Fluxes of nitrogen oxides and ozone measured by eddy correlation over a shortgrass prairie

Abstract

The fluxes of NO2, NOy and ozone were measured by eddy correlation at a grassland field site from March 5 to August 2, 1988. The nitrogen oxide fluxes were generally small and upward during the day. Overnight, the fluxes were zero. The average daytime NO2 flux increased from 2.2 ng (N) m−2 s−1 during March and April (Julian days 65 to 121) to 5.7 ng (N) m−2 s−1 in June and July (Julian days 153 to 193). The average NOy flux for the period June 21 to August 2, 1988, was 7.6 ng (N) m−2 s−1 (Julian days 173 to 215). The largest fluxes were observed after rainstorms from drying surfaces, when the flux increased exponentially with soil temperature. Fluxes from dry soils were small and were not a strong function of soil temperature. The deposition of ozone, measured as oxidant to correct for homogeneous gas phase chemistry, showed a strong diurnal variation. The average daytime deposition velocity was 0.36 cm s−1 and 0.22 cm s−1 overnight. Surface resistance calculations show that under conditions of adequate soil moisture, approximately half of the ozone was deposited on the inner surfaces of the foliage, the remainder being deposited on the soil, outer leaf walls, and to the dead plant material. When soil moisture is low, less ozone is deposited on the foliage presumably because of water‐stress‐induced stomatal closure.