1.1. Organic Carbon in Atmospheric Aerosol
 Atmospheric aerosols are important to fields of science ranging from Earth climate to human health. This study focuses on the organic carbon (OC) aerosols, including biological aerosols, in the atmospheric boundary layer, and on laser-induced fluorescence (LIF) spectral techniques [Pinnick et al., 2004] for studying this aerosol component. Extensive research over many years has aimed to characterize the OC fraction of atmospheric aerosol using a variety of techniques ranging from culturing of bacteria, to single-particle mass spectrometry (MS), to chromatographic studies of extracts of collected particles. Typically the various methods do not provide the same information, but each tends to provide a different view of the extremely complex mixture of atmospheric aerosol.
1.2. Brief Summary of Some Previous Studies of Atmospheric OC Aerosol
 OC in atmospheric aerosols ranges from small molecules such as oxalic acid or phenols, to PAH, to viable bacteria, fungi and spores (each of which is composed of many thousands of different kinds of organic molecules), to highly complex mixtures of decomposition products of biological materials (e.g., humic substances), to secondary OC aerosols formed initially by oxidation of volatile organics (studied extensively since the pioneering studies of Haagen-Smit  on smog, and Went  emphasizing biogenic terpenes). OC aerosols can include alkanes, alkenes, carboxylic acids, ketones, phenols, furans, terpenoids, PAH, aromatic polyacids, lignans, cellulose, humic and fulvic acids, humic-like substances (HULIS), pollens, bacteria, bacterial spores, viruses and fungal spores [Shah et al., 1986; Larson et al., 1989; White and Macias, 1989; Novakov and Penner, 1993; Rogge et al., 1993a, 1993b; Saxena and Hildeman, 1996; Penner and Novakov, 1996; Lighthart, 1997; Shaffer and Lighthart, 1997; Seinfeld and Pandis, 2006; Havers et al., 1998; Zappoli et al., 1999; Krivacsy et al., 2000; Decesari et al., 2002; Bauer et al., 2002a, 2002b, 2002c; Lighthart and Mohr, 1994; Kiss et al., 2002; Huang et al., 2006; Qin and Prather, 2006; Dzepina et al., 2007]. The relative contributions of primary organic aerosol (particles directly injected into the atmosphere) and secondary organic aerosol (particles formed by gas-to-particle conversion) is highly variable [Alfarra et al., 2004; Kanakidou et al., 2005; Qin and Prather, 2006; Robinson et al., 2007].
 Concentrations of OC aerosols are significant but highly variable in the lower troposphere [White and Macias, 1989, Saxena and Hildeman, 1996; Larson et al., 1989; Hitzenberger et al., 1999; Zappoli et al., 1999; Krivacsy et al., 2001; Fraser et al., 2002], both in urban environments [Shah et al., 1986; Huntzicker et al., 1986; Dasch and Cadle, 1989; Holler et al., 2002; Qin and Prather, 2006], where typical concentrations are 1–15 μg m−3, and in rural/remote/North Atlantic regions [Shah et al., 1986; Cavalli et al., 2004], where concentrations are 0.5–4 μg m−3. Meteorological factors can affect concentrations of biological aerosols [Jones and Harrison, 2004]. OC aerosols can show both diurnal cycles [Zhang et al., 2004; Qin and Prather, 2006; Dzepina et al., 2007] and seasonal dependence [Rogge et al., 1993b; Bae et al., 2006]. HULIS accounts for a significant fraction of urban OC aerosol mass [Samburova et al., 2005].
 Processes that generate OC aerosols include biomass combustion [Zappoli et al., 1999; Cao et al., 2006], residential wood and coal burning, cooking [Herckes et al., 2002], the nucleation and growth of new particles in urban environments [Zhang et al., 2004; Qin and Prather, 2006], microbial decay of plant matter [Rogge et al., 1993a; Havers et al.,1998; Robinson et al., 2007], emission of volatile OC by vegetation and bacteria followed by oxidation, and then condensation into secondary OC aerosols [Kanakidou et al., 2005; Went, 1960], microbial and biochemical degradation of organic debris in the soil [Gelencsér et al., 2002], wind action around dairy and cattle feedlots [Rogge et al., 2006], farming operations, and sewage wastewater treatment plants [Bauer et al., 2002c]. Humic-like substances in surfactant matter on the ocean surface are aerosolized by wave action [Oppo et al., 1999]. Bubble bursting processes are believed to be an important source of water soluble OC (WSOC) particles over the north Atlantic [Cavalli et al., 2004]. Most airborne bacteria are derived from plants, soil, and water bodies. Their injection into the atmosphere is thought to derive from wind action (on plants and soil) and from bubble-bursting [Lighthart, 1997]. Both primary OC and secondary OC aerosols may be aged in the atmosphere. Bioaerosols and humic materials tend to be extremely complex even before they undergo aging in the atmosphere. Even with far simpler types of primary OC aerosol, the chemical reactions that age OC aerosols and the resulting aged aerosols can be extremely complex. The chemical makeup of a significant mass fraction of OC aerosol is not well known [Samburova et al., 2005]. Collectively the above references suggest that much more needs to be learned about OC aerosol in the Earth's atmosphere.
1.3. Aerosol Diagnostic Techniques
 A variety of aerosol characterization methods are used to provide complementary, and sometimes overlapping, insights into compositions of particles. Analyses of OC aerosols are often performed on sample collections because a significant mass of aerosol is required for analysis, although many single-particle studies using mass spectroscopy (MS), and some studies using laser-induced-breakdown spectroscopy (LIBS) and LIF have been reported [e.g., Hettinger et al., 2005; Lithgow et al., 2004; Pinnick et al., 2004].
1.3.1. Analysis Techniques Suitable for Aerosol Collections
 Optical and electron microscopy can indicate particle shape, which can be definitive for some types of particles, for example, some types of pollens. The combination of x-ray fluorescence and electron microscopy can also provide information about single-particle elemental composition. Culturing of bacteria, other microorganisms, and viruses collected in air samples can in some cases provide definitive information relating to viability, and identification to the genus or species level [Hensel and Petzoldt, 1995; Shaffer and Lighthart, 1997; Lighthart and Tong, 1998]. Culturing may be a first step in obtaining sufficient material for more detailed biochemical analyses. Biochemical assays for specific proteins and DNA or RNA sequences [Hindson et al., 2005] can indicate the presence of specific bacteria or viruses, or specific allergens. These assays typically indicate very little about molecules for which probes were not used. Mass spectrometers have been used extensively to characterize organic and nonorganic aerosols, apportion sources, etc. In Aerodyne's Aerosol MS (AMS), a small fraction of the molecules within a particle is vaporized and ionized, and the mass-to-charge spectra of the resulting ions is detected and accumulated for a collection of particles [Jayne et al., 2000]. Nuclear magnetic resonance (NMR) has been used to estimate the fractions of hydrogen atoms bound to C=O, and to C-O. Collections of aerosol can be fractionated on the basis of physiochemical properties such as solubility, tendency to elute into specific solvents, retention times in chromatographic columns, electrophoretic mobility, etc., and the resulting fractions can be analyzed using a variety of techniques, such as MS, NMR, or infrared spectroscopy. Gas chromatography-MS (GC-MS) has been developed and used extensively [Mazurek and Simoneit, 1997; Simoneit et al., 1999; Rogge et al., 2006; Williams et al., 2006].
1.3.2. Non-LIF Single-Airborne-Particle Diagnostic Techniques
 Techniques for rapidly analyzing single aerosol particles, even if they are not as definitive as those used on bulk samples, can be useful for better understanding the sources, chemistry, and fate of OC aerosol. In collections of particles: (1) definitive inferences cannot be made about the compositions of individual particles; (2) particles that occur in low concentrations may be difficult to detect using some analytical techniques; and (3) rapid time variations of specific, low concentration aerosol species are often not detected, because aerosol must be collected long enough to obtain material for analysis.
 Three primary non-LIF single-airborne-particle analysis techniques (beyond particle size), being used and/or developed, are as follows.
 1. The laser-ablation aerosol time-of-flight MS (ATOFMS), particle analysis by laser mass spectrometry (PALMS), and the rapid single-particle MS (RSMS) provide elemental composition and masses of laser-ablated molecular fragments of single aerosol particles [Prather et al., 1994; Noble and Prather, 2000; Kane and Johnston, 2000; Lee et al., 2002; Murphy et al., 2003; Middlebrook et al., 2003; Sullivan and Prather, 2005]. Aerosol MS technology is the most widely used of the techniques listed here, and a commercially instrument is available (e.g., the ATOFMS from TSI Inc.).
 2. LIBS yields atomic emission and plasma emission spectra that can help classify OC aerosols [Radziemski et al., 1983; Hahn and Lunden, 2000; Carranza and Hahn, 2002; Hybl et al., 2006]. LIBS has been applied to atmospheric aerosols [Hettinger et al., 2005; Lithgow et al., 2004; Hybl et al., 2006].
 3. Angularly resolved elastic light scattering is a function of the shape and optical properties of a particle. Scattering patterns of single atmospheric particles have been measured in an attempt to classify particle morphologies [Kaye et al., 2000; Aptowicz et al., 2006].
1.3.3. Single-Particle Laser-Induced-Fluorescence Methods
 Instruments have been developed to measure LIF in one or two broadband wavelength channels [Pinnick et al., 1995; Hairston et al., 1997; Reyes et al., 1999; Seaver et al., 1999; Eversole et al., 1999; Kaye et al., 2000; Eversole et al., 2001; Ho, 2002], and such an instrument is available commercially, the UV-APS from TSI, Inc. Instruments have been developed to detect the dispersed LIF spectrum over an appreciable wavelength range [Hill et al., 1999; Pan et al., 2003a]. Pinnick et al.  assembled a Particle Fluorescence Spectrometer (PFS) that measures, on the fly, the fluorescence spectra of individual airborne particles in the atmosphere, and used it to measure moderate-resolution single-particle spectra of ambient atmospheric aerosol. The spectra were clustered using an unstructured hierarchical technique. Some of the clusters found had fluorescent spectra appearing similar to some materials that are known to exist as aerosols. For example, (1) one was similar to that of bacteria and proteins; (2) one was similar to spectra of collections of marine aerosol [Oppo et al., 1999]; (3) one was similar to some spectra of cellulose (pure cellulose should not fluoresce, but compounds in plant cell walls such as ferulic acid, have spectra similar to that of typical cellulose); and (4) one was similar to some spectra of fulvic or humic acids [De Souza Sierra et al., 2000; Krivacsy et al., 2000; Klapper et al., 2002; Goldberg and Weiner, 1994] or humic-like substances (HULIS) [Ouatmane et al., 2002].
 The LIF technique should be useful for measuring one or a few primary OC tracer molecules for different types of particles. Essentially all the fluorescence of aerosols is probably from aromatic OC, but only a small fraction of aromatics are strongly fluorescent. A LIF spectrum may be dominated by one or a few fluorescent moieties, even when the predominant fluorescent moiety contains a very small fraction of the mass. Also, many molecules that are fluorescent may not occur in sufficiently high concentrations in aerosol to contribute significantly to LIF spectra. For example, pure bacteria and bacterial spores, when illuminated with light in the 260 to 280 nm range, typically have fluorescence spectra that are strongly dominated by the aromatic amino acid tryptophan. Therefore tryptophan can serve as a LIF tracer for bacteria and proteins, similar to the way in which levoglucosan can be a tracer for particles from biomass burning when measured with GC-MS [Simoneit et al., 1999], and calcium dipicolinate can be a tracer for bacterial spores when measured with MS [Srivastava et al., 2005]. NADH and flavins are other strongly fluorescent biomolecules that occur in all cells, and which can be dominant fluorors from bacteria when the excitation wavelength is longer than, for example, 305 nm; however, when excited at 263 nm as in work by Pinnick et al.  their contribution is swamped by that of tryptophan. Many other aromatic moieties in cells are negligibly fluorescent, for example, the bases for DNA and RNA (adenine, guanine, thymine, cytosine, and uracil), which absorb well at 263 nm, have very low quantum efficiencies (adenine's efficiency is about 0.0001 as compared to tryptophan's 0.15). Little of the aromatic material in noncombustion aerosol that is biological or of recent biological origin (bacteria, proteins, pollens, lignins, and humic substances) is PAH, although OC in vegetation, for example, pine tar, can contain PAH from terpenes, and wind action may blow particles of these into the atmosphere. Emission rates of phosphorescent mineral aerosols are probably too small for single-shot measurements with the PFS, with its 1-μs measurement window during the 10-ns laser pulse. Previously we thought some nonaromatic OC compounds (e.g., chlorophyll) might contribute significantly to these fluorescence spectra, but were unable to find evidence to support this.
 A class of atmospheric OC material that may not be seen with the PFS includes secondary organic aerosol (SOA) generated from volatile terpenes (e.g., pinene), terpenoids and other volatiles emitted from vegetation. We are not aware of terpenoids formed by atmospheric reactions of volatile terpenes/terpenoids that would be detected by the PFS with excitation at 263 nm. Vegetation can emit volatile (e.g., benzaldehyde) and semivolatile (e.g., methyl salicylate, benzyl acetate, estragole [Dudavera et al., 2004]) aromatic OC, which may also react in air and/or condense with the SOA, and possibly these aromatic materials could be sufficiently fluorescent for the SOA formed only from vegetation to be measured with the PFS.
 The single-particle LIF spectral technique can provide additional and complementary information to that obtained by other techniques for characterizing OC, and is well-suited for volatile particles. Because a small fraction of molecules (or monomers that go into polymers such as proteins or lignans) are fluorescent, the fluorescent molecules can, in some cases, act as tracers for certain particle types. Because the LIF technique does not destroy the particles, it could be combined with other online instrumentation, for example, MS, or with an airborne particle sorter which collects selected particles for further analysis [Pan et al., 2004].
 Here improvements to the particle fluorescence spectrometer (PFS) are briefly summarized. Measurements of the fluorescence spectra of individual atmospheric aerosol particles at New Haven, CT and at Las Cruces, NM, and the results of clustering of these spectra are presented. A key result is that the main template spectra found earlier at Adelphi, MD [Pinnick et al., 2004] are found again in New Haven and in Las Cruces, although the fraction of fluorescent particles and the relative populations of particles in various clusters are different.