Significance Isoprene-derived hydroxy nitrates (IHNs) play a significant role in controlling the lifetime of NO _x . Here, we showcase isomer-specific measurements of IHN in ambient air, which have allowed us to quantify real-time changes in its isomer distribution. These measurements show that 1,2-IHN undergoes a rapid loss not experienced by the 4,3-IHN isomer. Laboratory studies suggest this sink is likely condensed-phase hydrolysis, forming HNO ₃ . To match observations, one-dimensional box-model simulations require additional loss rates (here, assigned to hydrolysis) similar in magnitude to the accounted loss processes (OH oxidation and dry deposition). Global simulations included in this process show reduced ozone levels and assign a significant fraction of HNO ₃ production to this mechanism within the lower atmosphere.