Fig. 2. Particle formation rates at 1.7 nm (J_1.7) versus ammonia concentration at 223 K and 25% relative humidity.

The chemical systems are HNO₃–NH₃ (black), H₂SO₄–NH₃ (red) and HNO₃–H₂SO₄–NH₃ (blue). The black diamond shows the CLOUD measurement of HNO₃–NH₃ nucleation at 1.5 × 10⁹ cm⁻³ HNO₃, 6.5 × 10⁸ cm⁻³ NH₃ and with H₂SO₄ below the detection limit of 5 × 10⁴ cm⁻³. The red solid curve is J_1.7 versus ammonia concentration at 4.0 × 10⁶ cm⁻³ sulfuric acid from a H₂SO₄–NH₃ nucleation parameterization on the basis of previous CLOUD measurements^18,19. The blue circles show the CLOUD measurements of HNO₃–H₂SO₄–NH₃ nucleation at 4.0 × 10⁶ cm⁻³ H₂SO₄, 1.5 × 10⁹ cm⁻³ HNO₃ and (1.6–6.5) × 10⁸ cm⁻³ NH₃. The data are fitted by a power law, J_1.7 = k[NH₃]^3.7 (blue dashed curve). The vertical grey dotted line separates ammonia concentrations measured in different regions in the upper troposphere⁵; the region to the right indicates the Asian monsoon conditions. The horizontal grey solid lines show J_1.7 upper limits for ion-induced nucleation resulting from the GCR ionization rate of around 2 ion pairs cm⁻³ s⁻¹ at ground level and 35 ion pairs cm⁻³ s⁻¹ in the upper troposphere. Among the three nucleation mechanisms, H₂SO₄–NH₃ nucleation dominates in regions with low ammonia (below around 1.0 × 10⁸ cm⁻³, or 12 pptv), whereas HNO₃–H₂SO₄–NH₃ nucleation dominates at higher ammonia levels characteristic of the Asian monsoon upper troposphere. The bars indicate 30% estimated total error on the particle formation rates. The overall systematic scale uncertainties are −33%/+50% for sulfuric acid and ±25% for nitric acid concentrations.

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