Measurement report: An exploratory study of fluorescence and cloud condensation nuclei activity of urban aerosols in San Juan, Puerto Rico

Many atmospheric aerosols are cloud condensation nuclei (CCN), capable of activating 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 environments are a source of these bioaerosols, some of which are naturally produced by the local flora or are transported from surrounding regions and others of which are a result of human activities. In the latter case, open sewage, uncovered garbage, mold or other products of such activities can be a source of PBAPs. There have been relatively few studies, especially in the tropics, where PBAPs and CCN have been simultaneously studied to establish a causal link between the two. The metropolis of San Juan, Puerto Rico, is one such urban area with a population of 2 448 000 people (as of 2020). To better understand the fluorescent characteristics and cloud-forming efficiency of aerosols in this region, measurements with a wideband integrated bioaerosol spectrometer (WIBS), a condensation nuclei (CN) counter and a CCN spectrometer were made at the University of Puerto Rico – Río Piedras Campus. Results show that the CCN / CN activation ratio and the fraction of fluorescing aerosol particles (FAPs) have repetitive daily trends when the FAP fraction is positively correlated with relative humidity and negatively correlated with wind speed, consistent with previous studies of fungi spores collected on substrates. The results from this pilot study highlight the capabilities of ultraviolet-induced fluorescence (UV-IF) measurements for characterizing the properties of FAPs as they relate to the daily evolution of PBAPs. The use of multiple 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 local fungal and pollen spores, have established a preliminary database of measurements that future, longer-term studies will build upon.