Extended Data Fig. 1. Enhancement of HNO3–NH3 particle formation by sulfuric acid.
a, Particle number concentrations versus time at mobility diameters >1.7 nm (magenta) and >2.5 nm (green). The solid magenta trace is measured by a PSM1.7 and the solid green trace is measured by a CPC2.5. The fixed experimental conditions are about 6.5 × 108 cm−3 NH3, 223 K and 25% relative humidity. b, Particle formation rate versus time at 1.7 nm (J1.7), measured by a PSM. c, Particle size distribution versus time, measured by an SMPS. d, Gas-phase nitric acid and sulfuric acid versus time, measured by an I− CIMS and a NO3− CIMS, respectively. We started the experiment by oxidizing NO2 to produce 1.6 × 109 cm−3 HNO3 in the presence of about 6.5 × 108 cm−3 ammonia. At time = 0 min, we turned off the high-voltage clearing field to allow the ion concentration to build up to a steady state between GCR production and wall deposition. The presence of ions (GCR condition) induces slow HNO3–NH3 nucleation, followed by relatively fast particle growth by nitric acid and ammonia condensation. We thus observe formation of both 1.7-nm and 2.5-nm particles by about one order of magnitude in about 3.5 h, with a slower approach to steady state because of the longer wall deposition time constant for the larger particles. Then, we increased H2SO4 in the chamber from 0 to 4.9 × 106 cm−3 by oxidizing progressively more injected SO2 after 211 min, with a fixed production rate of nitric acid and injection rate of ammonia. Subsequently, particle concentrations increase by three orders of magnitude within 30 min. The overall systematic scale uncertainties of ±30% on particle formation rate, −33%/+50% on sulfuric acid concentration and ±25% on nitric acid concentration are not shown.