Rate coefficient for the OH radical reaction with HONO2 under UT/LS conditions

Nitric acid (HONO2) is one of the most reactive nitrogen-containing species in our atmosphere and its oxidation chemistry plays an important role in controlling the O3 budget in the upper troposphere/lower stratosphere (UT/LS). The O3 budget in the UT/LS is largely controlled by the cycling of HOx (OH + HO2) and NOx (NO + NO2). The HONO2 reaction with OH is a net HOx radical sink and drives NOx/y partitioning chemistry by yielding NO3 radicals; therefore, understanding the rate coefficient of the OH + HONO2 reaction is critical for accurate prediction of the O3, HOx, and NOx budgets in the UT/LS. Previous studies have observed both negative temperature-dependent and pressure-dependent kinetics below, which results from the formation of a pre-reactive complex between OH and HONO2 ( 6 kcal mol-1). Currently, the JPL recommended uncertainty in kOH+HONO2 is 20% (±1σ) at STP, but under UT/LS conditions (low T and p), the uncertainty could increase to as much as ± 50%, which translates to 10 - 20% in model predictions of NOx/NOy partitioning. Previous groups have measured the concentration of HONO2 after or before the reaction cell but never in-situ, which could lead to greater uncertainties in the overall determination of the rate coefficient from heterogeneous uptake of the low volatility HONO2. Here, we present experimental results into the matrix of the temperature and pressure dependence of the OH + HONO2 rate coefficient. Specifically, our results represent the first determination of kOH+HONO2 using in-situ [HONO2] measurements coupled to a Light Induced Fluorescence (LIF) technique to probe OH radicals. Results and importance to UT/LS chemistry will be discussed. Copyright 2015, California Institute of Technology