Model calculations of night-time atmospheric OH

The distribution of OH in the troposphere at night was obtained with a time-dependent photochemical model. At night, the major source of OH in the troposphere is the reaction between O₃ and night-time HO₂. More than 16% of night-time HO₂ is produced by the thermal dissociation of HO₂NO₂. The sinks of OH at night are the same as during day-time. The globally and seasonally averaged night-time OH concentration is about 1.3 × 10⁴ molecules/cm³, which is about two orders of magnitude smaller than the globally and seasonally averaged day-time OH concentration. Night-time OH concentrations appear to be most important in winter in the middle troposphere at middle latitudes, where about 10% of removal of gases resulting from reactions with OH occurs at night. We also calculated the night-time OH concentrations over a wide range of climatic conditions, from the last ice age to the pre-industrial era, and to the present. We found that the day-time OH concentrations do not change much during these transitions but the night-time OH concentrations may have decreased significantly in the transitions between the ice ages and the pre-industrial times and between the pre-industrial times and the present.