With a global warming potential of nearly 300, N2O is a critically important greenhouse gas, contributing about 5 % of the US total GHG  emissions.  Agriculture soil management practices are the dominant source of anthropogenic N2O emissions, contributing nearly 75 % of US N2O emissions.  In urban areas, vehicle tailpipe emissions and waste water treatment plants are significant sources of N2O.  We report here a new mid-infrared laser-based cavity ring-down spectrometer (Picarro G5310) that was recently developed to measure sub-ppb ambient concentrations of two key greenhouse gas species, N2O and CO, simultaneously.  It combines a quantum cascade laser with a proprietary 3-mirror optical cavity.  The new optical analyzer was set up to monitor N2O and CO, along with CO2 and CH4, in ambient air obtained from a 10 meter tower in Santa Clara, California.  Evidence of contributions from traffic and a nearby sewage treatment facility were expected in the measurement data.