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Extended Data Fig. 3 | A typical measurement sequence. Nucleation was
carried out purely from nitric acid and ammonia, with no sulfuric acid
(measured to less than 5 × 10^4 cm−3 or 2 × 10−3 pptv), as a function of coordinated
universal time (UTC), at 60% relative humidity and −25 °C. a, Gas-phase
ammonia and nitric acid mixing ratios. The run started with injection of the
nitric acid and ammonia f low into the chamber to reach chosen steady-state
values near 30 pptv and 1, 500 pptv, respectively. The nitric acid f low was
increased at 5:53 on 14 November 2018 to prove consistency. b, Clearing-field
voltage and ion concentrations. Primary ions were formed from galactic
cosmic rays (GCR). The clearing-field high voltage (HV) was used to sweep out
small ions at the beginning of the run, and turned off at 05:21 on 14 November
2018 to allow the ion concentration to build up to a steady state between GCR
production and wall deposition. c, Particle concentrations at two cut-off sizes
(1.7 nm and 2.5 nm). Particles formed slowly in the chamber under ‘neutral’
conditions with the HV clearing field on and thus without ions present. The
presence of ions (‘GCR’ condition) caused a sharp increase in the
particle-number concentration by about one order of magnitude, with a slower
approach to steady state because of the longer wall-deposition time constant
for the larger particles. Particle numbers rose again with rising nitric acid.