Biogenic ice nuclei in boundary layer air over two U.S. High Plains agricultural regions

With 18% of the total U. S. landmass devoted to croplands, farmland and farming activities are potentially major sources of biogenic particles to the atmosphere. Farms harbor large populations of microbes both in the soil and on plant surfaces which, if injected into the atmosphere, may serve as nuclei for clouds. In this study, we investigated two farms as potential sources of biological ice nuclei (IN): an organic farm in Colorado and a cornfield in Nebraska. We used a continuous-flow diffusion chamber (CFDC) to obtain real-time measurements of IN at these farm sites. Total aerosol particles were also collected at the sites, and their temperature-dependent ice nucleating activity was determined using the drop freezing method. Quantitative polymerase chain reaction and DNA sequencing of 16S rDNA clone libraries were used to test aerosols and washings of local vegetation for abundance of the ina gene in ice nucleation active bacteria (from the well-known group within the gamma-Proteobacteria) and to identify airborne primary biological aerosol particles. The vegetation in each of these farms contained 6 x 10(5) to 2 x 10(7) ina genes per gram vegetation. In contrast to the vegetation, airborne ina gene concentrations at the organic farm were typically below detectable limits, demonstrating a disconnect between local vegetative sources and the air above them. However, for measurements made during combine harvesting at the Nebraska corn field, ina gene concentrations were 19 L-1, with maximum IN concentrations of 50 L-1 determined from the CFDC at -20 degrees C and above water saturation. At both farms, there was also an apparent biological contribution to the IN population which did not contain the ina gene.