Particle size distributions from 3 nm to 2 μm were measured at the Fresno, CA, Supersite from August 25, 2002 through July 31, 2003. Nanoparticle (3-10 nm) concentrations and the ratio of nanoparticle to total particle concentration were inversely related to particle surface areas from 50 to 1000 μm2 cm-3. Elevated nanoparticle concentrations were associated with motor vehicle emissions and with photochemical particle production. In contrast with Atlanta, GA, where concentrations of photochemically derived nanoparticles exceeded 105 cm-3, 5-min average nanoparticle concentrations in Fresno never exceeded 24,400 cm-3. While photochemical particle production occurs in Fresno, evidence of new particle production (i.e., an increase in number concentration with decreasing size below 10 nm) was not observed. This suggests that photochemical particle production may have occurred at a higher altitude followed by mixing to the surface, or that the fresh particle production rate was smaller with respect to the loss rate by coagulation than it was in Atlanta. Lower production rates in Fresno are more consistent with lower concentrations of sulfur precurors and low relative humidity in Fresno than they are in Atlanta.
Bibliographical noteFunding Information:
This work was supported by the California Air Resources Board (ARB), which supplied several instrument systems, field support, space, utilities, and data management. Supersite measurements were supported under cooperative agreements with the U.S. Environmental Protection Agency (EPA) and with EPA support through the Cooperative Institute for Atmospheric and Terrestrial Applications (CIATA) of the National Oceanic and Atmospheric Administration (NOAA). The authors thank Mr. Peter Ouchida and Mr. Scott Scheller of the ARB for their efforts in maintaining the monitoring instruments. Dr. John Bowen and Ms. Dana Trimble of DRI assisted in field coordination and data processing of Fresno Supersite measurements.