Photoacoustic optical properties at UV, VIS, and near IR wavelengths for laboratory generated and winter time ambient urban aerosols

摘要

We present the laboratory and ambient photoacoustic (PA) measurement ofaerosol light absorption coefficients at ultraviolet wavelength (i.e., 355 nm) and compare with measurements at 405, 532, 870, and 1047 nm.Simultaneous measurements of aerosol light scattering coefficients wereachieved by the integrating reciprocal nephelometer within the PA's acousticresonator. Absorption and scattering measurements were carried out forvarious laboratory-generated aerosols, including salt, incense, and kerosenesoot to evaluate the instrument calibration and gain insight on the spectraldependence of aerosol light absorption and scattering. Ambient measurementswere obtained in Reno, Nevada, between 18 December 2009 and 18 January 2010.The measurement period included days with and without strong ground leveltemperature inversions, corresponding to highly polluted (freshly emittedaerosols) and relatively clean (aged aerosols) conditions. Particulatematter (PM) concentrations were measured and analyzed with other tracers oftraffic emissions. The temperature inversion episodes caused very highconcentration of PM_2.5 and PM₁₀ (particulate matter withaerodynamic diameters less than 2.5 μm and 10 μm, respectively)and gaseous pollutants: carbon monoxide (CO), nitric oxide (NO), andnitrogen dioxide (NO₂). The diurnal change of absorption and scatteringcoefficients during the polluted (inversion) days increased approximately bya factor of two for all wavelengths compared to the clean days. The spectralvariation in aerosol absorption coefficients indicated a significant amountof absorbing aerosol from traffic emissions and residential wood burning.The analysis of single scattering albedo (SSA), ?ngstr?m exponent ofabsorption (AEA), and ?ngstr?m exponent of scattering (AES) forclean and polluted days provides evidences that the aerosol aging andcoating process is suppressed by strong temperature inversion under cloudyconditions. In general, measured UV absorption coefficients were found to bemuch larger for biomass burning aerosol than for typical ambient aerosols.