Characterisation and Modelling of Climatically Relevant Primary Biogenic Ice Nuclei in the BEACHON Southern Rocky Mountain Project

This proposal is for an invited UK contribution to an international multidisciplinary, multi-investigator study of the connections between biogeochemical cycling of carbon and water in semi-arid regions of the Western U.S comprising the Southern Rocky Mountain study region in 2011. The central hypothesis is that biogenic emission of reactive carbon gases from plants and soil, and wind transport of primary carbonaceous particles such as spores, pollen and bacterial cells, lead to the formation of cloud condensation nuclei (CCN) and ice nuclei (IN) which, in turn, influence regional precipitation patterns and thus link components of the carbon and water cycles. Conceptually, the potential linkages and feedbacks among biogenic emissions, the formation of CCN, and dynamics in the carbon and water cycles have been debate extensively and while various aspects of the coupled system have been studied in the past, e.g., emissions of biogenic volatile organic compounds (VOC) from vegetation and CCN formation from biogenic secondary organic aerosol (SOA), there are absolutely no studies that have traced the entire process in a quantitative, source to receptor fashion -- from emissions, through precipitation and back to emissions. A multidisciplinary research effort aimed at quantifying and assessing the relative importance of links among these processes has long been needed to enable regional coupled surface-atmosphere models. The strategic plans of the International Geosphere Biosphere Program - Integrated Land Ecosystem Atmosphere Processes Study (IGBP-iLEAPS) and the NCAR Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics, and Nitrogen (BEACHON) project both place a high priority on research efforts that will enhance understanding of the role of biogenic aerosols in linking and regulating the biogeochemical and water cycles and BEACHON SRM 2011provides an ideal opportunity to develop this integrative approach. The central aim of BEACHON-SRM is to use observations to assess the influence of ecosystem processes, and their responses to climate, on the number and composition of climatically-relevant biogenic aerosols and their potential to function as CCN and IN, and to modify precipitation, which feeds back to complete the loop and alter ecosystem processes. BEACHON-SRM will focus on the formation and modification of CCN and IN from biogenic sources. IN are those particles that initiate ice nucleation in the atmosphere. Although IN typically represent lessthan 1 in 10^5 of atmospheric aerosol particles, they are important in initiating ice formation at temperatures warmer than -36 C, a critical first step in most precipitation formation. In addition to dust, sources of IN include primary biological particles. The ubiquity of biological IN in precipitation from mid- to high-latitude locations has been well documented recently and biological IN have been measured directly from aerosol and cloud particle residuals. Biological IN, which again typically represent a small fraction of all primary biological particles, include ice nucleation active (INA) bacteria, fungal spores, pollen, leaf litter , and IN derived from bacterial decomposition of these particles. Biological IN are particularly efficient at warmer supercooled temperatures, and thus may play an especially important role in modestly supercooled clouds (warmer than -4.5 to - 10C. At the moment there is enormous uncertainty regarding number concentrations of INA biogenic particles in the atmosphere, and any estimates are limited due to the scarcity of data regarding the spatial and seasonal distributions of these types of particles. This proposal will contribute field measurement and modelling expertise to this ambitious project which will deliver quantitative and far reaching results for biogenic aerosol-cloud interactions and impacts on hydrological cycling for the first time.