The affordable and easily accessible sensing of greenhouse gases is a vital Smart City application, serving both climate targets, and digitization goals at a European level. Commonly used spectroscopic solutions, although reliable, remain bulky and costly. Integrated photonics can fill the need for ubiquitous air testing, offering robust, miniaturized systems, with low cost. We present a study of a MID-IR spectroscopy sensing system developed on a silicon photonics platform, utilizing a Bragg grating mirror cavity. For each targeted gas, an optical cavity is designed, allowing the use of multimode radiation in its dedicated spectral window, therefore increasing the interaction length in the silicon waveguide.