LAUSR

Abstract Particle size distributions in the 10–400 nm diameter size range were measured at a site south of Mexico City, during 30 non-consecutive days from October to December 2016 using a Scanning Mobility Particle Sizer, together with meteorological parameters and criteria pollutants concentration. Particles were classified into three modes depending on their diameter: D ‾ p ≤ 20 nm (M1, nucleation mode), 20 > D ‾ p > 100 nm (M2, Aitken mode), and D ‾ p ≥ 100 nm (M3, accumulation mode) to describe their concentration variability and discuss its connection with meteorology and atmospheric chemistry. In general, M1 and M3 concentrations showed maximum variability during the sampling period and were linearly anticorrelated. Concentrations of M3, black carbon, and PM2.5 showed a similar trend, which indicates that M3 represents the largest fraction of the particle mass concentration. The concentration fraction of M2 was relatively constant. The three modes showed a primary character during peak traffic hours in the morning; M2 also showed an important secondary character during the hours of highest photochemical activity. We only observed one nucleation event, which coincided with an unusually high concentration of SO2. On the other hand, we identified several days with typical condensational growth behavior at noon, and sometimes in the afternoon, associated with the highest values of O3, SO2, NO2, and PM2.5 concentrations, as well as of radiation and temperature.