Ion-mediated nucleation as an important global source of tropospheric aerosols

Aerosol nucleation events have been observed at a variety of locations worldwide, and may have signifi- cant climatic and health implications. While ions have long been suggested as favorable nucleation embryos, their sig- nificance as a global source of particles has remained uncer- tain. Here, an ion-mediated nucleation (IMN) mechanism, which incorporates new thermodynamic data and physical algorithms, has been integrated into a global chemical trans- port model (GEOS-Chem) to study ion-mediated particle for- mation in the global troposphere. The simulated annual mean results have been compared to a comprehensive set of data relevant to particle nucleation around the globe. We show that predicted annual spatial patterns of particle formation agree reasonably well with land-, ship-, and aircraft-based observations. Our simulations show that, globally, IMN in the boundary layer is largely confined to two broad latitude belts: one in the northern hemisphere ( 20 N-70 N), and one in the southern hemisphere ( 30 S-90 S). In the mid- dle latitude boundary layer over continents, the annual mean IMN rates are generally above 10 4 cm 3 day 1 , with some hot spots reaching 10 5 cm 3 day 1 . The zonally-averaged vertical distribution of IMN rates indicates that IMN is sig- nificant in the tropical upper troposphere, the entire middle latitude troposphere, and over Antarctica. Comparing the rel- ative strengths of particle sources due to IMN and due to pri- mary particle emissions demonstrates that IMN is significant on a global scale. Further research is needed to reduce mod- eling uncertainties and to understand the ultimate contribu- tion of freshly nucleated particles to the abundance of cloud condensation nuclei.