Understanding the Evolution of Cloud Condensation Nuclei In Urban Plumes: Effects of Aerosol Ageing and Mixing

The aerosol indirect effect is a major source of uncertainty for predicting anthropogenic influences on climate; constraining the concentration of particulate matter that can act as cloud condensation nuclei (CCN) is a key component for quantifying this uncertainty. Köhler theory, used to predict CCN activity is well established yet calculating ambient CCN concentrations requires a significant amount of information that characterizes the aerosol size distribution, chemical composition and mixing state. In addition, large uncertainties are associated with predicting these aerosol properties, especially for highly heterogeneous aerosol close to source regions. As a result, it is very important to quantify the CCN prediction uncertainty associated with simplifying assumptions taken in aerosol models. In this study, airborne aerosol and CCN measurements collected aboard the NOAA WP-3D platform during the 2006 Texas Air Quality Study/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS) are analyzed. From September 20th through October 10th, 2006 over ten flights characterize the freshly emitted and aged aerosol sampled in the vicinity of Houston, eastern/northeastern Texas and the northwestern Gulf of Mexico. The analysis focuses on the importance of chemical heterogeneity (size-dependant chemistry and mixing state) and aerosol size distribution function for achieving CCN closure, as a function of particle ageing. The relative contribution of particle ageing versus entrainment of background aerosol to the CCN concentration is also examined.