Aerosol optical properties in a rural environment near the mega-city Guangzhou, China: implications for regional air pollution, radiative forcing and remote sensing R. M. Garland1, H. Yang1, O. Schmid2, D. Rose1, A. Nowak3, P. Achtert3, A. Wiedensohler3, N. Takegawa4, K. Kita4, Y. Miyazaki4, Y. Kondo4, M. Hu5, M. Shao5, L. M. Zeng5, Y. H. Zhang5, M. O. Andreae1, and U. Pöschl1 1Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany 2Helmholtz Ctr. Munich, German Research Ctr. for Environmental Health, Inst. for Inhalation Biology, Neuherberg, Germany 3Leibniz Institute for Tropospheric Research, Leipzig, Germany 4RCAST, University of Tokyo, Tokyo, Japan 5State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China
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 (PM10)
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 average Ångström 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
Ångström 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 PM10 and PM1
concentrations derived from particle size distribution measurements were 2.8 m2 g−1
and 4.1 m2 g−1, respectively. The
Ångström exponent exhibited a wavelength dependence (curvature) that
was related to the ratio of fine and coarse particle mass (PM1/PM10) 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 between single scattering albedo and backscatter
fraction, and we found that it affects the impact that these parameters have on
aerosol radiative forcing efficiency and should be considered in model
studies of the PRD and similarly polluted mega-city regions.
Citation: Garland, R. M., Yang, H., Schmid, O., Rose, D., Nowak, A., Achtert, P., Wiedensohler, A., Takegawa, N., Kita, K., Miyazaki, Y., Kondo, Y., Hu, M., Shao, M., Zeng, L. M., Zhang, Y. H., Andreae, M. O., and Pöschl, U.: Aerosol optical properties in a rural environment near the mega-city Guangzhou, China: implications for regional air pollution, radiative forcing and remote sensing, Atmos. Chem. Phys., 8, 5161-5186, doi:10.5194/acp-8-5161-2008, 2008.