Abstract
Abstract. We present the laboratory and ambient photoacoustic (PA) measurement of aerosol 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 were achieved by the integrating reciprocal nephelometer within the PA's acoustic resonator. Absorption and scattering measurements were carried out for various laboratory-generated aerosols, including salt, incense, and kerosene soot to evaluate the instrument calibration and gain insight on the spectral dependence of aerosol light absorption and scattering. Ambient measurements were obtained in Reno, Nevada, between 18 December 2009 and 18 January 2010. The measurement period included days with and without strong ground level temperature inversions, corresponding to highly polluted (freshly emitted aerosols) and relatively clean (aged aerosols) conditions. Particulate matter (PM) concentrations were measured and analyzed with other tracers of traffic emissions. The temperature inversion episodes caused very high concentration of PM2.5 and PM10 (particulate matter with aerodynamic diameters less than 2.5 μm and 10 μm, respectively) and gaseous pollutants: carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxide (NO2). The diurnal change of absorption and scattering coefficients during the polluted (inversion) days increased approximately by a factor of two for all wavelengths compared to the clean days. The spectral variation in aerosol absorption coefficients indicated a significant amount of absorbing aerosol from traffic emissions and residential wood burning. The analysis of single scattering albedo (SSA), Ångström exponent of absorption (AEA), and Ångström exponent of scattering (AES) for clean and polluted days provides evidences that the aerosol aging and coating process is suppressed by strong temperature inversion under cloudy conditions. In general, measured UV absorption coefficients were found to be much larger for biomass burning aerosol than for typical ambient aerosols.
Highlights
Atmospheric aerosols impact air quality and Earth’s radiation balance
Our previous investigation (Gyawali et al, 2009) suggests that even non-absorbing coatings on Black carbon (BC) can be disguised as Brown carbon (BrC) or light absorbing organic aerosol (OA), so it is not always possible to separate the effects of coatings on amplifying aerosol absorption and intrinsic absorption by the coating (Jacobson et al, 2000)
Though our measurement techniques and approaches are for aerosol sampled close to ground level, it is worthwhile to compare our findings with other published data even if these data are mainly based on remote sensing and focus on the total atmospheric column
Summary
Atmospheric aerosols impact air quality and Earth’s radiation balance. Aerosol light scattering redistributes electromagnetic energy in the atmosphere, whereas light absorption transforms it into thermal energy by heating absorbing aerosols and their surroundings. While BC aerosols absorb strongly over the entire solar spectrum, some OA absorbs efficiently in the UV and blue regions (Barnard et al, 2008; Bergstrom et al, 2007; Bond, 2001; Chakrabarty et al, 2010; Jacobson, 1999; Kirchstetter et al, 2004; Martins et al, 2009; Roden et al, 2006) These organic materials appear yellowish/brownish and are known as “brown carbon” (Andreae and Gelencser, 2006). Our previous investigation (Gyawali et al, 2009) suggests that even non-absorbing coatings on BC can be disguised as BrC or light absorbing OA, so it is not always possible to separate the effects of coatings on amplifying aerosol absorption and intrinsic absorption by the coating (Jacobson et al, 2000)
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