No3 no2 no

Figure 4.14 Schematic of the partitioning between NO, NO,, and NO, (the sum of which is NO,). Each arrow is marked with the reactants that facilitate the conversion.

As with any chemical family, the reactions that interconvert NO, NO,, and NO, (reactions (4.32H4.38) and (4.6)) have no effect on the abundance of NO,. NO, is only lost when NO, N02, or NO, is converted to HNO„ ClONO,, BrONO,, or N,Os.

Figure 4.15 shows how the composition of NO, changes throughout the day. During daytime, NO, is almost entirely composed of NO and NO,, with the abundance of NO, practically zero. Because of the short lifetimes of both NO and NO,, these species are generally assumed to be in photochemical steady state during daylight. The daytime ratio of the abundance of NO to N02 can be approximated:

This has been well verified by atmospheric observations ¡98-100].

At sunset, the reactions that convert N02 to NO cease, allowing the reaction between NO and O, to convert virtually all NO to NO,. As a result, at night, the vast majority of NO, is in the form of NO,.

During the day, the abundance of NO, is set by a steady-state balance between loss of NO, through photolysis (reactions (4.37) and (4.38)) and formation of NO, through N0,+0, (reaction (4.36)). Because of the extremely fast photolysis of NO,, this turns out to be small: typical stratospheric daytime abundances of NO, are tens to hundreds of parts per quadrillion by volume (parts in 101'). After sunset, the abundance of NO, rises as production of NO, from N0,+0, (reaction (4.36)) is unopposed by photolysis. In the lower and mid-stratosphere, the abundance of NO, builds up until a steady-state balance is achieved between production of NO, through NO .+O, (reaction (4.36)) and loss of NO, through reaction with NO, to form N,(), (reaction (4.40)):

Note that Figure 4.15 shows NO, rising as a fraction of NO, throughout the night. This occurs despite a constant abundance of NO, because the abundance of NO, decreases throughout the night as it is converted to N ,05.

In the upper stratosphere, the rate of reaction between NO, and NO, to form N20, (reaction (4.40)) is slow enough that the system never reaches steady state; instead, NO, builds up throughout the night. Typical night-time values of NO, are a few parts per trillion by volume in the lower stratosphere, rising to a few tenths of a part per billion by volume in the upper stratosphere.

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