Fig. 2.

Neutralizing effect of ammonia on fine PM. (A) Equivalent ratio of NH₄⁺ to the sum of SO₄²⁻ and NO₃⁻ in Xi’an and Beijing. (B) Equivalent ratio of the total cations to anions in Xi’an and equivalent ratio of NH₄⁺ to the sum of SO₄²⁻, NO₃⁻, and Cl⁻ in Beijing. The blue, orange, and black colors correspond to the clean, transition, and polluted periods, respectively, as defined in Fig. 1.