Recent observations suggest a large and unknown daytime source of nitrous acid (HONO) to the atmosphere. Multiple mechanisms have been proposed, many of which involve chemistry that reduces nitrogen dioxide (NO2) on some timescale. We compare weekday and weekend measurements of NO2 and HONO in two U.S. cities to examine the NO2 dependence of the daytime HONO source. We find that daytime HONO is not proportional to that day's NO2, the local NO2 concentration at that time and several hours earlier. We discuss various published HONO formation pathways in the context of this constraint.