Tanner et al. (1989) also have used a dual channel ozone-chemiluminescence analyzer previously employed by Joseph and Spicer (1978) for the determination of HNO3 where this acid is selectively retained by nylon filters in the presence of other NOr compounds. With this approach, levels of HNO3 corresponded during daytime to those obtained with the diffusion denuder, but were higher during the night.