Fig. 1.

The pH evolution of simulated MEM particles with initial radii 25 μm (A and B) and 2 μm (C and D), at t = 0 in equilibrium with 99.4% relative humidity (RH) and 5% CO₂, subsequently injected into air with 50% RH and 400 ppm CO₂. (A and C) Laboratory air without minor trace gases (HNO₃, NH₃, HCl, CH₃COOH), and (B and D) typical indoor air containing trace gases. Colors indicate the pH inside the MEM droplets as a function of time. Dark gray regions are effloresced NaCl. At t = 0, pH is 7.4 as in the experiments of ref. 1, with particle composition 0.19158 m (molal) Na⁺, 0.17383 m Cl^–, 0.0057 m nonsodium and 0.0037 m nonchloride ions, 0.0168 m organics, and Na⁺/(CO_2,aq + HCO₃^– + CO₃^2–) = 9.15. Assumed trace gas concentrations of indoor air are 0.27 ppb HNO₃, 36 ppb NH₃, 0.23 ppb HCl, and 46.7 ppb CH₃COOH (3).