Primary marine aerosol physical flux and chemical composition during a nutrient enrichment experiment in mesocosms in the Mediterranean Sea

Abstract

Abstract. While primary marine aerosol (PMA) is an important part of global aerosol total emissions, its chemical composition and physical flux as a function of the biogeochemical properties of the seawater still remain highly uncharacterized due to the multiplicity of physical, chemical and biological parameters that are involved in the emission process. Here, two nutrient-enriched mesocosms and one control mesocosm, both filled with Mediterranean seawater, were studied over a 3-week period. PMA generated from the mesocosm waters were characterized in term of chemical composition, size distribution and size-segregated cloud condensation nuclei (CCN), as a function of the seawater chlorophyll a (Chl a) concentration, pigment composition, virus and bacteria abundances. The aerosol number size distribution flux was primarily affected by the seawater temperature and did not vary significantly from one mesocosm to the other. The aerosol number size distribution flux was primarily affected by the seawater temperature and did not vary significantly from one mesocosm to the other. Particle number and CCN aerosol fluxes increase by a factor of 2 when the temperature increases from 22 to 32 • C, for all particle submicron sizes. This effect, rarely observed in previous studies, could be specific to oligotrophic waters and/or to this temperature range. In all mesocosms (enriched and control mesocosms), we detected an enrichment of calcium (+500 %) and a deficit in chloride (−36 %) in the submicron PMA mass compared to the literature inorganic composition of the seawater. There are indications that the chloride deficit and calcium enrichment are linked to biological processes, as they are found to be stronger in the enriched mesocosms. This implies a non-linear transfer function between the seawater composition and PMA composition, with comPublished by Copernicus Publications on behalf of the European Geosciences Union. plex processes taking place at the interface during the bubble bursting. We found that the artificial phytoplankton bloom did not affect the CCN activation diameter (D p,50,average = 59.85 ± 3.52 nm and D p,50,average = 93.42 ± 5.14 nm for supersaturations of 0.30 and 0.15 % respectively) or the organic fraction of the submicron PMA (average organic to total mass = 0.31 ± 0.07) compared to the control mesocosm. Contrary to previous observations in natural bloom mesocosm experiments, the correlation between the particle organic fraction and the seawater Chl a was poor, indicating that Chl a is likely not a straightforward proxy for predicting, on a daily scale, PMA organic fraction in models for all types of sea and ocean waters. Instead, the organic fraction of the Aitken mode particles were more significantly linked to heterotrophic flagellates, viruses and dissolved organic carbon (DOC). We stress that different conclusions may be obtained in natural (non-enriched) or non-oligotrophic systems.

Temperature and Composition Dependence of Sea Spray Aerosol Production

“…However, above 9°C, the N p were relatively T w -independent, with Salter et al (2014) even reporting a slight increase in N p with T w above 9°C for particles with d p > 300 nm. Schwier et al (2017) also observed N p (reported as an estimated particle flux) increased with T w between 20 and 30°C for particles generated from seawater using a plunging jet. However, the magnitude of the increase (approximately a factor of 2) was much larger than observed here for the total N p .…”

“…To better understand the impact of temperature and composition on SSA particle production, we conducted T w -dependent (range À2 to 25°C) measurements of SSA Figure 1. Normalized temperature dependent SSA particle concentrations from laboratory studies including arctic seawater (black circles; , NaCl (dashed purple line; Zábori, Matisāns, et al, 2012), synthetic seawater (Salter et al, 2014) for D p > 10 nm (dashed green line) and D p > 300 nm (dashed blue line), and Mediterranean seawater (pink dotdashed line; Schwier et al, 2017). Also shown is that estimated from assessment of global ambient concentrations (dot-dashed grey line; Grythe et al, 2014), a parameterized flux based on ambient measurements (dot-dashed red line; Jaeglé et al, 2011) and a physically based parameterization (dotdashed light blue lines; Ovadnevaite et al, 2014).…”

“…To facilitate comparison of the MART and miniMART observations here with other wave breaking analogs, a set of experiments was performed wherein the MART was modified to use a continuous plunging jet instead of an intermittent plunging sheet. The jet design was modeled on Salter et al (2014) and is similar to that used by previous studies (Schwier et al, 2017;Zábori, Matisāns, et al, 2012).…”

“…However, the magnitude of the increase (approximately a factor of 2) was much larger than observed here for the total N p . Schwier et al (2017) did not perform measurements below 9°C, and thus, it is unknown whether they would have observed an increase in N p at lower T w . Overall, our observations-from MART and miniMART-are reasonably consistent with the previously observed behavior when T w > 9°C, but are in stark contrast to that reported at lower T w .…”

Preliminary results from the FARCE 2015 campaign: multidisciplinary study of the forests–gases–aerosols–clouds system on the tropical island of La Réunion

“…CC BY 4.0 License. primary marine aerosol (Schwier et al, 2017) indicating that further chloride depletion is likely not occurring after the aerosol has activated to cloud droplets.…”