NITROGEN CHEMISTRY OF STRATOSPHERE

N₂O which is relatively stable in the troposphere, usually moves into stratosphere and undergoes following photochemical reactions:

N₂O + hv (<260 nm) ————–> N₂ + O(¹D)

N₂O + O(¹D) ———-> N₂ + O₂

N₂O + O(¹D) ———-> 2NO

NO or NO₂ which may also move from troposhpere into stratosphere or are produced in the stratosphere, undergo following reactions there:

NO + O + M ——> NO₂ + M

NO₂ + OH + M —–> HNO3 + M

HNO3 + hv (<330 nm) ——> OH + NO₂

 Ozone layer in stratosphere absorbs sufficient amount of UV radiation so that at tropopause HNO₃ has photochemical lifetime of about 10 days. This time is long enough for much of it to cross the tropopause and come down with rainfall thus being removed from stratosphere.

CHLORINE CHEMISTRY OF STRATOSPHERE

Chief natural source of chlorine in atmosphere is probably methyl chloride from marine algae but it accounts for only 25% of the chlorine currently being transported across tropopause into the stratosphere. Other natural sources adding minor amounts are HCl acid from volcanoes and chlorine from sea sprays. In the past few decades, chlorofluorocarbons (mostly CFCl₃ i.e. Feron-11) and CF₂Cl₂ i.e. Feron-12) added to the atmosphere by human activities have become chief source of stratospheric chlorine. Ammonium perchlorate-aluminium solid rocket propellents are another anthropogenic source of atmospheric chlorine. These compounds absorb UV radiation in the range of 190 to 220 nm resulting in their photodissociation:

CH₃Cl + hv —–> CH₃ + Cl

CFCl₃ + hv ——> CFCl₂ + Cl

CF₂Cl₂ + hv —–> CF₂Cl + Cl

Free Cl atoms in stratosphere may undergo various reaction cycles:

1. Reaction with ozone: Free chlorine atoms in stratosphere react with ozone in catalytic manner and cause depletion of ozone:

Cl + O₃ —–> ClO + O₂

ClO produced may react with nitrogen compounds:

ClO + NO —–> Cl + NO₂

ClO + NO₂ + M ——–> ClNO₃ + M

ClNO₃ may be decomposed by UV radiation or by reaction with atomic oxygen:

ClNO₃ + hv —–> ClO + NO₂

ClNO3 + O —-> O₂ + ClO + NO

Reactions of ClO with NO or NO₂ are important because they effectively remove N- and Cl- species involved in ozone destroying cycles.

2. Reaction with methane and hydrogen: Free Cl may also react with CH₄ or H₂:

Cl + CH₄ —–> HCl + CH₃

Cl + H₂ ——> HCl + H

Some of the HCl may react with OH radical in stratosphere:

OH + HCl —–> H₂O + Cl

However, most of the HCl moves down to tropopause and is removed with rainfall as HCl acid.