Abstract
In the austral spring, biomass fires affect a vast area of South America each year. We combined in situ ozone (O-3) data, measured in the states of Sao Paulo and Parana, Brazil, in the period 2014-2017, with aerosol optical depth, co-pollutants (NOx, PM2.5 and PM10) and air backtrajectories to identify sources, transport and geographical patterns in the air pollution data. We applied cluster analysis to hourly O-3 data and split the investigation area of approximately 290,000 km(2) into five groups with similar features in terms of diurnal, weekly, monthly and seasonal O-3 concentrations. All groups presented a peak in September and October, associated with the fire activities and enhanced photochemistry. The highest mean O-3 concentrations were measured inland whilst, besides having lower concentrations, the coastal group was also associated with the smallest diurnal and seasonal variations. The latter was attributed to lower photochemical activity due to frequently occurring overcast weather situation. The mean annual regional contribution of O-3 over the area was 61 mu g/m(3), with large seasonal and intersite variabilities (from 35 to 84 mu g/m(3)). The long-range transport of smoke contributed with between 23 and 41% of the total O-3 during the pollution events. A pollution outbreak in September 2015 caused many-fold increases in O-3, PM2.5 and PM10 across the investigation area, which exceeded the World Health Organisation recommendations. We show that the regional transport of particulates and gas due to biomass burning overlays the local emissions in already highly polluted cities. Such an effect can outweigh local measures to curb anthropogenic air pollution in cities.