Megacities observing system for Los Angeles. In-situ sensors located around the LA basin provide continuous, high accuracy measurements of greenhouse gas (GHG) mixing ratios of CO2, CH4, and CO. A remote-sensing instrument on Mt. Wilson provides multiple scans per day of the basin to measure slant-column mixing ratios. Another remote-sensing instrument at Caltech provides continuous daytime measurements of column mixing ratios. Aircraft and mobile laboratories provide infrequent but intensive measurements of mixing ratios. Satellites are beginning to provide remote-sensing measurements of LA. Other instruments (not shown) measure winds and boundary layer height.