Direct link between the characteristics of atmospheric new particle formation and Continental Biosphere-Atmosphere-Cloud-Climate (COBACC) feedback loop

We investigated the role of atmospheric new particle formation (NPF) in the Continental Biosphere-Atmosphere-Cloud-Climate (COBACC) feedback mechanism, particularly the influences of NPF on condensation sink and particle number and cloud condensation nuclei (CCN) concentrations. Using atmospheric observations at the SMEAR II station in Hyytiälä, Finland, we showed that the growth rates of particles originating from NPF are relatively constant, depending only weakly on low-volatility vapor concentrations and having no apparent connection with the strength of NPF or the level of background pollution. We then constrained aerosol dynamic model simulations with these observations. We showed that under conditions typical for the boreal forest atmosphere, even the weakest NPF with a relatively low particle formation rate of 0.01 cm–3 s–1 at 5 nm (J5) is capable of enhancing the condensation sink (CS) to values larger than 0.001 s–1 after two days of particle growth. When J5 lies in a typical NPF range between about 0.1 and 0.5 cm–3 s–1 at SMEAR II, the growing-mode particles originating from NPF tend to dominate the total CS at this station. Increases in CS associated with NPF further increase the ratio of diffuse radiation to global radiation under clear sky by factors between about 1.5 and 2 at SMEAR II, which boosts the forest gross primary production and enhances forest carbon sink. Furthermore, when J5 is larger than about 0.1 cm–3 s–1, NPF frequently produces particle number concentrations larger than 1000 cm–3 into the size range relevant for cloud droplet activation. We conclude that outside the late autumn and winter periods when the frequency of NPF is low at SMEAR II, NPF followed by subsequent particle growth is able to give a dominant contribution to both CS and CCN concentration — the two most relevant aerosol-related quantities in the COBACC feedback mechanism. With some reservation, the same conclusion is likely to hold over large regions in the boreal forest zone.