A simple and sensitive method to determine parts per billion (ppb) of atmospheric formaldehyde in situ, using chromotropic acid, is described. A colorimetric sensor, coupled to a droplet of 15.5 µL chromotropic acid, was constructed and used to sample and quantify formaldehyde. The sensor was set up with two optical fibers, a light emitting diode (LED) and two photodiodes. The reference and transmitted light were measured by a photodetection arrangement that converts the signals into units of absorbance. Air was sampled around the chromotropic acid droplet. A purple product was formed and measured after the sampling terminated (typically 7 min). The response is proportional to the sampling period, analyte concentration and sample flow rate. The detection limit is ∼2 ppb and can be improved by using longer sampling times and/or a sampling flow rate higher than that used in this work, 200 mL min⁻¹. The present technique affords a simple, inexpensive near real-time measurement with very little reagent consumption. The method is selective and highly sensitive. This sensor could be used either for outdoor or indoor atmospheres.