Abstract
Significance
The high-altitude tropics constitute one of the world’s largest aerosol reservoirs, which may significantly affect clouds, radiation, and hydrological cycle by delivering the seeds on which clouds form to lower altitudes and maintaining the stratospheric aerosol background. However, the formation mechanisms of these aerosols remain a scientific mystery. Through a systematic experiment-based model representation of organic chemistry and new particle formation (NPF) combined with constraints from field measurements, we demonstrate that the NPF driven by extremely low volatility organics formed from biogenic emissions plays a key role in producing the large number of aerosols observed in the high-altitude Amazon. This organic NPF mechanism likely also prevailed during preindustrial times and hence may modulate anthropogenic aerosol forcing above the preindustrial baseline.
The large concentrations of ultrafine particles consistently observed at high altitudes over the tropics represent one of the world’s largest aerosol reservoirs, which may be providing a globally important source of cloud condensation nuclei. However, the sources and chemical processes contributing to the formation of these particles remain unclear. Here we investigate new particle formation (NPF) mechanisms in the Amazon free troposphere by integrating insights from laboratory measurements, chemical transport modeling, and field measurements. To account for organic NPF, we develop a comprehensive model representation of the temperature-dependent formation chemistry and thermodynamics of extremely low volatility organic compounds as well as their roles in NPF processes. We find that pure-organic NPF driven by natural biogenic emissions dominates in the uppermost troposphere above 13 km and accounts for 65 to 83% of the column total NPF rate under relatively pristine conditions, while ternary NPF involving organics and sulfuric acid dominates between 8 and 13 km. The large organic NPF rates at high altitudes mainly result from decreased volatility of organics and increased NPF efficiency at low temperatures, somewhat counterbalanced by a reduced chemical formation rate of extremely low volatility organic compounds. These findings imply a key role of naturally occurring organic NPF in high-altitude preindustrial environments and will help better quantify anthropogenic aerosol forcing from preindustrial times to the present day.