Compound heatwave-stagnation episodes amplify tropospheric ozone and fine particulate matter attributable mortality across the Mediterranean

The Mediterranean basin is one of the regions of the world most exposed to the joint pressure of rising temperatures and persistent tropospheric ozone (O₃) and fine particulate (PM2.5) air pollution. Heatwaves (HW), atmospheric stagnation (STG) and their compound occurrence (CE) are known to influence both pollutant accumulation and human vulnerability. However, the share of the population health burden that can be attributed specifically to each type of day, and to their compound occurrence, has not yet been quantified in a consistent way across the basin.

A high-resolution, end-to-end attribution framework for tropospheric (O₃) and PM₂.₅ mortality is presented under mutually exclusive daily event categories (no-event, HW-only, STG-only and CE) over the Mediterranean basin for the period 2013-2025. Air-quality fields are taken from the CAMS European reanalysis; meteorology from ERA5. HW are defined as maximum temperature exceeding the day-of-year 90^th percentile climatology for at least four consecutive days, and STG follows the Wang and Angell (1999) wind and precipitation thresholds. Concentration-response functions include a log-linear model, two non-linear alternatives (saturating and supralinear-log) and the Global Exposure Mortality Model (GEMM,) for PM2.5. Population fields are derived from the GHSL GHS-POP R2023A product and merged with Eurostat NUTS-3 demographics. Baseline mortality rates combine Eurostat NUTS-3 records with UN World Population Prospects 2024 single-age data, so that countries outside the European Union are also covered.

Over the wide Mediterranean (around 125 million inhabitants), O₃ concentrations on CE days reach 102 µg m^-3 (+38 % above no-event days), with annual O₃-attributable premature deaths  estimated at 2,500 (95 % CI 660–3,630) on CE days, versus 900 (95 % CI 240–1,320) on HW-only days and 820 (95 % CI 180–1,260) on STG-only days. The Relative Excess Risk due to Interaction (RERI), which quantifies whether the joint health effect of heatwaves and stagnation exceeds the sum of their individual effects on an additive risk scale, and the Non-Linear Response Effect (NLRE; Gao et al., 2020), which measures whether pollutant concentrations during CE depart from a linear addition of the HW-only and STG-only responses, displayed heterogeneous spatial patterns. Nonetheless, a localised super-additive behaviour was found over densely populated zones, with positive interaction values over 39% of grid cells in the Mediterranean region, implying that these local hotspots suffer strongly enhanced CE impacts. Alternative temporal definitions of CE further showed that the estimated burden is sensitive to how synchrony between HW and STG is defined: allowing STG to occur within ±1 or ±3 days of a HW increased CE day counts by factors of 1.22 and 1.32, respectively, and raised O₃-attributable mortality from 2,500 deaths yr⁻¹ under the same-day definition to 3,200 and 3,440 deaths yr⁻¹. These results provide a quantitative and reproducible baseline for compound heat-pollution risk in the Mediterranean, and support the design of integrated heat and air-quality early-warning systems for the region.