Ice nucleating particles measured in air, cloud and seawater at the Cape Verde Atmospheric Observatory (CVAO)

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, nor if all sources are known. In this work, we examined properties of INPs at Cape Verde from different environmental compartments: namely, the oceanic sea surface microlayer (SML), underlying water (ULW), cloud water and the atmosphere close to both sea 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 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 hour, the elevated NINP at the warm temperatures disappeared, indicating that these highly ice active INPs were most likely biological particles.

NINP 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 super-micron 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 non-cloud 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 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 super-micron particles during sea spray generation as reported in literature.