Chemical and optical characterization of aerosols measured in spring 2002 at the ACE-Asia supersite, Zhenbeitai, China

[1] In April 2002, aerosol characteristics have been recorded at Zhenbeitai (ZBT) near the city of Yulin (38degrees17'N, 109degrees43'E, Shaanxi province, China). One year earlier, ZBT had been one of the continental supersites of the ACE-Asia international experiment. In spring, this site, located on the southwestern fringe of the Mu Us desert, is at the crossing of the pathways followed by dust originating from the main Chinese dust sources. During the experiment a customized aerosol sampler has been used. It had been specially designed to ensure the best possible sampling isokineticity for all instruments and to minimize coarse particle losses. Its cutoff size (D-50) has been computed to be 9 mum. For particles smaller than this size (PM9), mass concentrations, number concentrations, size distributions, elemental composition, and scattering properties were measured at ground level. Vertically integrated characteristics such as aerosol optical thickness (AOT), or single scattering albedo ( (omega) over bar (0)), were also determined by the means of a Sun-tracking photometer. During the measurement period, several dust events, one of them a major dust storm when PM9 mass concentrations became as high as 4650 mug/m(3), were observed. At this time the aerosol scattering coefficient reached 2800 Mm(-1). These dust events were separated by periods when optical properties were altered, or even dominated, by anthropogenic aerosol of local origin. For these periods, PM9 was significantly less (always below 100 mug/m(3)) than during dust events, mass concentration in black carbon (BC) was between 0.9 and 6.7 mug/m(3), and the aerosol scattering coefficient between 7 and 800 Mm(-1). No difference in elemental composition could be detected between the various dust episodes. Measured Fe/Al (0.63 +/- 0.04) and Mg/Al (0.32 +/- 0.03) ratios are consistent with an aerosol source located in the "northwestern high desert sources.'' This result is also supported by the air mass back-trajectories coming from a west or northwest direction and by the fact that dust layers had generally already acquired a significant vertical extension (similar to1500 m) when reaching the measurement site. This transported dust had already incorporated an anthropogenic carbonaceous component upon reaching the measurement site. The size distribution of this transported mixture is relatively constant and is characterized by the presence of a particle population with a number mean diameter between 1 and 2 mum. The mass scattering efficiency of this aerosol is 1.05 +/- 0.13 m(2)/g, relative to PM9, and its Angstrom exponent is close to 0.19. The single scattering albedo determined by data inversion of Sun photometer measurements is found to increase from 0.89 at 441 nm to 0.95 at 873 nm. These relatively large (omega) over bar (0) values indicate that, though mixing of mineral dust and anthropogenic aerosols is the rule rather than the exception at ZBT, the aerosol is still not very absorbent during intense dust events. In consequence, the dust optical characteristics measured at ZBT during dust storms are probably representative of the ones of pure dust emitted from the "northwestern high desert'' sources.