The chemical systems are HNO3–NH3(black), H2SO4–NH3(red) and HNO3–H2SO4–NH3(blue). The black diamond shows the CLOUD measurement of HNO3–NH3nucleation at 1.5 × 109 cm−3HNO3, 6.5 × 108 cm−3NH3and with H2SO4below the detection limit of 5 × 104 cm...