Mechanisms of Surface Ozone's Chemical Response to High Temperatures: Differences Between Urban and Rural Areas

As climate change leads to more frequent and intense high-temperature events, elevated O₃ episodes during periods of extreme heat have raised widespread concerns. This research investigates how O₃'s chemical response to elevated temperatures varies between urban and rural areas, particularly focusing on different emission conditions defined by ozone precursor regimes.

Simulations are carried out using the UKCA Box Model under idealised meteorological conditions. It runs chemistry-only zero-dimensional experiments in a single grid cell with chemistry relevant to the troposphere and the stratosphere. Using the UKCA Box Model, we simulated conditions typical of summer 2022 at three global hotspots (Yangtze River Delta, England and California), analysing scenarios where O₃ precursors are either limited by VOCs in urban environments or by NO_x in rural settings. The simulations were conducted across a temperature range of 20°C to 40°C while controlling for relevant factors such as photolysis, humidity, emissions, and initial concentrations. To determine the O₃ precursors regimes, photochemical indicators such as NO_y, H₂O₂/HNO₃, H₂O₂/(O₃+NO₂) and HCHO/NO₂ were employed.

The results suggest a significant diversity of O₃’s chemical response to temperature in urban and rural areas. In urban areas characterised by VOC-limited conditions, O₃ levels exhibited a nearly linear increase with rising temperatures. In contrast, rural areas, where O₃ is typically NO_x-limited, displayed a more complex relationship where negative correlations were found. Additionally, humidity emerged as a critical factor influencing these chemical dynamics. The mechanism by which O₃ responds chemically to temperature changes will be examined by analysing O₃ production and destruction budgets.

Our findings highlight that the O₃ precursor regimes are crucial in evaluating the impact of temperature responses on ozone from a chemical perspective. This research contributes valuable insights into the mechanisms driving O₃ responses to temperature changes during extreme heat events. It underscores the importance of considering urban and rural differences in ozone studies and can inform future emission control strategies aimed at mitigating ozone pollution under varying temperature conditions.

Keywords: Surface ozone; Temperature response; UKCA Box model; O₃ precursor regimes; Urban and rural