Mobile monitoring was conducted with two Google Street View vehicles equipped with the Aclima mobile platform, which used research-grade instrumentation to provide 1 Hz measurements of BC, NO, NO2, and UFP (SI Appendix, Table ), and Global Positioning System coordinates with a nominal precision of ~1 m. Detailed description of measurement and calibration protocol is contained in .The inlet and sampling manifold, mounted on the roof of each mobile laboratory, was designed to minimize self-sampling (measurements of the vehicle's own exhaust) as well as particle and gas phase sampling losses, with an estimated particle sampling loss of <5%. Raw measurement data were reviewed and processed to exclude periods of poor instrument performance . Data collection occurred over 32 months from May 2015 through Dec 2017. The average non-highway driving speed of 9 m s -1 provides a nominal spatial resolution of ~10 m.