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

Gong, Xianda; Wex, Heike; van Pinxteren, Manuela; Triesch, Nadja; Fomba, Khanneh Wadinga; Lubitz, Jasmin; Stolle, Christian; Robinson, Tiera-Brandy; Müller, Thomas; Herrmann, Hartmut; Stratmann, Frank

Ice-nucleating particles (INPs) in the troposphere can form ice in clouds via heterogeneous ice nucleation. Yet, atmospheric number concentrations of INPs (inline-formulaNINP) 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 inline-formulaNINP 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.

inline-formulaNINP values in PMinline-formula10 sampled at Cape Verde Atmospheric Observatory (CVAO) at any particular ice-nucleation temperature spanned around 1 order of magnitude below inline-formula−15inline-formulaC, and about 2 orders of magnitude at warmer temperatures (inline-formula M7inlinescrollmathml > - normal 12 32pt10ptsvg-formulamathimga7f9da50f44ff0b2e27748f108bed98d acp-20-1451-2020-ie00001.svg32pt10ptacp-20-1451-2020-ie00001.png inline-formulaC). Among the 17 PMinline-formula10 samples at CVAO, three PMinline-formula10 filters showed elevated inline-formulaNINP at warm temperatures, e.g., above 0.01 Linline-formula−1 at inline-formula−10inline-formulaC. After heating samples at 95 inline-formulaC for 1 h, the elevated inline-formulaNINP at the warm temperatures disappeared, indicating that these highly ice active INPs were most likely biological particles.

INP number concentrations in PMinline-formula1 were generally lower than those in PMinline-formula10 at CVAO. About inline-formula83±22 %, inline-formula67±18 % and inline-formula77±14 % (medianinline-formula±standard deviation) of INPs had a diameter inline-formula>1inline-formulaµm at ice-nucleation temperatures of inline-formula−12, inline-formula−15 and inline-formula−18inline-formulaC, respectively. PMinline-formula1 at CVAO did not show such elevated inline-formulaNINP at warm temperatures. Consequently, the difference in inline-formulaNINP between PMinline-formula1 and PMinline-formula10 at CVAO suggests that biological ice-active particles were present in the supermicron size range.

inline-formulaNINP in PMinline-formula10 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 PMinline-formula10 filters at CVAO, they were not observed in PMinline-formula10 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 inline-formulaNINP 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 PMinline-formula10 filters, it can be concluded that sea spray aerosol (SSA) only contributed a minor fraction to the atmospheric inline-formulaNINP. This latter conclusion still holds when accounting for an enrichment of organic carbon in supermicron particles during sea spray generation as reported in literature.

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Gong, Xianda / Wex, Heike / van Pinxteren, Manuela / et al: 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. 2020. Copernicus Publications.

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