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

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 laboratorygenerated 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 PM 2.5 and PM 10 (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 (NO 2 ). 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), andÅngström 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.

Diurnal variations of aerosol optical properties in the North China Plain and their influences on the estimates of direct aerosol radiative effect

“…At dry conditions, the SSA reaches minimum in the morning and evening, and maximum at noon, with an average of 0.86. This result is similar to most previous studies on the diurnal variation of SSA for dry state aerosol (He et al, 2009;Fan et al, 2010;Junwei et al, 2012;Gyawali et al, 2012). For ambient aerosol, many of the aerosol components are hygroscopic and can take up water as a function of RH (Bian et al, 2014;Cheng et al, 2008), making the SSA change as the RH changes.…”

AERONET-based models of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth

“…Eck et al, 1999;Dubovik et al, 2002;Salinas et al, 2009;Bovchaliuk et al, 2013) or other techniques (e.g. He et al, 2009;Gyawali et al, 2012;McMeeking et al, 2012). On the one hand this means that optical models based on smoke may also be appropriate optical proxies for mixed smoke/urban or urban aerosols in some cases (although again commonality of optical properties should not be taken to imply commonality of microphysical properties or origin).…”

Classifying aerosol type using in situ surface spectral aerosol optical properties

“…SSA is the ratio of scattering to extinction (absorption + scattering) and provides information on aerosol darkness and composition and may determine the net sign of an aerosol's radiative forcing (Hansen et al, 1997). High SSA values near 1 indicate low-or nonabsorbing "white" aerosols, while low SSA values (below 0.85) indicate "darker" highly absorbing aerosols, and thus an SSA value can be used to characterize the aerosol type (Bergstrom et al, 2002;Russell et al, 2010;Gyawali et al, 2012). Equations for calculating these properties from extensive optical parameters are found in Sect.…”