Eddy correlation measurements of atmospheric fluxes of ozone, sulphur, and particulates during the Champaign Intercomparison Study

Abstract

Atmospheric fluxes of momentum, sensible heat, water vapor, ozone, and sulphur were measured on 10 days during the intercomparison study. Computed mean daytime (average z/L = −0.33) deposition velocities were 0.47 cm/s for ozone and 0.33 cm/s for sulphur, while for measurements during the early evening (average z/L = 0.27), the mean deposition velocities decreased to 0.35 and 0.21 cms, respectively. These deposition velocities have been corrected to account for the slow time response (time constant near 1.1 s) of the pollutant analyzers. The corrections were based on computed effects of a similar time response on temperature‐vertical velocity cospectra. These were found to be stability dependent and ranged from about an additional 20% in unstable atmospheres to factors of 2–3 for stable conditions. Eddy correlation measurements were also attempted for particulates using a laser particle spectrometer. Subsequent analysis revealed that for our combinations of sample flow rate and the ambient concentrations, deposition velocities were too small to be distinguished from the inherent Poisson uncertainty in particle counting. This implied that for particles of diameters 0.1–0.5 μm deposition velocities were less than 0.05 cm/s, assuming no source for particles below our measuring height.