The significant contribution of HONO to secondary pollutants during a severe winter pollution event in southern China

Abstract

Abstract. Nitrous acid (HONO) can strongly affect atmospheric photochemistry in polluted regions through the production of hydroxyl radicals (OHs). In January 2017, a severe pollution episode occurred in the Pearl River Delta (PRD) of China, with maximum hourly PM2.5, ozone, and HONO levels reaching 400 µg m−3, 150 ppb, and 8 ppb, respectively, at a suburban site. The present study investigated the sources and processes generating such high HONO concentrations and the role of HONO chemistry in this severe winter episode. Four recently reported HONO sources were added to the Community Multiscale Air Quality (CMAQ) model, including RH-dependent (relative humidity) and light-enhancing effects on heterogeneous reactions, photolysis of particulate nitrate in the atmosphere, and photolysis of HNO3 and nitrate on surfaces. The revised model reproduced the observed HONO and significantly improved its performance for O3 and PM2.5. The model simulations showed that the heterogeneous generation on surfaces (with RH and light effects) was the largest contributor (72 %) to the predicted HONO concentrations, with the RH-enhancing effects more significant at nighttime and the light-enhancing effects more important in the daytime. The photolysis of total nitrate in the atmosphere and deposited on surfaces was the dominant HONO source during noon and afternoon, contributing above 50 % of the simulated HONO. The HONO photolysis was the dominant contributor to HOx production in this episode. With all HONO sources, the daytime average O3 at the Heshan site was increased by 24 ppb (or 70 %), compared to the simulation results without any HONO sources. Moreover, the simulated mean concentrations of TNO3 (HNO3+ fine particle NO3-) at the Heshan site, which was the key species for this haze formation, increased by about 17 µg m−3 (67 %) due to the HONO chemistry, and the peak enhancement reached 55 µg m−3. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment.

Heterogeneous N<sub>2</sub>O<sub>5</sub> reactions on atmospheric aerosols at four Chinese sites: improving model representation of uptake parameters

“…2 and 3) was fitted as a linear function of [H 2 O], as (3.0 ± 0.4) × 10 4 × [H 2 O]. This value is in reasonable agreement with the values of (2.7-3.8) ×10 4 , ∼ 3.9 × 10 4 , and 2.6 × 10 4 M −1 s −1 determined from ammonium bisulfate, ammonium sulfate (Gaston and Thornton, 2016), and aqueous organic acid particles (Thornton et al, 2003), respectively. Compared to original BT09 (Eq.…”

“…S2). Different dimensionless K H values have been used in previous studies, e.g., ∼ 50 (e.g., Hallquist et al, 2003;Bertram and Thornton, 2009) or ∼ 120 (e.g., Gaston et al, 2014;Gaston and Thornton, 2016;Griffiths et al, 2009), which correspond to a Henry's law constant of 2 or 5 M atm −1 at 298 K. As γ N 2 O 5 in the parameterization is linearly dependent on the K H , an increase of K H value would proportionally increase the γ N 2 O 5 value, but it cannot account for the large variability of measured γ N 2 O 5 values. Given the lack of an explicit function of effective Henry's law constant for N 2 O 5 to include the different processes (e.g., "salting-in" effect and surface processes), we use the value of 51 suggested by Bertram and Thornton (2009) and enclose those effects from the aerosol composition in the last "chemical" term.…”

Evaluation and uncertainty investigation of the NO<sub>2</sub>, CO and NH<sub>3</sub> modeling over China under the framework of MICS-Asia III

“…Several potential factors were found to be responsible for the model deviation and differences, including the emission inventories, chemistry-meteorology coupling effects, bidirectional exchange of NH 3 and the NH 3 gas-aerosol partitioning, which are all important aspects with respect to the model improvements in future. Previous studies also suggest that the nitrous acid (HONO) chemistry plays an important role in the atmospheric nitrogen chemistry, which influences the simulations of NO 2 and NH 3 (Fu et al, 2019;Zhang et al, 2017Zhang et al, , 2016. Heterogeneous conversion from NO 2 to HONO (2NO 2(g) + H 2 O (l) → HONO (l) + HNO 3(l) ) is one of the dominant sources of HONO in the atmosphere, which has been considered in most models of MICS-Asia III, including CMAQ since version 4.7, NAQPMS, NHM-Chem and GEOS-Chem.…”

“…However, this method is more suited to deal with the uncertainty related to the continuous variables, such as input data or parameters in parameterization. The ensemble method, based on a set of different models, is an alternative approach to accounting for the range of uncertainties (Galmarini et al, 2004;Mallet and Sportisse, 2006). For example, the Air Quality Model Evaluation International Initiative (AQMEII) has been implemented in Europe and North America to investigate the model uncertainties of their regional-scale model predictions (Rao et al, 2011).…”

Revisiting the Agung 1963 volcanic forcing – impact of one or two eruptions

“…Differences of the zonal mean zonal wind in the extratropics, around ±10 %, are not significant and are mainly caused by a meridional shift of the higher-latitude wind systems. Punge et al (2009) showed that meridional transport in the tropics and subtropics depends on the QBO phase. Figure 2 shows that both eruption phases inject into easterly zonal wind.…”