Extended Data Fig. 7. Molecular composition of charged clusters with sulfuric acid, iodic acid and no acid during IP-OOM nucleation at −50 °C.
Mass defect (difference from integer mass) versus m/z (mass-to-charge ratio) during nucleation events for ions and negatively charged (a–d) and positively charged (e,f) molecular clusters measured with two APi-TOF instruments (see Methods), coloured by the number of carbon (a,b,e), sulfur (c), iodine (d) and nitrogen (f) atoms. The data are without NOx (a–e) and with NOx (f). In panel d, the (purple) clusters with no iodine contain a NO3− ion. The symbol area is proportional to the ion signal rate (cps, counts s−1). It should be noted that the APi-TOF used in this experiment is less sensitive than the MION2-APi-TOF and thus the absolute ion signals and cluster population should not be compared between the instruments. Together with Fig. 3, our results show that ion-induced nucleation occurs both with pure IP-OOM and with further H2SO4 and HIOx. The experimental conditions for a,c,e are: isoprene = 0.19 ppbv (5.9 × 109 cm−3), O3 = 93 ppbv (2.9 × 1012 cm−3), I2 = unmeasured, SO2 = unmeasured, OH = 3.0 × 106 cm−3, HO2 = unmeasured, RH = 63% and temperature = −49 °C. The concentrations of IP0N = unmeasured, H2SO4 = 3.0 × 105 cm−3 and HIOx = 2.8 × 104 cm−3. The experimental conditions for b,d are: isoprene = 0.09 ppbv (2.8 × 109 cm−3), O3 = 97 ppbv (3 × 1012 cm−3), I2 = 5.1 × 107 cm−3, SO2 = 0, OH = 3.4 × 106 cm−3, HO2 = 7.6 × 107 cm−3, RH = 63% and temperature = −49 °C. The concentrations of low-volatility vapours are IP0N = 1.3 × 107 cm−3, H2SO4 = 8.3 × 103 cm−3 and HIOx = 6.7 × 105 cm−3. The experimental conditions for f are: isoprene = 0.12 ppbv (3.7 × 109 cm−3), O3 = 1.2 ppbv (3.7 × 1010 cm−3), SO2 = 1.2 × 108 cm−3, OH = 6.2 × 107 cm−3, HO2 = 1 × 109 cm−3, RH = 42%, NO = 0.25 ppbv, NO2 = 0.78 ppbv and temperature = −48 °C. The concentrations of low-volatility vapours are IP0N = 1.2 × 107 cm−3, H2SO4 = 2.4 × 105 cm−3 and HIOx = 0 cm−3.