Intensive measurements of PM above the city of Naples (Southern Italy)
A super-site for the measurement of atmospheric pollutants from urban sources has been established in Naples (Campania, Southern Italy), where the complex layout of the coast and surrounding mountains favours the development of combined sea breeze upslope winds and the evolution of return flows with several layers of pollutants and subsidence. With a population of circa 4 million (around 1 million within the city), the metropolitan area of Naples has one of the highest densities in Europe, with consequent impacts in terms of emissions associated with diesel/gasoline exhaust, industrial emissions, agricultural burning and waste disposal problem. At the super-site, an eddy covariance tower has been installed on the rooftop of the Meteorological Observatory of Largo San Marcellino, situated in the historical city centre: a fast response ultrasonic anemometer (Gill WindMaster) has been mounted on a 10-m mast, alongside three insulated inlet lines through which the air is sampled for gaseous pollutants and particulate matter. The height of the terrace is on average 35 m above the irregular street level, resulting in an overall measuring height of 45 m. Size segregated aerosol are measured by gravimetric method at low frequency (hourly to daily) through a SWAM 5A Dual Channel (PM10 and PM2.5, FAI Instruments). A faster optical particle counter (4 Hz, FAI Instruments) allows the estimate of particle numbers of 22 classes of particles diameter, from 0.28 to 10?m; it also provides continuous concentration of fine and coarse fractions through an internal algorhythm, tuned with the gravimetric measurements. The fast analyser outputs are synchronised with the sonic anemometer through a common acquisition at 10 Hz using a CR3000 datalogger (Campbell Scientific). A full weather station is also available as ancillary measurements at the site including two webcams to record exceptional events to aid interpretation of the results. The fluxes are representative of varying footprint areas, covering the historical centre of Naples, the harbour, and some main traffic arteries of the city. Preliminary results show that in the period between 8 December 2014 and 8 January 2015, daily PM10 and PM2.5 levels kept below 59 ?g/m3 and 48 ?g/m3 respectively, with the maximum reached on Christmas day, during stable conditions characterised by a high pressure, lack of rain and weak winds. The eddy covariance system proved able to detect fluxes of particle numbers above the city: during the same month, the measured PM fluxes revealed a strong emission peak on New Year's Eve, in correspondence of the traditional fireworks display, coupled with a strong, short concentration pulse (lasting about one hour), that dispersed quickly due to the windy conditions. The year-long planned measurements will allow establishing relationships between the fluxes of PM and the other pollutant species measured, to investigate the controls of the exchange, and provide relative emission factors for the urban sources according to seasonality.