Particle Compositions and their Temporal Variations as Measured using Raman Spectroscopy near Washington, DC, USA: Comparison with Air-Mass Trajectories

Compositions of individual super-micron aerosol particles and submicron refractory particles are not commonly measured. Raman spectroscopy (RS) is useful for characterizing such particles but can be tedious, so that the numbers of particles examined with RS are often limited. Here we use an aerosol Raman spectrometer (ARS) to measure single-aerosol particle compositions including calcium sulfate, calcium oxalates, aragonite, sodium nitrate, quartz, rutile, hematite, magnetite and others. The majority of spectra include D/G peaks, which indicate black carbon (BC) but may also indicate humic-like substances. High numbers of BC particles are consistent with the urban sampling site (Adelphi, MD) which is near large freeways and other heavily-trafficked roads, and with BC having a relatively large Raman cross section. Fluorescence is also common in the spectra. It is separated from the Raman signals, then clustered, and in some cases used to surmise information about the particles. The most obvious time-variation, in numbers of spectra of different types of particles, is diurnal. Air-mass trajectories calculated with HYSPLIT are used to investigate questions such as: Why is sodium nitrate observed far more at some times than at others? The capability of measuring 30,000 spectra of particles per day at 15 minute time resolution with the ARS may prove useful in epidemiological studies of the effects of particles of different composition on health. Aerosol concentrations (PM2.5 and PM10) are strongly linked to cardiopulmonary diseases and cancers. Although much is known about the health effects of certain harmful materials in aerosols, e.g., PAH carcinogens or asbestos, there are many unknowns regarding the effects of particle composition on health. A step toward expanding the epidemiology of aerosols and health to include particle composition is to measure particle composition with sufficient temporal and spatial resolution to make improved epidemiological studies of particle composition possible.