Airborne observations of the effect of a cold front on the aerosol particle size distribution and new particle formation

Airborne measurements of condensation nuclei (CN), aerosol particles (APs) and sulphur dioxide (SO₂)were made near a cold front to examine new particle formation and the effects of the front on CN and SO₂ concentrations and the AP size distribution and concentration. Measurements were made in the boundary layer (BL) and free troposphere (FT) both preceding and following the passage of the front. Statistical analyses of CN and AP concentrations in the air masses around the front show that new particle formation was prevalent in the pre-frontal and post-frontal FT, however the post-frontal FT contained higher concentrations of ultrafine condensation nuclei (UCN). Mixing diagrams of conserved thermodynamic quantities, total water content and wet equivalent potential temperature were constructed for horizontal aircraft transects in the pre-frontal and post-frontal FT regions; they show that many of the fluctuations in UCN concentrations can be explained by mixing of air masses with differing initial concentrations of UCN. However, regions of enhanced UCN were found to be associated with mixing between air masses of distinct thermodynamic properties, suggesting that mixing between air masses with large gradients in temperature and relative humidity may have promoted new particle formation. Furthermore, analysis of fluctuations in UCN concentrations during vertical soundings show that low pre-existing AP surface area was not a necessary requirement for the production of new particles but, rather, turbulent mixing was a major mechanism for new particle production. Observations also suggest that species other than SO₂ were required for new particle production to occur.