Sources of formaldehyde and their contributions to photochemical O₃ formation at an urban site in the Pearl River Delta, southern China

Highlights

• Secondary formation was the greatest contributor to ambient HCHO, followed by vehicular exhaust and solvent usage.

• Alkenes was the most important group contributing to the secondary formation of HCHO.

• trans-2-Butene had the largest contribution to secondary HCHO formation, followed by i-butene, cis-2-butene and propene.

• Secondary formation contributed about 2.8 ppbv and 1.1 × 10⁷ molecule/cm³ to the production of O₃ and HO_x radical.

Abstract

Two models (the Positive Matrix Factorization (PMF) model and a photochemical box model with Master Chemical Mechanism (PBM-MCM)) were applied to analyze the formaldehyde (HCHO) data collected in July 2006 at an urban site (GPEMC) in the Pearl River Delta (PRD), southern China. Three major HCHO sources (secondary formation, vehicular exhaust, and solvent usage) were identified and they were found to contribute in average 53%, 31% and 16% respectively to the total HCHO loading at GPEMC. Alkenes was the most important group contributing to the secondary formation of HCHO, followed by aromatics and alkanes. Among them, trans-2-butene had the largest contribution to secondary HCHO formation, with the average percentage of 16 ± 4%, followed by i-butene, cis-2-butene, propene, isoprene and m,p-xylene. Secondary HCHO and HCHO emitted from vehicular emissions contributed comparably to ground-based measured O₃ and HO_x radical at GPEMC, higher than that from solvent usage (1.3 ± 0.1 ppbv and (4.1 ± 0.3) × 10⁶ molecule/cm³ for O₃ and HO_x radical). Our results highlight the importance of secondary HCHO formation for both photochemical formation of ozone and the oxidative capacity of the atmosphere in this region. It is hence critical for policy makers to propose strategies for controlling VOCs from vehicular emissions in order to reduce secondary HCHO formation. Our results also have important implication for improving the understanding of the source apportionments of HCHO and their contributions to photochemical pollution in the PRD region in China.

Introduction

Formaldehyde (HCHO) is an abundant carbonyl compound that plays an important role in atmospheric photochemistry and air quality. Oxidation or secondary formation of HCHO can provide radical sources (i.e., HO_x) that drive ozone formation and HCHO is hence a large component of the total VOC (Volatile Organic Compound) reactivity of the atmosphere (Steiner et al., 2008). Formaldehyde is also a hazardous pollutant that is listed as a human carcinogen highly associated with nasopharyngeal carcinoma and probably leukemia (WHO, 2000).

The primary direct sources of formaldehyde include biomass burning, fossil fuel combustion, solvent usage, industrial and biogenic emissions (Parrish et al., 2012). In addition, secondary formation through photo-oxidation of methane and non-methane hydrocarbons is another important source of HCHO in different environments (Atkinson et al., 2006). Taking propene (C₃H₆) as an example, secondary formation of HCHO can be represented by the following simplified mechanism:

OH + C₃H₆ → HYPROPO₂ (R1)

OH + C₃H₆ → IPROPOLO₂ (R2)

NO + HYPROPO₂ → HYPROPO + NO₂ (R3)

IPROPOLO₂ + NO → IPROPOLO + NO₂ (R4)

HYPROPO → CH₃CHO + HCHO + HO₂ (R5)

IPROPOLO → CH₃CHO + HCHO + HO₂ (R6)

Through oxidation of C₃H₆ initiated by the OH radical, propyl peroxy radical (HYPROPO₂ and IPROPOLO₂) could be formed (R1 and R2), which could further oxidize NO to produce propyl alkoxy radical (HYPROPO and IPROPOLO, R3 and R4). The propyl alkoxy radical could react with O₂, resulting in the formation of HCHO and HO₂ radical (R5 and R6). In addition, the oxidation of propene by other oxidants, i.e., O₃ and NO₃, could lead to the formation of HCHO. On the other hand, HCHO could be also formed from the scission of a C-C bond of alkoxy radicals through photolysis (Luecken et al., 2012).

Photolysis and reaction with OH radical are the major sinks of HCHO in the atmosphere (Ervens and Kreidenweis, 2007, Lei et al., 2009). Oxidation of HCHO can lead to the formation of HO₂ radical and ozone, playing important roles in local air pollution. Therefore, it is essential to identify the major sources of HCHO and to quantify their contributions on ozone formation in order to effectively control photochemical pollution in a given area. Source identification and apportionment are also critical for better understanding the formation of photochemical smog and assessing their impacts on human health and atmospheric chemistry based on the important roles of formation and removal of HCHO in photochemistry (Louie et al., 2012, Zhong et al., 2013).

As one of the most important developing regions in China, the Pearl River Delta (PRD) region is facing severe photochemical pollution in recent years (Wang et al., 2009, Zheng et al., 2010). Previous studies have been reported that O₃ formation in the urban areas in the PRD region is mainly VOC-limited, whereas it is NO_x-limited in the rural areas. For example, Zheng et al. (2010) found that photochemical O₃ production was controlled by VOCs in urban areas in the PRD region and possibly limited to NO_x in the northern or northeastern rural area in PRD based on the investigation of air monitoring data of 2006–2007 from the PRD regional air quality monitoring network. Similar results were found by investigating the VOCs/NO_x plots in the PRD region (Zou et al., 2015). Furthermore, by developing a speciated VOC emission inventory and calculating the O₃ formation potential of each species, Zheng et al. (2009) concluded that isoprene emitted from biogenic emission made the greatest contribution to O₃ formation in the PRD region. On the other hand, although many studies were conducted to investigate the source apportionments of HCHO with various methods, including the ratios between HCHO and other aldehydes, correlation between HCHO and tracers of other primary emissions, emission inventories, emission-based measurements and receptor models (Duan et al., 2008, Li et al., 2010, Wang et al., 2010, Ho et al., 2012, Louie et al., 2012), the contributions of primary emissions and secondary formation of HCHO still needed to be further quantified. Yuan et al. (2012a) analyzed qualitatively the influence of primary emissions and secondary formation on ambient HCHO by investigating the correlation between HCHO and tracers of primary emissions (CO) and secondary formation (O₃ + NO₂) in the PRD region. Based on in-situ measurements, Ho et al., 2006, Ho et al., 2012 estimated the abundance of HCHO from the emissions of commercial cooking and vehicular exhaust sources in Hong Kong. Dong et al. (2014) calculated the emission factors of HCHO from heavy-duty diesel vehicles with chassis dynamometer. Based on OH reactivity of HCHO different methods were adopted to quantify their contributions to ozone formation, including the maximum incremental reactivity (MIR) and propene-equivalent concentration of species methods. However, those methods only estimate ozone formation under optimum or ideal conditions (Dong et al., 2014, Duan et al., 2008, Louie et al., 2012, Shao et al., 2009). Although Lou et al. (2010) and Cheng et al. (2010) showed that HCHO was an important precursor of O₃ and the contribution to ozone formation was evaluated in the PRD region with observation-based model, they presented the level of O₃ formed from photo-oxidation of total measured HCHO only, not differentiating the contributions from different sources (Li et al., 2014). Hence the major sources of HCHO and their contributions to ozone formation and the oxidative capacity of the atmosphere in the PRD region currently remain unclear.

In this study, the relative contributions of primary emissions and secondary formation of HCHO were characterized using a receptor model (i.e., positive matrix factorization (PMF) model), based on the data collected from an extensive field measurement at an urban site in the PRD region in July 2006. By combining the PMF model with a photochemical box model, the contributions of the above sources, particularly the different primary emissions to photochemical O₃ formation were quantified. Though stringent control measures have been implemented in Guangdong Province to control emission from vehicles and industrial sources, the results of this study can provide reference data for the evaluation of the effectiveness of the control measures adopted after 2006, which is helpful for the development of future control strategies (Louie et al., 2012, Zheng et al., 2010).