Current particulate PM 2.5 emission control policy is based on measurements of total aerosol mass loading. Thus, there is a need for aerosol instrumentation which can quantitatively characterize atmospheric particles. We describe here real-time, chemically resolved, mass loading measurements of Houston aerosol obtained during the TEXAQS 2000 field campaign. These measurements were obtained with the Aerodyne Aerosol Mass Spectrometer (AMS). The AMS operates by sampling sub micron (.05 to 2 micron) ambient particles into vacuum where they are flash vaporized and chemically analyzed via electron impact (EI) quadrupole mass spectrometry. The separation of particle vaporization and vapor ionization steps is the key feature of the AMS that enables linear and quantitative chemical composition detection even for complex mixtures. During TEXAQS 2000, total aerosol loadings as well as chemically speciated mass loadings for sulfate, nitrate, ammonium and organic aerosol species were measured by the AMS. Total aerosol mass loading ranged from as low as 2 µ gm-3 to as high as 50 µ gm-3. In general, sulfate contributed to approximately 32% of the total mass loading while the mass loading contribution from organic species ranged from 30% to 50%. Trends in the measured AMS mass loading are compared with those obtained from conventional aerosol measurement techniques (TEOM, filters). The chemically speciated mass concentrations are compared with speciated filter measurements as well as with the ion chromatography based Georgia Tech PILS instrument. On-going efforts to characterize the sensitivity of AMS mass loading measurements to mass-spectral interferences, type of particle, and size of particle are discussed.