Effect of springtime thermal forcing over Tibetan Plateau on summertime ozone in Central China during the period 1950-2019

In recent years, ozone pollution has attracted wide attention from the scientific community, policymakers and the public. Previous studies have already confirmed the effects of meteorological conditions on regional ozone pollution events in the last two decades. However, due to the lack of historical ozone observations, the relationship of climate variability and ozone pollution has been less discussed. In our study, we use the NCEP/NCAR Reanalysis data and modeled historical surface ozone from CMIP6 (Coupled Model Intercomparison Project Phase 6) to explore the impact of climate variability related to Tibetan Plateau on summertime (June, July and August) ozone in Central China during 1950-2019. The results show that in interdecadal timescale, the annual summertime ozone over Central China is significantly correlated (R = -0.52; P-value <0.05) with the springtime (March, April and May) thermal forcing, indicated by total atmospheric energy (TPE), over Tibetan Plateau. In high TPE years, the East Asian monsoon (southerly wind flow) and vertical ascending motion have been enhanced in summer. It is conducive to both the diffusion of ozone and its precursors and occurrence of cloudy and high humidity weather. These processes lead to lower ozone concentrations up to 6.97 ppb (compared with values averaged during 1950-2019) over Central China. On the contrary, in low TPE years, summertime surface ozone concentrations have been increased by 2.96 ppb (the mean value is 49.00 ppb and the standard deviation is 11.75 ppb for 1950-2019) on average of Central China, due to the weakened East Asian monsoon and weakened vertical ascent, thus the stable weather with low humidity and small near surface wind. After the year 2000, the TPE has increased with a rapid growth rate of 3 x 104 hPa J kg-1 yr-1. However, similar to the reported surface ozone increments based on the modeling results and observations, the CMIP6 summertime surface ozone has shown an increasing trend in the last two decades (0.45 ppb yr-1 and 0.25 ppb yr-1 over Central China and entire China during 1950-2019, respectively). While a decline trend (0.18 ppb yr-1) of ozone from 1980 to 2019 over Central China is calculated after the removal of emission-attributed trend. These results indicate that the ozone increasing trend dependent on the changes of precursor emissions has offset the ozone mitigation attributed to the climate.

Automated summary

The study found that on an interdecadal timescale, summer ozone concentrations in Central China are significantly correlated with spring thermal forcing over the Tibetan Plateau. High thermal forcing leads to lower ozone concentrations due to enhanced monsoon and vertical ascent, while low thermal forcing results in increased ozone concentrations. The study also noted that while total atmospheric energy has been increasing, summertime ozone concentrations have shown an increasing trend over the past two decades, partially offsetting the ozone reduction attributed to climate factors.