In recent years, the intricate relationship between urban meteorology and atmospheric chemistry has drawn intense scientific scrutiny, especially given the increasing frequency and severity of compound environmental extremes such as heatwaves coupled with ozone pollution. A groundbreaking study led by Zhou and colleagues ventures deeply into this complex interaction, focusing on the sprawling urban clusters of eastern China, where rapid urbanization meets escalating climate stress. With a robust combination of long-term observation, advanced satellite retrievals, in situ atmospheric profiling, and cutting-edge modeling tools, this research delineates the mechanisms underpinning the co-occurrence of heat and ozone extremes, revealing pivotal insights with profound implications for urban environmental management.
The observational backbone of the study rests on comprehensive datasets, including more than 1,500 ground-based monitoring stations across China that capture nitrogen dioxide (NO₂) and ozone (O₃) concentrations over the past decade. These datasets allow for the analysis of spatial distributions and temporal trends in air quality, bridging the gap between localized observations and broader atmospheric patterns. Complementing these terrestrial records, tropospheric column densities of NO₂ and formaldehyde (HCHO) were derived from the Aura satellite’s Ozone Monitoring Instrument, providing high-resolution (~13 × 24 km²) snapshots that enrich the characterization of photochemical pollution processes over urban landscapes.
Temperature data, a crucial element given the study’s focus on heatwaves, were procured from the state-of-the-art ERA5 reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts. This dataset, with its fine temporal and spatial resolution, enabled calculation of daily maximum temperatures over extensive temporal windows—from historic records spanning 1969 to 2019 to contemporary and near-term future periods extending through 2023 and beyond. Such granularity facilitated the establishment of a dynamic, percentile-based threshold for defining heatwave days, a critical advancement that accounts for spatial and seasonal variability in temperature extremes rather than relying on fixed metrics.
To complement the observational efforts, the research team undertook an ambitious airship campaign over Nanjing, employing a tethered mega-balloon equipped with a suite of sophisticated instruments capable of capturing vertical profiles of key pollutants and meteorological parameters up to altitudes of 1,200 meters. This campaign, conducted during a pivotal warm-season window in 2023, amassed nearly two hundred vertical profiles, providing an unprecedented temporal and vertical resolution of pollutant and atmospheric structure variations during both normal and heatwave conditions. The instrumentation array measured a suite of gases, including ozone, nitrogen oxides (NO and NO₂), and volatile organic compounds (VOCs), alongside meteorological factors such as temperature, humidity, and wind patterns, capturing a holistic view of the atmospheric chemistry and dynamics shaping urban air quality.
Ground-level concurrent measurements were conducted at the nearby SORPES station, a representative urban monitoring site within the Yangtze River Delta region. By integrating continuous surface observations—including trace gases such as sulfur dioxide (SO₂), carbon monoxide (CO), and various VOC species—with the airborne vertical profiling, the researchers assembled a comprehensive three-dimensional depiction of atmospheric composition and the meteorological context, enabling refined interpretations of ozone formation mechanisms and pollutant transport processes under varying thermal regimes.
A notable facet of this study is the application of an observation-based zero-dimensional box model known as the Framework for 0-D Atmospheric Modelling (F0AM). This model, embracing the comprehensive Master Chemical Mechanism (MCM v3.3.1), simulates the intricate gas-phase chemical pathways involving thousands of species and reactions. By constraining the model with real-world measurements of meteorological parameters and ambient chemical species, the team generated detailed sensitivity analyses that elucidate the relative roles of VOCs and nitrogen oxides (NOₓ) in controlling ozone formation. This approach permitted the authors to construct empirical kinetic modeling approach (EKMA) isopleth diagrams, instrumental for identifying chemical regimes and optimal pollution control strategies in highly dynamic urban settings.
Understanding the vertical variability of VOC reactivity—a critical determinant of ozone formation—posed a particular challenge due to limited vertical VOC data. The researchers innovatively bridged this gap by leveraging the ratio of VOC concentrations measured by proton-transfer reaction time-of-flight mass spectrometry aboard the airship at different altitudes. Using surface measurements as a baseline, they extrapolated VOC hydroxyl radical reactivity to higher altitudes, thereby enabling a more accurate assessment of photochemical conditions in the lower troposphere during heatwave episodes.
Recognizing the complex interplay of meteorology and chemistry, Zhou et al. deployed the advanced Weather Research and Forecasting model coupled with Chemistry (WRF–Chem v3.9.1), which integrates physical atmospheric dynamics with comprehensive chemical transformations. This modeling framework was critical to disentangle the contributions of various meteorological and chemical drivers—such as vertical turbulent mixing, temperature-enhanced reaction kinetics, and biogenic VOC emissions—to the exacerbation of urban ozone pollution during heatwave events. By designing parallel numerical experiments differentiating between heatwave and normal conditions, the study quantified the magnitude of ozone enhancements attributable to increased turbulence-driven pollutant redistribution, accelerated photochemical reaction rates under elevated temperatures, and augmented biogenic emissions fostered by heat.
The use of a single-column model (SCM) within WRF–Chem was an elegant methodological choice, allowing isolation of vertical processes independent of complex three-dimensional dynamics. This facilitated a computationally efficient means to conduct sensitivity experiments, capturing key land-atmosphere interactions and boundary layer turbulence effects that often govern pollutant mixing and chemical transformations during extreme heat episodes. Incorporating observed radiosonde soundings to initialize these simulations enhanced their realism and relevance to urban meteorological conditions.
To project the future behavior of compound heatwave and ozone extremes under evolving climate and emissions pathways, the study integrated high-resolution downscaled climate projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) under the SSP 2-4.5 scenario. These projections, bias-corrected and statistically downscaled to 0.25-degree grids, provided robust temperature inputs for identifying future heatwave days up to 2060. Concurrently, anthropogenic emissions scenarios derived from the China-focused Global Change Assessment Model (GCAM) and the Dynamic Projection Model for Emissions in China (DPEC) offered nuanced perspectives on potential air quality outcomes under baseline and ambitious low-carbon, clean-air policy trajectories. This integration of climate and emissions scenarios is a critical advance in anticipating the intertwined challenges of heat and ozone extremes in megacities.
Emission modeling accounted for biogenic VOC sources using the Model of Emissions of Gases and Aerosols from Nature (MEGAN), which dynamically couples vegetation parameters with real-time meteorology to capture temperature-dependent emission responses. This coupling is essential for quantifying heat-induced increases in BVOC emissions, a crucial driver of urban photochemical ozone formation. Soil emissions of nitrogen oxides were similarly incorporated but acknowledged as potentially underestimated given the complex influences of soil moisture and microbial processes that vary with temperature and other variables.
Importantly, model validation exercises demonstrated that WRF–Chem simulations reproduced observed heat and ozone extremes with notable fidelity across spatial and temporal scales, including fine-grained day-to-day variations during the 2023 airship campaign period. Sensitivity analyses comparing different chemical mechanisms further established robustness in future ozone projections, bolstering confidence in the study’s findings and scenario assessments.
The study’s findings illuminate a vicious feedback loop in which intensified urban heatwaves amplify ozone formation through a combination of enhanced photochemical activity, stronger vertical mixing that redistributes pollutants, and increased biogenic emissions, all converging to deteriorate urban air quality in a warming climate. This coupling underscores the necessity of integrating meteorological and chemical considerations in urban environmental management, especially in megacity clusters facing compounding stressors. Notably, the study highlights that aggressive anthropogenic NOₓ and VOC emission controls can significantly mitigate future combined heat-ozone extremes, emphasizing the importance of policy interventions aligned with climate and air quality objectives.
By combining in-depth ground-based and airborne observations with sophisticated modeling frameworks, this research constitutes a milestone in elucidating the complex dynamics of urban atmospheric chemistry under climate change. It reveals how evolving meteorological conditions, in tandem with emission patterns, sculpt the frequency, intensity, and spatial extent of compound heat and ozone pollution events. These insights are paramount as urban populations worldwide confront escalating climate risks and strive for sustainable, healthy living environments.
In conclusion, Zhou et al.’s comprehensive investigation advances our understanding of urban meteorology-chemistry coupling during compound heat-ozone extremes in one of the world’s most rapidly developing regions. The integration of multi-platform observations, sophisticated mechanistic models, and forward-looking scenario analyses offers a blueprint for assessing and managing the health and environmental risks posed by intertwined climatic and air quality challenges. Their work sets a new benchmark for future research and urban climate resilience strategies, making it a must-read for atmospheric scientists, policymakers, and urban planners grappling with the realities of a warming planet.
Subject of Research: Urban meteorology-chemistry interactions driving compound heatwave and ozone pollution extremes in eastern China.
Article Title: Urban meteorology–chemistry coupling in compound heat–ozone extremes.
Article References:
Zhou, X., Li, M., Huang, X. et al. Urban meteorology–chemistry coupling in compound heat–ozone extremes. Nat Cities (2025). https://doi.org/10.1038/s44284-025-00302-1
Image Credits: AI Generated
