Governments restricted mobility and effectively shuttered much of the global economy in response to the COVID-19 pandemic. Six San Francisco Bay Area counties were the first region in the United States to issue a “shelter-in-place” order asking non-essential workers to stay home. Here we use CO2 observations from 35 Berkeley Environment, Air-quality and CO2 Network (BEACO2N) nodes and an atmospheric transport model to quantify changes in urban CO2 emissions due to the order. We infer hourly emissions at 900-m spatial resolution for 6 weeks before and 6 weeks during the order. We observe a 28% decrease in anthropogenic CO2 emissions during the order and show this decrease is primarily due to changes in traffic (-44%) with pronounced changes to daily and weekly cycles; non-traffic emissions show small changes (-8%). These findings provide a glimpse into a future with reduced CO2 emissions through electrification of vehicles. (submitted to GRL: 2020-07-24 16:49:19)