Effects of NH₃ on secondary aerosol formation from toluene/NOx photo-oxidation in different O₃ formation regimes

Highlights

• How NH₃ affects particle formation in different photochemical regimes was investigated.

• NH₃ does not affect O₃ formation but affect particle formation and composition.

• Higher O:C and N:C were observed after NH₃ injection in NOx-limited regime.

• Four factors contributing to the secondary aerosols were identified by PMF analysis.

Abstract

Ammonia (NH₃) is an important base gas in the atmosphere and has received extensive research interest due to its role on secondary aerosol formation. However, the effects of NH₃ on the photochemical smog oxidants formation including ozone and secondary aerosol did not attract much attention. In this study, a set of smog chamber experiments was conducted under toluene/NOx photo-oxidation base condition and adding NH₃ in the following photochemical regimes referred as hydrocarbon, NOx-limited and transitional regimes. Dedicated instruments including gas analysers, scanning mobility particle sizer (SMPS), aerosol mass spectrometry (HR-ToF-AMS) were used to measure the gaseous compounds, aerosol mass concentration and chemical composition. It was found that the presence of NH₃ did not affect the O₃ formation rate. However, the particle number concentration increased dramatically after NH₃ was injected in any photochemical regimes. This effect has been mainly attributed to the rapid formation of organic ammonium and ammonium nitrate. The mass spectra of organic aerosols from AMS showed that the most abundant fragments were at m/z 28, 29, 43 and 44, which became higher after the addition of NH₃ in the transition and NOx-limited regimes, indicating that NH₃ could enhance the formation of the compounds containing carbonyl and carboxylic acid functional groups. Compared to hydrocarbon-limited regime, higher atomic nitrogen-to-carbon (N:C) ratios were observed when NH₃ were injected into transition and NOx-limited regimes, and this is mainly due to more organic nitrate formation. Four factors, which were assigned to inorganic nitrate aerosols, carboxylic acid compounds, carbonyl compounds and N-containing organic compounds (NOC), were identified and interpreted by applying the positive matrix factorization (PMF) to the AMS dataset. These results provide new insights into the complex chemistry of toluene/NOx/NH₃ during different O₃ formation regimes, and imply the benefit of controlling NH₃ to mitigate PM_2.5 when O₃ formation is under VOC-limited regime.

Introduction

The ozone (O₃) formation is chemically affected by the abundance of its precursors, such as volatile organic compounds (VOCs), CO, NOx and sunlight, making the mitigation strategies difficult to be implemented due to the complexity of O₃ chemistry (Chen et al., 2018a; Fan et al., 2021; Han et al., 2019). The three well-known photochemical regimes, i.e., hydrocarbon-limited, NOx-limited and transition regimes were usually referred to assess the sensitivity of O₃ to its precursors (Tan et al., 2018; Wang et al., 2018a). For example, Li et al. (2017a) investigated the meteorological and chemical impacts on summertime O₃ in Hangzhou, and the results showed that the O₃ formation mostly occurred in the hydrocarbon-limited and transition regimes. The simulation results from chemical transport modelling of Wang et al. (2019) showed that the summertime O₃ formation in the areas of the west, central and south China were in the NOx-limited regime, while in the areas of north and east China were in the transition regime. The areas in the hydrocarbon-limited regime did not have such broad distribution and mainly located at urban cores and large city clusters. Thus, the strategies to mitigate VOCs and NOx should take different photochemical regimes into account to better control O₃ pollution.

Although many studies have focused on O₃ formation in different photochemical regimes, the particle formation was rarely investigated during different O₃ formation regimes, which was still not well understood and even controversial. Cheng et al. (2019) showed that the emission control measures, which was implemented to reduce PM_2.5, conversely enhanced the diurnal peak ozone concentration during the short-term emission reduction period of Asia-Pacific Economic Cooperation (APEC) in 2014 Beijing. However, in the areas such as Yangtze River Delta (YRD) and Pearl River Delta (PRD), the increased O₃ seemed to enhance the atmospheric oxidation and promoted secondary aerosol formation, and thus resulting in a positive correlation between PM_2.5 and O₃ (Fan et al., 2020; Feng et al., 2019; Li et al., 2013). All these phenomena indicated our incomplete understanding between the co-existed PM_2.5 and O₃ problems, and thus further studies on the interaction between the O₃ formation regimes and aerosol formation chemistry is highly needed in order to better explain the nature of O₃ and PM pollution events in China.

The formation of secondary organic aerosol (SOA) occurs during the photochemical smog activity. For instance, there are a variety of peroxy radicals (RO₂) being formed from the photo-oxidation process of volatile organic compounds. In addition to the RO₂-HO₂ reaction, these RO₂ radicals could also react with each other to form low-volatility compounds, which were considered to contribute SOA formation (Bianchi et al., 2019; Birdsall et al., 2010; Kroll and Seinfeld, 2008). Meanwhile, the RO₂ and HO₂ radicals can also react with the available NO to produce NO₂, which resulted into the net O₃ formation through NO₂ photolysis subsequently (Venecek et al., 2018). The relationship between O₃ and aerosol formation follows non-linear complex interactions that can be affected by relative humidity, NOx level and OH concentration, which had not yet been investigated extensively (Carlton et al., 2009). Therefore, any other substances (such as SO₂, NH₃) that reacted with the active radicals and NOx might finally affect the formation of O₃ and the intermediates of secondary aerosol (Li et al., 2018a; Na et al., 2006).

Ammonia (NH₃) is an important gaseous component that can contribute to new particle formation (NPF) and secondary inorganic aerosol formation in the atmosphere. The presence of a few parts per trillion by volume (pptv) NH₃ could greatly enhance the nucleation rate, and strengthen the aerosol-cloud radiative forcing (Dunne et al., 2016). Besides, NH₃ could neutralise nitric acid and sulfuric acid formed from NOx and SO₂ in the atmosphere and contribute to the secondary aerosol formation (Ding et al., 2019). The abundance of NH₃ in the atmosphere also offset the effects of reducing SO₂ and NOx to mitigate PM_2.5 pollution, suggesting that synchronously controlling NH₃ emission was needed, rather than merely controlling SO₂ and NOx emissions (Fu et al., 2017; Liu et al., 2019). Due to its vital role in nucleation and haze formation, NH₃ had received extensive research interest in recent years (Lehtipalo et al., 2018).

A few studies have reported the effects of NH₃ on SOA formation from biogenic and anthropogenic VOCs. For example, Babar et al. (2017) showed that NH₃ enhanced the aerosol formation from the photo-oxidation or ozonolysis of α-pinene. This phenomenon could be attributed to reaction of organic acids with NH₃, which would form organic ammonium salts, and thus changing the particle size, chemical composition and optical properties of SOA (Babar et al., 2017; Hao et al., 2020; Na et al., 2007). Similar results were also observed for the photo-oxidation of aromatic hydrocarbon and gasoline vapour in the presence of NH₃, in which the particle number concentration and mass concentration increased significantly (Chen et al., 2019; Huang et al., 2018a; Liu et al., 2015). The aerosol composition also changed due to the reactions of NH₃ with nitric acid and some oxidised products from parent VOCs (Wang et al., 2018b; Yang et al., 2021). Nevertheless, the influence of NH₃ on aerosol formation in different O₃ formation regimes is not clear, which consequently motivates this work.

In this study, we performed a set of smog chamber experiments to investigate how NH₃ affects O₃ formation, or the radicals formed in each photochemical regime, which would eventually affect the particle formation and composition. The obtained data and results collected from this study can provide an improved understanding of the correlation between O₃ and particle formation in the presence of NH₃. To our best knowledge, this study was the first attempt to investigate the effects of NH₃ on particle formation in different photochemical regimes.