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 PM2.5 filters from 4 sites in Southern England were obtained during summer (21st July – 23rd August) and winter (6th Jan–11th 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 (OPAA and OPGSH, expressed per unit mass of PM) were then determined following incubation in a synthetic respiratory tract lining fluid (4 hours at 37oC, pH 7.0) using a novel on filter methodology.
Results In terms of the OPAA metric only PM2.5 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 PM2.5 OPGSH was significantly (P < 0.001) lower in the summer period at the urban background site. Summer increases in OPGSH were observed at the rural sites, with evidence that summer PM2.5 OPGSH 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 PM2.5 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.