Abstract
Abstract. The aerosol size-segregated chemical composition was analyzed at an urban (Bologna) and a rural (San Pietro Capofiume) site in the Po Valley, Italy, during June and July 2012, by ion-chromatography (major water-soluble ions and organic acids) and evolved gas analysis (total and water-soluble carbon), to investigate sources and mechanisms of secondary aerosol formation during the summer. A significant enhancement of secondary organic and inorganic aerosol mass was observed under anticyclonic conditions with recirculation of planetary boundary layer air but with substantial differences between the urban and the rural site. The data analysis, including a principal component analysis (PCA) on the size-resolved dataset of chemical concentrations, indicated that the photochemical oxidation of inorganic and organic gaseous precursors was an important mechanism of secondary aerosol formation at both sites. In addition, at the rural site a second formation process, explaining the largest fraction (22 %) of the total variance, was active at nighttime, especially under stagnant conditions. Nocturnal chemistry in the rural Po Valley was associated with the formation of ammonium nitrate in large accumulation-mode (0.42–1.2 µm) aerosols favored by local thermodynamic conditions (higher relative humidity and lower temperature compared to the urban site). Nocturnal concentrations of fine nitrate were, in fact, on average 5 times higher at the rural site than in Bologna. The water uptake by this highly hygroscopic compound under high RH conditions provided the medium for increased nocturnal aerosol uptake of water-soluble organic gases and possibly also for aqueous chemistry, as revealed by the shifting of peak concentrations of secondary compounds (water-soluble organic carbon (WSOC) and sulfate) toward the large accumulation mode (0.42–1.2 µm). Contrarily, the diurnal production of WSOC (proxy for secondary organic aerosol) by photochemistry was similar at the two sites but mostly affected the small accumulation mode of particles (0.14–0.42 µm) in Bologna, while a shift to larger accumulation mode was observed at the rural site. A significant increment in carbonaceous aerosol concentration (for both WSOC and water-insoluble carbon) at the urban site was recorded mainly in the quasi-ultrafine fraction (size range 0.05–0.14 µm), indicating a direct influence of traffic emissions on the mass concentrations of this range of particles.
Highlights
The knowledge of the size-segregated chemical composition of atmospheric aerosols, i.e., the chemical composition as a function of particle size, is key to the understanding of several important characteristics of particles such as optical properties, hygroscopicity and reactivity, which affect the atmospheric radiation budget, cloud formation and human health
The PEGASOS summer campaign was characterized by the occurrence of different meteorological patterns, with the first part characterized by days of very perturbed weather followed by stable anticyclonic conditions and the second part experiencing more variable meteorological conditions
We focused on the time trends and size distributions of major carbonaceous and inorganic ionic species and we concluded that at least two secondary formation processes were active in the Po Valley: the first, probably photochemical, is active throughout the campaign at both stations, affecting the concentrations of all species and in the condensation mode during stagnation periods; the second one is associated with deliquesced particles and is selectively important for nitrate and to a lesser extent for sulfate and watersoluble organic carbon (WSOC) in droplet-mode particles
Summary
The knowledge of the size-segregated chemical composition of atmospheric aerosols, i.e., the chemical composition as a function of particle size, is key to the understanding of several important characteristics of particles such as optical properties, hygroscopicity and reactivity, which affect the atmospheric radiation budget, cloud formation and human health. The size distribution of inorganic and organic components reflects their origin, and provides a wealth of information about aerosol formation mechanisms and atmospheric processing, including secondary formation (Seinfeld and Pandis, 1998). The resulting size distribution is bimodal with the small mode accounting for secondary aerosols produced uniquely by gas-to-particle conversion (“condensation mode”) and the large mode containing particles that underwent cloud processing (“droplet mode”) (Hering and Friedlander, 1982; John et al, 1990). Knowledge about the concentrations of aerosol organic and inorganic compounds in size-segregated aerosol samples provides information on the nature of secondary formation processes. Measurements of the size-segregated chemical composition of aerosols are traditionally performed using multi-stage impactors followed by offline chemical analysis. An example of comparison of aerosol chemical measurements performed using a five-stage impactor and AMS is provided by our previous study focusing on the 2009 field campaign in the Po Valley, Italy (Decesari et al, 2014)
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