Excitation-Emission Spectra and Fluorescence Quantum Yields for Fresh and Aged Biogenic Secondary Organic Aerosols

Certain biogenic secondary organic aerosols (SOA) become absorbent and fluorescent when exposed to reduced nitrogen compounds such as ammonia, amines, and their salts. Fluorescent SOA may potentially be mistaken for biological Particles by detection methods relying on fluorescence. This work quantifies the spectral - 'distribution' and effective quantum yields of fluorescence of water-soluble SOA; generated from two monoterpenes, limonene and alpha-pinene, and two different oxidants, ozone (O-3) and hydroxyl radical (OH). The SOA was generated in a smog chamber, collected on substrates, and aged by exposure to similar to 100 ppb ammonia in air saturated with water vapor. Absorption and excitation-emission matrix (EEM) spectra of aqueous extracts of aged and control SOA samples were measured, and the effective absorption coefficients and fluorescence quantum yields (similar to 0.005 for 349 nm excitation) were determined from the data. The strongest fluorescence for the limonene-derived SOA was observed for lambda(excitation) = 420 +/- 50 nm and lambda(emission) = 475 +/- 38 nm. The window of the strongest fluorescence shifted to lambda(excitation) = 320 +/- 25 nm and lambda(emission) = 425 +/- 38 nm for the alpha-pinene-derived SOA. Both regions overlap with the EEM spectra of some of the fluorophores found in aerosols. Despite the low quantum yield, the aged SOA particles may have sufficient fluorescence intensities to interfere with the fluorescence detection of common bioaerosols.