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Abstract. Measurements during episodes of African dust, made with two wideband integrated bioaerosol spectrometers (WIBSs), one on the northeastern coast of Puerto Rico and the other in the city of León, Spain, show unmistakable, bioaerosol-like fluorescing aerosol particles (FAPs) that can be associated with these dust episodes. The Puerto Rico event occurred during a major incursion of African dust during June 2020. The León event occurred in the late winter and spring of 2022, when widespread, elevated layers of dust inundated the Iberian Peninsula. Satellite and back-trajectory analyses confirm that dust from northern Africa was the source of the particles during both events. The WIBSs measure the size of individual particles in the range from 0.5 to 30 µm, derive a shape factor, and classify seven types of fluorescence from the FAPs. In general, it is not possible to directly determine the specific biological identity from fluorescence signatures; however, measurements of these types of bioaerosols in laboratory studies allow us to compare ambient fluorescence patterns with whole microbial cells measured under controlled conditions. Here we introduce some new metrics that offer a more quantitative approach for comparing FAP characteristics derived from particles measured under different environmental conditions. The analysis highlights the similarities and differences at the two locations and reveals differences that can be attributed to the age and history of the dust plumes, e.g., the amount of time that the air masses were in the mixed layer and the frequency of precipitation along the air mass trajectory. 

Abstract. Measurements during episodes of African dust, made with two Wideband Integrated Bioaerosol Spectrometers (WIBS), one on the northeastern coast of Puerto Rico and the other in the city of León, Spain, show unmistakable, bioaerosol-like fluorescing aerosol particles (FAP) that can be associated with these dust episodes. The Puerto Rico events occurred during a major incursion of African dust during June 2020. The León events occurred in the late winter and spring of 2022 when widespread, elevated layers of dust inundated the Iberian Peninsula. Satellite and back trajectory analyses confirm that dust from Northern Africa was the source of the particles during both events. The WIBS measures the size of individual particles in the range from 0.5 µm to 30 µm, derives a shape factor and classifies seven types of fluorescence from the FAP. In general it is not possible to directly determine the specific biological identity from fluorescence signatures, however, measurements of these types of bioaerosols in laboratory studies allow us to compare ambient fluorescence patterns with whole microbial cells measured under controlled conditions. Here we introduce some new metrics that offer a more quantitative approach for comparing FAP characteristics derived from particles measured under different environmental conditions. The analysis highlights the similarities and differences at the two locations and reveals differences that can be attributed to the age and history of the dust plumes, e.g., the amount of time that the air masses were in the mixed layer and the frequency of precipitation along the air mass trajectory. 

Abstract. Many atmospheric aerosols are cloud condensation nuclei (CCN), capable ofactivating as cloud droplets when the relative humidity exceeds 100 %.Some primary biological aerosol particles (PBAPs), such as plant spores,pollen, or bacteria, have been identified as such CCN. Urban environmentsare a source of these bioaerosols, some of which are naturally produced by thelocal flora or are transported from surrounding regions and others of whichare a result of human activities. In the latter case, open sewage, uncoveredgarbage, mold or other products of such activities can be a source of PBAPs.There have been relatively few studies, especially in the tropics, wherePBAPs and CCN have been simultaneously studied to establish a causal linkbetween the two. The metropolis of San Juan, Puerto Rico, is one such urbanarea with a population of 2 448 000 people (as of 2020). To betterunderstand the fluorescent characteristics and cloud-forming efficiency ofaerosols in this region, measurements with a wideband integrated bioaerosolspectrometer (WIBS), a condensation nuclei (CN) counter and a CCNspectrometer were made at the University of Puerto Rico – Río PiedrasCampus. Results show that the CCN / CN activation ratio and the fraction offluorescing aerosol particles (FAPs) have repetitive daily trends when theFAP fraction is positively correlated with relative humidity and negativelycorrelated with wind speed, consistent with previous studies of fungi sporescollected on substrates. The results from this pilot study highlight the capabilities ofultraviolet-induced fluorescence (UV-IF) measurements for characterizing theproperties of FAPs as they relate to the daily evolution of PBAPs. The use ofmultiple excitation and emission wavelengths, along with shape detection,allows the differentiation of different PBAP types. These measurements,evaluated with respect to previous, substrate-based analysis of the localfungal and pollen spores, have established a preliminary database ofmeasurements that future, longer-term studies will build upon. 

Abstract. We report on fall speed measurements of raindrops in light-to-heavyrain events from two climatically different regimes (Greeley,Colorado, and Huntsville, Alabama) using the high-resolution(50 µm) Meteorological Particle Spectrometer (MPS) anda third-generation (170 µm resolution) 2-D videodisdrometer (2DVD). To mitigate wind effects, especially for thesmall drops, both instruments were installed within a 2∕3-scaleDouble Fence Intercomparison Reference (DFIR) enclosure. Two casesinvolved light-to-moderate wind speeds/gusts while the third casewas a tornadic supercell and several squall lines that passed overthe site with high wind speeds/gusts. As a proxy for turbulentintensity, maximum wind speeds from 10 m height at theinstrumented site recorded every 3 s were differenced withthe 5 min average wind speeds and then squared. The fall speedsvs. size from 0.1 to 2 and >0.7 mm were derived from theMPS and the 2DVD, respectively. Consistency of fall speeds from thetwo instruments in the overlap region (0.7–2 mm) gaveconfidence in the data quality and processing methodologies. Ourresults indicate that under low turbulence, the mean fall speedsagree well with fits to the terminal velocity measured in thelaboratory by Gunn and Kinzer from 100 µm up toprecipitation sizes. The histograms of fall speeds for 0.5, 0.7, 1and 1.5 mm sizes were examined in detail under the sameconditions. The histogram shapes for the 1 and 1.5 mm sizeswere symmetric and in good agreement between the two instrumentswith no evidence of skewness or of sub- or super-terminal fallspeeds. The histograms of the smaller 0.5 and 0.7 mm dropsfrom MPS, while generally symmetric, showed that occasionaloccurrences of sub- and super-terminal fall speeds could not beruled out. In the supercell case, the very strong gusts andinferred high turbulence intensity caused a significant broadeningof the fall speed distributions with negative skewness (for drops of1.3, 2 and 3 mm). The mean fall speeds were also found todecrease nearly linearly with increasing turbulent intensityattaining values about 25–30 % less than the terminalvelocity of Gunn–Kinzer, i.e., sub-terminal fall speeds.