximum instantaneous wind velocity (59.4 m/s, the corresponding wind velocity in 1 h was 43.7 m/s) during Typhoon Haishen (2020), and such wind speeds were generally captured by the modelling system.These evaluations of the meteorological parameters showed that the present modelling system reproduced the meteorological field well, especially the features when the typhoons passed.The modelling performance for O 3 concentration in Nagasaki Prefecture is shown in Fig. 4. The O 3 concentration was lower during two typhoon periods (purple shading, see also Fig. 3).After Typhoon Maysak (2020) passed on 3 September, 2020, the O 3 concentration was slightly increased to 60-80 ppbv at all sites in Nagasaki Prefecture.Subsequently, due to Typhoon Haishen (2020) passing, O 3 concentration was decreased to 20 ppbv on 7 September, 2020.There was a sharp increase in O 3 concentration after Typhoon Haishen (2020) passed, and the O 3 concentration reached its peak from 8 to 9 September, 2020.This maximum concentration was close to 100 ppbv in cities in the main parts of Nagasaki Prefecture, whereas on the remote islands of Nagasaki Prefecture (Tsushima, Iki, and Goto) it was close to 120 ppbv, which is the alert level for severe O 3 pollution.Of these three remote sites, only Goto Island issued an O x alert, and the present modelling system captured this high O 3 concentration exceeding 120 ppbv well.The statistical metrics scores (see the "Methods" section for their definitions) are also shown in Fig. 4. The correlation coefficient (R) was from 0.61 to 0.91, the normalized mean bias (NMB) was from + 5.8% to + 31.2%, and the normalized mean error (NME) was from + 15.9% to 31.2%.Except for the grid corresponding to Nagasaki city (#3 in Fig. 4), these metrics met the model performance criteria.These evaluations of O 3 concentration also demonstrated that the present modelling system reproduced O 3 pollution in Nagasaki Prefecture.In addition to these validations in Nagasaki prefecture, the vertical O 3 profiles are compared in Supplementary Fig. S4 and show that O 3 concentration from the surface to the middle stratosphere (approximately 20 km) was also captured well by the modelling system.The NO 2 column density observed by

Discussion
To identify the reasons for the high O 3 concentration that triggered the alert, the horizontal distribution pattern from 7 to 9 September, 2020 is shown in Fig. 5.At midnight on 7 September (Fig. 5a), high O 3 concentration (70-90 ppbv; yellow to orange, which exceeds the Japanese EQS) was seen over the East Asian ocean adjacent to the Asian continent.In addition, the typhoon was over Goto Island, and westerly or north-westerly wind fields prevailed over the East China Sea.At this time, the O 3 concentrations over Goto Island and other sites in Nagasaki Prefecture were low (20 ppbv; light green, see also Fig. 4).The high O 3 concentrations stretched into the East China Sea (Fig. 5b), and O 3 was produced during the daytime and the concentration increased on 7 September (Fig. 5c).During the night of 7 September (Fig. 5d), the high O 3 concentration over mainland China was decreased to a low concentration by NO titration and deposition, whereas the high O 3 concentration (120 ppbv; dark red, alert level) remained over the East China Sea.During this time, Typhoon Haishen (2020) was over the northern part of the Korean peninsula, and southerly and south-westerly wind fields transported this high O 3 concentration to Kyushu Island (Fig. 5e and f).During the daytime on 8 September (Fig. 5g), this air mass with O 3 higher than 120 ppbv reached Goto Island on the western edge of Kyushu Island; hence, the O x alert was issued.Then, the air mass was transported to the Tsushima Strait (between Kyushu Island and the Republic of Korea) due to the south-westerly wind affected by Typhoon Haishen (2020) in Liaoning Province, northeastern China (Fig. 5h and i).This analysis of the simulated O 3 spatial distribution suggested transboundary O 3 transport from the Asian continent to the western part of Japan, and the wind system caused by Typhoon Haishen (2020) was important in determining the air mass transport.Based on the analysis at the top of the boundary layer (approximately 750 hPa) shown in Supplementary Fig. S5, this high transboundary O 3 transport was limited near the surface level.The transboundary O 3 transport after the typhoon passing was clarified, and mitigation of this severe pollution is a concern.Finally, the effects of emission regulations are discussed based on the sensitivity simulations.The changes in O 3 concentration when 20% reductions in Chinese VOC and NO x emissions ( C V in Eq. ( 4) and C N in Eq. ( 5), see "Methods" section) were applied are shown in Figs. 6 and 7, respectively, and those for simultaneous VOC and NO x emission reductions are shown in Supplementary Fig. S6.The reduction in anthropogenic Chinese VOC emissions always helped to reduce O 3 concentration during the analysis period (Fig. 6).In contrast, the reduction in anthropogenic Chinese NO x emissions had complex effects because it caused both O 3 decreases and increases (Fig. 7).The increase in O 3 was observed only over mainland China from night to early morning and was associated with the weakening NO titration effect due to the NO x emission reduction.During the daytime, Chinese NO x emission reduction led to an O 3 decrease, and moreover, it always led to an O 3 decrease over the downwind region of Japan and this impact was greater than Chinese VOC emission regulation.Therefore, NO x emission regulations in China should consider these negative aspects.Recently, the aggravating effect of NO x emission regulations on O 3 pollution has been reported in China 23,24 , and the present results are also consistent with these findings.
Due to global warming, the frequency of typhoons will decrease, whereas their intensity will increase 25,26 .Similar future changes are also reported for the western North Pacific, and will lead to destructive threats by www.nature.com/scientificreports/stronger wind fields and intense precipitation 27,28 .As shown in the present study, high levels of O x over East Asia could be related to the meteorological fields associated with typhoons.Although precursor emission reduction could mitigate both global warming and air quality 29 , the behaviour of air pollutants related to the meteorological field should be continuously paid attention in future studies.

Observations
Ground-based observation of air pollutants that are regulated by EQS (carbon monoxide (CO), sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), O x , suspended particulate matter (SPM), and particulate matter with an aerodynamic diameter less than 2.5 μm (PM 2.5 )) are routinely measured by the Atmospheric Environmental Regional Observation System (AEROS) (https:// soram ame.env.go.jp).AEROS is divided into monitoring of ambient air quality by ambient air pollution monitoring stations (APMSs) and of air pollution, particularly from automobiles, by roadside air pollution monitoring stations (RAPMSs).In this study, APMSs data was used in Nagasaki Prefecture.The O x measurements were obtained by the neutral-buffered potassium iodide method or ultraviolet absorption spectrometry.Although ultraviolet absorption spectrometry only detects O 3 , it was approved in 1996 as the standard method for measuring O x concentrations.For the consistency of the measurement dataset, O x concentrations based on the ultraviolet absorption spectrometry at APMSs were used in this study.Nagasaki Prefecture, which is the focus of this study, is on the western coast of Japan (Fig. 1).Previous studies have clarified transboundary air pollution using measurement data here because of its location, especially at remote island sites (e.g., Goto Island) where domestic emissions are small [30][31][32] .There are 17 sites in total in Nagasaki Prefecture, and

Figure 1 .
Figure 1.Record of the O x warning days for severe concentrations (see, text for this definition) from 2000 to 2021 over seven prefectures on Kyushu Island, Japan.The colours indicate the month.At the start of this study, the confirmed values of AEROS were reported up to 2021 12 , and no days were reported in 2000-2005 and 2021 over all prefectures in Kyushu Island.The maps were generated with GMT (https:// docs.gener ic-mappi ng-tools.org/ dev/ index.html).

Figure 4 .
Figure 4. Comparison of O 3 concentrations from observations (red) and the model (black) at all 17 sites in Nagasaki Prefecture.The purple shading indicates the days affected by typhoons (Fig. 3).The dark-red shading indicates the period of O x alerts.When the measurement sites were allocated in the same model grid, the averaged values were used to evaluate the model, and the minimum and maximum concentrations are indicated by red shading.The statistical scores are shown in the box.The maps were generated with GMT (https:// docs.gener ic-mappi ng-tools.org/ dev/ index.html).

Figure 5 .
Figure 5. Simulated spatial distributions of O 3 concentration at the surface level over East Asia before, during, and after the O x alert on Goto Island, Nagasaki Prefecture.The maps were generated with gtool3 (http:// www.gfd-dennou.org/ libra ry/ gtool/ index.htm.en).