Typical PM emission-factor studies for dairies and feedyards have involved measuring ground-level concentration as close to the plume centerline as weather predictions permit, followed by dispersion modeling using a single-valued emission factor to match the 24-hour average concentration. Using AERMOD as our dispersion-modeling platform, we confirm that the 24-hour average emission factor cannot reproduce the hourly average concentrations throughout the day and vastly underpredicts the magnitude of the evening dust peak. Moreover, matching a single concentration measurement near the plume centerline is not as rigorous a dispersion-modeling test as attempting to match an entire transverse cross-section of the plume along the downwind boundary. We report preliminary results of two intensive monitoring campaigns at an open-lot dairy and a cattle feedyard during summer 2009 in which seven real-time PM10 monitors were distributed across the downwind boundary of the open-lot area. Using AERMOD with a time-varying emission flux to match predicted and measured concentrations along the full, transverse cross-section of the aerosol plume. We show that, granting the constraints of the Gaussian modeling algorithm, the emission flux from open lots cannot be assumed time-invariant within a 24-hour period. Moreover, the time-variant emission fluxes estimated from one-hour average concentration data appear to confirm that (a) dust-emitting animal behaviors increase during a three- to four-hour interval around sundown in the feedyard and (b) vehicle traffic and the animals' feeding behavior during the morning hours contribute significantly to the daily average PM10 flux.