Fluorescence spectra and elastic scattering characteristics of atmospheric aerosol in Las Cruces, New Mexico, USA: Variability of concentrations and possible constituents and sources of particles in various spectral clusters

The UV-excited laser-induced-fluorescence (UV-LIF) spectra of single atmospheric particles and the three-band integrating-nephelometer elastic scattering of atmospheric aerosol were measured during four approximately 24-h periods on May 2007 in Las Cruces, New Mexico, USA. Aerosol scattering measurements in the nephelometer red channel (50-nm band centered at 700-nm) ranged from around 3–10 times the molecular (Rayleigh) scattering background. On average 22.8% of particles with size greater than about 1 μm diameter have fluorescence above a preset fluorescence threshold. A hierarchical cluster analysis indicates that most of the single-particle UV-LIF spectra fall into about 10 categories (spectral clusters) as found previously at other geographic sites (Pinnick et al., 2004; Pan et al., 2007). The clusters include spectra characteristic of various humic/fulvic acids, humic-like-substances (HULIS), chemically aged terpenes, fungal spores, polycyclic aromatic hydrocarbons, bacteria, cellulose/pollens, and mixtures of various organic carbon compounds. By far the most populated cluster category is similar to those of chemically aged terpenes/humic-materials; on average this population comprises about 62% of fluorescent particles. Clusters with spectra similar to that of some HULIS aerosol contain on average 10.0% of particles; those characteristic of some fungal spores (or perhaps mixtures of aromatic organic compounds) 8.4% of particles; bacteria-like spectra 1.6% of particles; and cellulose/pollen-like spectra 0.8% of particles. Measurements of fluorescent particles over relatively short (24 min) periods reveal that the concentrations of particles in the most populated clusters are highly correlated, suggesting that the particles populating them derive from the same region; these particles might be composed of crustal material coated with secondary organic carbon. On the other hand, concentrations of particles having cellulose-like spectra are generally uncorrelated with those in any other cluster. No clear distinction in fluorescent aerosol characteristics can be seen for different air mass trajectories arriving at the sampling site, suggesting that fluorescent aerosol particles are primarily of local origin. Integrations of the single-particle UV-LIF spectra over approximate 24 h time intervals reveal two broad peaks around 350 nm and 450 nm (for 263 nm excitation); the 450 nm peak is somewhat similar to that measured previously for water soluble organic carbon derived from aerosol collections. The 350 nm peak apparently has not been seen before in measurements of aerosol collections and may derive from non-soluble primary biological aerosol particles such as fungal spores. Further measurements are needed to investigate in more detail the generality of these results.