EFFECTS OF ULTRAVIOLET SPECTRAL DISTRIBUTION ON THE PHOTOCHEMISTRY OF SIMULATED POLLUTED ATMOSPHERES

We have observed enhanced photochemical smog manifestations in an irradiated hydrocarbon-NOx-air system under simulated atmospheric conditions upon incrementally increasing the intensity of the ultraviolet (u.v.) photolyzing radiation near the atmospheric cutoff (in the region of λ > ∼ 290-330 nm). This study employed a 25 kW (xenon arc) solar simulator which provides radiation down to 260 nm, a 5800-l. evacuable, thermostated, environmental chamber with quartz end windows which admit radiation at λ > 200 nm, and a set of Pyrex pane filters with 50%T at λ = 325,305, and 300 nm, respectively. In these experiments the rates of ozone and PAN formation, hydrocarbon consumption, and NOx oxidation increased substantially as the filter cutoff wavelength decreased. For example, irradiation of mixtures of propene (0.4 ppm), n-butane (2 ppm), and NOx = 0.5 ppm ( NO2 NOx = 0.2) in air employing the solar simulator filter with 50% T at λ = 325 nm resulted in the ozone maximum of 0.6 ppm occurring after 5-6.5 h of photolysis, while irradiation of similar mixtures with the 50% T wavelength reduced to 300 nm caused the ozone maximum to increase to 0.7 ppm, and to occur after only 2.5 h of photolysis. Calculations using a detailed kinetic mechanism were in reasonable agreement with these results and showed that the observed increase in reactivity was due to increased photolysis rates for ozone, aldehydes and ketones. The calculations included a significant but unknown photolytic radical source characteristic of environmental chambers, but this radical source apparently does not increase when the relative intensity of the short wavelength photolysing radiation is increased. This confirmation that the manifestations of photochemical smog are affected by the relative intensity of the u.v. component of the photolyzing radiation has implications with respect to the effects of stratospheric ozone variations, changes in latitude, and changes in altitude on tropospheric air quality problems. © 1979.