Vehicle Tailpipe Emissions, Secondary Aerosol Formation and Pertinent Health Implications

Introduction: Road transport emissions are key sources of primary exhaust pollutants and secondary organic aerosol (SOA) precursors, with important health implications in photochemically active regions. The Eastern Mediterranean, characterised by an aging diesel fleet, high vehicle ownership, and intense solar radiation, provides a suitable case study. Therefore, this study aimed to quantify traffic emissions and their contribution to cardiovascular and respiratory mortality in Cyprus.

Methods: Daily mortality data (2012–2023), hourly pollutant concentrations (PM_2.5, PM₁₀, NOₓ, SO₂, CO, O₃, VOCs, C₆H₆; 1990–2023), traffic counts, and national fleet statistics were collected for two urban cities and one rural area in Cyprus. The COVID-19 lockdown served as a natural experiment to isolate transport-related emission changes. Primary exhaust emissions were estimated using COPERT software with local fleet composition, activity data, and fuel sales as inputs. Secondary PM_2.5 was quantified using three cross-validated approaches: literature-derived secondary emission factors applied to fleet data, precursor-to-particulate conversion ratios, and a bottom-up fuel-based formulation. Health impacts were estimated using concentration–response functions linking annual mean pollutant levels to mortality.

Results: PM_2.5 concentrations declined significantly until 2014 but plateaued thereafter, while indicators of SOA formation increased. During the 2020 lockdown, traffic reductions (13–62%) corresponded to decreases of up to 86% in NOₓ, 61% in VOCs, and 48.5% in O₃. Secondary aerosol formation contributed approximately 86 t of road-traffic-related PM_2.5 in 2020. NO₂ exposure was associated with the highest cardiovascular mortality burden (~100 deaths per 100,000 inhabitants per 10 μg/m³). Approximately 43% of PM_2.5-related premature cardiovascular deaths were attributable to secondary PM_2.5. Limiting annual mean PM_2.5 to 12 μg/m³ was associated with a 42% reduction in cardiovascular mortality.

Conclusion: Secondary aerosol formation substantially amplifies the health burden of road transport emissions, highlighting the need for integrated primary and precursor emission control strategies.