NARSTO_PAC2001_CASSIAR_TUNNEL_GAS_PM_DATA is the North American Research Strategy for Tropospheric Ozone (NARSTO) Pacific 2001 Air Quality Study (PAC2001) Cassiar Tunnel Gaseous and Particle Mass and Composition Data product. This data product was obtained from August 8-15, 2001 during the PAC2001.The Cassiar Tunnel site is located at 49 17' 01.9 N and 123 01' 54.2 W, at 40 m above sea level (a.s.l.). The tunnel is used mostly by light duty traffic with peak traffic volumes at rush hours. The goal of measurements at this site was to reduce the uncertainty in mobile source inventory for gas and particle emissions from light duty traffic sources, emphasizing the emissions of the precursors to Particulate Matter (PM) formation and primary PM emissions for comparison with tailpipe emissions data from traditional testing conducted in laboratory on mobile source emissions, particularly for mass emission rates and chemical profiles. For several measurements, different sampling and analytical techniques were used as a check on the accuracy of the measurements. For most gas measurements, two sets of instruments were deployed, one at each end of the tunnel, whereas most of the PM measurements were conducted at the exit end of the tunnel. Gas measurements included the typical pollution gases (SF6 as the tracer, NOx, N2O, CO, CO2, methane, SO2, VOCs, carbonyls, organic acids, NH3, Graham and Gray, 2002). The PM chemical and physical properties were measured in great details. For physical properties, particle number size distributions from 10 nm to 3 um were measured, and hygroscopic properties were measured at two sizes (Prenni et al., 2002). Chemical measurements characterized the mass, inorganic and carbonaceous compositions of the primary particles. The PAC2001 was conducted from August 1 until September 31, 2001 in the Lower Fraser Valley (LFV), British Columbia, Canada. The study consisted of individual research projects organized to address several issues on ambient particulate matter and ozone that are important to policy makers. A special issue of Atmospheric Environment [Vol. 38(34), Nov 2004] described specific study objectives (Li, 2004) and presented a series of results papers from the field study. The ground sampling sites during the study were (1) Cassiar Tunnel, (2) Slocan Park, (3) Langley Ecole Lochiel, (4) Sumas Eagle Ridge, and (5) Golden Ears Provincial Park. Aloft measurements were taken from a Convair 580 and a Cessna 188. Selected measurement data were compiled for each site and aircraft and are archived as site-specific data sets.NARSTO, which has since disbanded, was a public/private partnership, whose membership spanned across government, utilities, industry, and academe throughout Mexico, the United States, and Canada. The primary mission was to coordinate and enhance policy-relevant scientific research and assessment of tropospheric pollution behavior; activities provide input for science-based decision-making and determination of workable, efficient, and effective strategies for local and regional air-pollution management. Data products from local, regional, and international monitoring and research programs are still available.