Abstract

Peroxyacetylnitrate (PAN) and ozone are considered typical photochemical secondary pollutants and, as such, are expected to reach higher concentrations in periods of high irradiance (summer). However, in Santiago, PAN highest values are reached in autumn. The average of daily maximums are: 4.0 ± 1.7 ppb in autumn (May and June); 0.9 ± 0.5 ppb in winter (July and August); 1.9 ± 1.0 ppb in spring (October and November); 3.1 ± 1.4 ppb in summer (January and March). In autumn the integrated daily values are also considerably higher than in other periods (averages of 3 8 ppb h in May and 13 ppb h in January). This behavior is related to a lower PAN's thermal stability in summer. This factor, together with an increased atmospheric stability and lower height of the mixing layer in autumn explains the high levels measured in periods of relatively low solar irradiance.