High atmospheric NO_x levels and multiple photochemical steady states

Previous zero-dimensional photochemical calculations indicate that multiple tropospheric steady states may exist, in which different NO_x(≡NO+NO₂) levels could be supported by the same source of NO_x. To investigate this possibility more closely, a one-dimensional photochemical model has been used to estimate the rate of removal of atmospheric NO_x compounds at different NO_x levels. At low NO_x levels NO_x is photochemically converted to HNO₃, which is removed by either wet or dry deposition. At high NO_x levels formation of HNO₃ is inhibited, and NO_x is removed by a variety of other processes, including rainout of N₂O₄ and N₂O₅, surface deposition of NO and NO₂, and direct dissolution of NO and NO₂ in rainwater. Multiple steady states are possible if surface deposition of NO_x is relatively inefficient. The NO_x source required to trigger high atmospheric NO_x levels is approximately 10 to 15 times the present global emission rate-less than half the source strength predicted by the zero-dimensional model. NO_x mixing ratios in excess of 10^-7 would cause severe damage to the ozone layer and could result in either a climatic warming or cooling, depending upon the amount of NO₂ present.