Particle water and pH in the eastern Mediterranean: source variability and implications for nutrient availability

Abstract

Abstract. Particle water (liquid water content, LWC) and aerosol pH are important parameters of the aerosol phase, affecting heterogeneous chemistry and bioavailability of nutrients that profoundly impact cloud formation, atmospheric composition, and atmospheric fluxes of nutrients to ecosystems. Few measurements of in situ LWC and pH, however, exist in the published literature. Using concurrent measurements of aerosol chemical composition, cloud condensation nuclei activity, and tandem light scattering coefficients, the particle water mass concentrations associated with the aerosol inorganic (Winorg) and organic (Worg) components are determined for measurements conducted at the Finokalia atmospheric observation station in the eastern Mediterranean between June and November 2012. These data are interpreted using the ISORROPIA-II thermodynamic model to predict the pH of aerosols originating from the various sources that influence air quality in the region. On average, closure between predicted aerosol water and that determined by comparison of ambient with dry light scattering coefficients was achieved to within 8 % (slope = 0.92, R2 = 0.8, n = 5201 points). Based on the scattering measurements, a parameterization is also derived, capable of reproducing the hygroscopic growth factor (f(RH)) within 15 % of the measured values. The highest aerosol water concentrations are observed during nighttime, when relative humidity is highest and the collapse of the boundary layer increases the aerosol concentration. A significant diurnal variability is found for Worg with morning and afternoon average mass concentrations being 10–15 times lower than nighttime concentrations, thus rendering Winorg the main form of particle water during daytime. The average value of total aerosol water was 2.19 ± 1.75 µg m−3, contributing on average up to 33 % of the total submicron mass concentration. Average aerosol water associated with organics, Worg, was equal to 0.56 ± 0.37 µg m−3; thus, organics contributed about 27.5 % to the total aerosol water, mostly during early morning, late evening, and nighttime hours.The aerosol was found to be highly acidic with calculated aerosol pH varying from 0.5 to 2.8 throughout the study period. Biomass burning aerosol presented the highest values of pH in the submicron fraction and the lowest values in total water mass concentration. The low pH values observed in the submicron mode and independently of air mass origin could increase nutrient availability and especially P solubility, which is the nutrient limiting sea water productivity of the eastern Mediterranean.

The Potential Impact of Saharan Dust and Polluted Aerosols on Microbial Populations in the East Mediterranean Sea, an Overview of a Mesocosm Experimental Approach

“…The bioavailability of nutrients and trace metals from aerosols is related to both, the original aerosol chemical and mineralogical composition and the interactions and chemical transformations during atmospheric transport Baker and Jickells, 2006;Mackey et al, 2015). Recent studies have shown that the exposure to acid processes in the atmosphere can also change the phosphorus (P) bioavailability of dust/aerosols (Nenes et al, 2011;Bougiatioti et al, 2016).…”

Bioavailable atmospheric phosphorous supply to the global ocean: a 3-D global modeling study

“…Therefore, for HAP dissolution kinetics, we use the dissolution rates of FAP after correcting them to account for the differences between HAP and FAP dissolution kinetics as a function of pH and T, as reported by Palandri and Kharaka (2004). For this, we consider the different dissolution rates for a pH range of 0 to 7-8, which is the range of acidity encountered by atmospheric particles, including dust (e.g., Bougiatioti et al, 2016;Weber et al, 2016). At the strongly acidic limit (25 • C and pH = 0), the dissolution rate of HAP is assumed here to be about 27 % (i.e., 10 −0.56 times) slower than that of FAP, but for neutral and basic conditions (and 25 • C) HAP dissolves 2 orders of magnitude faster than FAP (Palandri and Kharaka, 2004).…”

Global impact of mineral dust on cloud droplet number concentration

“…Anthropogenic NO − 3 and SO 2− 4 mainly contribute to the chemical aging of dust over continents, while sea-salt-derived Cl − is more important over oceans (Sullivan et al, 2007;Fountoukis et al, 2009;Dall'Osto et al, 2010;Tobo et al, 2010;Karydis et al, 2011b;Bougiatioti et al, 2016b;Weber et al, 2016). Apart from the gas-phase composition, the chemical processing of dust also depends on its chemical composition and thus on the source region (Sullivan et al, 2009;Karydis et al, 2016).…”