Deposition ice nucleation on soot at temperatures relevant for the lower troposphere

The ice nucleating efficiency of many important atmospheric particles remains poorly understood. Here we investigate the ice nucleation properties of a range of soot types including soot that has been treated with atmospherically relevant amounts of ozone. We focus on deposition nucleation below water saturation and at temperatures ranging from 243 to 258 K. For our experimental conditions, ice nucleation never occurred at temperatures above 248 K and below water saturation. Below 248 K, ice occasionally formed in our experiments with no indication of the formation of water droplets prior to ice formation. However, even at these temperatures the relative humidity with respect to ice (RHi) was close to water saturation when ice nucleation was observed, suggesting water nucleation may have occurred first followed by ice nucleation during the condensation process. We also performed a complimentary set of experiments where we held soot particles at 248 K and RHi = 124 +/- 4%, which is just below water saturation, for a period of 8 hours. From these measurements we calculated an upper limit of the heterogeneous ice nucleation rate coefficient of 0.1 cm(-2) s(-1). If the number of soot particles is 1.5 x 10(5) L-1 in the atmosphere ( which corresponds to urban-influenced rural areas), then the number of ice particles produced below water saturation at these conditions is at most 0.1 particles L-1 on the basis of our upper limit. We conclude from our studies that deposition nucleation of ice on most types of soot particles is not important in the Earth's troposphere above 243 K and below water saturation.