Nucleation and growth of new particles in the rural atmosphere of Northern Italy—relationship to air quality monitoring

This study investigates the relationship between aerosols number size distribution on the one hand, and air quality in terms of particulate matter (PM) mass concentrations (as usually monitored in the air quality networks) on the other hand. For this purpose, time series of trace gases levels, submicron aerosol size distributions, both recorded at a rural site in Northern Italy (ISPRA), and of trace gas levels and PM mass concentrations, recorded in the air quality network operating in this region, have been compared and interpreted. Because of the regional nature of the PM pollution events, the daily mean levels of the aerosol volume (V), surface area (S) and black carbon (BC) concentrations at ISPRA rural site are well correlated with the daily mean levels of PM mass concentrations recorded at the other air quality monitoring sites. At ISPRA, the submicron aerosol size distribution is strongly influenced by two main competing processes: nucleation of new particles and condensation of gas-phase components onto pre-existing particles (resulting in particles growth). These processes influence on the daily, seasonal and day-to-day variations of the submicron aerosol features. Because increasing aerosol S concentrations favour condensation and hinder nucleation (and vice versa) the ‘mean’ particle size DpN (mode of the dN/dlog D size distribution) increases with increasing PM concentrations (e.g. 45 nm for V=4μm3cm-3 and 110 nm for V=45μm3cm-3). Owing to this, time series of aerosol DpN   and VV, SS, mass and BC concentrations are strongly anti-correlated with those of the smallest ultrafine particle number concentration (N, 5–10 and 10–20 nm). Nucleation episodes occur under the clean air conditions prompted by the North-Föehn meteorology. This anti-correlation between submicron aerosol mass and N<20 nm concentrations (prompted by the low contribution of the ultrafine particles to the aerosol mass) has important implications for a proper air quality monitoring: the parameters classically used for the air quality assessing (e.g. PM2.5) are not suitable for monitoring of this ultrafine PM pollution and consequently a specific monitoring of the ultrafine PM number concentration should be performed. The significance of this specific ultrafine PM number concentration monitoring is supported by facts already proven: a significant fraction of the current urban PM emissions occurs in the ultrafine PM fraction and exposure to ultrafine PM is associated with adverse effects on human health.