Hygroscopic and chemical characterisation of Po Valley aerosol

Abstract

Abstract. Continental summer-time aerosol in the Italian Po Valley was characterized in terms of hygroscopic properties and the influence of chemical composition therein. The campaign-average minima in hygroscopic growth factors (HGFs) occurred just before and during sunrise from 03:00–06:00, but more generally, the whole night shows very low hygroscopicity, particularly in the smaller particle sizes. The average HGFs increased from 1.18 for the smallest sized particles (35 nm) to 1.38 for the largest sizes (165 nm) for the lowest HGF period while during the day, the HGF gradually increased to achieve maximum values in the early afternoon hours from 12:00–15:00, reaching 1.32 for 35 nm particles and 1.46 for 165 nm particles. Two contrasting case scenarios were encountered during the measurement period: Case 1 was associated with westerly air flow moving at a moderate pace and Case 2 was associated with more stagnant, slower moving air from the north-easterly sector. Case 1 exhibited low diurnal temporal patterns and was associated with moderate non-refractory aerosol mass concentrations (for 50% size cut at 1 μm) of the order of 4.5 μg m−3. For Case 1, organics contributed typically to 50% of the mass. Case 2 was characterized by > 9.5 μg m−3 total mass (< 1 μm) in the early morning hours (04:00), decreasing to ∼ 3 μg m−3 by late morning (10:00) and exhibited strong diurnal changes in chemical composition, particularly in nitrate mass but also in total organic mass concentrations. Organic growth factors (OGFs) exhibited a minimum around 15:00, 1–2 h after the peak in HGF. Particles sized 165 nm exhibited moderate diurnal variability in HGF, ranging from 80% at night to 95% of "more hygroscopic" growth factors (i.e. GF = 1.35–1.9) around noon. The diurnal changes in HGF progressively became enhanced with decreasing particle size, decreasing from 95% "more hygroscopic" growth factor fraction at noon to 10% fraction at midnight, while the "less hygroscopic" growth factor fraction (1.13–1.34) increased from 5% at noon to > 60% and the "barely hygroscopic" growth factor fraction (1.1–1.2) increased from less than 2% at noon to 30% at midnight. OGFs were generally anti-correlated to HGF and also total organic mass as measured by the aerosol mass spectrometer due to a high sulphate/organics ratio. Surprisingly, the lowest HGFs occurred for periods when nitrate mass reached peak concentrations. This may suggest formation of organonitrates and organosulphates, which significantly decreased the OGF. Coincident with the peak in nitrate was a peak in Hydrocarbon-like Organic Aerosol (HOA) and Semi-Volatile Oxygenated Organic Aerosol (SV-OOA) and analysis of the HGF probability distribution function (PDF) reveals a transformation of a predominant "More Hygroscopic" (MH) mode with HGF of 1.5 around noon, into two modes, one with a "less hygroscopic" (LH) HGF of 1.26, and another with a "barely hygroscopic" (BH) mode of 1.05. The analysis points to an internal mixture of larger size inorganic species, mainly nitrates, coated with a hydrophobic organic layer which suppresses water uptake. In addition, a new, externally mixed BH ultrafine mode appears and persists through the night.