The role of relative humidity in continental new particle formation

Relative humidity (RH) has been observed to be anticorrelated with continental new particle formation. Several reasons have been proposed for this rather surprising finding, but no firm conclusions have been drawn so far. Here we study several of the proposed reasons: Enhanced coagulational scavenging of sub-3 nm clusters at high RH, diminished solar radiation at high RH leading to diminished gas phase oxidation chemistry, and increased condensation sink (CS) of condensable gases due to hygroscopic growth of the preexisting particles. Our theoretical calculations indicate that the increase of coagulational scavenging plays a relatively small role in the inhibition of nucleation at high RH. On the other hand, field data show that the maximum observed gas phase sulphuric acid concentrations are limited to RHs below 60%. The field data also indicate that this is likely due to low OH concentrations at high RH. This finding is also supported by aerosol dynamics model simulations. The model was used to find out whether this is mainly due to decreased source (solar radiation) or increased sink (CS) terms at the elevated RH. The simulation results show that the decreased source term at high RH limits H₂SO₄ levels in the air, and therefore high new particle formation rates (above ~1 cm₋₃ s₋₁) rarely occur above 80% RH.