NO can also be oxidised by molecular oxygen:

2 NO +O2 Þ 2 NO2

but this reaction is normally too slow to make significant contributions to ambient

NO2 levels. However, the reaction has a negative temperature coefficient and during

persistent winter-time episodes at low temperature, when NO2 levels may rise to >650

mg.m-3 (300 ppb) over several days, this process may lead to considerable

contributions (Bower et al, 1994 ,Grennfelt, P. 1996).

NO2 is removed from the atmosphere by several processes, mainlydry deposition and

the oxidation to HNO3 by OH@ radicals:

NO2 + OH@ Þ HNO3

HNO3 deposits rapidly on surfaces and also reacts easily with NH3 giving ammonium

nitrate (NH4NO3) in the particulate phase. HNO3 is removed through dry and wet

deposition processes.

NO2 is also removed by heterogeneous reaction with water occurring on surfaces,

generating HNO2 and HNO3.

2NO2 + H2O Þ HNO3 + HNO2

Photolysis only occurs during day. At night heterogeneous reaction between water and

N2O5 produced via the NO3 radical, is more important:

 

 

 

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