Sources and sinks of ozone in rural areas

Abstract

Based on data collected at rural sites in South Dakota, Louisiana and Virginia during the summers of 1978–1980, the factors controlling the diurnal behavior of O 3 near the surface were determined. At all three sites, the diurnal O 3 profile consisted of a period of O 3 increase from sunrise until mid-afternoon, and a period of O 3 decrease from late afternoon until sunrise. The daily O 3 increase was comprised of two approximately equal portions: one due to downmixing of O 3 from aloft (0600–1000 h), and the other due to photochemistry and possibly some further downmixing (1000–1400 h). Even assuming that all of the O 3 increase during the 1000–1400 h period was due to local photochemistry, only very small amounts of O 3 were formed i.e. about 6 ppb of O 3 per ppb of nitrogen oxides (NO x). Both bag irradiation and photochemical modeling studies suggested similar, but slightly higher O 3 formation. Comparing the ambient data with the bag irradiation and modeling results, it was concluded that rural photochemistry was not controlled exclusively by NO x, but depended on hydrocarbons as well. The O 3 decrease (1800 to 0600 h) generally occurred while nocturnal inversions isolated the lower 50 m of the atmosphere from the air aloft. Since the O 3 loss-rate could be parameterized equally well by either zero- or first-order rate laws, we could not distinguish between the two mechanisms. However, concentrations of gaseous species such as terpenes and NO x that might lead to pseudo zero-order O 3 loss were generally of minor significance. Consequently, it is concluded that O 3 loss was controlled by surface deposition with deposition velocities ranging from 0.06 to 0.34cms −1 at the three sites.