We present a spectral imager capable of measuring the 2-D distribution of NO₂ above well-delimited emission sources (power plant, city, etc.) with an unprecedent spatiotemporal resolution. Tests at a coal-fired power plant demonstrated its capability to observe dynamic processes such as the conversion from NO to NO₂ in the early plume. Potential applications are pollution sources monitoring, reactive plume chemistry models validation, ships and volcanic emissions tracking, etc.