About the Cover: To assess diesel vehicle particulate mass emissions in a timely manner as function of engine operating conditions and driving mode, instruments having a time resolution of about 1 second are needed. This is important because much greater particulate mass emissions occur during transient operation, such as hard acceleration, than during steady driving conditions. Despite this requirement, commonly used tests depend on instruments yielding results taking anywhere from minutes to hours to obtain and that often provide poor temporal resolution during transient engine operating conditions. As discussed in "Time Resolved Characterization of Diesel Particulate Emissions", H. Moosmüller and colleagues at the Desert Research Institute, in Reno, Nevada and J. Collins and colleagues at the University of California, Riverside address this problem by investigating and reporting their evaluation of five promising fast response instruments.