PNNL

Abstract

The relationship between atmospheric particulate matter and the formation of clouds is among the most uncertain aspects of our current understanding of climate change1. One specific question that remains unanswered is how anthropogenic particulate emissions are affecting the nucleation of ice crystals. Satellites show ice clouds cover more than a third of the globe2 and models suggest that ice nucleation initiates the majority of terrestrial precipitation3. It is therefore not possible to adequately understand either climate change or the global water cycle without understanding ice nucleation. Here we show that lead-containing particles are among the most efficient ice nucleating substances commonly found in the atmosphere. Field observations were conducted with mass spectrometry and electron microscopy at two remote stations on different continents, far removed from local emissions. Laboratory studies within two cloud chambers using controlled experimental conditions support the field data. Because the dominate sources of particulate lead are anthropogenic emissions such as aviation fuel, power generation, smelting, and the re-suspension of residue from tetra-ethyl leaded gasoline4, it is likely that cloud formation and precipitation have been affected when compared to pre-industrial times. A global climate model comparing pre-industrial and anthropogenically perturbed conditions shows that lead-containing particles may be increasing the outgoing longwave radiation by 0.2 to 0.8 W m-2, thereby offsetting a portion of the warming attributed to greenhouse gases1.