Arctic sea ice , Eurasia teleconnection pattern and summer surface ozone pollution in North China

Summer surface O3 pollution has rapidly intensified recently, damaging human and ecosystem health. In 2017, the 10 summer mean maximum daily average 8 h concentration of ozone was greater than 150 μg/m3 in North China. Here, we show that in addition to anthropogenic emissions, the Eurasia teleconnection pattern (EUTP), a major globally significant atmospheric teleconnection pattern, influences surface O3 pollution in North China. The local meteorological conditions associated with the EUTP positive phase supported intense and efficient photochemical reactions to produce more surface O3. The associated southerlies over North China transported surrounding O3 precursors to superpose local emissions. Solar 15 radiation and high temperature dramatically enhanced O3 photochemical reactions. Furthermore, due to the close connection between the preceding May Arctic sea ice and summer EUTP, approximately 60% of the interannual variability of summer surface O3 pollution was attributed to Arctic sea ice to the north of Eurasia. This finding aids in the seasonal prediction of O3 pollution.


Introduction
Along with social and economic development, air pollution has been increasing in China (Chen, 2013;Watts et al., 2018).The major air pollution types in China are haze pollution in winter (Yin et al., 2015;Wang, 2018) and surface ozone (O 3 ) pollution in summer (Ma et al., 2016;Tang et al., 2018).Due to the low visibility it caused and its obvious unusual smell, haze pollution easily causes warning and are being controlled in recent years (The environmental statistics unit of stat-center in Perking 25 University, 2018).However, surface O 3 pollution has always occurred on clear and sunny days (Wang et al., 2017), so it is not visible to humans.The features and causes of O 3 pollution in China, especially the impacts of climate variability, have not been sufficiently studied.Europe has benefitted from its rigorous air protection act and maintained good air quality, but the surface ozone levels still showed significant increases during 1995-2012 (Yan et al., 2017).In the major urban areas in China, the surface O 3 concentrations have exceeded the ambient air quality standard by 100-200% (Wang et al., 2017), especially in the The 1°×1° ERA-Interim data used here included the geopotential height (Z), zonal and meridional wind, relative humidity, vertical velocity, air temperature at different pressure levels, boundary layer height (BLH), surface air temperature (SAT) and wind, downward UV radiation, downward solar radiation, low and medium cloud cover and precipitation (Dee et al. 2011).
The daily and monthly ERA-Interim data were analyzed in this study.Furthermore, the daily and monthly reanalysis datasets 65 supported by the National Oceanic and Atmospheric Administration (NOAA) were also employed and denoted as NOAA data.The 2.5°×2.5°geopotential height (Z), zonal and meridional wind, relative humidity, vertical velocity, air temperature at different pressure levels, SAT and wind, downward UV radiation, downward solar radiation, low and medium cloud cover were downloaded from the National Center for Environmental Prediction and the National Center for Atmospheric Research (Kalnay et al. 1996).The BLH from 1979 to 2014 in the NOAA data was derived from the NOAA-CIRES 20th Century 70 Reanalysis version 2c (Giese et al., 2016).The daily precipitation data was from the CPC global analysis of the daily precipitation dataset (Chen et al., 2008).The calculation procedure for the Eurasia teleconnection pattern (EUTP) index was consistent with that of Wang et al. (2015).

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where H500 represents the geopotential height at 500 hPa, and overbars denote the area average.The SDZ MDA8 significantly covaried with the MDA8 in the north of China., especially in North China.The variation of summer SDZ MDA8 is presented in Figure S2.The diurnal difference in MDA8 was large, which contradicts the quasi-constant emission of ozone precursors.Therefore, the impacts of meteorological conditions were significant.According 95 to the Technical Regulation on Ambient Air Quality Index in China (The Ministry of Environmental Protect of China, 2012), the thresholds of moderate surface O 3 pollution (MOP) and nonsurface O 3 pollution (NOP) are 215 μg/m³ and 100 μg/m³ , respectively.During the years 2007-2017, there were 126 NOP days and 155 MOP days.The maximum number of MOP days was 26 days in 2015, and the mean number of MOP days was 14 days (Figure S3).The interannual variation in MOP (NOP) days was significant, without an obvious long-term trend.

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The meteorological conditions were composited for the MOP and NOP days (Figure 1).The local and surrounding weather conditions were significantly different.The anomalous southerlies, higher boundary layer height (BLH), less rainfall, warmer surface air temperature, and cooler temperature in the high troposphere favored surface O 3 pollution and vice versa.
Anomalous southerlies from the Yangtze River transported O 3 precursors (that were discharged in the economically developed Yangtze River Delta) and superposed them with the local high emissions in North China (Figure 1a).When the anomalous 105 winds reversed, i.e., northerlies, the O 3 precursors in North China was dispersed, and the surface O 3 concentration was reduced (Figure 1b).The correlation coefficient between the SDZ O 3 concentration and the area-averaged meridional wind at 10 m (35-50oN, 110-122.5oN,denoted as V10mI) was 0.39, exceeding the 99% confidence level.The moisture environment (i.e., high relative humidity) and cloudy skies were essential conditions for precipitation, which weakened the photochemical reaction by influencing exposure to ultraviolet rays.In addition, precipitation was also an important indicator of the wet 110 removal efficiency (Figure 1f).In summer, a day without rain represents efficient solar radiation, in favor of the occurrence of surface O 3 pollution (Figure 1e).The correlation coefficient between the area-averaged precipitation (37.5-42.5°N,112-127.5°N,denoted as PI) and the SDZ O 3 concentration was -0.35 (above the 99% confidence level), indicating that sufficient precipitation was connected with more NOP days.
In contrast, the high temperature near the surface enhanced the photochemical reaction and resulted in a higher surface O 3 115 concentration (Figure 1 g).The sky without clouds not only provided strong solar radiation (i.e., warmer surface air temperature) but also resulted in weak absorption of long-wave radiation in the upper air (i.e., cooler temperature at 200 hPa).
The correlation coefficient between the area-averaged difference in the temperature at the surface and 200 hPa (surface air temperature minus temperature at 200 hPa, 37.5-47.5°N,110-122.5°N,denoted as DTI) and the SDZ O 3 concentration was 0.49.Furthermore, due to the strengthening of solar radiation, the near-surface turbulence was enhanced, and the boundary 120 layer was lifted (Figure 1c).The downwash of atmospheric ozone from the upper air enlarged the surface O 3 concentration (An et al., 2009).The correlation coefficient between the SDZ O 3 concentration and the area-averaged BLH (37.5-47.5°N,112.5-120°N, denoted as BI) was 0.40.Therefore, the anomalous southerlies, high surface temperature, above average BLH, and sunny skies were favorable environments for severe surface O 3 pollution.To confirm the robustness of the link between meteorological conditions and the MOP and NOP days over North China, the above composite analysis was repeated with 125 NOAA reanalysis data, and identical results were obtained (Figures S4,5).
To assess the interannual variation of surface O 3 pollution and its relationship with climate variability (Cai et al., 2017), we tried to fit an O 3 weather index (OWI) based on long-term meteorological observations.Here, we defined the OWI as OWI=normalized V10mI+normalized BI-normalized PI+normalized DTI.For comparison, the multiple regression equation was built between the MDA8 and associated weather indices (Figure S6).Our analysis indicated that the observed MDA8 was 130 well fit by the multiple regression equation (Figure S6).The correlation coefficient was 0.61 between the fit and daily measured MDA8 during 2007-2017 (i.e., 92 days × 11 years).In contrast, the correlation coefficient between the observed MDA8 and daily OWI was also 0.61 for the 11 year period.Thus, the OWI was easily constructed by accumulating the normalized weather index and was selected to represent the variation in surface O 3 pollution.A total of 90.3% of the MOP events were in the range of OWI > 0, and correspondingly, 90.5% of the NOP events were linked with OWI < 0 (Figure 2).The 135 correlation coefficients between the OWI and observed MDA8 at the other sites were calculated (Figure S7).The significantly positive correlations were distributed in North China.Thus, it is reasonable to analyze the variation in surface O 3 pollution in North China using the OWI, which also extends the study period to the historical period before 2007 and the projected future.

Impacts of EUTP on the interannual variation of surface ozone
After 1979, the quality of the reanalysis data was improved to support studies of climate variability and change.Here, the daily 140 OWI was calculated with both ERA-Interim and NOAA reanalysis data, and the monthly mean OWI was computed.During 2007-2017, the constructed JJA mean OWI varied similarly with the observed MDA8 and captured the extremes (Figure 3).In The atmospheric circulation associated with summer mean OWI, indicated by the correlation coefficients, are displayed in Figure 4-5.In the mid-upper troposphere, cyclonic and anticyclonic anomalies were alternately distributed over the north-central Siberian Plateau (-), North China and Mongolia (+), and the Yellow Sea and Japan Sea (-) (Figure 4a).These 155 three atmospheric centers, propagated from the polar region to the mid-latitudes, appeared to be the EUTP.This Rossby wave-like train, i.e., the EUTP, could also be recognized in the surface air temperature.The correlation coefficient between the EUTP index and OWI was 0.44 (after detrending and above the 99% confidence level), indicating that the strengthening of the EUTP positive phase contributed to the severe surface O 3 pollution in North China.The EUTP is considered to be the main reason for the variability of the severe drought in North China, i.e., resulting in hot and dry climate extremes (Wang and He, 160 2015).To a certain extent, the severe drought environment promoted the formation of surface ozone.After 2007, the EUTP index and the observational SDZ MDA8 synchronously changed (Figure S8).More than 80% of the SDZ MDA8 anomalies showed the same mathematical sign as the anomalous EUTP index.Furthermore, the intensified EUTP anomalies (i.e., the |EUTP index| > 0.8 × its standard deviation) always induced homodromous surface ozone pollution.
Under barotropic anticyclonic circulation over North China, i.e., one of the active centers of the positive EUTP, the significant 165 descending air flows indicated efficient adiabatic heating (resulting in high temperatures near the surface) and dry air (i.e., less cloud cover) below 300 hPa.Furthermore, over North China, the air temperature (relative humidity) anomalies were negative (positive) at 200 hPa but positive (negative) below 300 hPa (Figure 4c).The barotropic anticyclonic circulation associated with surface ozone pollution (Figure 4b) was similar to the positive EUTP (Figure 4c) and led to sunny days, i.e., hot temperatures (Figure 4a), strong downwards solar radiation and UV radiation (Figure 5c-d), less low and medium cloud cover (Figure 5d), 170 and dry conditions (Figure 5b-c).Without the cover of low and medium clouds, the short wave solar radiation, especially the UV radiation, penetrated straight to the land surface.The photochemical reaction of the O 3 precursor was enhanced, generating more O 3 near the surface.The dry atmosphere near the surface, i.e., less precipitation and lower relative humidity, accelerated the photochemical reaction but restricted the wet clearing of the stocked O 3 in the atmosphere.A higher BLH (Figure 5b), resulting from the strengthening of solar radiation, facilitated the downward transportation of O 3 from aloft.Near the surface, 175 the western part of these anticyclonic anomalies manifested as significant southerlies (Figure 5a

Roles of the Arctic sea ice
The positive EUTP enhanced the local anticyclonic circulation over North China and facilitated the processes leading to the formation of surface ozone.The EUTP originated from the Arctic region; thus, the role of Arctic sea ice on the OWI was also considered in this study.The interannual variation of OWI was significantly correlated with May sea ice conditions to the north of Eurasia, especially near the Gakkel Ridge, the Canada Basin and the Beaufort Sea (Figure 6a).The area-averaged (green 185 boxes in Figure 6a) sea ice in May was calculated as the SI index, whose linear correlation coefficient with JJA OWI was 0.67 (after detrending) from 1979 to 2017.During 2007-2017, 73% of the May SI anomalies may be followed by observational SDZ MDA8 anomalies with the same mathematical sign (Figure 6b).Furthermore, the linear and nonlinear relationships were both introduced using the generalized additive model (Figure S11), and the contribution of May sea ice to the interannual variability of OWI was approximately 60%.

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These positive sea ice anomalies could induce EUTP-like responses in the subsequent summer (Figure 6c).The excited atmospheric and thermal centers were located over the Central Siberian Plateau, North China and Mongolia, and the Yellow Sea.Similarly, the local meteorological responses, such as anomalous southerlies and less precipitation (Figure 6d), less cloud and strong solar radiation (Figure 6e) were also closely connected with the positive sea ice anomalies in May.Thus, the preceding May sea ice positively modulated the EUTP, and then, this Rossby wave train transported the impacts from the polar 195 region and strengthened the anti-cyclonic anomalies over North China.Finally, suitable meteorological conditions, including hot-dry air, anomalous southerlies and intense sunshine, were induced to intensify the photochemical production of surface ozone pollution.To confirm the roles of Arctic sea ice and associated physical mechanisms, the above analysis was repeated with the NOAA data, and identical results were obtained (Figure S12).

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Recently, the summer surface O 3 concentrations and the number of O 3 observation stations have steadily increased.Spatially, the O 3 concentrations in North China were substantially higher than those in South China.To reveal the climatic driver and improve the potential of seasonal prediction of summer surface O 3 pollution in North China, a daily OWI (i.e., surface O 3 weather index) was constructed based on long-term meteorological observations.The robustness of this index (i.e., OWI) was verified by the ERA-Interim and NOAA reanalysis datasets and surface O 3 measurements.May Arctic sea ice was found to be 205 a preceding and efficient climatic driver.In the historical period, variation in Arctic sea ice can explain approximately 60% of the interannual variability of the summer surface O 3 pollution in North China.Currently, the Arctic region has been warming approximately twice as much as the global average (Huang et al., 2017;Zhou, 2017), indicating accelerated change in the sea ice.Thus, understanding the role of Arctic sea ice may contribute to the seasonal forecasting of O 3 pollution.
Due to increased surface O 3 pollution in China, the number of O 3 measurement stations has dramatically increased since 2014 (Figure S1 a, c, e, g).During 2006-2014, O 3 concentrations were only observed in the most developed regions in China, i.e., the Beijing-Tianjin-Hebei area, the Yangtze River Delta, and the Pearl River Delta.Since 2015, O 3 concentrations have been 80 measured in most areas in eastern China.O 3 concentrations in the high-mid latitudes were higher than those in the lower latitudes, which appeared to be bordered by the Yangtze River.The O 3 concentrations in North China were rather high in 2014; the summer mean maximum daily average 8 h concentration of ozone (MDA8) in North China was higher than 120 μg/m³.Since that time, the O 3 polluted region has expanded approximately yearly.In 2017, the areas with summer mean MDA8 > 120 μg/m³ were visibly enlarged.In North China, the summer mean MDA8 observations were larger than 150 μg/m³, and the 85 maximum MDA8 was almost higher than 265 μg/m³ (i.e., the threshold of severe surface O 3 pollution).South of the Yangtze River, the O 3 concentrations were distinctly lower and decreased progressively towards the Pearl River Delta.The time span of O 3 observations limited the possibility of determining the role of climate variability in the interannual O 3 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1127Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 19 November 2018 c Author(s) 2018.CC BY 4.0 License.variations in North China.Thus, we examined the representativeness of the O 3 measurements at Shangdianzi station (SDZ: one of the three regional background air-monitoring stations in China), with observations from 2006-2017.The correlation 90 coefficients between SDZ MDA8 and the observed MDA8 at the other sites were calculated and are shown in Figure S1 (b, d, f, h).Similar to the O 3 concentrations, these correlation coefficients were also oppositely distributed south and north of 30oN.
the above composite analysis and OWI construction processes, the range of the data used was 2007-2017; thus, the datasets in 2006 were independently verified samples.The JJA mean OWI in 2006 successfully reflected the variation in observed MDA8; even the MDA8 in 2006 was a staged minimum.Derived from two different reanalysis datasets, the OWI-ERA and 145 OWI-NOAA varied consistently, confirming the robustness of the monthly OWI.In the following study, the monthly OWI from ERA-interim data and associated physical mechanisms were analyzed.Before the mid-1990s, the OWI was below zero, with a slightly decreasing trend and insignificant interannual variation.Since then, the OWI has significantly increased; furthermore, the intensity of interannual variation has strengthened.The emissions of O 3 precursors increased persistently and Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1127Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 19 November 2018 c Author(s) 2018.CC BY 4.0 License.pollution was significantly related to climate variability.Thus, the impacts of the large-scale atmospheric circulations were analyzed.
), which transported the O 3 precursors from the economically developed Yangtze River Delta.The extraneous O 3 precursor, superposed with local emissions, supported a harsh and efficient photochemical reaction of O 3 .To confirm the robustness of the atmospheric circulation and associated physical mechanisms, the above analysis was repeated with the NOAA data, and identical results were obtained (Figure S9-S10).

Figure 1 .
Figure 1.Composite of the meteorological conditions associated with different O 3 events during 2007-2017.Results for MOP (a, c, e, g) and NOP (b, d, f, h) events included (a-b) surface wind (arrow) and v-wind (shading), (c-d) BLH, (e-f) precipitation, (g-h) SAT, and temperature at 200 hPa.The black dots denote the composite results passed the 95% 330

Figure 5 .
Figure 5.The associated meteorological conditions.(a) The correlation coefficients between the JJA mean OWI and v wind at 10 m (shading), surface wind (arrow), (b) relative humidity near the surface (shading), boundary layer height (contour), (c) precipitation (shading), downward UV radiation at the surface (contour), (d) downward solar radiation at the surface (shading), sum of low and medium cloud cover (contour) from 1979 to 2017.The black dots indicate that the CC with 345

Figure 6 .
Figure 6.The role of the Arctic sea ice.(a) The correlation coefficients between the JJA mean OWI and May sea ice, (b) The variation of the May SI index (red bar, area-averaged sea ice of the green boxes in panel a), JJA mean EUTP index (blue bar) and JJA mean observational SDZ MDA8 (black bar) from 2007 to 2017.(c) The correlation coefficients between the May SI 350

Figure 5 .
Figure 5.The associated meteorological conditions.(a) The correlation coefficients between the JJA mean OWI and v wind at 10 m (shading), surface wind (arrow), (b) relative humidity near the surface (shading), boundary layer height (contour), (c) precipitation (shading), downward UV radiation at the surface (contour), (d) downward solar radiation at the surface (shading), sum of low and medium cloud cover (contour) from 1979 to 2017.The black dots indicate that the CC with temperature was above the 95% 385