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

Abstract

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 ~20% of each other at background sites, and at ~50% of each other in traffic conditions. <br><br> 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 ~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 ~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 ~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. <br><br> 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 μg 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 μg m⁻³. Both analyses indicate the annual average contrib...

EARLINET: potential operationality of a research network

“…This section aims at illustrating how SCC-2 products, namely the vertical profiles of extinction coefficient, can be used to examine the performance of dust transport models to predict the 4-D evolution of mineral dust during a dust intrusion event. Literature is available on the subject (Gobbi et al, 2013;Mona et al, 2014;Binietoglou et al, 2015, among others), so that, again, the emphasis is put more on the potential of SCC-2 products for the evaluation of aerosol transport models than on the proper evaluation of the selected dust transport models during the exercise. Two models participated in this evaluation: BSC DREAM8b v2 (Pérez et al, 2006a, b;Basart et al, 2012) and NMMB/BSC-DUST (Nonhydrostatic Multiscale Meteorological Model on the B grid/Barcelona Supercomputing Center -Dust; Janjic et al, 2011;Pérez et al, 2011).…”

“…Model and observation comparisons can be performed for many different quantities and in many different ways (Gobbi et al, 2013;Mona et al, 2014;Binietoglou et al, 2015;Granados-Muñoz et al, 2015, among others). In order to avoid applying additional algorithms on the profiles, we discarded the comparison of structural parameters, like the center of mass or the dust layer thickness (which, in addition, are not SCC products).…”

WRF-Chem model simulations of a dust outbreak over the central Mediterranean and comparison with multi-sensor desert dust observations

“…It has been estimated that about half of the global total natural dust emissions are generated in the Sahara and its surroundings (Goudie, 2009;Huneeus et al, 2011;Ginoux et al, 2012;Shao et al, 2011b). Deep convection produced by the strong surface heating can uplift mineral dust particles for several kilometers into the free troposphere, where they are finally advected over large distances at the continental and intercontinental scales (Goudie and Middleton, 2001;Engelstaedter et al, 2006).…”

“…Deep convection produced by the strong surface heating can uplift mineral dust particles for several kilometers into the free troposphere, where they are finally advected over large distances at the continental and intercontinental scales (Goudie and Middleton, 2001;Engelstaedter et al, 2006). Saharan dust is mainly transported along four trajectories patterns (D'Almeida, 1986;Shao, 2011b).…”

Investigating the quality of modeled aerosol profiles based on combined lidar and sunphotometer data

“…Currently, the daily operational model evaluation includes satel-lites (MODIS and MSG -Meteosat Second Generation) and AERONET sun photometers. Some comparisons between lidar and forecast model profiles were performed in terms of aerosol vertical distribution for specific Saharan dust events in the Mediterranean Basin (e.g., Balis et al, 2004;Pérez et al, 2006a;Amiridis et al, 2009;Mona et al, 2012;Gobbi et al, 2013;Amiridis et al, 2013;Mona et al, 2014). In addition, Binietoglou et al (2015) include BSC-DREAM8b as one of the models that participate in their analysis, validating its performance against LIRIC retrievals at 10 EAR-LINET/AERONET stations.…”