Size-resolved hygroscopicity and ambient hydration state measured downwind of Mexico City during the MILAGRO campaign were used to evaluate the relationship between light extinction and relative humidity. Of specific interest is the contribution of deliquescent particles and the assumptions used to link their hydration state to ambient humidity. A pair of modified tandem differential mobility analyzers (TDMAs) was used to provide the needed data. In the first of these, the relative humidity between the two DMAs was scanned between roughly 30 and 90% during sequential measurements that characterized the upper and lower legs of any hygroscopic growth hysteresis loop. The hydration state of any particles that did exhibit such hysteresis was determined using an ambient state TDMA, which measures the response of an initially monodisperse aerosol to alternate humidity profiles.