Background Reactions of ambient particulate matter (PM) with atmospheric radicals and oxidising gases such as nitrogen dioxide (NO₂) and ozone (O₃) may alter their toxicity – often referred to as particle ageing. As concentrations of O₃ dominate in the warmer months, with NO₂ episodes more common in the winter, we investigated whether seasonal differences in the PM oxidative potential (the capacity of inhaled PM to cause damaging oxidation reactions at the air-lung interface) could be attributed to their interaction with these gases.

Methods Daily PM₁₀ and PM_2.5 filters from 4 sites in Southern England were obtained during summer (21^st July – 23^rd August) and winter (6^th Jan–11^th Feb) campaigns in 2012: two sites in London, roadside (Marylebone Road) and urban background (North Kensington) locations, plus two rural sites to the West (Harwell) and East (Detling) of the city. Ascorbate and glutathione-dependent oxidative potential (OP^AA and OP^GSH, expressed per unit mass of PM) were then determined following incubation in a synthetic respiratory tract lining fluid (4 hours at 37^oC, pH 7.0) using a novel on filter methodology.

Results In terms of the OP^AA metric only PM_2.5 measured at the urban background location demonstrated a seasonal increase (p < 0.05) during the summer, with equivalent seasonal OPs across the remaining 3 sites and no differences apparent in PM₁₀. In contrast, PM₁₀ and PM_2.5 OP^GSH was significantly (P < 0.001) lower in the summer period at the urban background site. Summer increases in OP^GSH were observed at the rural sites, with evidence that summer PM_2.5 OP^GSH was increased at the roadside site. This heterogeneous pattern of seasonal contrasts could not be simplistically related to the differences in period, or daily NO₂ or O₃ concentrations.

Conclusions No consistent seasonal pattern in PM₁₀ or PM_2.5 OP was observed across the four sites and the associations between the measured particulate OPs with O₃ and NO₂ at these locations did not fit with the particle ageing hypothesis. Rather high OP days were often found to be associated with wind speed and direction, suggesting that the differences were driven by a combination of long and short range sources.