Atmospheric aerosol measurements by cavity ringdown turbidimetry

We have investigated monitoring the ambient air extinction coefficient as a sensitive indicator of micrometer sized airborne particles through cavity ringdown spectroscopy (CRDS) at 2 wavelengths. Scatter and absorption of light by the airborne particles induced a measureable decrease in the ringdown decay time of our ringdown cell. When a copper vapor laser operating at 8-10 kHz was employed and 1,500 individual ringdowns were averaged on an oscilloscope, minimal detectable extinction coefficients of 10^-6 m^-1 could be achieved. We have used our ringdown instrument to detect a correlation between the observed ringdown extinction coefficient and particulate mass concentrations (μg/m³ of air). The correlation we have observed allows estimation of suspended mass concentrations in a matter of minutes, rather than a sampling time of several hours or days encountered in traditional gravimetric approaches. Additionally, we have constructed a variable cut diameter inertial impactor for use with our ringdown system. By varying the cut diameter while making ringdown measurements, it was possible to extract a size resolved extinction spectrum of ambient atmospheric aerosol in a matter of minutes.