Nighttime NO_x loss and ClNO₂ formation in the residual layer of a polluted region: Insights from field measurements and an iterative box model

Highlights

• N₂O₅ uptake coefficient and ClNO₂ yield were 0.004 ± 0.003 and 0.4 ± 0.3, respectively.

• NO₃ + VOCs, N₂O₅ + H₂O, and N₂O₅ + Cl⁻ contributed to 47%, 27% and 23% of NO₃ loss.

• NO_x in the air masses was removed by 70% ± 10% during the nighttime.

• The ClNO₂ of up to 6 ppbv was produced over the entire night.

Abstract

The heterogeneous reaction of dinitrogen pentoxide (N₂O₅) on aerosols is an important sink of nitrogen oxides (NO_x) in the polluted boundary layer, and the production of nitryl chloride (ClNO₂) can have significant effects on the atmospheric oxidative capacity. However, the heterogeneous loss of N₂O₅ and the formation of ClNO₂ are still not well quantified, especially in China. In a previous study, we measured ClNO₂ and N₂O₅ concentrations in several air masses at a high-elevation site in Hong Kong, and found the highest levels ever reported at one night. The present study employed an iterative box model to investigate five N₂O₅/ClNO₂-laden nights. We first estimated the N₂O₅ uptake coefficient and ClNO₂ yield and then calculated the relative importance of N₂O₅ heterogeneous reactions to NO_x loss and the accumulated ClNO₂ production over the entire night. The average uptake coefficient was 0.004 ± 0.003, and the average yield was 0.42 ± 0.26. As the air masses aged, the accumulated ClNO₂ reached up to 6.0 ppbv, indicating significant production of ClNO₂ in the polluted air from the Pearl River Delta. ClNO₂ formation (N₂O₅ + Cl⁻), N₂O₅ hydrolysis (N₂O₅ + H₂O), and NO₃ reactions with volatile organic compounds (NO₃ + VOCs) consumed 23%, 27%, and 47% of the produced NO₃, respectively, as the average for five nights. A significant portion of the NO_x in the air masses (70% ± 10%) was removed during the night via NO₃ reactions with VOCs (~ 40%) and N₂O₅ heterogeneous loss (~ 60%).

Introduction

The heterogeneous uptake of dinitrogen pentoxide (N₂O₅) on chlorine-containing aerosols is an important nighttime loss pathway of nitrogen oxides (NO_x) and leads to the production of nitryl chloride (ClNO₂) (Brown et al., 2006b, Osthoff et al., 2008). ClNO₂ is a major source of chlorine radical (Cl) in the morning and affects atmospheric oxidation capacity in the boundary layer (Phillips et al., 2012, Riedel et al., 2014, Sarwar et al., 2014, Thornton et al., 2010, Young et al., 2012). The chemical removal of nighttime NO_x is initialized by the production of nitrate radicals (NO₃) via the oxidation of nitrogen dioxide (NO₂) by ozone (O₃) (see R1). The reaction between NO₂ and NO₃ radicals is reversible and quickly reaches equilibrium with N₂O₅ (see R2). NO₃ is an efficient nighttime oxidant of volatile organic compounds (VOCs, see R3), especially alkenes (Brown and Stutz, 2012, Ng et al., 2017). N₂O₅ further reacts with aerosol water and chloride to form nitrate and ClNO₂ after accommodation into the aerosol.

$$\left(\mathrm{R}1,k_1\right){\mathrm{NO}}_2+{\mathrm{O}}_3\to {\mathrm{NO}}_3{k}_1=1.40\times {10}^{-13}\times {\mathrm{e}}^{\left(-2470/\text{TEMP}\right)}$$

$$\left(\mathrm{R}2,K_{\mathrm{eq}}\right){\mathrm{NO}}_2+{\mathrm{NO}}_3\leftrightarrow {\mathrm{N}}_2{\mathrm{O}}_5{K}_{\mathrm{eq}}=2.7\times {10}^{-27}\times {\mathrm{e}}^{\left(11000/\text{TEMP}\right)}$$

$$\left(\mathrm{R}3,k_{\mathrm{NO}3}\right){\mathrm{NO}}_3+\text{VOCs}\to \text{products}$$

$$\left(\mathrm{R}{4}_{\mathrm{a}},k_{\mathrm{N}2\mathrm{O}5}\right){\mathrm{N}}_2{\mathrm{O}}_5+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{het}\right)\to 2\left(1-\varphi \right){{\mathrm{NO}}_3}^{-}\left(\mathrm{aq}\right)$$

$$\left(\mathrm{R}{4}_{\mathrm{b}},k_{\mathrm{N}2\mathrm{O}5}\right){\mathrm{N}}_2{\mathrm{O}}_5+{\mathrm{Cl}}^{-}\left(\mathrm{het}\right)\to \varphi {{\mathrm{NO}}_3}^{-}\left(\mathrm{aq}\right)+\varphi {\text{ClNO}}_2\left(\mathrm{g}\right)$$

The heterogeneous uptake of N₂O₅ can, in general, be described as R4_a and R4_b, with k_N2O5 as the rate coefficient and ϕ as the yield of ClNO₂. The rate coefficient can be calculated with the uptake coefficient of N₂O₅ (γ_N2O5), the mean molecular speed of N₂O₅ (V_N2O5), and aerosol surface density (S_a), as shown in Eq. (1), when the gas-phase diffusive effect is negligible (Tang et al., 2014).

$$k_{\mathrm{N}2\mathrm{O}5}=\frac{1}{4}{\gamma }_{\mathrm{N}2\mathrm{O}5}{\mathrm{V}}_{\mathrm{N}2\mathrm{O}5}{\mathrm{S}}_{\mathrm{a}}$$

The heterogeneous reactions of N₂O₅ are influenced by both the composition of the aerosols and meteorological conditions (Brown and Stutz, 2012, Chang et al., 2011, Phillips et al., 2016), and considerable uncertainties exist in constraining different pathways of the nighttime loss of NO₃ and NO_x. To better understand the nighttime chemistry, field observations of N₂O₅ and ClNO₂ have been extensively conducted in North America and Europe (Brown and Stutz, 2012 and references therein; Chang et al., 2011 and references therein; Phillips et al., 2016) and more recently in China (Brown et al., 2016, Tham et al., 2014, Tham et al., 2016, Wang et al., 2017a, Wang et al., 2016b, Wang et al., 2017c, Wang et al., 2017b). Based on the field measurements, two important parameters for characterizing nighttime nitrogen chemistry, γ_N2O5 and ϕ_ClNO2, have been derived. In previous studies, the γ_N2O5 ranged from 0.0005 to ~ 0.1, and ϕ_ClNO2 varied widely from 0.01 to unity (Brown and Stutz, 2012, Phillips et al., 2016). The contributions of different pathways to the nighttime loss of NO₃ and NO_x have been quantified by combining the measured data with numerical modeling. The loss of NO₃ was dominated by reactions with anthropogenic alkenes (> 70%) in Houston, Texas, where large petrochemical industries were located (Stutz et al., 2010), and by nitrate formation through N₂O₅ uptake in Los Angeles (~ 90%) (Tsai et al., 2014) and in Boulder, Colorado (~ 80%) (Wagner et al., 2013). The nighttime loss of NO_x was responsible for up to 60% of the total NO_x loss in a 24 h period in the lower atmosphere (Tsai et al., 2014, Wagner et al., 2013). These studies suggested that the relative importance of each NO₃ and NO_x loss pathway could differ from place to place due to different chemical and meteorological conditions.

The Pearl River Delta (PRD) region of China (including Hong Kong) suffers from severe photochemical pollution with high levels of NO_x and O₃ (Wang et al., 1998, Wang et al., 2001, Wang et al., 2009, Wang et al., 2017d, Zhang et al., 2008a). Hazy days with high aerosol loadings also occur from time to time (Deng et al., 2008, Fan et al., 2014, Huang et al., 2016, Wang et al., 2016a, Zhang et al., 2008b). In a previous field campaign at Mt. Tai Mo Shan (TMS, 957 a.s.l.) in Hong Kong, we observed the highest levels ever reported for N₂O₅ and ClNO₂ (7.7 ppbv and 4.7 ppbv, respectively) in one well-processed air mass from the inland PRD region (Brown et al., 2016, Wang et al., 2016b). The ClNO₂ of 4.7 ppbv enhanced O₃ concentration by 16% at the peak time and led to a 41% increase in daytime O₃ production when the air mass moved out to the open sea (Wang T. et al., 2016). Elevated concentrations of N₂O₅ or ClNO₂ (up to 3.5 ppbv and 1.5 ppbv, respectively) were also observed on several other occasions just after sunset during that study. These N₂O₅/ClNO₂-laden air masses were brought to the site by northerly winds from the inland PRD region. Similar winds were present at the time when the highest concentrations were observed. As the air masses aged, high concentrations of ClNO₂ may be present in downwind locations and may have significant effects on the O₃ production in the following day.

In this follow-up study, we applied an iterative box model to investigate NO_x loss and ClNO₂ formation over the entire night for N₂O₅/ClNO₂-laden air masses in the TMS campaign. We estimated the uptake coefficient of N₂O₅ and the yield of ClNO₂, and determined the relative importance of NO_x loss pathways in the observed air masses. We demonstrated that high levels of ClNO₂ can be frequently produced in the locations downwind of TMS. This study adds new insights into the nighttime nitrogen chemistry in the residual layers of polluted regions.