Accurate assessment of the potential impact of greenhouse gases and aerosols on the Earth system can be enhanced by a global monitoring network and can be facilitated by the development of compact, portable optical instruments for field use. The more important of these gases, e.g., methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O), have strong absorptions at wavelengths between 2 and 5 micrometers ; however, this spectral region is heavily dominated by absorption by water (H2O) which is itself an important contributor to radiative transfer at these wavelengths. To achieve the desired reduction in instrument size, it is often necessary to relax wavelength resolution requirements which in turn affects the accuracy and precision of the retrieved column abundances. To address these measurement problems, an infrared sun photometer has been constructed for application to trace-gas detection and analysis techniques are being developed to extract column abundances from the spectrally congested data. The current instrument design is based on a circular variable filter (CVF) with wavelength coverage from 1.2 to 5 micrometers . Preliminary measurements with this instrument are presented and electro-optical alternatives to the CVF as the tuning element are discussed.