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. Observations of the long-range transport of biologicalparticles in the tropics via dust vectors are now seen as fundamental tothe understanding of many global atmosphere–ocean biogeochemical cycles,changes in air quality, human health, ecosystem impacts, and climate.However, there is a lack of long-term measurements quantifying theirpresence in such conditions. Here, we present annual observations ofbioaerosol concentrations based on online ultraviolet laser-inducedfluorescence (UV-LIF) spectrometry from the World MeteorologicalOrganization – Global Atmospheric Watch (WMO-GAW)Cape Verde Atmospheric Observatory on São Vicente. We observethe expected strong seasonal changes in absolute concentrations ofbioaerosols with significant enhancements during winter due to the strongisland inflow of air mass, originating from the African continent.Monthly median bioaerosol concentrations as high as 45 L−1 werefound with 95th percentile values exceeding 130 L−1 during strongdust events. However, in contrast, the relative fraction ofbioaerosol numbers compared to total dust number concentrationshows little seasonal variation. Mean bioaerosolcontributions accounted for 0.4 ± 0.2 % of total coarse aerosolconcentrations, only rarely exceeding 1 % during particularly strongevents under appropriate conditions. Although enhancements in the medianbioaerosol fraction do occur in winter, they also occur at other times ofthe year, likely due to the enhanced Aeolian activity driving dust events atthis time from different sources. We hypothesise that this indicates therelative contribution of bioaerosol material in dust transported acrossthe tropical Atlantic throughout the year is relatively uniform, comprisedmainly of mixtures of dust and bacteria and/or bacterial fragments. Weargue that this hypothesis is supported from analysis of measurements alsoat Cabo Verde just prior to the long-term monitoring experiment where UV-LIFsingle particle measurements were compared with laser ablation aerosolparticle time-of-flight mass spectrometer (LAAP-ToF) measurements. Theseclearly show a very high correlation between particles with mixedbiosilicate mass spectral signatures and UV-LIF biofluorescent signaturessuggesting the bioaerosol concentrations are dominated by these mixtures.These observations should assist with constraining bioaerosol concentrations for tropical globalclimate model (GCM) simulations. Note that here we use the term “bioaerosol” toinclude mixtures of dust and bacterial material.

atmospheric chemistry and physics

Quantifying bioaerosol concentrations in dust clouds through online UV-LIF and mass spectrometry measurements at the Cape Verde Atmospheric Observatory