Recent progress in understanding particle nucleation and growth

In the past half decade, several new tools have become available for investigating particle nucleation and growth. A number of joint field and laboratory studies exploiting some of these new measurement capabilities will be described and new insights shared. The ability to measure OH, SO₂, H₂SO₄ and aerosol number and size distributions has made possible a comparison between H₂SO₄ production and loss onto particles in continental air masses. In regions remote from urban emissions, agreement is typically quite good. In contrast, joint field measurements of nucleation precursors such as gas phase H₂SO₄ and ultrafine particles suggest that classical bimolecular nucleation theory may not properly described the tropospheric nucleation process. An alternative mechanism, possibly involving ammonia as a stabilizing agent for H₂SO₄/H₂O molecular clusters is discussed. Finally, ultrafine particle measurements are shown to offer new opportunities for studying particle growth rates. Preliminary results suggest that in a remote continental air mass, gas phase H₂SO₄ uptake is far too slow to explain observed growth rates.