Air quality monitoring as well as comfort or overall energy performance require accurate information on airflow patterns, although they are particularly difficult to assess in existing buildings. We present an innovative instrumentation system using gas tracers, based on the combined use of several highly sensitive emerging micro-gas chromatographs ($\mu$GC) as pollutant sensors and an identification method for data processing. The strategy used is based on the optimal control theory in which only the time evolution of the components of the velocity field needs to be reconstructed thanks to the use of a proper orthogonal decomposition (POD) method. We present numerical results showing evidence of the performance of the approach in the case of a unimodal flow.