The regional distributions of black carbon aerosol (soot) are calculated and the impacts of black carbon aerosol on photolysis frequencies and ozone concentrations are evaluated in Houston, Texas, using a regional chemical transport model. Compared with measurements of the fine particle matter in the Houston area, the simulated daily mean black carbon aerosol concentration is in agreement with observations in the urban sites. Black carbon aerosol reduces the photolysis frequencies of J3(1D)> and J2> in the planetary boundary layer by 10--30% when air pollution is highest in the Houston area. As a result, the ground level ozone concentration in the Houston area decreases by 5--20%. Our study demonstrates that the impacts of black carbon aerosol on photochemistry are important in polluted urban atmospheres.