We examine here the conditions under which atmospheric nitric acid produced by oxidation of NOx is incorporated into the aerosol. Consider an air parcel at T=290K containing a fixed gasphase HNO₃(g) concentration of 1ppbv at 1atm in equilibrium with an otherwise pure-water aerosol. The relevant equilibria are:
HNO₃(g)⇌HNO₃(aq), K₁=2.1×10⁵ M atm⁻¹
HNO₃(aq)⇌NO₃⁻+H⁺, K₂=12M

(a) Calculate the pH of the resulting aerosol. You should find pH=1.3. Is aerosol nitrate present mainly as NO₃⁻ or as HNO₃ (aq) ?