Identification of long-term evolution of ozone sensitivity to precursors based on two-dimensional mutual verification

Tropospheric ozone pollution has been continuously worsening in China during the past decade. Identification of long-term evolution of ozone sensitivity to precursors is essential to evaluating the impact of emission reduction measures on ozone pollution. Traditional observation-based model and 3-d numerical model are not suitable for analyzing long-term variation of ozone sensitivity to precursors. In this study, by transforming the conventional ozone isopleth plot into a VOCR isopleth plot in the functional space of NOx and ozone concentrations, we developed a novel approach to identify ozone sensitivity to precursors by simply using long-term monitoring data of ozone, NOx and temperature. This approach estimated ozone formation regimes (OFR) by ozone sensitivity to NOx and temperature separately, and the convergence of OFR serves as a way of mutual verification. We found that ozone formation was generally in the VOCR-limited or transitional regime in Shanghai, the largest metropolitan area in China. However, OFR was shifted to NOx-limited at Pudong station during 2017-19 due much to the stringent NOx emission control. OFR was also shifted to NOx-limited along with the increasing temperature. When temperature was over 30 degrees C, Shanghai was mostly in a NOx-limited OFR. This highlights that the NOx emission controlmeasures need to be strengthened to reduce peak ozone levels more efficiently. Jinshan station exhibited a different trend with OFR shifted to VOCR-limited in 2017-19, which proved the effectiveness of VOCs emission control on petrochemical sector. However, OFR was shifted to NOx-limited when temperature was over 30 degrees C, suggesting more stringent VOCs emissions control should be targeted on days with higher temperature. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The study revealed that ozone formation in cities like Shanghai is generally limited by VOCR or NOx, with a shift towards NOx limitation when temperatures rise above 30 degrees Celsius. Strengthening NOx emission control is necessary for more efficiently reducing peak ozone levels.