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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 $\mathrm{N}{\mathrm{O}}_2+\mathrm{N}{\mathrm{O}}_3+\mathrm{M}\rightleftharpoons {\mathrm{N}}_2{\mathrm{O}}_5+\mathrm{M}$ 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.

A planetary boundary layer is the part of an atmosphere strongly affected by exchanges of momentum, heat and material with the underlying surface on relatively short timescales. Surface friction generates shear, while heating or cooling changes buoyancy and hence vertical mixing. Its thickness and internal structure vary with the forcing rather than being fixed by one universal altitude.

A sun-heated surface can drive vigorous convection, whereas nocturnal cooling can create stable density stratification and suppress mixing. This distinction matters for interpreting a weather station: a near-surface wind or temperature observation need not represent the air above the boundary layer. Moisture and suspended material are likewise sensitive to the strength of vertical transport.