Increasing trends (2001–2018) in photochemical activity and secondary aerosols in Santiago, Chile

Despite the decline in partially (PM₁₀) and fully (PM_2.5) inhalable particles observed in recent decades, Santiago in Chile shows high levels of particle and ozone pollution. Attainment plans have emphasized measures aimed at curbing primary and, to some extent, secondary particles, but little attention has been paid to photochemical pollution. Nevertheless, ozone hourly mixing ratios in Eastern Santiago regularly exceed 110 ppbv in summer, and in winter maximum mixing ratios often reach 90 ppbv. Moreover, the sum of ozone and nitrogen dioxide shows an increasing trend of more than 3.5 ppbv per decade at 5 out of 8 stations. This trend is driven by increasing NO₂, possibly associated with increasing motorization but also with changes in photochemistry. To estimate the fraction of secondary particles in PM_2.5 and due to the lack of long-term speciation data for particles, we use carbon monoxide as a proxy of primary particles and ozone daily maxima as a proxy for secondary particle formation. We find a growing fraction of secondary particles due to an increase in the oxidizing capacity of Santiago’s atmosphere. This stresses the need for new curbing measures to tackle photochemical pollution. This is particularly needed in the context of a changing climate.