= Solution
<Peroxyacetyl nitrate> forms reversibly from a peroxyacetyl radical and NO$_2$. It stores reactive nitrogen while intact, then thermal dissociation can release it again. It can therefore transport nitrogen through colder free-tropospheric air and recycle it more rapidly in a warmer <planetary boundary layer>. Deposition or further chemistry can compete, but reversible storage is its reservoir role.
<Dinitrogen pentoxide> is likewise a reservoir because $\mathrm{NO_2+NO_3+M\rightleftharpoons N_2O_5+M}$ can return reactive species. Conversion to nitric acid makes it a sink from the active <nitrogen oxides> family, especially through heterogeneous uptake on wet <aerosol> or cloud particles, followed by deposition or washout. <Water> vapour alone and wet-particle uptake are different mechanisms; they should not be assigned the same <rate constant>. <Temperature>, particle surface area and removal are often very different between the boundary layer and the free troposphere, so a reservoir can be transported in one environment and chemically removed in another.
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