Summer ozone variation in North China based on satellite and site observations

Compared with other regions, air pollution in North China is very serious, especially its levels of fine particulate matter, which are closely associated with the concentrations of polluting gases, such as nitrogen oxides, sulfur oxides, organic gases, and ozone. Fine particle pollution has been studied in-depth, but there is less known about ozone. This paper focuses on 10 the interannual variability of tropospheric ozone in North China and identifies its influential factors. Our analysis relies on satellite observations (ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide and formaldehyde concentrations) and nearsurface data (carbon monoxide, sulfur dioxide, nitrogen dioxide, fine particulate concentrations, temperature, and humidity). Studies have shown that the tropospheric ozone column in North China has been at a high level for the past 3 years, with the similar time series for temperature and formaldehyde. However, trends in ozone are opposite to those of sulfur dioxide and 15 nitrogen dioxide over this 3-year period. This indicates that the increase in ozone in North China was mainly caused by the increase in temperature and an increase in organic gas content, rather than by nitrogen oxides. Over both temporal and spatial scales, the production rate of ozone appears to be most sensitive to temperature change, as ground observations in Beijing have suggested.


Introduction 20
Ozone (O3) in the atmosphere is mostly distributed within the stratosphere, where it absorbs ultraviolet radiation and maintains atmospheric temperature.Consequently, the O3 content in the troposphere is relatively small, being about ten times less.
However, tropospheric O3 has more direct and important impacts on human health and ecosystems.In recent years, North China has experienced severe air pollution, especially related to fine particles of pollution gases.
A large range in spatial distribution and long-term temporal changes of O3 are observed in satellite data.Typically, O3 pollution 25 is closely related to other air pollution components, such as NOx and volatile organic compounds (VOCs) (Sillman et al., 2003), as well as temperature and humidity.Although previous studies have mostly been based on a case study of the O3-VOC-NOx system sensitivity, there are few large scale studies of range or change (Carrillo-Torres et al., 2017).In this study, the horizontal transport of O3 was not considered.We only considered vertical transport and photochemical reactions because in summer Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-537Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 21 June 2018 c Author(s) 2018.CC BY 4.0 License.

Temporal and spatial distribution of tropospheric ozone
Figure 2 shows the monthly average O3 distributions for the study area; these statistics cover the period from 2005 to 2016.
North China is situated within the monsoon area of East Asia.The dominant airflow in winter is a dry cold northwest airstream originating from Siberia, while the dominant airflow in summer is a southeast airstream from warm and humid air masses over the oceanic/coastal sea areas.As a result, the region's temperature and humidity are closely linked to season.Figure 2 shows 5 that the distribution of O3 is also closely related to season.Throughout the year, O3 levels are lowest in the winter (December/January data), with highest levels in summer (June to August).Spring and autumn are transitional periods.The most ozone-intense period is July.This pattern is consistent with the seasonal distributions of temperature.July O3 levels rose from 2005 to 2016, indicating that O3 levels in North China are continuing to rise annually, following a longer-term trend with values rising from 44.9 DU in 2006 to 52.6 DU in 2015.This trend is not apparent in winter.Table 1 lists the percentage of 10 tropospheric O3 per month from 2005 to 2016.These data confirm that the lowest O3 levels are in January.For example, lowest O3 levels in January 2006 accounted for only 5.65 % of annual amounts.Similar low values are often recorded in the December to February period, for example, levels accounted for 5.5% and 5.87 % of total amounts in February 2012 and December 2015.
In contrast, levels in July 2016 accounted for 11.91 % of the whole year.Because of the increase in O3 pollution incidents in North China, now occurring during most summers, we focused our study on the summer period.In addition to O3 distributions, 15 we also discuss several other relevant pollutant gases and meteorological factors.Their potential to affect O3 is analyzed to evaluate their various contributions to its spatial and temporal changes in O3 distribution.

Nitrogen dioxide
There are many types of nitrogen oxides (NOx), several of them causing air pollution, especially nitric oxide (NO) and NO2 20 (Brown et al., 2003;Foy et al., 2015).Except for NO2 , most nitrogen oxides are extremely unstable.Under intense light, moisture or heat, they are converted into NO2 (or NO, which in turn is converted into NO2).Therefore, the nitrogen oxides in the atmosphere are mainly these two species, having a final form of NO2.Therefore, over long periods, the amount of NO2 and the total amount of nitrogen oxides are basically the same.According to previous research, there is a positive correlation between atmospheric NOx and O3 concentrations (Scholz and Rabl, 2006).The formation of tropospheric O3 requires a series 25 of complex chemical reactions.NO and NO2 act as catalysts, yielding the final chemical reaction equation: O + O2 → O3, et al., 2008).According to satellite observations, NO2 pollution in North China is more serious than in adjacent areas (Gu et al., 2014).The distributions of this pollution and northern industrial areas are well correlated.Annual 2011 to 2016, as did concentrations in June and August.However, in July, trends in NO2 and O3 are not consistent.Therefore, the increase in O3 over this period is not caused by an increase in nitrogen oxides.

Tropospheric sulfur dioxide
O3 and SO2 participate in photochemical reactions (Xie et al., 2005).Oxidation of SO2 by O3 in the presence of water forms sulfate (Ullerstam et al., 2002).O3 has a clear impact on SO2 levels.We conclude that the reduction in SO2 content may reflect 5 an increase in atmospheric oxidants and a reduction in sulfide emissions.Enhanced oxidation properties of the atmosphere, related to increasing O3 content, would increase the sinks of SO2.The annual trend of SO2 in North China is shown in Fig. 4.
Quantitative calculations indicate that the overall trend is downward from 2007 to 2011.In July, the average content of SO2 decreased by 43.2 % compared with 2005 values, while June values decreased by 49.1 %, and August values by 43.6 %.

10
CO emission from biomass burning was estimated by fire counts (Duncan et al., 2003).The diffusion and migration of CO in the atmosphere is controlled by wind speed (Liu et al., 2003).The solubility of CO in water is very low, and the effect of removing it from the air through wet deposition is not significant.The reaction between CO and oxygen yields: CO+2O2→CO2+O3 (Jin, 2008).When the concentration of NO is high, one molecule of CO produces one molecule of O3 (Heald et al., 2003).When the concentration of NO is low, there is no increase in O3; there may even be a loss of O3.In 15 contaminated areas, CO is one of the important precursors to photochemical production of O3.There is a positive correlation between vertical profiles of O3 and CO (Fishman and Seiler, 1983;Choi et al., 2017), where the main fluctuations in concentrations are similar.The positive correlation between CO and O3 suggests that tropospheric O3 is mainly derived from photochemical processes (Tang et al., 2006;Suthawaree et al., 2008;Chin et al., 1994).The spatial distribution of CO is basically like those of NO2 and SO2, all of which have elevated levels in North China.However, interannual trends of CO did 20 not show a clear downward trend.This indicates there is no marked reduction in CO emissions over the study period.

Summer ozone concentrations and its relationship to other factors
The order of magnitude of each component in the atmosphere varies greatly and is typically normalized for ease of comparison (Ji et al., 2016).The standardization method used in this study involved normalizing to [0, 1]: , where   is the input data,   and   are the largest and the smallest values in the series (Karanwal et al., 2010;Patel and Shah, 2015).

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Figure 6 shows temporal trends of various parameters (formaldehyde, temperature and surface solar radiation downwards) in July throughout North China.To compare these parameters, their time series were normalized.Since 2010, the trends in O3 and temperature have been consistent.Meanwhile, HCHO content is positively correlated with VOC content, suggesting these has been consistent changes in all VOC species (Seinfeld and Pandis, 2006).Over the past 12 years, HCHO content has increased, with a slight decrease following 2012.We conclude that although the concentration of nitrogen oxides decreased over this period, the concentration of O3 did not decrease because VOCs have continued to increase (Duncan et al., 2010).
Clearly, temperature and solar radiation are also important factors (Tang et al., 2006).

The relationships among ozone concentration and other factors from ground observations
In cities, there are many components that co-occur with O3.Whether there is chemical correlation between these species is 5 investigated herein.Figure 7 displays the correspondence between O3 content and several other components in Beijing during summer periods of 2013-2016.Clearly, precursor species and meteorological factors affect O3 concentrations, including temperature, humidity, and solar radiation levels.Herein, we have selected two factors to discuss in detail.
Clearly, CO contributes to the formation of O3.But there are other atmospheric components, such as fine particulate matter PM2.5 (Fig. 7a) that may play a role.As the concentration of fine particles has increased, O3 concentration also has rapidly 10 increased.Typically, when the concentration of CO is stable, O3 concentration rises rapidly as SO2 concentrations increase.
However, as discussed in sect.3.2.2,SO2 concentration declines in July, suggesting that any increase in O3 in July is not caused by the decrease in SO2.When the mass ratio of CO and NO2 are close to 1/20, O3 concentrations reach a peak.Meanwhile, when relative humidity reaches 40 %-60 %, CO concentration is greater than 2 μg m -3 and NO2 concentration is greater than 40 μg m -3 , the O3 concentration also peaks.However, the effect of temperature is most pronounced, showing a strong positive 15 correlation with O3 levels (Fig. 7e) (Duncan et al., 2009).

Conclusions
Based long-term statistics, the tropospheric O3 content of North China is highest in summer (from June to August), accounting for about 33 % of annual amounts, with highest levels in July (Table 1).Tropospheric O3 column concentrations for July are increasing annually in North China.Near the ground, such periods have been associated with increasing O3 pollution incidents.

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In contrast, the winter period (from December to February) has the lowest O3 content, accounting for about 18 % of the whole year.
Here, we analyzed temporal and spatial changes of several key atmospheric components to evaluate their impact on O3 formation.Changes in NO2, SO2, and CO were not consistent with changes in O3 levels in July.Acid gas emissions (NO2 and SO2) decreased annually, while O3 has been increasing annually, suggesting that NO2 and SO2 are not causing O3 pollution to 25 increase.Similarly, there was no obvious increase related to levels of CO.One of the main uncertainties linked to O3 formation is the impact of the emission of organic gases.We considered both CO and HCHO in this study.The impact of reductions of CO was not obvious on O3 levels, which appear to maintain relatively stable levels.HCHO levels observed by satellite show greater fluctuation and are more volatile over this time.However, it is not certain that CO or HCHO are causing the increase in summer O3 in North China in recent years.

Figure 1 :
Figure 1: The focus area of this study is within block, as the North China area referred to in the article.

Figure 2 :
Figure 2: Monthly average ozone vertical column density(VCD)distribution in the troposphere(2005 -2016)and monthly time series of ozone in North China, as the area in the box in Figure 1 (unit: DU).

Figure 3 :
Figure 3: Average nitrogen dioxide vertical column density distribution in July(2005 -2016)in the troposphere and summer (from June to August) time series of nitrogen dioxide in North China, as the area in the box in Figure 1 (unit: 10 15 molec cm -2 ).

Figure 4 :
Figure 4: Average sulfur dioxide column density distribution in July (2005 -2016)in the troposphere and summer (from June to August) time series of sulfur dioxide in North China, as the area in the box in Figure 1 (unit: DU).

Figure 5 :
Figure 5: Average carbon monoxide column density distribution in July in the troposphere and summer (from June to August) time series of carbon monoxide (unit: 10 18 molec cm -2 ) in North China, as the area in the box in Figure 1.