A method of generating a spectral absorption map for detecting emission of gas comprising one or more compounds of interest, the method comprising: (a) positioning an instantaneous field of view ( ifov) of an optical sensor toward a reflector installment mounted about a site to be monitored and within a target region; (b) directing a beam of illumination from an illumination source and toward the reflector installment; (c) scanning the beam of illumination across at least a portion of the reflector installment, thereby illuminating a plurality of sampled locations on the reflector installment; (d) detecting, with one or more detectors of the optical sensor, light within one or more spectral bands of interest, at least a portion of which overlap with one or more spectral features associated with the one or more compounds of interest, wherein the detected light comprises: (i) reflected illumination light corresponding to light from the beam of illumination having been reflected from the plurality of sampled locations on the reflector installment and captured within the ifov of the optical sensor, and (ii) reflected ambient light having been reflected from a plurality of additional sampled locations and captured within the ifov of the optical sensor, the plurality of additional sampled locations within the target region, but not on the reflector installment; (e) receiving and/or accessing, by a processor of a computing device, data corresponding to the detected light, comprising both (i) the reflected illumination light and (ii) the reflected ambient light; and (f) determining, by the processor, for each of (i) at least a portion of the plurality of sampled locations on the reflector installment and (ii) at least a portion of the plurality of additional sampled locations, an absorption level associated with at least one of the one or more spectral features using the detected light, thereby generating a spectral absorption map comprising a plurality of absorption levels, each associated with a: (i) a particular sampled location on the reflector installment or a particular additional sampled location and (ii) a spectral feature. (a) positioning an instantaneous field of view ( ifov) of an optical sensor toward a reflector installment mounted about a site to be monitored and within a target region; (b) directing a beam of illumination from an illumination source and toward the reflector installment; (c) scanning the beam of illumination across at least a portion of the reflector installment, thereby illuminating a plurality of sampled locations on the reflector installment; (d) detecting, with one or more detectors of the optical sensor, light within one or more spectral bands of interest, at least a portion of which overlap with one or more spectral features associated with the one or more compounds of interest, wherein the detected light comprises: (i) reflected illumination light corresponding to light from the beam of illumination having been reflected from the plurality of sampled locations on the reflector installment and captured within the ifov of the optical sensor, and (ii) reflected ambient light having been reflected from a plurality of additional sampled locations and captured within the ifov of the optical sensor, the plurality of additional sampled locations within the target region, but not on the reflector installment; (i) reflected illumination light corresponding to light from the beam of illumination having been reflected from the plurality of sampled locations on the reflector installment and captured within the ifov of the optical sensor, and (ii) reflected ambient light having been reflected from a plurality of additional sampled locations and captured within the ifov of the optical sensor, the plurality of additional sampled locations within the target region, but not on the reflector installment; (e) receiving and/or accessing, by a processor of a computing device, data corresponding to the detected light, comprising both (i) the reflected illumination light and (ii) the reflected ambient light; and (f) determining, by the processor, for each of (i) at least a portion of the plurality of sampled locations on the reflector installment and (ii) at least a portion of the plurality of additional sampled locations, an absorption level associated with at least one of the one or more spectral features using the detected light, thereby generating a spectral absorption map comprising a plurality of absorption levels, each associated with a: (i) a particular sampled location on the reflector installment or a particular additional sampled location and (ii) a spectral feature.