Photochemical reactions provide the driving force for much of the chemistry in the atmosphere. The in situ rates of these photolysis reactions are important in understanding production and loss terms for ozone, cycling of atmospheric nitrogen oxide species, and odd hydrogen production. Recent developments in instrumentation have provided in situ photolysis frequency determinations of many molecules important in tropospheric photochemistry with 10 second time response. Anthropogenic aerosols can have large effects on the actinic radiation transmitted to the surface, thus perturbing clear sky boundary layer photochemistry. Ground based photolysis frequencies have been determined using Scanning Actinic Flux Spectroradiometry (SAFS) in the urban plume of Houston, Texas. These measurements are compared with clear sky radiative transfer model outputs, with and without incorporating overhead aerosol burden determined from wavelength dependent aerosol optical depth measurements.