Seasonal cycles of fluorescent biological aerosol particles in boreal and semi-arid forests of Finland and Colorado

Abstract

Abstract. Biological aerosol particles have become increasingly important for atmospheric study, but continuous measurements at high time and size resolution have not been available until recently. Here we report seasonal cycles of fluorescent biological aerosol particles (FBAP) from the boreal forest in Hyytiälä, Finland (18 months) and the semi-arid Manitou Experimental Forest, Colorado (10 months). FBAP at both locations were observed to be highest in summer and lowest in winter, increasing by factors of 12 and 5 between these seasons, respectively. In addition to the low temperatures and reduced sunlight during winter, we suggest that snow cover inhibited FBAP release from local terrestrial surfaces and that more extensive snow cover at the Finland site contributed to lower winter FBAP concentrations. Average size distributions at each site exhibited peaks between 1.5 and 6 μm in aerodynamic diameter. The Finland site consistently showed a dominant, narrow FBAP peak at ~ 3 μm in addition to discreet modes at ~ 1.5 and ~ 5 μm, whereas the Colorado site showed broader peaks at 1.5 and 5 μm, suggesting different modes of biological particles at the two sites. FBAP concentrations in both locations were shown to correlate with daily patterns of relative humidity (RH) during each season. Also during summer at each site, average FBAP concentration scaled with RH, but at the Finland site RH values above ~ 82% led to a significant decrease in FBAP concentration. We hypothesize that this is due to dew formation that inhibits bioparticle release. Lastly we show that rain during summer at each location led to pronounced increases in both fluorescent and total particle concentrations with FBAP peak particle size at ~ 2 μm and concentration scaling with rain intensity. We suggest that these particles are primarily fungal spores and other bioparticles lofted from splashing of rain droplets hitting soil and leaf surfaces. During the summer at the Colorado site we consistently observed a mode of ~ 4 μm particles appearing several hours after rain events that we suggest are fungal spores actively emitted when ambient conditions are most advantageous for spread and germination. The pronounced patterns of fluorescent bioparticles observed here suggest that parameterizations of both daily and seasonal cycles will be important to accurately reflect bioparticle emissions in future studies of atmospheric bioaerosols and their potential effects on clouds and precipitation.

Characterization of free amino acids, bacteria and fungi in size-segregated atmospheric aerosols in boreal forest: seasonal patterns, abundances and size distributions

“…As could be expected (Manninen et al, 2014;Schumacher et al, 2013), the lowest concentration levels of FAAs and microorganisms were detected during winter, when the air and soil temperatures were below 0 • C and the ground was covered by snow. In February, particles below 2.5 µm accounted for 89 % of the total FAA concentration.…”

Evaluation of machine learning algorithms for classification of primary biological aerosol using a new UV-LIF spectrometer

“…Nonetheless, the practicality of long-term, continuous, real-time monitoring and discrimination of at least some of these properties for the more common types has already been demonstrated, e.g. at rural and semi-rural background sites in Germany, Ireland and Finland (Healy et al, 2014;Toprak and Schnaiter, 2013;Schumacher et al, 2013).…”

Airborne soil organic particles generated by precipitation

“…Large super-micrometre fluorescent particles are commonly attributed to biogenic spores, bacteria, fungi, and so on. However, because of their SOM nature, ASOP would also have a strong fluorescence signal 29 and some of these sub-micrometre fluorescent particles 27,28 may have been ASOP. Furthermore, elevated concentrations of ice nucleating particles detected 27,28 after rainfall might be also partially related to ASOP.…”

“…However, because of their SOM nature, ASOP would also have a strong fluorescence signal 29 and some of these sub-micrometre fluorescent particles 27,28 may have been ASOP. Furthermore, elevated concentrations of ice nucleating particles detected 27,28 after rainfall might be also partially related to ASOP. Finally, a slightly counterintuitive observation 30 that stronger and more widespread rain, in a tropical rural location in India, resulted in a smaller reduction and faster recovery of the aerosol optical depth than less intense rainfall, may again be in line with the ASOP formation.…”

“…8), it is reasonable to suggest that those particles were perhaps ASOP. Also, numerous studies have reported enhancement of fluorescent particles after rain events [25][26][27][28] . Large super-micrometre fluorescent particles are commonly attributed to biogenic spores, bacteria, fungi, and so on.…”