Aerosol optical extinction measurements in the exhaust plume of diesel vehicles using a pulsed laser cavity-ring down transmissometer

Airborne particulate matter (PM) comes from both natural and anthropogenic sources. The impact of PM on the atmosphere includes environmental issues affecting human health, visibility, and climate. Although interest in the role of PM in the atmosphere has grown, real-time measurements of PM optical properties have been an ongoing challenge. A recently developed portable instrument, the pulsed cavity ring-down transmissometer/nephelometer tandem (CRDT/N), is used to directly determine the optical extinction and scattering of the atmospheric PM. The results of laboratory and field studies of PM optical extinction, scattering, and single scattering albedo are presented.; Validation studies of the instrument for controlled PM types and concentrations were carried out in our laboratory. The CRDT/N measures the optical extinction at the parts-per-million level corresponding to an extinction coefficient (bext) of ∼1 Mm-1. It provides the single scattering albedo (o0) with an accuracy of 1 to 5 percent and a precision of absorption measurement around 1 percent. The extinction coefficients at two different wavelengths (532 nm/1064 nm) measured by the instrument and calculated from Mie theory for uniform polystyrene spheres (0.5 mum diameter) agree to within 4 percent.; Measurements taken at a location, 100 m from the highway are in close agreement with the reported o0 value for black particles, which is 0.31. As black particles travel, their optical properties can change causing an increase in o0. This expected increase was represented by the CRDT/N measurements from a separate location, 1 km from the highway, where the o0 was 0.49.; In addition to monitoring the optical properties of ambient PM, the concentration of O3, NO, and NO2, and environmental conditions were also measured during the field studies. The correlation between absorption of PM and these factors was then investigated. At the first location during rush hour ∼68 percent of the CRDT/N PM absorption measurements correlate with [NO] in combination with less strongly correlated environmental factors. A weaker correlation (∼33%) was observed at the second location. The field studies show that the CRDT/N instrument will give much needed insight into the penetration of diesel exhaust into neighborhoods near highways.