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

Abstract

Abstract. Observations of the long-range transport of biological particles in the tropics via dust vectors are now seen as fundamental to the 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 their presence in such conditions. Here, we present annual observations of bioaerosol concentrations based on online ultraviolet laser-induced fluorescence (UV-LIF) spectrometry from the World Meteorological Organization – Global Atmospheric Watch (WMO-GAW) Cape Verde Atmospheric Observatory on São Vicente. We observe the expected strong seasonal changes in absolute concentrations of bioaerosols with significant enhancements during winter due to the strong island inflow of air mass, originating from the African continent. Monthly median bioaerosol concentrations as high as 45 L−1 were found with 95th percentile values exceeding 130 L−1 during strong dust events. However, in contrast, the relative fraction of bioaerosol numbers compared to total dust number concentration shows little seasonal variation. Mean bioaerosol contributions accounted for 0.4 ± 0.2 % of total coarse aerosol concentrations, only rarely exceeding 1 % during particularly strong events under appropriate conditions. Although enhancements in the median bioaerosol fraction do occur in winter, they also occur at other times of the year, likely due to the enhanced Aeolian activity driving dust events at this time from different sources. We hypothesise that this indicates the relative contribution of bioaerosol material in dust transported across the tropical Atlantic throughout the year is relatively uniform, comprised mainly of mixtures of dust and bacteria and/or bacterial fragments. We argue that this hypothesis is supported from analysis of measurements also at Cabo Verde just prior to the long-term monitoring experiment where UV-LIF single particle measurements were compared with laser ablation aerosol particle time-of-flight mass spectrometer (LAAP-ToF) measurements. These clearly show a very high correlation between particles with mixed biosilicate mass spectral signatures and UV-LIF biofluorescent signatures suggesting the bioaerosol concentrations are dominated by these mixtures. These observations should assist with constraining bioaerosol concentrations for tropical global climate model (GCM) simulations. Note that here we use the term “bioaerosol” to include mixtures of dust and bacterial material.

Measurement Report: Comparative Analysis of Fluorescing African Dust Particles in Spain and Puerto Rico

“…Techniques that use Ultra-violet Laser Induced Fluorescence (UV-LIF), such as the Wideband Integrated Bioaerosol Spectrometer (WIBS), provide detailed information on the size, shape and fluorescence intensity of individual particles in real time (Kaye et al, 2004). The recent investigation by Morrison et al (2020) The measurements reported here, in our current study, complement those of Morrison et al (2020) with results from locations farther down-wind than their study site of Cape Verde. Our objectives are…”

“…These artifacts cannot be completely removed from the analysis but can be minimized by removing from the processing any particles whose fluorescence falls below a preset threshold. As described in the supplementary material, we follow the methodology of Perring et al, (2015) and Morrison et al (2020) by creating daily frequency histograms of the FL1, FL2 and FL3 type FAPs and use a threshold that is the mode of the frequency distribution plus nine standard deviations (9 σ) as the minimum threshold that has to be exceeded before a fluorescing event is accepted as valid. The aforementioned uncertainty due to electronic deadtime is associated with the eight milliseconds that is required to recharge after each Xenon lamp flash.…”

“…The analysis methodology that is selected to evaluate the WIBS data needs to be tailored to the specific questions that are being addressed. Much progress has been made in the past 10 years in the use of cluster analysis to identify features of the FAP, which are indirectly related to the type of bioaerosol (Robinson et al, 2013;Crawford et al, 2015;Morrison et al,2020). In our study, however, we are asking a different set of questions, where knowing the type of bioaerosol is not as important as understanding how the FAPs are transported and their properties transformed while in AD plumes.…”

Assessing the global contribution of marine, terrestrial bioaerosols, and desert dust to ice-nucleating particle concentrations

“…Secondly, FBAP may also contain non-PBAP (e.g., mineral dust particles, secondary organic aerosol particles) that contribute to fluorescence (Bones et al, 2010;Lee et al, 2013;Morrison et al, 2020). The number fractions of FBAP to the total aerosol particles in the super-micron size range could be relatively similar to those of PBAP under dry conditions (Tobo et al, 2013).…”

Airborne Bioaerosol Observations Imply a Strong Terrestrial Source in the Summertime Arctic

“…It is important to consider potential non-biological interferents in the context of these results. Previous studies have shown that dust contributes a substantial fraction of fluorescent particle loadings in remote environments (Crawford et al, 2016;D. Morrison et al, 2020).…”