Passive monitoring is an important tool for measuring, troubleshooting, and protecting modern wireless networks. To date, WiFi monitoring has focused primarily on indoor settings or ephemeral outdoor studies though wardriving. We present Argos, the first urban-scale wireless sensor network designed explicitly to support measurement of ambient WiFi traffic across an entire city. Urban-scale wireless monitoring presents unique challenges due to limited packet-capture ability, heterogeneous traffic loads, and limited backhaul capacity between sensor nodes. Argos addresses these through in-network traffic merging and processing, plus an intelligent approach to coordinated channel sampling by multiple sniffers. Argos provides a rich query interface allowing users to study the complex dynamics of ambient wireless traffic. We present a detailed evaluation of a 26-node Argos network deployed on streetlights and rooftops around a city, demonstrating its ability to detect and classify wireless ...