The role of HONO in O<sub>3</sub> formation and insight into its formation mechanism during the KORUS-AQ Campaign

Abstract

Abstract. Photolysis of nitrous acid (HONO) has long been recognized as an early morning source of OH radicals in urban air, but the detailed mechanism of its formation is still unclear. During the Korea-US Air Quality (KORUS-AQ) campaign, HONO was measured using Quantum Cascade Tunable Diode Laser Absorption Spectroscopy (QC-TDLAS) at Olympic Park in Seoul from 17 May to 10 June, 2016. HONO concentrations ranged from 0.07 ppbv to 3.46 ppbv with an average of 0.93 ppbv. HONO remained high at night from 1 am to 5 am, during which the mean concentration was higher in high-O3 episodes (1.82 ppbv) than non-episode (1.20 ppbv). In the morning, OH budget due to HONO photolysis were higher by 50 % (0.95 pptv) during high-O3 episodes compared to non-episode. Diurnal variations of HOx and O3 simulated by the F0AM model demonstrated a difference of ~ 20 ppbv in daily maximum O3 between the two periods. The HONO concentration increased with relative humidity (RH) until 80 %, of which the highest HONO was associated with the top 10 % NOx, confirming that NOx is a crucial precursor of HONO and its formation is facilitated by humidity. The conversion ratio of NOx to HONO was estimated to be 0.86 × 10−2 h−1 at night and also increased with RH. As surrogate for the catalyst surface, the mass concentrations of black carbon (eBC) and the surface areas of particles smaller than 120 nm showed a tendency for RH similar to conversion ratio. Using an Artificial Neuron Network (ANN) model, HONO concentrations were successfully simulated with measured variables (r = 0.8 for the best suite), among which NOx, surface area, and RH were found to be main factors affecting ambient HONO concentrations with weigh values of 26.2 %, 11.9 %, and 10.6 %, respectively. This study demonstrates the coupling of HONO with HOx-VOCs-O3 cycle in Seoul Metropolitan Areas (SMA) and provides practical evidence for heterogeneous formation of HONO by employing the ANN model to atmospheric chemistry.

Characterization of Atmospheric PM2.5 Inorganic Aerosols Using the Semi-Continuous PPWD-PILS-IC System and the ISORROPIA-II

“…− shows a better correlation with the observed HONO during the nighttime (from 6 p.m. to 6 a.m.) with R 2 of 0.41, while a poorer correlation is found during the daytime (from 7 a.m. to 5 p.m.) in the NH 3 -rich environment (3.90 ± 3.02 µg/m 3 ). This result is because HONO is easily consumed in the daytime (up to 80% reduction) by photolysis (Equation (6)) [66,67] as: HONO + hv→NO + OH…”

Exploration of the atmospheric chemistry of nitrous acid in a coastal city of southeastern China: Results from measurements across four seasons

“…Hydroxyl radicals play an important role in triggering the oxidation of volatile organic compounds and therefore determine the fate of many anthropogenic atmospheric pollutants (Lei et al, 2018). Recent research results have shown that HONO production is the cause of an increase in secondary pollutants (Li et al, 2010;Gil et al, 2019;Fu et al, 2019). Though extensive studies have been conducted in the four decades since the first clear measurement of HONO (Perner and Platt, 1979), the HONO formation mechanisms are still elusive, especially during the daytime, when there is a large difference between measured concentrations and those calculated from known gas-phase chemistry (Sörgel et al, 2011).…”

Answer for 'Review of manuscript'

“…Please see the response 6 above. 2) and 3) The detailed measurement techniques and information can be found elsewhere, including (Kim et al, 2020;Jordan et al, 2020) prepared for the KORUS-AQ special issue in Elementa. Lines 397-419: While the authors have used an interesting ANN tool, it is not clear what the results have taught us.…”