Analysis of tropospheric ozone concentration and their predictors in mainland Portugal

Abstract

Tropospheric ozone is a secondary air pollutant with a photochemical origin, commonly resulting from nitrogen oxides and volatile organic compounds in the presence of short-wave radiation. Portugal has high solar radiation exposure and, consequently, near-surface ozone levels tend to reach, or even exceed, the threshold defined by the European Union air quality warning system during summer. Furthermore, episodes with high radiation, temperature, and ozone concentration values are expected to increase owing to more recurrent and persistent heatwaves and wildfires driven by climate change. Exposure to high ozone concentrations has negative effects on human health and vegetation. The present study aims to understand the atmospheric patterns associated with high ozone levels and identify the main ozone precursors on mainland Portugal. The analysis was carried out for daily ozone concentrations averaged between 0900 and 2100 UTC, from 2004 to 2022. A stepwise regression analysis was applied to the potential predictors: fire radiative power, thermal amplitude, maximum temperature, boundary layer height, NO₂ concentration, radiation, and time of the year. The obtained model shows high accuracy and robustness, even when tested with the Monte Carlo method, and indicates that the predictors have a joint and different contribution to ozone formation across the Portuguese regions. The variables that have greater coefficients as a response from the stepwise regression model were temperature and fire radiative power, with fire radiative power being more significant for Norte and Centro regions, which are the most affected by wildfires. Radiation and temperature are more relevant in the Alentejo and Sul regions, where radiation and temperatures are climatologically higher.