Explicit modeling of background HCHO formation in southern China
Introduction
Formaldehyde (HCHO) is an important reactive carbonyl that has a critical influence on atmospheric chemistry. HCHO can be removed from the atmosphere mainly via photolysis with a lifetime to be of the order of few hours (Mellouki et al., 2015; Zhou et al., 2007). Especially, the photolysis of HCHO is a significant source of radicals (i.e., HOx) and hence plays an essential role in ozone (O3) formation (Edwards et al., 2014; Ling et al., 2017; Xue et al., 2016). Furthermore, HCHO has mutagenic and carcinogenic properties, which can directly threaten the health of exposed people and has been listed as a dangerous carcinogen by the World Health Organization (WHO) (WHO, 2000). Therefore, pollution arising from HCHO has attracted much attention in recent years with regard to its significant role in air quality and health risks.
In the troposphere, HCHO is discharged directly into the atmosphere from anthropogenic emissions (e.g., motor exhaust and industrial emissions) and natural emissions (e.g., live and decaying plant emission, biomass burning and sea water) (da Silva et al., 2016; Liu et al., 2009; Luecken et al., 2012; Mellouki et al., 2015). Another important HCHO source is secondary sources formed through the degradation of hydrocarbons, i.e., the OH/NO3/O3-initiated degradation of VOCs, produced HCHO in multiple steps during VOCs oxidation (Atkinson and Arey, 2003; Lin et al., 2012; Liu et al., 2015). What's more, previous studies confirmed that the secondary formation of HCHO may be larger than primary emission (Li et al., 2014a; Liu et al., 2009; Wang et al., 2015). Hence, more efforts are needed to shed light on the formation mechanisms of HCHO. The secondary formation process of HCHO is very complicated, here taking ethene (C2H4) as an example.
Eqs. (R1), (R2), (R3), (R4), (R5), (R6), (R7) show the simplified mechanism of HCHO formation from C2H4. The OH-initiated degradation of C2H4 creates an ethyl peroxy radical (R1) which can oxidize NO to NO2, creating ethyl alkoxy radical (R2). The ethyl alkoxy radical could react with O2 to form HCHO (R3). The NO3-initiated oxidation of C2H4 produces a nitryl peroxy radical, followed by fast subtraction of oxygen to nitryl peroxy radical forms a nitryl alkoxy radical, which can produce HCHO (R4 to R6). The oxidation of C2H4 by O3 could lead to HCHO formation (R7). In addition, alkoxy radicals photolysis reaction can produce HCHO (Ling et al., 2017; Luecken et al., 2012). Therefore, HCHO can be produced via the oxidation reaction chains of various hydrocarbons, and the dominant primary precursors are quite inhomogeneous between different study regions. For instance, Li et al. (2014b) report that methane (CH4) oxidation contributed most to HCHO production in remote areas, whereas isoprene (C5H8) degradation was the dominant source of HCHO in semirural areas. In comparison, anthropogenic alkenes are usually important precursors of HCHO in urban areas of Beijing (Liu et al., 2015). Accordingly, it is essential to understand the chemistry formation mechanisms and the precursors of HCHO for making effective control measures to mitigate photochemical air pollution in a particular region.
In fact, numerous studies have reported on the serious HCHO pollution in China, most of which mainly focused on either its pollution levels or potential sources in the polluted regions of China (Li et al., 2010; Ling et al., 2017; Yang et al., 2017; Yang et al., 2019). In recent,limited studies have focused on the formation mechanisms of HCHO in severe polluted regions of China, such as Beijing (Liu et al., 2015; Yang et al., 2018b) and Pearl River Delta (PRD) (Li et al., 2014b; Ling et al., 2017). Hong Kong, which adjacent to PRD, has been undoubtedly experiencing severe HCHO and O3 pollution owing to its fast expansion of economy and urbanization (Cheng et al., 2014; Guo, 2009; Louie et al., 2013). Yet, to the best of our knowledge, HCHO budget in the Hong Kong region remain unclear currently.
As a rural coastal site of Hong Kong, Hok Tsui (HT) has been widely used to be an ideal regional background station and captures the “regional air pollution” in the PRD and Hong Kong (Wang et al., 2001; Zha et al., 2014). To understand the characteristics and HCHO process on the regional scale, intensive field observations were conducted at this coastal site in late summer, autumn and winter of 2012. In the following sections, the data we will first describe chemical compositions of the measured carbonyls. We then explore the processes affecting the HCHO levels by examining time series. The effects of carbonyls on O3 formation are assessed by calculating the OH loss rates (LOH) of individual VOCs. Finally, we analyze the photochemical formation processes of HCHO, with the application of an observation-based chemical box model constrained by a full suite of observed chemical species and meteorological parameters. Through this detailed chemical analysis, we have aimed to explicitly explore the dominant formation pathways. Sensitivity experiments were also conducted to diagnose the formation regimes of HCHO.
Section snippets
Experiments
The field measurements were carried out at the Hong Kong Polytechnic University's air monitoring station at Hok Tsui (HT, 22.22°N, 114.25°E, 60 m above sea level), which is located in the southeastern tip of Hong Kong Island (see Fig. 1). The station is a typical coastal site with a 270° view of the South China Sea, and has been used as a regional background site in many previous studies. Under the influence of the Asian monsoon, this site receives varying air masses including clean marine air,
Characteristics of observed carbonyls
Averaged values of the major carbonyls together with Formaldehyde/Acetaldehyde Ratio (F/A) are summarized in Table 2. The total carbonyls concentration was 2.80 ± 1.35 ppbv. Of the ten carbonyls, HCHO was the most abundant carbonyl, ranging from 0.98 to 5.25 ppbv with an average of 2.00 ± 1.04 ppbv and contributed to as high as 71% of the observed carbonyls. CH3CHO was the second abundant carbonyl species and its concentration was 0.59 ± 0.28 ppbv, with a maximum of 1.32 ppbv. HCHO and CH3CHO
Summary
Our field experiments were performed at a background site of Hong Kong over the period from August 20 to December 22, 2012. Severe photochemical air pollution accompanied by high concentrations of HCHO and O3 during the most early stages of the investigation period and reasonable air quality with lower concentrations of O3 and HCHO during the later period provided us an opportunity to probe the photochemistry characteristics of the typical background atmosphere in Hong Kong. During the
Acknowledgments
The authors thank Tao Wang, Shuncheng Lee, Likun, Xue, Steven Poon, Xiao-Cui Chen for their contributions to the field study and writing guide. We are grateful to the University of Leeds for providing the Master Chemical Mechanism (v3.3). The field measurement data used in this study were collected with support from the Hong Kong Polytechnic University. This study is supported by the Doctoral Research Fund of Shandong Jianzhu University (X19036ZZ).
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