Analyses and assessment on the influence of sanddust activities in the Hexi Corridor on the PM<sub>10</sub> concentration in
Lanzhou city from 2001 to 2005 lead to the following conclusions of this paper. Lanzhou PM<sub>10</sub> concentrations (LPC)
possess a double-peak annual distributive character with one peak in December and the other in next March both in the
same winter half-year, however, the number of the sanddust activities in the corridor shows a single-peak annual
variation with the peak in April (9.8 times/month). It is known from comparing the distributions of the both that the peak
value of the number of Hexi sanddust activities (HSA) corresponds to the sub-peak of LPCs in spring, and it is held that
the winter particular boundary layer condition is mainly responsible for the peak of LPC in December. The number of
spring HSA is positively/significantly correlated with the LPC in the same time period, i.e. the frequent period of HSA
corresponds to the sub-peak period of LPCs. The quantitative analyses of sanddust indices indicate that 15.7 %, 37.8%,
and 49.5% of the annual, march, April LPCs are associated with the impact of HSAs, respectively, and 8%, 12.7%,
19.8% of the annual, march, April LPCs arise from the PM<sub>10</sub> particles transfer of HSAs. The HSAs lead to the
multiplication of the LPC in successive impacting day, but the impacting extent is different in different time periods.
The helicity of a severe sandstorm happened in Northwest China was analyzed by using global reanalysis grid data of NCEP/NCAR for 4 times a day. As an important physical parameter in analyzing and predicting severe convective weather, the helicity also has good indication in the forecasting of sandstorm. The distribution of helicity over the sandstorm area was negative at higher levels and positive at lower levels. There was definite relationship between the evolvement of helicity's negative value at higher levels and the occurrence of sandstorm.