Atmos. Chem. Phys., 18, 17515–17527, 2018 https://doi.org/10.5194/acp-18-17515-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Nitrate formation from heterogeneous uptake of dinitrogen pentoxide during a severe winter haze in southern China Hui Yun1, Weihao Wang1, Tao Wang1, Men Xia1, Chuan Yu1,2, Zhe Wang1, Steven C. N. Poon1, Dingli Yue3, and Yan Zhou3 1Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China 2Environment Research Institute, Shandong University, Jinan, China 3Guangdong Environmental Monitoring Center, State Environmental Protection Key Laboratory of Regional Air Quality Monitoring, Guangzhou, China Correspondence: Tao Wang ([email protected]) Received: 10 July 2018 – Discussion started: 6 August 2018 Revised: 20 November 2018 – Accepted: 25 November 2018 – Published: 11 December 2018 − Abstract. Nitrate (NO3 ) has become a major component of 1 Introduction fine particulate matter (PM2:5) during hazy days in China. However, the role of the heterogeneous reactions of dinitro- Severe haze in China has been a major concern of the reg- gen pentoxide (N2O5) in nitrate formation is not well con- ulatory and scientific communities in recent years. Nitrate strained. In January 2017, a severe haze event occurred in has also been identified as an important component of PM2:5 the Pearl River Delta (PRD) of southern China during which during hazy days in both summer and winter (e.g., Huang −3 high levels of PM2:5 (∼ 400 µg m ) and O3 (∼ 160 ppbv) et al., 2014; Li et al., 2018; Pathak et al., 2009; Zhang et were observed at a semi-rural site (Heshan) in the western al., 2015). Moreover, the proportion of nitrate in PM2:5 has PRD. Nitrate concentrations reached 108 µg m−3 (1 h time steadily increased over the last decade due to the lagged con- resolution), and the contribution of nitrate to PM2:5 was trol of NOx emissions compared to SO2 (Fu et al., 2014; − Geng et al., 2017; Qu et al., 2017; Reuter et al., 2014; X. nearly 40 %. Concurrent increases in NO3 and ClNO2 (with a maximum value of 8.3 ppbv at a 1 min time resolution) Wang et al., 2016). As a result, the concentrations of nitrate were observed in the first several hours after sunset, indi- in PM2:5 = PM1:0 have been even higher than those of sulfate cating an intense N2O5 heterogeneous uptake by aerosols. during some haze events (Ge et al., 2017; Li et al., 2017; Liu − et al., 2015; Yang et al., 2017; Yue et al., 2015). The formation potential of NO3 via N2O5 heterogeneous re- − actions was estimated to be between 29.0 and 77.3 µg m 3 Nitrate is formed from NOx during both the daytime and in the early hours (2 to 6 h) after sunset based on the mea- nighttime. During the day, nitric acid (HNO3) is produced surement data, which could completely explain the mea- through the gas-phase reaction between OH and NO2 (Re- − action R1), while this pathway is insignificant at night due sured increase in the NO3 concentration during the same time period. Daytime production of nitric acid from the gas- to very low OH concentrations (e.g., Seinfeld and Pandis, phase reaction of OH C NO2 was calculated with a chemi- 2016). Nitric acid can react with ammonia (NH3) to form cal box model built using the Master Chemical Mechanism ammonium nitrate (NH4NO3), and an equilibrium can be (MCM v3.3.1) and constrained by the measurement data. reached for these three compounds between the gas phase The integrated nocturnal nitrate formed via N2O5 chemistry and the particle phase (Reactions R2, R3). During nighttime, was comparable to or even higher than the nitric acid formed the heterogeneous uptake of N2O5, which is formed from during the day. This study confirms that N2O5 heterogeneous reactions involving O3, NO2, and NO3, becomes a source chemistry was a significant source of aerosol nitrate during of nitrate and also produces gaseous ClNO2 when chloride- hazy days in southern China. containing aerosol is present (Reactions R4–R7; Finlayson- Pitts et al., 1989). This nitrate formation pathway is only im- portant at night due to the fast photolysis of NO3 during the Published by Copernicus Publications on behalf of the European Geosciences Union. 17516 H. Yun et al.: Nitrate formation from heterogeneous uptake of dinitrogen pentoxide day. Compared to the relatively well-understood formation time nitrate formation. Based on the observed covariation of of aerosol nitrate via the OHCNO2 reaction, the contribution nitrate and RH, Wang et al. (2009) speculated that N2O5 re- from N2O5 heterogeneous reactions has been poorly quanti- actions dominated the nitrate formation on polluted days with fied due to the limited knowledge of key factors controlling high NO2 and O3 concentrations in Shanghai. Neither N2O5 the heterogeneous processes, such as the N2O5 uptake coef- nor ClNO2 was measured during these early observation- ficient (γN2O5 ) and ClNO2 yield (φClNO2 ) (Brown and Stutz, based studies. A recent study (H. Wang et al., 2017) inferred 2012; Chang et al., 2011). γN2O5 from the measured N2O5 on 4 days in urban Beijing and estimated the lower limit of the formation potential of OH C NO2 C M ! HNO3 C M (R1) aerosol nitrate assuming a unity φClNO2 because ClNO2was HNO3.g/ C NH3.g/ $ NH4NO3.s/ (R2) not measured. Their result showed a comparable contribution C $ C C − to nitrate formation from the N O heterogeneous chemistry HNO3.g/ NH3.g/ NH4 .aq/ NO3 .aq/ (R3) 2 5 and from the daytime pathway of the OH C NO2 reaction. NO2 C O3 ! NO3 (R4) In the present study, N O , ClNO , and the related chemi- C C $ C 2 5 2 NO2 NO3 M N2O5 M (R5) cal and meteorological parameters were measured at a semi- NO3 C VOCs ! products (R6) rural site in the Pearl River Delta of southern China from 2 − N2O5 C H2O or Cl .aq/ to 15 January 2017. A severe haze event was observed dur- −3 ! − − C ing the field study with PM2:5 reaching 400 µg m and O3 .2 φ/NO3 .aq/ φClNO2.g/ (R7) peaking at 160 ppbv. ClNO2, which is only known to be pro- Model studies initially treated γN2O5 as a constant (0.03 duced from N2O5 heterogeneous uptake, reached 8.3 ppbv, to 0.1) (Dentener and Crutzen, 1993; Makar et al., 1998; which is the largest reported value to date and revealed ex- Munger et al., 1998; Schaap et al., 2004; Wen et al., 2015; tremely active N2O5 chemistry during the abovementioned Xue et al., 2014), and later utilized several parameterization episode. The concurrent measurements of N2O5, ClNO2, schemes of γN2O5 and φClNO2 based on laboratory investiga- and aerosol nitrate provide better constraints for elucidating tions of their dependence on aerosol composition and aerosol nighttime NO3 = N2O5 chemistry and aerosol nitrate forma- water content (Anttila et al., 2006; Bertram and Thornton, tion. An overview of the measurement data was first pre- 2009; Davis et al., 2008; Evans and Jacob, 2005; Riemer et sented. The nighttime processes that led to the formation of al., 2003, 2009; Roberts et al., 2009). However, recent stud- nitrate (e.g., the production of NO3 and N2O5,N2O5 uptake ies have found a significant discrepancy between the field- coefficient, and ClNO2 yield) were analyzed. The nighttime derived and the parameterized γN2O5 and φClNO2 (McDuffie formation potential of nitrate was estimated based on these et al., 2018; Phillips et al., 2016; Tham et al., 2018; X. Wang data and compared to the measured increase in nitrate. The et al., 2017; Z. Wang et al., 2017; Zhou et al., 2018). These daytime production of nitric acid via the OH C NO2 reac- findings suggest that N2O5 uptake is more complicated than tion was calculated based on a box model using the Master previously thought, and a better understanding of the uptake Chemical Mechanism (MCM v3.3.1) and compared to the process is needed to improve the prediction of nitrate and nighttime formation potential of nitrate. haze. In addition to the modeling approach, field measurements of trace gases and aerosol composition have been used to 2 Methods infer the contribution of N2O5 heterogeneous chemistry to nitrate formation. Pathak et al. (2009) postulated the im- 2.1 Site description portance of N2O5 heterogeneous reactions with respect to the high aerosol nitrate observed in Beijing and Shanghai The field observation was conducted at the Guangdong atmo- in summertime by examining the variation of nitrate with spheric supersite, a semi-rural site located at Hua Guo Shan the change in relative humidity (RH) and the equilibrium (HGS, 22.728◦ N, 112.929◦ E) southwest of the city of He- between anions and cations in PM2:5. Pathak et al. (2011) shan from 2 to 15 January 2017. As shown in Fig. 1, HGS further investigated nitrate formation using a coupled aque- is a hill with a height of 60 m a.s.l. All measurement instru- ous phase radical mechanism (CAPRAM) and a gas-phase ments were located on the fourth floor of a four-story build- chemistry mechanism (RACM, without ClNO2chemistry). ing on the top of the hill. The observation site was located By constraining the uptake coefficient of N2O5 in the range in the western PRD where the economic activity and popu- from 0.001 to 0.1, they reproduced the observed enhance- lation density are much lower than the central PRD. There ment of nitrate and suggested that N2O5 uptake by aerosols are five main roads near the HGS site, including three na- contributed to 50 % to 100 % of the nighttime increase in tional roads (G325, G94, and G15), and two provincial roads nitrate.