Hourly composition of gas and particle phase pollutants at a central urban background site in Milan, Italy
Introduction
The interactions between gaseous and aerosol phase pollutants have long been studied due to their impact on air quality (Penkett et al., 1979, Ravishankara, 1997) and on human health (World Health Organization, 2006). In order to investigate processes leading to atmospheric pollutants formation and ageing in densely populated areas with large emission load (i.e. hotspots), time resolved composition of both gas and particle phase atmospheric compounds are needed. Po valley (Northern Italy) is one of the most important hotspot regions in Europe (Putaud et al., 2010), with Milan metropolitan area exhibiting one of the poorest air qualities within the valley (Bigi and Ghermandi, 2014).
Very few 1-hour time resolution campaigns were accomplished in Milan urban area. Three noteworthy studies resulted from the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono (LOOP/PIPAPO) campaign held in May and June 1998 in Milan urban background (Neftel et al., 2002). In one of these studies Baltensperger et al. (2002) analysed data by several continuous instruments sampling aerosol physical properties (number size distribution, volatility, hygroscopicity, mass), aerosol chemical composition (BCE, nitrate, sulphate) (NH3, HNO3). This same study showed the large contribution to airborne particles smaller than 40 nm by primary emissions rich in soot content and the increase in secondary and hygroscopic aerosol for particles larger than 50 nm. The second study within PIPAPO, Putaud et al. (2002) collected samples of size-segregated aerosol with 4- and 7-hour time resolution and analysed them for elemental carbon (EC), organic carbon (OC), particulate organic matter (POM) and major ionic species; their results showed the large contribution (¿30%) to PM mass by POM, by ammonium nitrate (29% of PM mass) and ammonium sulphate (22% of PM mass). Putaud et al. (2002) found also a diurnal and weekly pattern for traffic-related pollutants (e.g. EC and resuspended mineral dust), the influence of traffic emissions on nitrate formation and of industrial emissions on sulphate formation. In the third study of PIPAPO Alicke et al. (2002) investigated hydroxyl radical formation by measuring several gas phase pollutants by DOAS (differential optical absorption spectroscopy): HCHO, HONO, NO2, NO, O3 and SO2. Their results identified HCHO as the primary source of OH. radicals (up to 40% of total OH. on clear days), while photolysis of nitrous acid and of ozone provides similar contribution to atmospheric OH. (15–30% of total OH. each), with the former compound dominating during early morning and the latter during the afternoon. In addition to the above mentioned studies, the aerosol elemental composition and sources were investigated with hourly resolution in Milan by D’Alessandro et al. (D’Alessandro et al., 2003, D’Alessandro et al., 2004) during wintertime and summertime 2001 evidencing quasi-periodical and episodic pollution sources.
Several sources influence the sampling site: Bernardoni et al. (2011) used positive matrix factorization to apportion 4-hour PM10 measurements and showed the diurnal pattern in the relative contribution by resuspended dust, construction works and industry, which altogether account for 48% to total PM10 in summer. These results were confirmed by the a detailed source apportionment exercise in Milan urban background by Perrone et al. (2012), where three years of daily PM2.5 and PM10 samples were analysed. This latter study also showed how 60% of summer daily PM2.5 derives from traffic and secondary inorganic ions (sulphate, nitrate and ammonium) and contribution of resuspended dust to summer daily PM2.5 is only to 3.8%. Consistently emission inventory for the only municipality of Milan assessed Road Traffic (SNAP 7) to be the main source of NOx and EC for the city of Milan, SNAP 2 (non-industrial combustion) is the main source of OC and the second most important of NOx, and SNAP 6 (solvent use) is the main source of NM-VOC. Notwithstanding these studies, in Milan there is no available analysis of simultaneous characterization of atmospheric pollutants in both gas and particle phase sampled at a 1-hour resolution. The present article is based on a thorough analysis of measurements of several gas phase pollutants and main chemical composition of PM2.5 sampled at 1-hour time resolution. Formation process of PM2.5 in Milan will be presented along with the influence of meteorological conditions and air mass trajectories. Observations include HCl, HONO, HNO3 and NH3, i.e. the first published measurement of hydrochloric acid in the Po valley and the first 1-hour resolution measurements of nitrous and nitric acids in the last 15 years in Milan. Details on the instrumentation and methods used are presented in Section 2. Results and conclusions are found in 3 Results and discussion, 4 Conclusions respectively.
Section snippets
Data and methods
Milan (45° 28′N; 9° 13′E) urban area counts about 1,500,000 inhabitants and is the second largest town in Italy, after Rome, and considering the whole Milan province the population rises up to about 3.1 millions inhabitants.
The data here presented were collected on the roof of the Department of Chemistry, University of Milan, at a ∼ 10 m a.g.l. within the University campus, a site representative of central urban background conditions for the city. Sampling was performed from June 5th until July
Results and discussion
In the following the data will be presented and discussed in specific subsections for each common species, including their pattern, formation and removal processes (if known) and their potential source. The analysis investigates the average concentration pattern over the whole sampling period, and the variability of a subset of pollutants under different atmospheric dispersion conditions.
Conclusions
The article presented the most recent and complete analysis of 1-hour resolution observations of gaseous pollutants and main chemical composition of PM2.5 in Milan. Ozone and nitrogen oxides pattern are consistent with intense photochemical activity under strong solar radiation, heavy emission sources and recirculation of pollutants, along with re-entrainment from the residual layer. HONO mixing rations, compared to 1998, exhibited a decrease, consistently with the reduction in NO2 atmospheric
Acknowledgments
Thanks to Luca Lombroso for the description of the synoptic meteorological conditions.
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Now at ARPA Puglia, Bari, Italy.
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