Development of a dual-wavelength photoacoustic instrument for measurement of light absorption and scattering by aerosol and gases

A dual-wavelength photoacoustic instrument has been developed for measurement of light absorption and scattering by aerosol and gases. The novel instrumentation allows for in situ measurement of optical properties of atmospheric constituents simultaneously at two wavelengths. Two compact diode lasers, operating at 405 nm and 870 nm, are employed, providing for assessment of spectral variation in aerosol optical properties from near-UV to near-IR wavelengths. Measurements of light scattering by reciprocal nephelometry within the instrument resonator accompany photoacoustic absorption measurements and allow for calculation of extinction. The dual-wavelength photoacoustic builds on previous single-wavelength photoacoustic instrumentation. The theory of instrument operation is described in detail, including separation and measurement of noise from signal and acoustic considerations for single and dual-wavelength absorption measurements. Also described and implemented is a simple method of instrument calibration. The calibration process utilizes NO2 gas, salt aerosol, and kerosene-flame soot aerosol, and so covers a range of optical properties relevant to atmospheric constituents.; The dual-wavelength photoacoustic instrument was used during the 2006 Fire Lab at Missoula Experiment (FLAME) to measure optical properties of wood smoke from a variety of biomass fuels. A range of single scattering albedo values was measured for different fuel types, and the spectral dependence of absorption was quantified and parameterized using the Angstrom exponent of absorption. Increased absorption at shorter wavelengths, characterized by an Angstrom exponent greater than one, was found for many highly scattering aerosols. The measurements show conclusively that light absorbing organic material is present in wood smoke. Spectral properties of this organic material, which preferentially absorbs UV and near-UV radiation, indicate that casual use of the inverse wavelength dependence of aerosol light absorption in remote sensing and modeling applications can introduce large errors at UV and visible wavelengths when extrapolated from near-IR absorption for certain types of wood smoke. The spectral variation in optical properties provides insight into differentiation of aerosols from mobile or industrial sources versus those from biomass burning. Urban aerosol measurements made on the University of Nevada, Reno campus complement laboratory wood smoke measurements and confirm instrument stability and sensitivity for ambient measurements.