In Situ Measurements of Black Carbon Aerosol During the 2006 Texas Air Quality Study

In situ measurements of black carbon (BC) aerosol were acquired on 17 flights during the 2006 Texas Air Quality Study (TexAQS) with a Single-Particle Soot Photometer (SP2) operating aboard the NOAA WP-3D research aircraft. BC mass mixing ratios decrease by about 3 orders of magnitude from the polluted boundary layer to the clean middle troposphere, based on the particle size range of 0.1 to 0.6 μm volume-equivalent diameter, representing approximately 90% of the BC mass distribution. BC spans the range from 1000 ng/kg in the polluted boundary layer to less than 1 ng/kg in the clean middle troposphere. Furthermore, BC varies by about 2 orders of magnitude at any given altitude over the course of the mission. Simultaneous measurements of BC and carbon monoxide (CO) are positively correlated throughout the lower and middle troposphere. While the relationship between BC and CO is linear and compact for background tropospheric air, the correlation changes near BC source regions, with different signatures for the Houston urban, ship channel and biomass burning plumes. We examine the large variance in BC mass loadings, particularly at higher values of CO. A major implication of this work is that measurements of CO cannot be reliably used to estimate BC mass loadings in polluted urban environments.