Characterization of aerosol particles at Cabo Verde close to sea level and at the cloud level – Part 2: Ice-nucleating particles in air, cloud and seawater

Abstract

Abstract. Ice-nucleating particles (INPs) in the troposphere can form ice in clouds via heterogeneous ice nucleation. Yet, atmospheric number concentrations of INPs (NINP) are not well characterized, and, although there is some understanding of their sources, it is still unclear to what extend different sources contribute or if all sources are known. In this work, we examined properties of INPs at Cabo Verde (a.k.a. Cape Verde) from different environmental compartments: the oceanic sea surface microlayer (SML), underlying water (ULW), cloud water and the atmosphere close to both sea level and cloud level. Both enrichment and depletion of NINP in SML compared to ULW were observed. The enrichment factor (EF) varied from roughly 0.4 to 11, and there was no clear trend in EF with ice-nucleation temperature. NINP values in PM10 sampled at Cape Verde Atmospheric Observatory (CVAO) at any particular ice-nucleation temperature spanned around 1 order of magnitude below −15 ∘C, and about 2 orders of magnitude at warmer temperatures (>-12 ∘C). Among the 17 PM10 samples at CVAO, three PM10 filters showed elevated NINP at warm temperatures, e.g., above 0.01 L−1 at −10 ∘C. After heating samples at 95 ∘C for 1 h, the elevated NINP at the warm temperatures disappeared, indicating that these highly ice active INPs were most likely biological particles. INP number concentrations in PM1 were generally lower than those in PM10 at CVAO. About 83±22 %, 67±18 % and 77±14 % (median±standard deviation) of INPs had a diameter >1 µm at ice-nucleation temperatures of −12, −15 and −18 ∘C, respectively. PM1 at CVAO did not show such elevated NINP at warm temperatures. Consequently, the difference in NINP between PM1 and PM10 at CVAO suggests that biological ice-active particles were present in the supermicron size range. NINP in PM10 at CVAO was found to be similar to that on Monte Verde (MV, at 744 m a.s.l.) during noncloud events. During cloud events, most INPs on MV were activated to cloud droplets. When highly ice active particles were present in PM10 filters at CVAO, they were not observed in PM10 filters on MV but in cloud water samples instead. This is direct evidence that these INPs, which are likely biological, are activated to cloud droplets during cloud events. For the observed air masses, atmospheric NINP values in air fit well to the concentrations observed in cloud water. When comparing concentrations of both sea salt and INPs in both seawater and PM10 filters, it can be concluded that sea spray aerosol (SSA) only contributed a minor fraction to the atmospheric NINP. This latter conclusion still holds when accounting for an enrichment of organic carbon in supermicron particles during sea spray generation as reported in literature.

Marine organic matter in the remote environment of the Cape Verde islands – an introduction and overview to the MarParCloud campaign

“…The most prominent pigment throughout the campaign was zeaxanthin, suggesting cyanobacteria as the dominant group in this region. This is in good agreement with the generally low biomass in the waters of the Cape Verde region and in line with previous studies reporting the dominance of cyanobacteria during the spring and summer seasons (Franklin et al, 2009;Hepach et al, 2014;Zindler et al, 2012). However, once the biomass increased, cyanobacteria were repressed by diatoms as indicated by the relative increase in fucoxanthin.…”

Characterization of aerosol particles at Cabo Verde close to sea level and at the cloud level – Part 1: Particle number size distribution, cloud condensation nuclei and their origins

“…Mineral dust from deserts contributes largely to tropospheric aerosols and impacts the air quality of several regions, even of the globe (Ginoux et al, 2001;Huang et al, 2006;Tanaka and Chiba, 2006). The largest dust source is located in the Northern Hemisphere in the Sahara and Sahel regions (Goudie and Middleton, 2001;Prospero et al, 2002;Ginoux et al, 2012), with millions of tonnes of mineral dust being transported to Europe and the Middle East, as well as to the Americas, yearly (including the Caribbean and the Amazon basin) (Swap et al, 1992;Salvador et al, 2013;Wex et al, 2016). Mineral dust aerosol in the atmosphere can affect the Earth's radiative budget by directly scattering and absorbing solar and infrared radiation (Goudie and Middleton, 2001;Shao et al, 2011).…”

“…The largest dust source is located in the Northern Hemisphere in the Sahara and Sahel regions (Goudie and Middleton, 2001;Prospero et al, 2002;Ginoux et al, 2012), with millions of tonnes of mineral dust being transported to Europe and the Middle East, as well as to the Americas, yearly (including the Caribbean and the Amazon basin) (Swap et al, 1992;Salvador et al, 2013;Wex et al, 2016). Mineral dust aerosol in the atmosphere can affect the Earth's radiative budget by directly scattering and absorbing solar and infrared radiation (Goudie and Middleton, 2001;Shao et al, 2011). On the other hand, it can modify cloud properties, i.e., serve as CCN or ice-nucleating particles (INPs) (Sassen et al, 2003;DeMott et al, 2003).…”

review

“…line 48: "three main groups" -There is actually a quite new paper in which SML and airborne concentrations were connected (including cloud water): Gong et al (2020). For air masses that were continentally and/or mineral dust influenced (Cape Verde), INP concentrations in the SML did not explain atmospheric INP concentrations -that does not say anything about the remote oceans, but is a piece in the puzzle, nevertheless, which is worth mentioning, as this helps closing a gap between ocean and atmosphere.…”

“…But there are results you can summarize! Table 1: As you try to give an overview here, including the papers I referred to above makes sense (McCluskey et al, 2018a,b;Creamean et al, 2019;Gong et al, 2020). And there is one more for coastal Mexico by Ladino et al, (2019) which may fit.…”