Seven Years of Data Reveal How Ground-Level Ozone Shifted Across Henan

Ground-level ozone in Henan Province rose sharply through the mid-2010s, peaked in 2018, and then declined and stabilized, according to a new analysis of seven years of real-time monitoring data published in the journal Discover Atmosphere. The study, which drew on hourly measurements from the China National Environmental Monitoring Centre covering all 17 prefecture-level cities in the province except Jiyuan, found that the annual average ozone concentration climbed steadily from 2015 to a maximum of 124 micrograms per cubic meter in 2018, before easing and levelling off by 2021.

The researchers, led by Jiandong Shang and colleagues, also tracked how often ozone breached China’s air quality standard. Standard exceedance days were most numerous in 2019, when the province recorded 1,609 days on which ozone exceeded the limit, corresponding to a standard exceedance rate of 26.00 percent. By contrast, 2015 saw the fewest exceedance days, 778 in total, at a rate of 12.71 percent. The daily maximum eight-hour moving average ozone concentration was used as the daily evaluation indicator, in line with China’s Ambient Air Quality Standards, with concentrations above 160 micrograms per cubic meter classified as exceeding the standard.

Seasonal patterns were pronounced. Ozone concentrations ranked from highest to lowest as summer, spring, autumn, and winter across the study period. Summer 2018 recorded the highest seasonal average of the entire seven years, at 173 micrograms per cubic meter, while the lowest seasonal value was 61 micrograms per cubic meter in the winter of 2015. Monthly concentrations followed a unimodal, irregular inverted-V shape: values climbed from January to a June peak, declined slowly through September, where a secondary peak sometimes appeared, and then fell to their December minimum. June 2018 produced the highest monthly reading of the whole period, 205.5 micrograms per cubic meter, and the average June concentration across the seven years was 179 micrograms per cubic meter, against a December average of 55 micrograms per cubic meter.

Geographically, the analysis revealed persistent contrasts between regions of the province. Western Henan consistently recorded low ozone concentrations, while Central and North Henan were higher, and East and South Henan moved in parallel with each other. In 2015, a high-concentration zone appeared in Central Henan, with Pingdingshan at 117 and Luohe at 121 micrograms per cubic meter, while Anyang in the north recorded the province’s lowest value at 83 micrograms per cubic meter. By 2018, concentrations had risen across the board, ranging from 118.1 to 129.2 micrograms per cubic meter province-wide, and areas that had previously been relatively clean, in East, South, and North Henan, became high-concentration zones. North Henan led that year, with Anyang at 126, Jiaozuo at 128, Hebi at 127, Xinxiang at 129, and Puyang at 128 micrograms per cubic meter. From 2019 to 2021, concentrations fell in all cities.

The study also examined the 90th percentile of the daily maximum eight-hour average, known as O3-8 h-90%, as the annual evaluation indicator. This measure was highest across all cities in 2017, when Xinxiang recorded the provincial maximum of 222 micrograms per cubic meter, and lowest overall in 2015, when Zhoukou registered 133 micrograms per cubic meter. Zhengzhou, Anyang, and Jiaozuo all reached 220 micrograms per cubic meter in 2017. The ranking of years by this indicator, from highest to lowest, was 2017, 2018, 2019, 2020, 2021, and 2015, broadly mirroring the annual average concentrations.

Exceedance statistics varied widely between cities. Jiaozuo recorded the highest single-year figures, with 124 standard exceedance days and a rate of 33.97 percent in 2019, while Zhoukou had the lowest, with just 3 exceedance days and a rate of 0.82 percent in 2015. Over the full seven years, Zhengzhou, Jiaozuo, and Xinxiang topped the cumulative exceedance tally, with Zhengzhou highest at 654 exceedance days, while Xinyang, Zhoukou, and Sanmenxia had the fewest, Xinyang lowest at 334 days.

Beyond describing concentrations, the study applied spatial autocorrelation analysis, using global and local Moran’s I statistics with ArcGIS and GeoDa, to test whether ozone levels in neighboring cities cluster together. The results showed a clear evolution over the seven years: from 2015 to 2018 the annual ozone concentrations displayed a weak negative spatial correlation, and from 2018 to 2021 a strong positive one, with 2018 as the watershed. The autocorrelation in 2019 was weak, which the authors suggest may relate to the Three-Year Action Plan for Pollution Prevention and Control in Henan Province issued at the end of 2018, under which cities adopted differing pollution control plans suited to their areas. Local Moran’s I values for all seven years passed the significance test at the 5 percent level.

The cluster analysis pinpointed where high and low values grouped. High-value ozone areas concentrated in Central and North Henan, while low-value areas clustered in West and South Henan. The negative correlation was strongest in 2017, when high-high value areas appeared in Jiaozuo and Zhengzhou, with annual averages of 122 and 121 micrograms per cubic meter respectively, the province’s highest that year, while Sanmenxia in West Henan formed a low-high area at 113 micrograms per cubic meter, far below its surroundings. The positive correlation was strongest in 2021, when Xinxiang in North Henan formed a high-high area with an annual average of 115 micrograms per cubic meter despite sitting only in the second concentration echelon, because its neighbors were in the first. Low-low areas that year lay across South Henan, with Nanyang, Xinyang, and Zhumadian at 106, 107, and 108 micrograms per cubic meter respectively, and Luohe in Central Henan stood out as a high-low area at 112 micrograms per cubic meter, a pattern the authors link to rising industrial and vehicle emissions, wind direction, and the urban heat island effect.

Seasonal spatial patterns shifted year by year. Spring showed the greatest change in spatial aggregation over the seven years, with no consistent distribution of high or low values. In summer, North Henan was generally the high-value area and South Henan the low-value area; in the summer of 2018, six cities, Anyang, Hebi, Puyang, Xinxiang, and Jiaozuo in the north plus Zhengzhou in the center, recorded the highest concentrations, while in the summer of 2020 five cities, Nanyang, Zhumadian, Xinyang, Luohe, and Zhoukou, formed low-low clusters. Autumn’s low-concentration zone lay in West Henan and its high zone in East Henan, with Zhumadian and Zhoukou forming high-high areas. Winter reversed the summer pattern, with South Henan generally high and North Henan low, and Zhumadian forming a high-high winter cluster. The authors conclude that ozone pollution in the province has followed a certain regularity over the seven years, but that because spatial characteristics differ by time and place, each city should formulate its own ozone prevention and control measures according to local conditions.

Subject of Research: Spatiotemporal variation of near-ground ozone pollution in Henan Province, China, from 2015 to 2021

Article Title: Spatiotemporal variation of near-ground ozone in Henan province 2015–2021

Article References: Shang, J., Bao, H., Guo, H., Wang, L., Wang, N., & Zhang, D. (2026). Spatiotemporal variation of near-ground ozone in Henan province 2015–2021. Discover Atmosphere, 4(1), Article 25. https://doi.org/10.1007/s44292-026-00090-w

Image Credits: AI Generated

DOI: 10.1007/s44292-026-00090-w

Keywords: ozone, Henan Province, air quality, spatial autocorrelation, Moran's I, China National Environmental Monitoring Centre, seasonal variation, NOx, VOCs, LISA cluster analysis, ground-level ozone, Spatiotemporal