The maximum NO2 concentration in the lower atmosphere is limited by the presence

of oxidising agent, mainly ozone to no more than about 100 Tg/m; (50 ppb) for most

of the year although summer-time “ozone episodes” can lead to higher concentrations

if they coincide with high NOx concentrations. Direct emission of NO2, e.g. from

tailpipe, can also lead to higher NO2 concentration.

Ozone episodes occur when sunlight initiates a complicated photochemically driven

chain of reactions in air polluted with VOCs and nitrogen oxides. The hydrocarbons

are attacked by hydroxyl (OH) free radicals to form organic peroxy (RO2@) radicals

which oxidise NO to NO2; this, in turn, is photochemically decomposed back to NO

with the subsequent formation of O3. Photochemical smog which can build up, often

some distance downwind of the source area, may also contain increased

concentrations of hydrogen peroxide (H2O2), peroxyacetyl-nitrate (PAN), other

hydrocarbon reaction products and particulates.

 

 

 

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