Observation Overview

The EXACT (Ensembled Experiments on Atmospheric oxidation Capacity in the Troposphere) program is a comprehensive stereoscopic observational campaign on atmospheric radical chemistry, led by the College of Environmental Sciences and Engineering of Peking University. This program focuses on the key scientific issue of quantitatively characterizing and effectively regulating atmospheric oxidation capacity. It systematically investigates this issue, targeting the Beijing-Tianjin-Hebei (BJJH) region and its surroundings, which face the most severe O₃ and PM₂.₅ pollution problems in China.During 2024-2025, the campaign will conduct intensive, closed-loop field observations of atmospheric radicals and their precursors across all four seasons. These observations will target specific underlying surfaces and address characteristic pollution episodes: severe winter haze, summer ozone pollution, and simultaneous high levels of both pollutants in spring and autumn.By integrating multiple observational platforms—including ground-based stations, tower-based and mountain-top boundary layer vertical profiling, and aircraft surveys—the program synergizes cross-spatiotemporal-scale laboratory kinetic studies and numerical modeling. It further incorporates a suite of modeling tools such as box models, regional air quality models, and global chemical models, the campaign aims to clarify the sources and transformation mechanisms of daytime and nocturnal atmospheric radicals at the molecular level, quantify the composition, intensity, and key sources of atmospheric oxidation capacity in key regions of China, reveal the evolution patterns and driving mechanisms of tropospheric atmospheric oxidation capacity and atmospheric self-cleaning capacity, and propose chemical principles for the coordinated response to secondary pollution and climate change centered on the regulation of atmospheric oxidation capacity and the enhancement of atmospheric self-cleaning capacity.

Research Background

Since the implementation of the "Air Pollution Prevention and Control Action Plan", China has achieved rapid improvement in PM2.5 pollution, but O3 pollution has not decreased and has instead increased, still not yet entering an effective control track. O3 pollution prevention and control is an international challenge and remains a focus for air quality improvement in Europe and the United States. Simultaneously, the continuous rise of greenhouse gases like CH4 is accelerating global warming. Conducting coordinated prevention and control of short-lived climate forcers such as O3, PM2.5, and CH4 is a major medium- to long-term strategic national need. Atmospheric radicals determine the production of tropospheric O3 and PM2.5 and the removal of CH4. Research on atmospheric oxidation capacity and self-cleaning capacity, with atmospheric radical chemistry as its scientific core, is the key theoretical foundation for achieving the synergy between pollution reduction and carbon emission reduction. However, current research on atmospheric radical chemistry faces several major shortcomings: 1) Radical chemistry research relies on the direct and accurate measurement of atmospheric radicals. Limited by a lack of measurement techniques, most current studies focus on single radical species, lacking closed-loop observational campaigns and closure studies targeting full-spectrum atmospheric radical chemistry, making it difficult to capture the interconnected full-chain radical reaction processes. 2) Current observational studies are mainly concentrated in summer and at ground level where photochemistry is strong. There is a lack of systematic research on key radical chemical mechanisms and controlling factors across different seasons, different underlying surfaces, and different heights in polluted regions, making it difficult to support comprehensive studies of atmospheric radical chemistry across seasons and throughout the boundary layer. 3) Existing atmospheric chemical mechanisms were primarily developed and established by European and North American countries based on cleaner atmospheric environments. These mechanisms struggle to accurately describe the complex atmospheric radical chemistry processes in megacity regions. There is currently a lack of closure studies at the molecular level on the sources and transformation mechanisms of different RO2 radicals. 4) Current research also lacks model studies related to regional-urban scales and long-term trends in atmospheric oxidation capacity, and their synergistic consideration with atmospheric self-cleaning capacity. Therefore, there is an urgent need to carry out systematic research on the evolution patterns of atmospheric oxidation capacity and the driving mechanisms of atmospheric self-cleaning capacity in key regions of China, to provide key scientific support for the coordinated prevention and control of O3, PM2.5, CH4, and other greenhouse gases in China.

Scientific Objectives

1.Establish the world's most comprehensive observational dataset of atmospheric radicals.

2.Identify key chemical processes of atmospheric radicals and develop a universally applicable global atmospheric chemical mechanism to enhance the accuracy of model simulations.

3.Assess the evolution trend of the global atmospheric self-cleaning capacity and accurately predict the concentration changes of greenhouse gases such as CH4 and HFCs.

4.Identify the dominant factors controlling regional atmospheric oxidation capacity and propose governance principles for regulating atmospheric oxidation capacity to support regional air quality improvement.

Research Content

1. Intercomparison Experiments for Full-Spectrum Atmospheric Radical Measurement Instruments

Building upon the core measurement systems for OH, HO2, RO2, NO3, and Cl radicals and their key precursors independently developed by the team, systematically optimize instrument performance—including temporal resolution, detection limits, and measurement uncertainties—to meet the research requirements for achieving closed-loop observation of atmospheric radicals. Based on field observations and outdoor smog chamber experiments, conduct intercomparison experiments for various atmospheric radical and reactive precursor measurement instruments under typical air mass conditions. Implement rigorous quality control of the measurement data to ensure the consistency and accuracy of instrument responses during multi-site synchronous observations.

2. Closure Studies on the Sources, Cycling, and Sinks of Atmospheric Radicals

Aiming to address issues such as the underestimation of atmospheric oxidation capacity on a regional scale and missing radical sources during heavy air pollution episodes, and based on the research concept of full-chain radical closure, a closed-loop radical observation platform centered on the measurement of OH, HO₂, RO₂, NO₃, and Cl radicals will be established. This platform will encompass the entire process of radical initiation, cycling, and termination. Focusing on the key Beijing-Tianjin-Hebei region, 12 closed-loop field observation experiments for atmospheric radicals will be conducted across the four seasons (spring, summer, autumn, and winter) at monitoring sites representing three environmental types: urban, regional, and background. High-quality observational datasets of radicals and key precursors, each with a continuous duration of over one month per campaign, will be obtained. Full-chain closure studies on atmospheric OH, HO₂, RO₂, NO₃, and Cl radicals and their reactive precursors will be carried out to test and develop the latest international atmospheric radical chemical mechanisms. The sources, cycling, and sink mechanisms of daytime and nocturnal radicals near the ground will be accurately quantified at the molecular level, and key radical reaction processes and dominant controlling factors during typical pollution episodes in different seasons will be identified.

3. Mobile Laboratory Observations for Stereoscopic Profiling

During the closed-loop field observation campaigns for atmospheric radicals, typical PM₂.₅ and O₃ pollution episodes will be selected to conduct mobile laboratory observations between the three monitoring sites. These observations will involve real-time measurements of parameters such as temperature, humidity, wind speed, atmospheric pressure, and trace gas concentrations. The aim is to precisely and dynamically track atmospheric pollution processes, providing critical data and technical support for pollution attribution analysis.

4. Key Sources and Transformation Mechanisms of Atmospheric Radicals in the Boundary Layer

Aiming to address the challenge of achieving vertically-resolved closure observations of radical budget processes within the atmospheric boundary layer, a technical system for vertical profiling of atmospheric oxidation capacity will be established, utilizing coordinated multi-platform observations including tall towers, mountain stations, tethered balloons/airships, and ground-based remote sensing. Based at urban and rural sites, vertical observation experiments encompassing atmospheric radicals, meteorological parameters, and chemical components will be conducted during typical seasons such as summer and winter. These campaigns will systematically characterize the vertical distribution features of radicals and their key precursors. The vertical variations in the budget processes of daytime and nocturnal OH, HO₂, RO₂, NO₃, and Cl radicals, along with their key precursors, will be elucidated, leading to the development of a parameterization scheme for the vertical structure of atmospheric oxidation capacity. A novel methodology for model simulation of radical gradient observation data will be developed to quantitatively resolve and identify the key radical production and loss processes in different atmospheric layers under varying stability conditions. The coupling mechanisms of daytime and nocturnal radical chemistry across vertical scales will be clarified. Finally, the contribution of atmospheric oxidation capacity to the formation of secondary pollutants like ozone and nitrate within the boundary layer, along with the dominant controlling factors, will be assessed.

5. Conducting Demonstrative Supersite Observations of National-Scale Atmospheric Oxidation Capacity to Quantitatively Characterize the Atmospheric Oxidation Capacity and Self-Cleaning Capacity of Typical Urban Agglomerations

Aiming to address the quantitative analysis of the causes and sources of widespread summertime ozone pollution across China, demonstrative national-scale supersite observations of atmospheric oxidation capacity, centered on closed-loop measurement of atmospheric radicals, will be conducted. These observations will leverage the team's long-term operational supersites in key regions including the Beijing-Tianjin-Hebei region, the Yangtze River Delta, the Pearl River Delta, and the Chengdu-Chongqing region (e.g., supersites at Peking University, Shanghai Academy of Environmental Sciences, Hefei Institutes of Physical Science, Peking University Shenzhen Graduate School, Sun Yat-sen University, and Chengdu Academy of Environmental Sciences). Building upon this demonstrative observation network, a national atmospheric oxidation capacity observation-modeling platform and data center will be established to promote the operationalization of atmospheric oxidation capacity monitoring. The spatiotemporal evolution patterns of OH, HO₂, RO₂, NO₃, and Cl radicals will be systematically extracted, comparing the similarities and differences in radical concentration variations across different sites and seasons. The dominant controlling factors influencing radical concentration changes across different timescales (minute-to-hour, diurnal, pollution episode, and seasonal) will be identified. Based on the synchronized radical observation results, the composition, sources, dominant controlling factors of atmospheric oxidation capacity, and the drivers of atmospheric self-cleaning capacity in China's typical urban agglomerations will be systematically quantified.

6. Model Quantification of the Evolution of Atmospheric Oxidation Capacity and Self-Cleaning Capacity

An air quality model for quantitatively analyzing atmospheric oxidation capacity will be constructed by integrating field observations, kinetic experimental data, and satellite retrieval products. Research will be conducted to quantify the main components, trends, and driving factors of atmospheric oxidation capacity and self-cleaning capacity. This will include quantifying the oxidative contributions driven by OH, HO₂, RO₂, NO₃, and Cl radicals under different temporal and spatial conditions. Observation-based models will be utilized to estimate atmospheric radical concentrations and investigate their long-term evolution trends and seasonal distribution characteristics. Furthermore, retrieval datasets for major atmospheric components influencing atmospheric oxidation capacity will be independently developed based on domestic and international multi-source satellite remote sensing. The spatiotemporal variation patterns of these key components affecting atmospheric oxidation capacity will be clarified. Finally, the study aims to reveal the dominant controlling factors of atmospheric oxidation capacity and self-cleaning capacity during different pollution episodes in key regions of China.

Initiating Team

EXACT currently brings together outstanding scientists from top domestic universities and the Chinese Academy of Sciences in the field of atmospheric oxidation capacity, with future plans to engage in global collaboration with world-leading international teams in the field.

Steering Committee: Zhang Yuanhang, Liu Wenqing, Chen Jianmin, Wang Yuesi, Steven Brown, Andreas Wahner, Dwayne Heard

Core Members:Lu Keding (Principal Investigator), Wang Weigang, Liu Cheng, Yuan Bin, Zhang Lin, Wang Hongli, Tan Zhaofeng, Kajii, Hu Renzhi, Lou Shengrong, Wang Haichao, Tong Shengrui, Zhao Weixiong, Ma Xuefei, Liu Zirui, Tang Guiqian, Tan Yujun, Dong Huabin, Chen Shiyi, Zheng Jun, Lu Xiao, Chen Xiaorui, Feng Miao, Zhang Chengxin