Historical Changes in Seasonal Aerosol Acidity in the Po Valley (Italy) as Inferred from Fog Water and Aerosol Measurements

Abstract

Acidity profoundly affects almost every aspect that shapes the composition of ambient particles and their environmental impact. Thermodynamic analysis of gas-particle composition datasets offers robust estimates of acidity, but they are not available for long periods of time. Fog composition datasets, however, are available for many decades; we develop a thermodynamic analysis to estimate the ammonia in equilibrium with fog water and to infer the pre-fog aerosol pH starting from fog chemical composition and pH. The acidity values from the new method agree with the results of thermodynamic analysis of the available gasparticle composition data. Applying the new method to historical (25 years) fog water composition at the rural station of San Pietro Capofiume (SPC) in the Po Valley (Italy) suggests that the aerosol has been mildly acidic, with its pH decreasing by 0.5−1.5 pH units over the last decades. The observed pH of the fog water also increased 1 unit over the same period. Analysis of the simulated aerosol pH reveals that the aerosol acidity trend is driven by a decrease in aerosol precursor concentrations, and changes in temperature and relative humidity. Currently, NO x controls would be most effective for PM 2.5 reduction in the Po valley both during summer and winter. In the future, however, seasonal transitions to the NH 3 -sensitive region may occur, meaning that the NH 3 reduction policy may become increasingly necessary.

An organic crystalline state in ageing atmospheric aerosol proxies: spatially resolved structural changes in levitated fatty acid particles

“…lower bulk diffusion coefficients and higher viscosities, compared to their precursors and other components in the particle. The higher-molecular-weight products may therefore impede reactant diffusion and act as a "crust", as proposed in previous modelling studies (Pfrang et al, 2011;Zhou et al, 2019). This is consistent with the continued slow ozonolysis after the acid-soap complex degradation we report here.…”

“…We observed a clear phase gradient in a humidifying levitated acid-soap complex consistent with impeded diffusion of water through highly viscous (in this case, solid and semisolid) aerosols (Koop et al, 2011;Mikhailov et al, 2009;Pfrang et al, 2011;Reid et al, 2018). This is the first time that a spatially resolved phase gradient throughout a levitated humidifying aerosol particle has been reported using an X-ray-based technique.…”

“…2h). The fact that (i) the Raman laser was focussed on the bulk of the sample, showing only disordered oleic acid by the end of the reaction, and (ii) ozonolysis does not speed up after the acid-soap SAXS signal has gone means it is most likely that a core-shell morphology prevails and that the reaction becomes limited by this inert shell (Pfrang et al, 2011). This also fits with the previous suggestion that viscous and large particles are not well mixed and that the reaction occurs primarily in the surface layers of the particle (Moise and Rudich, 2002).…”

“…As pH is variable in atmospheric aerosols (Paglione et al, 2021), so too would the ratio of oleic acid and sodium oleate be. SAXS data from a 2 : 1 wt (oleic acid : sodium oleate) levitated particle, representing more acidic condi-tions, demonstrate that an inverse micellar phase forms at ∼ 50 % H (Fig.…”

Polycyclic aromatic hydrocarbons and their oxygenated derivatives in urban aerosol: levels, chemical profiles, and contribution to PM2.5 oxidative potential

“…1 , where a map of the Bologna area gives a view of the surrounding densely populated region ( ENEA 2017 ). Po Valley is regarded as an air pollution hot spot in Europe, with frequent severe PM air pollution events, as a consequence of intense anthropogenic emissions from industry, agriculture, and traffic, as well as high photochemical activity in summertime (Paglione et al 2021 ; Ricciardelli et al 2017 ). The sampling urban background site was located in the surrounding of the historical center of Bologna (inset of Fig.…”