Changes in particulate matter physical properties during Saharan advections over Rome (Italy): a four-year study, 2001-2004

Particulate matter mass concentrations measured in the city of Rome (Italy) in the period 2001-2004 have been cross-analysed with concurrent Saharan dust advection events to infer the impact these natural episodes bear on the standard air quality parameter PM10 observed at two city stations and at one regional background station. Natural events such as Saharan dust advections are associated with a definite health risk. At the same time, the Directive 2008/50/EC allows subtraction of PM exceedances caused by natural contributions from statistics used to determine air quality of EU sites. In this respect, it is important to detect and characterise such advections by means of reliable, operational techniques. To assess the PM10 increase we used both the regional-background method suggested by EC Guidelines and a local background method, demonstrated to be most suited to this central Mediterranean region. In terms of exceedances, the two approaches provided results within similar to 20% of each other at background sites, and at similar to 50% of each other in traffic conditions. The sequence of Saharan advections over the city has been either detected by Polarization Lidar (laser radar) observations or forecast by the operational numerical regional mineral dust model BSC-DREAM8b of the Barcelona Supercomputing Centre. Lidar observations were also employed to retrieve the average physical properties of the dust clouds as a function of height. Over the four-year period, Lidar measurements (703 evenly distributed days) revealed Saharan plumes transits over Rome on 28.6% of the days, with minimum occurrence in wintertime. Dust was observed to reach the ground on 17.5% of the days totalling 88 episodes. Most (90 %) of these advections lasted up to 5 days, averaging to similar to 3 days. Median time lag between advections was 7 days. Typical altitude range of the dust plumes was 0-6 km, with the centre of mass at similar to 3 km a.g.l. BSC-DREAM8b model simulations (1461 days) predicted Lidar detectable (532 nm extinction coefficient >0.005 km(-1)) dust advections on 25.9% of the days, with ground contacts on 13% of the days. As in the Lidar case, the average dust centre of mass was forecast at similar to 3 km. Along the 703 day Lidar dataset, model forecast and Lidar detection of the presence of dust coincided on 80% of the cases, 92% coincidences are found within a +/- 1 day window. Combination of the BSC-DREAM8b and Lidar records leads to about 21% of the days being affected by presence of Saharan dust at the ground. This combined dataset has been used to compute the increase in PM with respect to dust-unaffected previous days. This analysis has shown Saharan dust events to exert a meaningful impact on the PM10 records, causing average increases of the order of 11.9 mu g m(-3). Conversely, PM10 increases computed relying only on the Lidar detections (i.e., presence of dust layers actually observed) were of the order of 15.6 mu g m(-3). Both analyses indicate the annual average contribution of dust advections to the city PM10 mass concentrations to be of the order of 2.35 mu g m(-3). The number of exceedances attributable to Saharan advections at the three station types addressed in this study (urban traffic, urban background and regional background) were found to be 25%, 30% and 43 %, respectively. These results confirm Saharan advections in the central Mediterranean as important modulators of PM10 loads and exceedances.