We report the first online measurements of the chemical composition of atmospheric aerosol in the 6-20 nm diameter range. These measurements were performed using the recently developed Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), and were made possible by recent sensitivity enhancements resulting from (a) the development of a unipolar charger optimized for high aerosol flow rates, and (b) an improved flow system in the TDCIMS sample inlet. Measurements of atmospheric aerosol in Boulder, CO revealed large concentration variations in most detected compounds. The most dominant observed compounds in the negative ion TDCIMS spectra were nitrate and sulfate, while in the positive ion spectra ammonium dominated all other observed compounds. Comparison with laboratory data suggests that particles are composed primarily of ammonium sulfate during times of relatively low ambient aerosol concentration.
Bibliographical noteFunding Information:
Received 5 May 2003; accepted 12 August 2003. The authors gratefully acknowledge the financial support of the DOE through grant No. DE-FG02-98R62556. Address correspondence to J. N. Smith, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, USA. E-mail: firstname.lastname@example.org