At the top of the Williams Tower high rise building in west Houston, six-hourly size-segregated aerosol composition measurements were made beginning 15 August 2000 for one month. The objective was (i) to describe changes in elemental composition of PM2.5 aerosols as a function of particle size and time and (ii) to gain insight into the origin of aerosols and their change during a typical daily cycle of vertical mixing. Aerosols were collected on mylar film using a three-stage rotating drum cascade impactor in size ranges: (i) coarse: 2.5 to 1.15 ìm; medium:0.34 to 1.15 ìm and fine 0.07 to 0.34 ìm. Subsequent chemical analysis was done non-destructively by XRF, PIXE, PESA and STIM for mass, hydrogen and fourteen other elements including Al, Cl, Cu, Ni, S, Si and V. Sub-micron S peaked from the 15 to 28 August with a secondary maximum from 3 to9 September. A maximum in the coarse fraction of S and of Cl was evident initially from 16 to 18 August. The variation in sub-micron Ni was different than that of S aerosol even though they tended to be highest in the polluted periods indicated by sub-micron S(above). Anthropogenic Ni and V indicate aerosol variability and morning mixing. Based on Ni to V content, there are three distinct fine particle aerosol types: (i) low Ni and high V; (ii) high Ni and low V (iii) similar concentrations typical of oil combustion. The distribution of the ratio Ni to V in fine particles shows clear evidence of type (iii) aeorosols in the morning period during vertical mixing of ground pollution up to the sampling level. In contrast, the other types occurred only at night. Further data analysis using mass apportionment and multivariate techniques taking into account atmospheric mixing and air parcel trajectories will be used to investigate sources and origin of aerosols. Together with parallel measurements of single particle characteristics made by other investigators, this data provides valuable insight into aerosol origin, formation and mixing processes at this elevated location.