Sub-mm particle size distributions in a suburban Mediterranean area. Aerosol populations and their possible relationship with HONO mixing ratios

This study presents, for the first time, long-term data of sub-mm particle number size distributions in the particle size range of 14e670 nm in the Mediterranean area of Rome. Data are analysed in terms of possible aerosol populations. A particular “event” including also measurements of nitrous acid mixing ratios in the atmosphere is further analysed. High statistical correlations are established: At daytime, between the unknown HONO formation rate and the product of NO2, JNO2 , relative humidity, and sub-mm particle surface area concentration. At nighttime, between the volume concentration of the small Aitken mode particles and the HONO mixing ratios. The study would provide information on fine and ultrafine aerosol particle size distribution down to 14 nm in the Mediterranean suburban area of Rome. Besides, it would support previous works which have already provided several evidences for the existence of relations between the sub-mm aerosol populations and HONO. Our conclusions include speculations on possible daytime HONO formation mechanisms on particle surfaces in the accumulation mode size range, and nighttime mass formation mechanisms in common for the small Aitken mode particles and HONO.