Extrapolating Ground-Based Aircraft Engine Exhaust Emissions to Cruise Conditions: Lessons From the 2013 ACCESS Chase Plane Experiment

Aircraft engine emissions constitute a tiny fraction of the global black carbon mass, but can have a disproportionate climatic impact because they are emitted high in the troposphere and in remote regions with otherwise low aerosol concentrations. Consequently, these particles are likely to strongly influence cirrus and contrail formation by acting as ice nuclei (IN). However, the ice nucleating properties of aircraft exhaust at relevant atmospheric conditions are not well known, and thus, the overall impact of aviation on cloud formation remains very uncertain. While a number of aircraft engine emissions studies have previously been conducted at sea level temperature and pressure (e.g., APEX, AAFEX-1 and 2), it unclear the extent to which exhaust emissions on the ground translate to emissions at cruise conditions with much lower inlet gas temperatures and pressures. To address this need, the NASA Alternative Fuel Effects on Contrails and Cruise Emissions (ACCESS) was conducted in February-April, 2013 to examine the aerosol and gas emissions from the NASA DC-8 under a variety of different fuel types, engine power, and altitude/meteorological conditions. Two different fuel types were studied: a traditional JP-8 fuel and a 50:50 blend of JP-8 and a camelina-based hydro-treated renewable jet (HRJ) fuel. Emissions were sampled using a comprehensive suite of gas- and aerosol-phase instrumentation integrated on an HU-25 Falcon jet that was positioned in the DC-8 exhaust plume at approximately 100-500m distance behind the engines. In addition, a four-hour ground test was carried out with sample probes positioned at 30 m behind each of the inboard engines. Measurements of aerosol concentration, size distribution, soot mass, and hygroscopicity were carried out along with trace gas measurements of CO2, NO, NO2, O3, and water vapor. NOx emissions were reconciled by employing the well-established Boeing method for normalizing engine fuel flow rates to STP; however, comparison of aerosol emissions between ground and altitude is less straight forward. The implications of these factors for developing new aviation emissions factors / inventories related to aerosol species will be discussed.