Connections between atmospheric sulphuric acid and new particle formation during QUEST III–IV campaigns in Heidelberg and Hyytiälä

Abstract. This study investigates the connections between atmospheric sulphuric acid and new particle formation during QUEST III and BACCI/QUEST IV campaigns. The campaigns have been conducted in Heidelberg (2004) and Hyytiälä (2005), the first representing a polluted site surrounded by deciduous forest, and the second a rural site in a boreal forest environment. We have studied the role of sulphuric acid in particle formation and growth by determining 1) the power-law dependencies between sulphuric acid ([H₂SO₄]), and particle concentrations (N_3–6) or formation rates at 1 nm and 3 nm (J₁ and J₃); 2) the time delays between [H₂SO₄] and N_3–6 or J₃, and the growth rates for 1–3 nm particles; 3) the empirical nucleation coefficients A and K in relations J₁=A[H₂SO₄] and J₁=K[H₂SO₄]², respectively; 4) theoretical predictions for J₁ and J₃ for the days when no significant particle formation is observed, based on the observed sulphuric acid concentrations and condensation sinks. In both environments, N_3–6 or J₃ and [H₂SO₄] were linked via a power-law relation with exponents typically ranging from 1 to 2. The result suggests that the cluster activation theory and kinetic nucleation have the potential to explain the observed particle formation. However, some differences between the sites existed: The nucleation coefficients were about an order of magnitude greater in Heidelberg than in Hyytiälä conditions. The time lags between J₃ and [H₂SO₄] were consistently lower than the corresponding delays between N_3–6 and [H₂SO₄]. The exponents in the J₃∝[H₂SO₄ ]^n_J3-connection were consistently higher than or equal to the exponents in the relation N_3–6∝[H₂SO₄ ]^n_N36. In the J₁ values, no significant differences were found between the observed rates on particle formation event days and the predictions on non-event days. The J₃ values predicted by the cluster activation or kinetic nucleation hypotheses, on the other hand, were considerably lower on non-event days than the rates observed on particle formation event days. This study provides clear evidence implying that the main process limiting the observable particle formation is the competition between the growth of the freshly formed particles and their loss by scavenging, rather than the initial particle production by nucleation of sulphuric acid. In general, it can be concluded that the simple models based on sulphuric acid concentrations and particle formation by cluster activation or kinetic nucleation can predict the occurence of atmospheric particle formation and growth well, if the particle scavenging is accurately accounted for.