Ultrafine particles in the urban area of Rome

Measurements of particle number size distribution in the range 3-117 nm have been made in a street characterized by high density of autovehicular traffic in downtown Rome, in the period April-May 2008. Hourly averaged bimodal or trimodal size distributions were measured. Total particle number concentration followed the same trend as carbon monoxide, evidencing their same origin associated to autovehicular traffic. Differences in the trends of the two pollutants were observed during period of high solar radiation, when total particle number concentrations reached peak values and the number median mobility diameter dropped, due to the formation of particles in the nucleation mode from the gas phase. Nitrous acid measurements evidenced the importance of radical photooxidative activity, since particle formation in the nucleation mode appeared to be favoured when the rate of photolysis was maximal and availability of radical OH was greater.