On November 18, 1997, above-road particulate matter (PM) lidar (light detec
tion and ranging) signals and heavy-duty (HD) and light-duty (LD) vehicle c
ounts were simultaneously collected for 894 10-sec sampling periods at the
Caldecott Tunnel in Orinda, CA, for the purpose of measuring the relative c
ontributions of LD and HD vehicles to the PM lidar signal under real-world
driving conditions.