Atmospheric gaseous organic acids in winter in a rural site of the North China Plain

Abstract

Organic acids are important contributors to the acidity of atmospheric precipitation, but their existence in the Chinese atmosphere is largely unclear. In this study, twelve atmospheric gaseous organic acids, including C₁-C₉ alkanoic acids, methacrylic acid, pyruvic acid, and benzoic acid, were observed in the suburb of Wangdu, Hebei Province, a typical rural site in the northern China plain from 16^th December, 2018 to 22^nd January, 2019, using a Vocus® Proton-Transfer-Reaction time-of-flight mass spectrometer (Vocus PTR-ToF). The quantification of C₂-C₄ alkanoic acids by the Vocus PTR-ToF was calibrated according to the titration of a NaOH solution by C₂-C₄ alkanoic acids from home-made permeation sources, and the other organic acids except for formic acid were quantified based on the k_cap-sensitivity linearity in the Vocus PTR-ToF, whereas formic acid was not quantified because our instrument setting led to a significant underestimation in its concentration. The average total concentration of eleven gaseous organic acids was 6.96 ± 5.20 ppbv (parts per billion by volume). The average concentration of acetic acid was the highest (3.86 ± 3.00 ppbv), followed by propanoic acid, butyric acid, and methacrylic acid. Domestic straw burning was likely the dominant source of the observed gaseous organic acids according to the good correlations between acetonitrile and organic acids and between particulate K⁺ and organic acids, and traffic emissions could also have contributed. During episodes with continuously high concentrations of organic acids, short-distance transport dominated in Wangdu according to the backward trajectory analysis. Baoding, Shijiazhuang, and Hengshui areas were the main source areas based on potential source contribution function and concentration weighing track analysis.

Introduction

Organic acids contribute to the acidity of atmospheric precipitation and are involved in the formation of secondary organic aerosols, thus affecting the visibility, climate, and human health (Chebbi and Carlier, 1996; Fisseha et al., 2006; Pye et al., 2020). A variety of organic acids exist in the troposphere, and a wide concentration range of organic acids have been detected in the gas phase, aqueous phase, and particulate matters in the urban, rural, marine, and polar atmospheres (Bastidas and La Iglesia, 2007; Chebbi and Carlier, 1996; Nolte et al., 1999). For example, Kawamura et al. (1985) detected gaseous C₁-C₁₀ alkanoic acids in the atmosphere of Los Angeles with formic acid and acetic acid being the most abundant, reaching levels of a few ppbv (parts per billion by volume), followed by propanoic acid with a concentration of about 0.1 ppbv. Satsumabayashi et al. (1989) detected acetic acid, propanoic acid, and butyric acid in the ranges of 4.0–8.0, 0.3–0.7, and 0.1–0.3 ppbv, respectively, in Japan's atmosphere (Satsumabayashi et al., 1989). In addition to alkanoic acids, pyruvic acid was widely detected in the atmospheres of the Amazon forest, the eastern United States, and the marine atmosphere (Talbot et al., 1990). Formic acid, acetic acid, and propanoic acid have also been detected in the tropospheric rainwater, cloud water, fog water, snow water, and even polar ice samples (Pye et al., 2020). Compared to monocarboxylic acids with relatively higher volatilities, dicarboxylic acids are more likely to exist in the atmosphere in the form of particulate phase (Mochizuki et al., 2017).The most common organic acids in atmospheric particulates are oxalic acid, succinic acid, malonic acid, maleic acid, and phthalic acid according to studies in Southeast Asia and East Asia, urban and rural areas of central Europe, the United States, Africa, the marine atmosphere, and the Arctic region (Andreae et al., 1988; Mochizuki et al., 2016; Nolte et al., 1999; Veres et al., 2011; Zemankova and Brechler, 2010).

Atmospheric organic acids can be directly emitted or secondary formed in the atmosphere. The direct emissions of gaseous organic acids consist of biogenic and anthropogenic ones. Common primary biogenic sources of atmospheric gaseous acids include direct emissions from soil, plant, metabolic activities of bacteria and microorganisms, and ants (Chebbi and Carlier, 1996). Primary sources of anthropogenic emissions include traffic emissions, biomass burning, and stationary emissions (Bastidas and La Iglesia, 2007; Nolte et al., 1999). It is worth noting that the anthropogenic primary emission is an essential source of organic acids in the urban, industrial, and concentrated source areas, and one of the most important sources of organic acids in the non-urban atmosphere (Chebbi and Carlier, 1996).

Photochemical formation is another important source of atmospheric organic acids in summer (Kawamura et al., 2005; Orzechowska et al., 2005; Orzechowska and Paulson, 2005). Veres et al. (2011) demonstrated that gaseous organic acids could be rapidly generated in Pasadena's urban atmosphere through photooxidation reactions. Mattila et al. (2018) detected formic acid, propanoic acid, butyric acid, valeric acid, and pyruvic acid in the atmosphere of the Colorado Front Range using high-resolution time-of-flight chemical ionization mass spectrometry (HR-ToF-CIMS), and showed that the main sources of gaseous organic acids were direct transportation, agricultural activities, and photochemical oxidation based on the correlations between atmospheric tracers and organic acids.

There are very sparse systematic studies on atmospheric gaseous organic acids in China. Mochizuki et al. (2017) used Gas Chromatography-Mass Spectrometry (GC-MS) to detect gaseous and particulate organic acids near Mountain Tai, and found 0.0004–1.5650 ppbv of gaseous C₁-C₁₀ alkanoic acids and benzoic acid. On the other hand, atmospheric gaseous organic acids were more often reported in studies of volatile organic compounds (VOCs) or oxygenated volatile organic compounds (OVOCs). For example, Han et al. (2019) detected 7.60 ppbv of formic acid and 5.23 ppbv of acetic acid, respectively, in summer 2014 in Wangdu, and 0.67 ppbv of formic acid and 0.57 ppbv of acetic acid, respectively, in summer 2016 in YangMeiKeng in the suburb of Shenzhen, when VOCs were measured by a proton transfer reaction-mass spectrometer (PTR-MS). In 2018, Zhu et al. (2019) detected formic acid (1.21 ppbv) and acetic acid (1.68 ppbv) in the atmosphere of Shenzhen, when using a PTR-MS to study the atmospheric VOCs (Zhu et al., 2019). Clearly, more attention has been paid to formic acid and acetic acid that are of higher concentrations in China. Other gaseous organic acids were seldom reported, likely due to the challenges for detection of gaseous organic acids with lower concentrations.

In this study, a Vocus® proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-ToF, Tofwerk AG/Aerodyne Research, Inc, Switzerland/USA) was used to characterize atmospheric gaseous organic acids in Wangdu, Hebei Province, a rural area in the northern China plain that is located within the “Beijing-Tianjin-Hebei urban agglomeration”. Meteorological parameters, concentrations of atmospheric trace gases, and particulate K⁺ were recorded and compared with the profiles of organic acids to elucidate their potential sources.