Aerosol optical properties in a rural environment near the mega-city Guangzhou, China: implications for regional air pollution, radiative forcing and remote sensing

Abstract

Abstract. The scattering and absorption of solar radiation by atmospheric aerosols is a key element of the Earth's radiative energy balance and climate. The optical properties of aerosol particles are, however, highly variable and not well characterized, especially near newly emerging mega-cities. In this study, aerosol optical properties were measured at a rural site approximately 60 km northwest of the mega-city Guangzhou in southeast China. The measurements were part of the PRIDE-PRD2006 intensive campaign, covering the period of 1-30 July 2006. Scattering and absorption coefficients of dry aerosol particles with diameters up to 10 µm (PM 10 ) were determined with a three-wavelength integrating nephelometer and with a photoacoustic spectrometer, respectively.Averaged over the measurement campaign (arithmetic mean ± standard deviation), the total scattering coefficients were 200±133 Mm −1 (450 nm), 151±103 Mm −1 (550 nm) and 104±72 Mm −1 (700 nm) and the absorption coefficient was 34.3±26.5 Mm −1 (532 nm). The averageÅngström exponent was 1.46±0.21 (450 nm/700 nm) and the average single scattering albedo was 0.82±0.07 (532 nm) with minimum values as low as 0.5. The low single scattering albedo values indicate a high abundance, as well as strong sources, of light absorbing carbon (LAC). The ratio of LAC to CO concentration was highly variable throughout the campaign, indicating a complex mix of different combustion sources. The scattering and absorption coefficients, as well as the Correspondence to: R. M. Garland (garland@mpch-mainz.mpg.de)Å ngström exponent and single scattering albedo, exhibited pronounced diurnal cycles, which can be attributed to boundary layer mixing effects and enhanced nighttime emissions of LAC (diesel soot from regulated truck traffic). The daytime average mid-visible single scattering albedo of 0.87 appears to be more suitable for climate modeling purposes than the 24-h average of 0.82, as the latter value is strongly influenced by fresh emissions into a shallow nocturnal boundary layer. In spite of high photochemical activity during daytime, we found no evidence for strong local production of secondary aerosol mass.The average mass scattering efficiencies with respect to PM 10 and PM 1 concentrations derived from particle size distribution measurements were 2.8 m 2 g −1 and 4.1 m 2 g −1 , respectively. TheÅngström exponent exhibited a wavelength dependence (curvature) that was related to the ratio of fine and coarse particle mass (PM 1 /PM 10 ) as well as the surface mode diameter of the fine particle fraction. The results demonstrate consistency between in situ measurements and a remote sensing formalism with regard to the fine particle fraction and volume mode diameter, but there are also systematic deviations for the larger mode diameters. Thus we suggest that more data sets from in situ measurements of aerosol optical parameters and particle size distributions should be used to evaluate formalisms applied in aerosol remote sensing. Moreover, we observed a negative correlation be...

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Truncation and Angular-Scattering Corrections for Absorbing Aerosol in the TSI 3563 Nephelometer

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“…Anderson et al (2003) reportedå sp = 2.1 for pollution-dominated airmasses and 1.5 for air with primarily fine-mode aerosol in the ACE-Asia field experiment. Garland et al (2008) foundå sp = 1.5 in Guangzhou, China. Values toward the right side of the graph are similar to atmospheric aerosol; we include lower values ofå sp because we have observed them in the laboratory for submicron, brown carbon aerosol.…”