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This study investigates the connections between atmosphericsulphuric acid and new particle formation during QUEST III andBACCI/QUEST IV campaigns. The campaigns have been conducted inHeidelberg (2004) and Hyytiälä (2005), the firstrepresenting a polluted site surrounded by deciduous forest, andthe second a rural site in a boreal forest environment. We havestudied the role of sulphuric acid in particle formation andgrowth by determining 1) the power-law dependencies betweensulphuric acid ([H2SO4]), and particle concentrations(N3–6) or formation rates at 1 nm and 3 nm (J1 andJ3); 2) the time delays between [H2SO4] andN3–6 or J3, and the growth rates for 1–3 nm particles;3) the empirical nucleation coefficients A and K in relationsJ1=A[H2SO4] andJ1=K[H2SO4]2, respectively; 4) theoreticalpredictions for J1 and J3 for the days when nosignificant particle formation is observed, based on the observedsulphuric acid concentrations and condensation sinks. In bothenvironments, N3–6 or J3 and [H2SO4] werelinked via a power-law relation with exponents typically rangingfrom 1 to 2. The result suggests that the cluster activationtheory and kinetic nucleation have the potential to explain theobserved particle formation. However, some differences between thesites existed: The nucleation coefficients were about an order ofmagnitude greater in Heidelberg than in Hyytiäläconditions. The time lags between J3 and [H2SO4]were consistently lower than the corresponding delays betweenN3–6 and [H2SO4]. The exponents in the J3∝[H2SO4 ]nJ3-connection wereconsistently higher than or equal to the exponents in the relationN3–6∝[H2SO4 ]nN36. In theJ1 values, no significant differences were found between theobserved rates on particle formation event days and thepredictions on non-event days. The J3 values predicted by thecluster activation or kinetic nucleation hypotheses, on the otherhand, were considerably lower on non-event days than the ratesobserved on particle formation event days. This study providesclear evidence implying that the main process limiting theobservable particle formation is the competition between thegrowth of the freshly formed particles and their loss byscavenging, rather than the initial particle production bynucleation of sulphuric acid. In general, it can be concluded thatthe simple models based on sulphuric acid concentrations andparticle formation by cluster activation or kinetic nucleation canpredict the occurence of atmospheric particle formation and growthwell, if the particle scavenging is accurately accounted for

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