Photochemistry of minor constituents in the troposphere

Abstract

Altitude profiles for the number densities of NO, NO 2, NO 3, N 2O 5, HNO 2, CH 3O, CH 3O 2, H 2CO, OH, and HO 2 are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H 2O, CH 4, H 2, CO, O 3, and the sum of NO and NO 2 are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO 3, H 2O 2, and CH 3O 2H. The major nitrogen compound HNO 3 may have a number density approaching 5 × 10 11 molecules cm −3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value. The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 10 7 molecules cm −3 at noon, while the number density of HO 2 decreases throughout the lower troposphere from its noontime value of 8 × 10 8 molecules cm −3 at the surface. H 2O 2 and H 2CO both have number densities in the ppb range in the lower troposphere. Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 10 8 molecules cm −3 for OH and 3 × 10 7 molecules cm −3 for HO 2 at the tropopause. The related minor species show even sharper decreases with increasing altitude. The primary path for interconverting OH and HO 2 serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr. Another reaction cycle, the oxidation of CH 4, is quite important in the lower troposphere and leads to the production of H 2CO along with the destruction of CH 4 for which a tropospheric lifetime of ~2 yr is estimated. The destruction of H 2CO that was produced in the CH 4 oxidation cycle provides the major source of CO and H 2 in the atmosphere.