# Pollution Characteristics of Atmospheric Carbonyl Compounds in a Large City of Northern China

Authors: Jinhe Wang; Shan Chen; Xiaoguo Qiu; Wenya Niu; Ouyang Li; Chao Zhu; Xi Zhang; Xue Yang; Guiqin Zhang
Journal: Journal of Chemistry
 (2022)

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## Abstract

To better understand the pollution characteristics and formation mechanisms of atmospheric carbonyl compounds, continuous measurements of carbonyl compounds in Jinan were taken for one month at a sampling frequency of 2 h. The sources, pollution characteristics, and concentration changes of carbonyl compounds during the summers of 2018 and 2020 were compared. The total concentrations of carbonyl compounds were 10.51 ± 0.13 ppbV and 6.30 ± 1.08 ppbV in 2018 and 2020, respectively. In both years, formaldehyde, acetone, and acetaldehyde were the major carbonyls. Diurnal variations and correlation analyses showed that exhaust emissions from motor vehicles during peak traffic periods significantly contributed to the concentrations of carbonyl compounds in Jinan, with formaldehyde exhibiting net production. The ratio of formaldehyde/acetaldehyde (C1/C2) was 2.64 in 2018 and 2.03 in 2020, indicating that carbonyl compounds are jointly affected by anthropogenic sources and photochemical reactions. Master Chemical Mechanism model analyses showed that the formation of formaldehyde in Jinan was controlled by RO + O2 reactions, and formaldehyde was mainly consumed via photolysis and its reaction with the hydroxyl radical. In situ photochemistry can further promote formaldehyde production. The comparison of the reactivities of different carbonyl compounds revealed that formaldehyde, acetaldehyde, butyraldehyde, and propionaldehyde play an important role in hydroxyl radical reactions and ozone generation. Among all the measured carbonyl compounds, benzaldehyde contributed the most to secondary organic aerosols (SOAs). Overall, this study provides new insights into the formation mechanisms of carbonyl compounds as well as their pollution characteristics.


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## Body

## 1. Introduction

Atmospheric carbonyl compounds, including aldehydes and ketones, are a group of oxygenated volatile organic compounds (OVOCS), which are both important precursors for and components of photochemical smog [1, 2]. Carbonyl compounds can react with the hydroxyl radical (·OH) to generate HO2 and RO2 radicals, which can oxidize NO to NO2 and further promote the formation of O3 in the troposphere [3]. Carbonyl compounds are also important intermediates in the formation of secondary aerosols and widely distributed in the troposphere [4]. The primary source of atmospheric carbonyl compounds is direct emissions from human activities and natural processes, such as fossil fuel combustion, incomplete combustion of biomass, and vegetation emissions [5], and the secondary source is generation from volatile organic compounds from anthropogenic or natural emissions through atmospheric oxidation reactions [6]. The oxidation of carbonyl compounds could produce formic acid, acetic acid, and other acidic substances, which may enhance atmospheric acidity, intensify the formation of acid rain, and harm the environment [7]. In addition, most carbonyl compounds are highly irritating and toxic. In particular, formaldehyde poses serious risks to the human eyes, skin, and respiratory system, and has been confirmed as a first-class human carcinogen by the International Agency for Research on Cancer (IARC) [8]. Acetaldehyde is also considered as a potential carcinogen [9].At present, the monitoring of atmospheric carbonyl compounds is a hotspot in the field of atmospheric environmental studies [10]. However, the accurate measurement of carbonyl compounds is difficult and demanding because of their low concentrations in the atmosphere, wide range of variation, high reactivity, unstable chemical properties, and short atmospheric life. In China, research on atmospheric carbonyl compounds has been focused on Guangzhou, Shanghai, Beijing, Hong Kong, and other large cities, with other cities in different regions largely being neglected. Huang et al. [11] selected two sampling sites to analyze the pollution characteristics of carbonyl compounds in Shanghai and found that the concentrations of C1-C4 carbonyl compounds were high. Wang et al. [12] studied carbonyl compounds at different sampling points in Guangdong Province and found that vehicle exhaust emissions are a significant contributor in Guangdong. Rao et al. [3] explored the relationship between carbonyl compounds and photochemical smog in Beijing. As the capital of Shandong Province, Jinan is a developing national central city characterized by advanced chemical and manufacturing industries. Jinan is also one of the cities with the most serious air pollution in China [13, 14]. In recent years, O3 concentration in Jinan has shown an obvious increasing trend, which is closely related to volatile organic compounds (VOCS) in the atmosphere [3]. Carbonyls are an important component of VOCS [2]. Therefore, the observation of VOCS, especially carbonyl compounds and investigation of their influence on O3 pollution control have attracted much attention. Our group previously studied the pollution characteristics of atmospheric carbonyls in autumn in Jinan [14]. However, long-term observational studies on carbonyls in Jinan in summer, during which photochemical pollution is the most serious, at high temporal resolution have not been reported in the literature.In this study, field measurements of atmospheric carbonyls in Jinan at high temporal resolution were taken continuously in the summers of 2018 and 2020. The concentration levels, pollution characteristics, sources, and influencing factors of atmospheric carbonyls were explored, and the primary contributors to ·OH radical reactions and secondary organic aerosol (SOA) formation were determined.


## 2. Material and Methods

### 2.1. Sampling Site and Time

Samples were collected at the campus of Shandong Jianzhu University in Jinan in the summers of 2018 and 2020 (Figure1). The sampling site is located on the roof (about 20 m above the ground) of a six-story building (117.19°E, 36.68°N). Multilane main roads with heavy traffic and rich vegetation run along two sides of the campus. Except for a large refinery (Sinopec Jinan Refining and Chemical Company) 2 km to the northwest, the campus is mostly surrounded by residential areas. Each sample was collected for 2 h, and the sample collection continued for 24 h every day. Jinan belongs to the temperate monsoon region. The hottest period occurs in the mid and late July, which is the typical summer period in Jinan. Because the carbonyl compounds are difficult to collect and the experimental cost is high, many previous studies considered a sampling period of only 3–10 days for carbonyl compounds [15–18]. In this study, the sampling time was from July 17 to July 31, 2018, and from July 17 to August 1, 2020, which represents the typical summer periods in Jinan.Figure 1
Location of Jinan sampling site.


### 2.2. Sampling Method and Analysis

The sample collection and analysis methods were strictly in accordance with the US EPA Method TO-11A [19]. Air samples were collected using 2, 4-dinitrophenylhydrazine (DNPH) silica column sampling tubes (Waters Sep-Pak), with a KI adsorption column set in front to eliminate the interference of O3 on the collection of carbonyl compounds. The samples were collected once every 2 h with the sample flow set to 1.2 L/min; meanwhile, one laboratory blank and one field blank were collected every 2 days for quality assurance. After collection, the sampling tube was sealed and stored at a constant temperature of 4°C in a refrigerator, and the samples were then analyzed using high-performance liquid chromatography (HPLC). The detection wavelength of the ultraviolet absorption detector was set to 360 nm, the C18 reverse phase column (4.6 × 250 mm) was selected for the chromatographic column, and the column temperature was set at 30°C. Details of the mobile phase selection of acetonitrile/water binary solvent system, gradient elution, and elution program are shown in Table 1. The HPLC standard curves of 15 types of carbonyls were plotted using 8 standard samples with known concentrations ranging from 0.025 to 1.50 μg/mL, which fully covered the actual concentration range of carbonyls in the atmosphere. Details of the analysis method can be found in the literature [10, 20]. The observation-based model (OBM) is a useful tool for explicit simulation of atmospheric chemistry, which contains not only homogeneous chemistry but also physical processes, i.e., dry deposition and dilution mixing within the boundary layer. The OBM was simulated using the latest version of the Master Chemical Mechanism (MCM, V3.3; https://mcm.leeds.ac.uk/MCM/) chemical sector as the core. Specifically, the HCHO photolysis frequencies appropriate for Jinan were simulated using a two-stream isotropic scattering model, which defined HCHO photolysis frequencies as a function of solar zenith angle (SZA) under clear sky conditions [21]. The boundary layer height in the model was assumed to be 300 m at night and linearly rise to 1500 m in the early afternoon. Sensitivity model runs with other J-values (reducing photolysis rates by 20%) as well as other boundary layer heights (e.g., 1000 m and 2000 m) indicated that the impacts on the modeling results are negligible and they do not affect our conclusions. We constrained the model by inputting observed O3, SO2, CO, NO, NO2, CH4, C2-C10 NMHCs, carbonyls, H2O, and temperature, and simulated the formation of carbonyls. The model was run at least five times to stabilize the simulation of the unconstrained compounds, i.e., OH, NO3, and other short-lived species. Generally due to time constraints, we conducted the model calculations for unique days, when O3 existed at relatively high concentrations and detailed measurements of carbonyls were available. The complete operation steps, configuration, and verification processes are described in the previous literature [22, 23].Table 1
Elution procedure for HPLC.
Time (min)Flow rate (ml/min)Mobile phase A (%)Mobile phase B (%)00.866.733.380.866.733.3120.87525180.8100019.50.81000210.866.733.3240.866.733.3


## 2.1. Sampling Site and Time

Samples were collected at the campus of Shandong Jianzhu University in Jinan in the summers of 2018 and 2020 (Figure1). The sampling site is located on the roof (about 20 m above the ground) of a six-story building (117.19°E, 36.68°N). Multilane main roads with heavy traffic and rich vegetation run along two sides of the campus. Except for a large refinery (Sinopec Jinan Refining and Chemical Company) 2 km to the northwest, the campus is mostly surrounded by residential areas. Each sample was collected for 2 h, and the sample collection continued for 24 h every day. Jinan belongs to the temperate monsoon region. The hottest period occurs in the mid and late July, which is the typical summer period in Jinan. Because the carbonyl compounds are difficult to collect and the experimental cost is high, many previous studies considered a sampling period of only 3–10 days for carbonyl compounds [15–18]. In this study, the sampling time was from July 17 to July 31, 2018, and from July 17 to August 1, 2020, which represents the typical summer periods in Jinan.Figure 1
Location of Jinan sampling site.


## 2.2. Sampling Method and Analysis

The sample collection and analysis methods were strictly in accordance with the US EPA Method TO-11A [19]. Air samples were collected using 2, 4-dinitrophenylhydrazine (DNPH) silica column sampling tubes (Waters Sep-Pak), with a KI adsorption column set in front to eliminate the interference of O3 on the collection of carbonyl compounds. The samples were collected once every 2 h with the sample flow set to 1.2 L/min; meanwhile, one laboratory blank and one field blank were collected every 2 days for quality assurance. After collection, the sampling tube was sealed and stored at a constant temperature of 4°C in a refrigerator, and the samples were then analyzed using high-performance liquid chromatography (HPLC). The detection wavelength of the ultraviolet absorption detector was set to 360 nm, the C18 reverse phase column (4.6 × 250 mm) was selected for the chromatographic column, and the column temperature was set at 30°C. Details of the mobile phase selection of acetonitrile/water binary solvent system, gradient elution, and elution program are shown in Table 1. The HPLC standard curves of 15 types of carbonyls were plotted using 8 standard samples with known concentrations ranging from 0.025 to 1.50 μg/mL, which fully covered the actual concentration range of carbonyls in the atmosphere. Details of the analysis method can be found in the literature [10, 20]. The observation-based model (OBM) is a useful tool for explicit simulation of atmospheric chemistry, which contains not only homogeneous chemistry but also physical processes, i.e., dry deposition and dilution mixing within the boundary layer. The OBM was simulated using the latest version of the Master Chemical Mechanism (MCM, V3.3; https://mcm.leeds.ac.uk/MCM/) chemical sector as the core. Specifically, the HCHO photolysis frequencies appropriate for Jinan were simulated using a two-stream isotropic scattering model, which defined HCHO photolysis frequencies as a function of solar zenith angle (SZA) under clear sky conditions [21]. The boundary layer height in the model was assumed to be 300 m at night and linearly rise to 1500 m in the early afternoon. Sensitivity model runs with other J-values (reducing photolysis rates by 20%) as well as other boundary layer heights (e.g., 1000 m and 2000 m) indicated that the impacts on the modeling results are negligible and they do not affect our conclusions. We constrained the model by inputting observed O3, SO2, CO, NO, NO2, CH4, C2-C10 NMHCs, carbonyls, H2O, and temperature, and simulated the formation of carbonyls. The model was run at least five times to stabilize the simulation of the unconstrained compounds, i.e., OH, NO3, and other short-lived species. Generally due to time constraints, we conducted the model calculations for unique days, when O3 existed at relatively high concentrations and detailed measurements of carbonyls were available. The complete operation steps, configuration, and verification processes are described in the previous literature [22, 23].Table 1
Elution procedure for HPLC.
Time (min)Flow rate (ml/min)Mobile phase A (%)Mobile phase B (%)00.866.733.380.866.733.3120.87525180.8100019.50.81000210.866.733.3240.866.733.3


## 3. Results and Discussion

### 3.1. Concentration Levels of Carbonyl Compounds in the Atmosphere

A total of 15 carbonyl compounds were determined in the ambient air of Jinan in summer: formaldehyde, acetaldehyde, acetone, butyraldehyde, crotonaldehyde, benzaldehyde,o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, acrolein, hexaldehyde, 2, 5-dimethylbenzaldehyde, propionaldehyde, valeraldehyde, and isovaleraldehyde. Crotonaldehyde, acrolein, and p-tolualdehyde were determined only in samples from 2020. The total concentrations of carbonyl compounds in Jinan in 2018 and 2020 were 10.51 ± 0.13 ppbV and 6.30 ± 1.08 ppbV, respectively. Time series diagrams of atmospheric carbonyls and relevant parameters during these two sampling periods are shown in Figures 2 and 3.Figure 2
Time series of atmospheric carbonyls and relevant parameters in Jinan in the summer of 2018. RH: relative humidity; T: temperature; O3 ClassII NAAQS-CN: secondary standard of the China’s National Ambient Air Quality Standard.Figure 3
Time series of atmospheric carbonyls and relevant parameters in Jinan in the summer of 2020. RH: relative humidity; T: temperature; O3 ClassII NAAQS-CN: secondary standard of the China’s National Ambient Air Quality Standard.As shown in Figure2, the concentration of total carbonyl compounds increased rapidly from the night of the 22nd to the early morning of the 23rd, with the highest concentration reaching up to 24.9 ppbV. The second peak of carbonyl compounds appeared on the 27th, accompanied by a CO peak. In the later period of observation, the concentrations of carbonyl compounds as well as O3 (average concentration of 86.88 ppbV) were seriously high. During this time, the wind speed was low (1.00 m/s on average), which is not conducive to the diffusion of pollutants. O3 levels exceeded the secondary standard (per hour average concentration of 93 ppbV) of the China’s national ambient air quality standard (GB3095-2012) on six days in 2018, accounting for 37.5% of the total number of sampling days.As shown in Figure3, the concentration of carbonyls in 2020 was significantly lower than that in 2018, and the total carbonyl concentration in 2020 (6.30 ± 1.08 ppbV) was 40.1% lower than that in 2018 (10.51 ± 0.13 ppbV). Compared with 2018, the carbonyl concentration fluctuated within a small range, which was significantly related to changes in meteorological factors during the observation period. Throughout the observation period, the study area experienced mainly cloudy and rainy weather. The average humidity was 72.8%, the average temperature was 26.6 ± 3.4°C, 2.72°C lower than that in 2018, and the wind speed was low (1.12 m/s on average). At the same time, the average O3 concentration was 45.87 ppbV, 47.2% lower than that in 2018.The concentrations of carbonyls during the observation period in Jinan in 2018 and 2020 are shown in Table2. Formaldehyde exhibited the highest concentration among these measured carbonyls during two sampling periods with 3.94 ± 2.25 ppbV and 2.51 ± 1.17 ppbV in 2018 and 2020, respectively, accounting for 37.49% and 39.99% of the total carbonyl concentration (Figure 4). The concentration of formaldehyde was higher than the concentration of formaldehyde observed by Zhang et al. [12] in the autumn of 2017 in Jinan, indicating abundant sources of formaldehyde in summer and that atmospheric photochemical reactions significantly affected the generation of formaldehyde. In this study, formaldehyde, acetone, and acetaldehyde were the most abundant carbonyl compounds in the atmosphere of Jinan in summer. This finding is basically consistent with international and domestic studies [24,25]. Excluding formaldehyde, acetone, and acetaldehyde, the concentrations of propionaldehyde and butyraldehyde in these two periods were much higher than those of other carbonyl compounds, accounting for 10.37% and 6.35% of total carbonyls in 2018 and 2020, respectively. Studies have shown that propionaldehyde is mainly synthesized by ethylene, while butyraldehyde is the photochemical oxidation product of n-butene [26]. Their higher concentration levels may be attributable to local sources, such as refineries near the sampling sites (producing gasoline, diesel, and other chemical products), because raw materials for organic synthesis are mass-produced in this factory. Previous studies reported higher concentrations of acetaldehyde than those of acetone [11, 25]. However, in our study, we found that the concentration of atmospheric acetone was higher than that of acetaldehyde in Jinan. The reason may be that it takes much longer time for acetone to be removed from the atmosphere through ·OH reactions (about 53 days) than that for acetaldehyde through ·OH reactions (about 8.8 hours) [23, 25], leading to the longer lifetime of acetone and the “accumulation effect” of acetone released from various sources. Production processes of the large chemical plant nearby the sampling site can lead to the release and accumulation of acetone. Consequently, the atmosphere around the sampling site has a relatively high acetone concentration.Table 2
Concentration of carbonyl compounds in Jinan during the summers of 2018 and 2020 (ppbV).
Compound2018Mean ± SD2020Mean ± SDFormaldehyde3.94 ± 2.252.51 ± 1.17Acetaldehyde1.71 ± 0.721.28 ± 0.47Acetone2.60 ± 0.971.88 ± 0.71Butyraldehyde0.62 ± 0.260.24 ± 0.10Crotonaldehyde—0.08 ± 0.03Isovaleraldehyde0.20 ± 0.130.02 ± 0.02Propionaldehyde0.47 ± 0.190.16 ± 0.06o-Tolualdehyde0.26 ± 0.160.006 ± 0.01m-Tolualdehyde0.12 ± 0.110.01 ± 0.01p-Tolualdehyde—0.003 ± 0.01Valeraldehyde0.19 ± 0.090.02 ± 0.01Hexaldehyde0.11 ± 0.070.002 ± 0.01Benzaldehyde0.25 ± 0.180.01 ± 0.01Acrolein—0.04 ± 0.01Dimethylbenzaldehyde0.04 ± 0.020.03 ± 0.04Total10.51 ± 0.136.30 ± 1.08Note: “—” means the concentrations are below the detection limits. “SD” means standard deviation.Figure 4
Compositions of carbonyl compounds in Jinan in 2018 (a) and 2020 (b).
(a)(b)The concentrations of the main carbonyl compounds (formaldehyde, acetone, and acetaldehyde) observed in this study were compared with those in other countries and regions (Table3). The concentration of carbonyl compounds in Jinan appears to be far lower than that in Beijing, where photochemical pollution is more serious [23]. Compared with southern China’s large cities, such as Shanghai and Guangzhou [12, 27], the concentrations of carbonyls were far lower in Jinan than in Shanghai in 2018, except for acetaldehyde. In addition, the concentration of carbonyl compounds in Jinan was higher than that in Hong Kong [28] and other cities, such as Madrid in Spain [15, 24]. Compared with cities in southwestern China, the concentration of carbonyls in Jinan is significantly lower than that in Guiyang [29], a mountainous area similar to Jinan. Compared with the Zhangjiajie National Forest Park [30], which has a low pollution level, the formaldehyde concentration in Jinan was slightly lower in 2020, but the concentrations of the other two carbonyls (acetone and acetaldehyde) were much higher. The concentration of acetaldehyde is generally lower than that of formaldehyde in many countries and regions, including Jinan. However, the concentration of acetaldehyde is relatively high in Brazil because ethanol is widely used as vehicle fuel in this country and acetaldehyde is the atmospheric oxidation product of ethanol [31]. The comparative analysis shows that the level of atmospheric carbonyl concentration in Jinan is not as high as that in Beijing [23], Shanghai [27], and Guangzhou [12], where photochemical pollution is serious. However, it is higher than that in other areas, such as Hong Kong and Guiyang, indicating relatively severe carbonyl pollution in Jinan.Table 3
Comparison of formaldehyde, acetaldehyde, and acetone concentrations at various observation sites in China and other countries (ppbV).
LocationFAAAACC1/C2PeriodReferencesJinan3.941.712.602.64July 2018This study2.511.281.882.03July 2020Beijing11.175.276.982.12July-August 2008[23]Shanghai4.711.716.102.75July-August 2014[27]Guangzhou6.934.314.181.12July 2016[12]Hong Kong1.700.60—2.83June-August 2013[28]Guiyang4.805.705.100.84September 2008[29]Xi’an6.472.062.801.35July 2009[17]Mount Tai3.481.273.572.74July-August 2014[26]Zhangjiajie Forest Park3.220.481.166.71August 2014[30]Orléans, France3.081.042.002.96June 2011[15]Madrid, Spain0.641.012.110.63October 2010[24]Rio de Janeiro, Brazil14.6938.71—0.38July-October 2013[31]FA: formaldehyde; AA: acetaldehyde; AC: acetone (AC); C1/C2: formaldehyde/acetaldehyde.


### 3.2. Diurnal Variation of Atmospheric Carbonyls

Figure5 shows the daily variation trends of formaldehyde, acetaldehyde, and acetone and O3 in the atmosphere in summer in Jinan. The diurnal variation trends of formaldehyde and acetaldehyde were similar between the two observation periods, and the diurnal variation trends were much smoother in 2020, which was attributable to the rainy weather. Both formaldehyde and acetaldehyde peaked in the morning and evening traffic rush hours, indicating that motor vehicle exhaust emissions significantly contributed to formaldehyde and acetaldehyde concentrations. Formaldehyde concentration began to rise in the morning, and the first peak appeared around 8 : 00 in the morning, followed by a rapid decline; from 10 : 00 to 14 : 00, it started to rise again slowly. This trend indicated an emission reduction after the morning traffic peak and that the diffusion effect of atmospheric turbulence led to a rapid decline in formaldehyde concentration. With the enhancement of solar radiation at noon, the formation rate of formaldehyde exceeded its photolysis rate, thus leading to accumulation [32]. The diurnal variation trend of formaldehyde indicates that formaldehyde is not only affected not only by primary emissions, but also by secondary generation through photochemical reactions. In contrast, acetone was maintained at a high concentration at night, which may be attributed to the higher ambient temperature during the day, which is conducive to the volatilization of acetone and other organic solvents used in the production processes of the large refinery near the study site. Daytime O3 levels were similar in 2018 and 2020, but nighttime O3 levels were relatively high in 2020. In both years, O3 concentration gradually increased with increasing light intensity after sunrise, peaked at 14 : 00, and lagged behind formaldehyde, acetaldehyde, and acetone, suggesting that carbonyls have a promoting effect on O3 production.Figure 5
Diurnal variation trends of formaldehyde, acetaldehyde, acetone, and O3 in Jinan in 2018 and 2020.


### 3.3. Concentration Ratios and Correlation Analysis

The ratio of formaldehyde/acetaldehyde (C1/C2) can be used to roughly determine the main source of atmospheric carbonyl compounds [33]. The C1/C2 ratio is generally believed to be 1–2 in the urban atmosphere and around 10 in the suburbs [34]. The ratio of formaldehyde/carbon monoxide (FA/CO) can be used to determine whether the source of carbonyl compounds has changed [35]. Figure 6 shows the daily variation trends of the ratios of C1/C2 and FA/CO at the sampling site in 2018 and 2020. The minimum value appeared at 10 : 00 in 2018 and 2020, indicating that the photolysis of formaldehyde leads to a smaller concentration difference with acetaldehyde at that time, which indirectly indicates that atmospheric photochemical reactions contribute to the secondary generation of atmospheric carbonyl compounds to a certain extent.Figure 6
Daily variation trends of C1/C2 and FA/CO ratios in Jinan in 2018 and 2020.In this study, the C1/C2 ratios at Jinan were 2.64 in 2018 and 2.03 in 2020, which are higher than those in urban areas, such as Beijing (2.12) [3], Guangzhou (1.12) [12], and Xi’an (1.35) [17]. The average ratios are similar to those in Orléans, France (2.96) [15], and Mount Tai (2.74) [26], which is a typical mountain site of China, but they are still much lower than those in Zhangjiajie National Forest Park (6.71) [30] and other forest areas under the influence of natural sources. These findings suggest that carbonyl compounds in Jinan were largely contributed by anthropogenic emissions and photochemical reactions.To further study the source characteristics of carbonyl compounds, Pearson correlation analysis was carried out on carbonyl compounds at the sampling site. Table4 lists the correlation coefficients between carbonyl compounds in Jinan in 2018 and 2020. The correlation coefficients of formaldehyde and acetaldehyde were found to be 0.592 and 0.677, respectively. Compared with the observation results of Wang et al. [14] in Jinan in autumn, the correlation between the two carbonyls has significantly increased, indicating that they were more likely to have the same source and sink in summer than in autumn. Acetaldehyde showed a good correlation with propionaldehyde in 2018 and 2020. Propionaldehyde is related to emissions from petrochemical industries [36], indicating that the industrial emissions may also contribute to acetaldehyde in the atmosphere. Acetone and most carbonyl compounds exhibited moderate to strong correlation in 2018, but acetone and other carbonyl compounds exhibited poorer correlation in 2020. It is worth noting that acetone and butyraldehyde showed strong correlation in both years. Studies have shown that butyraldehyde originates from exhaust emissions of gasoline and diesel vehicles [36], suggesting that motor vehicle exhaust emissions may be an important source of acetone in the atmosphere. CO exhibited good correlation with formaldehyde, acetaldehyde, and acetone, which also shows that motor vehicle exhaust may be an important source of carbonyls.Table 4
Correlation of atmospheric carbonyl compounds in the summers of 2018 (top) and 2020 (bottom) in Jinan city.
Compound(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)Formaldehyde (1)1.000Acetaldehyde (2)0.592∗∗1.000Acetone (3)0.229∗∗0.644∗∗1.000Propionaldehyde (4)0.1430.570∗∗0.367∗∗1.000Butyraldehyde (5)−0.1100.385∗∗0.842∗∗0.227∗∗1.000Benzaldehyde (6)−0.318∗∗−0.172∗0.338∗∗−0.267∗∗0.455∗∗1.000Isovaleraldehyde (7)−0.115−0.160−0.0080.0410.1450.559∗1.000Valeraldehyde (8)0.1170.422∗∗0.257∗0.306∗∗0.518∗∗−0.087−0.697∗1.000o-Tolualdehyde (9)−0.217∗∗−0.1450.307∗∗0.0010.272∗∗0.544∗∗0.328−0.279∗1.000Hexaldehyde (10)0.592∗∗0.607∗∗0.281∗∗0.213∗∗0.294∗∗−0.259∗∗−0.3280.478∗∗−0.456∗∗1.000Dimethylbenzaldehyde (11)0.322∗0.501∗∗0.1390.120−0.173−0.1850.2240.031−0.0460.1111.000CO (12)0.388∗∗0.530∗∗0.330∗∗0.390∗∗0.088−0.313∗∗−0.4290.220∗−0.159∗0.280∗∗0.1761.000Formaldehyde (1)1.000Acetaldehyde (2)0.677∗∗1.000Acetone (3)−0.228∗∗0.0491.000Propionaldehyde (4)0.658∗∗0.650∗∗−0.0681.000Butyraldehyde (5)0.227∗∗0.440∗∗0.564∗∗0.305∗∗1.000Benzaldehyde (6)0.319∗∗0.532∗∗0.1710.421∗∗0.436∗∗1.000Isovaleraldehyde (7)−0.0070.0500.024−0.0640.0650.2261.000Valeraldehyde (8)0.2850.430∗−0.1760.449−0.2540.433∗0.2411.000o-Tolualdehyde (9)0.1420.1210.1840.0530.0700.327∗−0.0330.484∗1.000Hexaldehyde (10)−0.309−0.105−0.575∗∗−0.136−0.415∗0.1700.2230.671∗0.2441.000Dimethylbenzaldehyde (11)0.0260.243∗−0.0920.213−0.261∗0.590∗∗0.2660.888∗∗0.319∗0.729∗∗1.000CO (12)0.313∗∗0.339∗∗−0.179∗0.446∗∗0.170∗−0.0340.116−0.041−0.2800.459∗−0.1361.000The boldface is used to emphasize the importance of these values as they represent higher correlation coefficient values and they have no additional meanings.


### 3.4. Photochemical Reaction

Carbonyl compounds play an important role in the formation of photochemical smog, and their effect on photochemical reactions mainly depends on their photochemical activity. To evaluate the contributions of different carbonyl compounds to atmospheric photochemical reactions and O3 generation, the rate of hydroxy radical (·OH) consumption (LOH) and ozone formation potential (OFP) was calculated using:(1)LOH=VOCi×KiOH,(2)OFP=VOCi×MIRi,where [VOC]i is the molecule concentration of VOC species i (molecule/cm3) and KiOH denotes the rate constant (unit: cm3/(molecule·s)) of [VOC]i reacting with ·OH radical at 298 K, with values from Atkinson’s work [1, 2]. MIRi represents the maximum incremental reaction coefficient of the carbonyl compound i, and the specific value was referred from the literature [37, 38], Table 5 lists the specific values.Table 5
MIR and KOH values of carbonyl compounds.
CompoundMIR (g O3/g VOCS)KOH∗ 1012 (cm3/(molecule·s))OFP (μg·m−3)LOH (s−1)2018202020182020Formaldehyde9.469.3745.6457.581.091.15Acetaldehyde6.5415.0020.0818.730.630.59Acetone0.360.172.220.70.010.003Butyraldehyde5.2824.009.625.460.370.21Crotonaldehyde——Isovaleraldehyde4.4227.002.02—0.13—Propionaldehyde7.0820.007.887.520.230.22o-Tolualdehyde—18.00——0.120.06m-Tolualdehyde—17.00——0.050.08p-Tolualdehyde——Valeraldehyde4.4228.002.942.550.130.11Hexaldehyde3.799.581.71.150.030.02Benzaldehyde−0.5712.00−0.61−0.250.070.03Acrolein7.45——0.57——Dimethylbenzaldehyde——————Figure7 shows the calculated LOH and OFP of atmospheric carbonyl compounds at the sampling site. The results showed that formaldehyde, acetaldehyde, butyraldehyde, and propionaldehyde all played a major role in ·OH consumption in 2018 and 2020, and the four species accounted for 89.6% and 95.7% of the total ·OH consumption of carbonyl compounds in 2018 and 2020, respectively. In 2020, the LOH of formaldehyde was 0.63 s−1, accounting for 42.9% of the total LOH. Therefore, formaldehyde is the main carbonyl compound affecting atmospheric oxidation in Jinan. In addition, the contributions of acetaldehyde, butyraldehyde, and propionaldehyde cannot be ignored. In terms of the potential of O3 formation, formaldehyde has a leading position, with generation rates of 45.69 μg m−3 and 31.96 μg m−3 in 2018 and 2020, respectively. O3 formation can be significantly limited by inhibiting secondary generation from formaldehyde through the control of emissions of formaldehyde and its precursors. Acetaldehyde, butyraldehyde, and propionaldehyde showed large contributions to OFP. The OFP values of the four species accounted for more than 90% of the total OFP of carbonyl compounds. As shown in Figures 2 and 3, clear time series variation patterns of OFP and carbonyl concentrations with nearly simultaneous peaks were observed, which demonstrates the contribution of carbonyl photochemical reactions to ozone formation.Figure 7
LOH and OFP of carbonyl compounds in Jinan in 2018 and 2020.


### 3.5. In Situ Formation Pathway of Carbonyl Compounds

To further explore the secondary generation mechanism of atmospheric carbonyl compounds in Jinan, the main formation and consumption pathways of formaldehyde were analyzed using the MCM photochemical box model based on the observations. The two days of July 29 and July 30, 2020, during which severe photochemical pollution (high concentrations of carbonyl compounds and O3) was observed, were selected as typical cases of photochemical pollution for analysis. The specific simulation analysis process has been described in detail by Yang et al. [22, 23].As shown in Figure8, the formation and consumption pathways of formaldehyde in Jinan on July 29 and July 30 were basically the same. The formation reactions of formaldehyde were mainly controlled by the reaction of RO + O2. Among the reactions of RO and O2, the oxidation reaction of CH3O + O2 significantly contributed to the secondary generation of formaldehyde, accounting for 86% of the total generation rate of formaldehyde. The oxidation reaction was followed by the RO + O2 (alkenes) reaction (7%), while other reactions, such as free radical chain transfer, OH + OVOCS, and OVOCS photolysis, contributed little to the formation of formaldehyde, accounting for 7% in total. The main route of formaldehyde consumption is OH + HCHO and formaldehyde photolysis reactions, which account for 99% of the total consumption rate.Figure 8
Main formation and consumption pathways of formaldehyde determined through model simulation.The production and consumption rates of formaldehyde both exhibited peaks around noon, and the production rate remained higher than the consumption rate, showing net production. The daily average of the net production rates was 0.59 ppb/h and 0.63 ppb/h, lower than the simulated results of Yang et al. for Hong Kong [23], but both findings indicate that in situ photochemical reactions promoted the accumulation of formaldehyde. On the whole, the OBM for investigating atmospheric oxidative capacity and photochemistry simulation highlights the important role of CH3O + O2 in the secondary formation of formaldehyde, which is consistent with the results of previous studies [23].


### 3.6. Effects of Carbonyl Compounds on Secondary Aerosols

Carbonyl compounds have strong water solubility and can be absorbed into clouds and fog to react with ·OH to form oligomers, which promote the formation of SOA [39]. We used the secondary organic aerosol potential (SOAP), which was developed by Grosjean et al. [40, 41] to reflect the propensity of individual organic compound to form SOA and evaluate the contribution of these compounds to SOA through their calculated emission inventory of toluene-based compounds. SOAP was calculated using :(3)SOAP=VOCi×FACi,where [VOC]i is the mass fraction of VOC species i (unit: μg·m−3) and FACi denotes the aerosol formation coefficient (%). The value of FAC was proposed by Grosjean et al. [40, 41], based on a large number of smog chamber experiments and atmospheric chemical dynamics data. Among them, formaldehyde, acetaldehyde, acetone, butyraldehyde, and benzaldehyde were found to be the major contributors to SOA. As shown in Figure 9, under the influence of different SOAP values of different carbonyls, the change trends of the contribution of different carbonyls to SOA were different from the trends of their concentration change. Among the SOAP values of different carbonyl compounds, benzaldehyde was found to contribute the most to SOA. Although with a low concentration, benzaldehyde contributed the most to SOA in 2018 and 2020, even reaching 54 ppbV in 2018. Formaldehyde was the second largest contributor, but the contribution was less than 3 ppbV in both years. The change trends of the contributions of carbonyl compounds to SOA followed the same pattern in both years: benzaldehyde > formaldehyde > acetaldehyde > acetone > propionaldehyde > butyraldehyde.Figure 9
Comparison of SOAP values of atmospheric carbonyls in Jinan between 2018 and 2020.


## 3.1. Concentration Levels of Carbonyl Compounds in the Atmosphere

A total of 15 carbonyl compounds were determined in the ambient air of Jinan in summer: formaldehyde, acetaldehyde, acetone, butyraldehyde, crotonaldehyde, benzaldehyde,o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, acrolein, hexaldehyde, 2, 5-dimethylbenzaldehyde, propionaldehyde, valeraldehyde, and isovaleraldehyde. Crotonaldehyde, acrolein, and p-tolualdehyde were determined only in samples from 2020. The total concentrations of carbonyl compounds in Jinan in 2018 and 2020 were 10.51 ± 0.13 ppbV and 6.30 ± 1.08 ppbV, respectively. Time series diagrams of atmospheric carbonyls and relevant parameters during these two sampling periods are shown in Figures 2 and 3.Figure 2
Time series of atmospheric carbonyls and relevant parameters in Jinan in the summer of 2018. RH: relative humidity; T: temperature; O3 ClassII NAAQS-CN: secondary standard of the China’s National Ambient Air Quality Standard.Figure 3
Time series of atmospheric carbonyls and relevant parameters in Jinan in the summer of 2020. RH: relative humidity; T: temperature; O3 ClassII NAAQS-CN: secondary standard of the China’s National Ambient Air Quality Standard.As shown in Figure2, the concentration of total carbonyl compounds increased rapidly from the night of the 22nd to the early morning of the 23rd, with the highest concentration reaching up to 24.9 ppbV. The second peak of carbonyl compounds appeared on the 27th, accompanied by a CO peak. In the later period of observation, the concentrations of carbonyl compounds as well as O3 (average concentration of 86.88 ppbV) were seriously high. During this time, the wind speed was low (1.00 m/s on average), which is not conducive to the diffusion of pollutants. O3 levels exceeded the secondary standard (per hour average concentration of 93 ppbV) of the China’s national ambient air quality standard (GB3095-2012) on six days in 2018, accounting for 37.5% of the total number of sampling days.As shown in Figure3, the concentration of carbonyls in 2020 was significantly lower than that in 2018, and the total carbonyl concentration in 2020 (6.30 ± 1.08 ppbV) was 40.1% lower than that in 2018 (10.51 ± 0.13 ppbV). Compared with 2018, the carbonyl concentration fluctuated within a small range, which was significantly related to changes in meteorological factors during the observation period. Throughout the observation period, the study area experienced mainly cloudy and rainy weather. The average humidity was 72.8%, the average temperature was 26.6 ± 3.4°C, 2.72°C lower than that in 2018, and the wind speed was low (1.12 m/s on average). At the same time, the average O3 concentration was 45.87 ppbV, 47.2% lower than that in 2018.The concentrations of carbonyls during the observation period in Jinan in 2018 and 2020 are shown in Table2. Formaldehyde exhibited the highest concentration among these measured carbonyls during two sampling periods with 3.94 ± 2.25 ppbV and 2.51 ± 1.17 ppbV in 2018 and 2020, respectively, accounting for 37.49% and 39.99% of the total carbonyl concentration (Figure 4). The concentration of formaldehyde was higher than the concentration of formaldehyde observed by Zhang et al. [12] in the autumn of 2017 in Jinan, indicating abundant sources of formaldehyde in summer and that atmospheric photochemical reactions significantly affected the generation of formaldehyde. In this study, formaldehyde, acetone, and acetaldehyde were the most abundant carbonyl compounds in the atmosphere of Jinan in summer. This finding is basically consistent with international and domestic studies [24,25]. Excluding formaldehyde, acetone, and acetaldehyde, the concentrations of propionaldehyde and butyraldehyde in these two periods were much higher than those of other carbonyl compounds, accounting for 10.37% and 6.35% of total carbonyls in 2018 and 2020, respectively. Studies have shown that propionaldehyde is mainly synthesized by ethylene, while butyraldehyde is the photochemical oxidation product of n-butene [26]. Their higher concentration levels may be attributable to local sources, such as refineries near the sampling sites (producing gasoline, diesel, and other chemical products), because raw materials for organic synthesis are mass-produced in this factory. Previous studies reported higher concentrations of acetaldehyde than those of acetone [11, 25]. However, in our study, we found that the concentration of atmospheric acetone was higher than that of acetaldehyde in Jinan. The reason may be that it takes much longer time for acetone to be removed from the atmosphere through ·OH reactions (about 53 days) than that for acetaldehyde through ·OH reactions (about 8.8 hours) [23, 25], leading to the longer lifetime of acetone and the “accumulation effect” of acetone released from various sources. Production processes of the large chemical plant nearby the sampling site can lead to the release and accumulation of acetone. Consequently, the atmosphere around the sampling site has a relatively high acetone concentration.Table 2
Concentration of carbonyl compounds in Jinan during the summers of 2018 and 2020 (ppbV).
Compound2018Mean ± SD2020Mean ± SDFormaldehyde3.94 ± 2.252.51 ± 1.17Acetaldehyde1.71 ± 0.721.28 ± 0.47Acetone2.60 ± 0.971.88 ± 0.71Butyraldehyde0.62 ± 0.260.24 ± 0.10Crotonaldehyde—0.08 ± 0.03Isovaleraldehyde0.20 ± 0.130.02 ± 0.02Propionaldehyde0.47 ± 0.190.16 ± 0.06o-Tolualdehyde0.26 ± 0.160.006 ± 0.01m-Tolualdehyde0.12 ± 0.110.01 ± 0.01p-Tolualdehyde—0.003 ± 0.01Valeraldehyde0.19 ± 0.090.02 ± 0.01Hexaldehyde0.11 ± 0.070.002 ± 0.01Benzaldehyde0.25 ± 0.180.01 ± 0.01Acrolein—0.04 ± 0.01Dimethylbenzaldehyde0.04 ± 0.020.03 ± 0.04Total10.51 ± 0.136.30 ± 1.08Note: “—” means the concentrations are below the detection limits. “SD” means standard deviation.Figure 4
Compositions of carbonyl compounds in Jinan in 2018 (a) and 2020 (b).
(a)(b)The concentrations of the main carbonyl compounds (formaldehyde, acetone, and acetaldehyde) observed in this study were compared with those in other countries and regions (Table3). The concentration of carbonyl compounds in Jinan appears to be far lower than that in Beijing, where photochemical pollution is more serious [23]. Compared with southern China’s large cities, such as Shanghai and Guangzhou [12, 27], the concentrations of carbonyls were far lower in Jinan than in Shanghai in 2018, except for acetaldehyde. In addition, the concentration of carbonyl compounds in Jinan was higher than that in Hong Kong [28] and other cities, such as Madrid in Spain [15, 24]. Compared with cities in southwestern China, the concentration of carbonyls in Jinan is significantly lower than that in Guiyang [29], a mountainous area similar to Jinan. Compared with the Zhangjiajie National Forest Park [30], which has a low pollution level, the formaldehyde concentration in Jinan was slightly lower in 2020, but the concentrations of the other two carbonyls (acetone and acetaldehyde) were much higher. The concentration of acetaldehyde is generally lower than that of formaldehyde in many countries and regions, including Jinan. However, the concentration of acetaldehyde is relatively high in Brazil because ethanol is widely used as vehicle fuel in this country and acetaldehyde is the atmospheric oxidation product of ethanol [31]. The comparative analysis shows that the level of atmospheric carbonyl concentration in Jinan is not as high as that in Beijing [23], Shanghai [27], and Guangzhou [12], where photochemical pollution is serious. However, it is higher than that in other areas, such as Hong Kong and Guiyang, indicating relatively severe carbonyl pollution in Jinan.Table 3
Comparison of formaldehyde, acetaldehyde, and acetone concentrations at various observation sites in China and other countries (ppbV).
LocationFAAAACC1/C2PeriodReferencesJinan3.941.712.602.64July 2018This study2.511.281.882.03July 2020Beijing11.175.276.982.12July-August 2008[23]Shanghai4.711.716.102.75July-August 2014[27]Guangzhou6.934.314.181.12July 2016[12]Hong Kong1.700.60—2.83June-August 2013[28]Guiyang4.805.705.100.84September 2008[29]Xi’an6.472.062.801.35July 2009[17]Mount Tai3.481.273.572.74July-August 2014[26]Zhangjiajie Forest Park3.220.481.166.71August 2014[30]Orléans, France3.081.042.002.96June 2011[15]Madrid, Spain0.641.012.110.63October 2010[24]Rio de Janeiro, Brazil14.6938.71—0.38July-October 2013[31]FA: formaldehyde; AA: acetaldehyde; AC: acetone (AC); C1/C2: formaldehyde/acetaldehyde.


## 3.2. Diurnal Variation of Atmospheric Carbonyls

Figure5 shows the daily variation trends of formaldehyde, acetaldehyde, and acetone and O3 in the atmosphere in summer in Jinan. The diurnal variation trends of formaldehyde and acetaldehyde were similar between the two observation periods, and the diurnal variation trends were much smoother in 2020, which was attributable to the rainy weather. Both formaldehyde and acetaldehyde peaked in the morning and evening traffic rush hours, indicating that motor vehicle exhaust emissions significantly contributed to formaldehyde and acetaldehyde concentrations. Formaldehyde concentration began to rise in the morning, and the first peak appeared around 8 : 00 in the morning, followed by a rapid decline; from 10 : 00 to 14 : 00, it started to rise again slowly. This trend indicated an emission reduction after the morning traffic peak and that the diffusion effect of atmospheric turbulence led to a rapid decline in formaldehyde concentration. With the enhancement of solar radiation at noon, the formation rate of formaldehyde exceeded its photolysis rate, thus leading to accumulation [32]. The diurnal variation trend of formaldehyde indicates that formaldehyde is not only affected not only by primary emissions, but also by secondary generation through photochemical reactions. In contrast, acetone was maintained at a high concentration at night, which may be attributed to the higher ambient temperature during the day, which is conducive to the volatilization of acetone and other organic solvents used in the production processes of the large refinery near the study site. Daytime O3 levels were similar in 2018 and 2020, but nighttime O3 levels were relatively high in 2020. In both years, O3 concentration gradually increased with increasing light intensity after sunrise, peaked at 14 : 00, and lagged behind formaldehyde, acetaldehyde, and acetone, suggesting that carbonyls have a promoting effect on O3 production.Figure 5
Diurnal variation trends of formaldehyde, acetaldehyde, acetone, and O3 in Jinan in 2018 and 2020.


## 3.3. Concentration Ratios and Correlation Analysis

The ratio of formaldehyde/acetaldehyde (C1/C2) can be used to roughly determine the main source of atmospheric carbonyl compounds [33]. The C1/C2 ratio is generally believed to be 1–2 in the urban atmosphere and around 10 in the suburbs [34]. The ratio of formaldehyde/carbon monoxide (FA/CO) can be used to determine whether the source of carbonyl compounds has changed [35]. Figure 6 shows the daily variation trends of the ratios of C1/C2 and FA/CO at the sampling site in 2018 and 2020. The minimum value appeared at 10 : 00 in 2018 and 2020, indicating that the photolysis of formaldehyde leads to a smaller concentration difference with acetaldehyde at that time, which indirectly indicates that atmospheric photochemical reactions contribute to the secondary generation of atmospheric carbonyl compounds to a certain extent.Figure 6
Daily variation trends of C1/C2 and FA/CO ratios in Jinan in 2018 and 2020.In this study, the C1/C2 ratios at Jinan were 2.64 in 2018 and 2.03 in 2020, which are higher than those in urban areas, such as Beijing (2.12) [3], Guangzhou (1.12) [12], and Xi’an (1.35) [17]. The average ratios are similar to those in Orléans, France (2.96) [15], and Mount Tai (2.74) [26], which is a typical mountain site of China, but they are still much lower than those in Zhangjiajie National Forest Park (6.71) [30] and other forest areas under the influence of natural sources. These findings suggest that carbonyl compounds in Jinan were largely contributed by anthropogenic emissions and photochemical reactions.To further study the source characteristics of carbonyl compounds, Pearson correlation analysis was carried out on carbonyl compounds at the sampling site. Table4 lists the correlation coefficients between carbonyl compounds in Jinan in 2018 and 2020. The correlation coefficients of formaldehyde and acetaldehyde were found to be 0.592 and 0.677, respectively. Compared with the observation results of Wang et al. [14] in Jinan in autumn, the correlation between the two carbonyls has significantly increased, indicating that they were more likely to have the same source and sink in summer than in autumn. Acetaldehyde showed a good correlation with propionaldehyde in 2018 and 2020. Propionaldehyde is related to emissions from petrochemical industries [36], indicating that the industrial emissions may also contribute to acetaldehyde in the atmosphere. Acetone and most carbonyl compounds exhibited moderate to strong correlation in 2018, but acetone and other carbonyl compounds exhibited poorer correlation in 2020. It is worth noting that acetone and butyraldehyde showed strong correlation in both years. Studies have shown that butyraldehyde originates from exhaust emissions of gasoline and diesel vehicles [36], suggesting that motor vehicle exhaust emissions may be an important source of acetone in the atmosphere. CO exhibited good correlation with formaldehyde, acetaldehyde, and acetone, which also shows that motor vehicle exhaust may be an important source of carbonyls.Table 4
Correlation of atmospheric carbonyl compounds in the summers of 2018 (top) and 2020 (bottom) in Jinan city.
Compound(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)Formaldehyde (1)1.000Acetaldehyde (2)0.592∗∗1.000Acetone (3)0.229∗∗0.644∗∗1.000Propionaldehyde (4)0.1430.570∗∗0.367∗∗1.000Butyraldehyde (5)−0.1100.385∗∗0.842∗∗0.227∗∗1.000Benzaldehyde (6)−0.318∗∗−0.172∗0.338∗∗−0.267∗∗0.455∗∗1.000Isovaleraldehyde (7)−0.115−0.160−0.0080.0410.1450.559∗1.000Valeraldehyde (8)0.1170.422∗∗0.257∗0.306∗∗0.518∗∗−0.087−0.697∗1.000o-Tolualdehyde (9)−0.217∗∗−0.1450.307∗∗0.0010.272∗∗0.544∗∗0.328−0.279∗1.000Hexaldehyde (10)0.592∗∗0.607∗∗0.281∗∗0.213∗∗0.294∗∗−0.259∗∗−0.3280.478∗∗−0.456∗∗1.000Dimethylbenzaldehyde (11)0.322∗0.501∗∗0.1390.120−0.173−0.1850.2240.031−0.0460.1111.000CO (12)0.388∗∗0.530∗∗0.330∗∗0.390∗∗0.088−0.313∗∗−0.4290.220∗−0.159∗0.280∗∗0.1761.000Formaldehyde (1)1.000Acetaldehyde (2)0.677∗∗1.000Acetone (3)−0.228∗∗0.0491.000Propionaldehyde (4)0.658∗∗0.650∗∗−0.0681.000Butyraldehyde (5)0.227∗∗0.440∗∗0.564∗∗0.305∗∗1.000Benzaldehyde (6)0.319∗∗0.532∗∗0.1710.421∗∗0.436∗∗1.000Isovaleraldehyde (7)−0.0070.0500.024−0.0640.0650.2261.000Valeraldehyde (8)0.2850.430∗−0.1760.449−0.2540.433∗0.2411.000o-Tolualdehyde (9)0.1420.1210.1840.0530.0700.327∗−0.0330.484∗1.000Hexaldehyde (10)−0.309−0.105−0.575∗∗−0.136−0.415∗0.1700.2230.671∗0.2441.000Dimethylbenzaldehyde (11)0.0260.243∗−0.0920.213−0.261∗0.590∗∗0.2660.888∗∗0.319∗0.729∗∗1.000CO (12)0.313∗∗0.339∗∗−0.179∗0.446∗∗0.170∗−0.0340.116−0.041−0.2800.459∗−0.1361.000The boldface is used to emphasize the importance of these values as they represent higher correlation coefficient values and they have no additional meanings.


## 3.4. Photochemical Reaction

Carbonyl compounds play an important role in the formation of photochemical smog, and their effect on photochemical reactions mainly depends on their photochemical activity. To evaluate the contributions of different carbonyl compounds to atmospheric photochemical reactions and O3 generation, the rate of hydroxy radical (·OH) consumption (LOH) and ozone formation potential (OFP) was calculated using:(1)LOH=VOCi×KiOH,(2)OFP=VOCi×MIRi,where [VOC]i is the molecule concentration of VOC species i (molecule/cm3) and KiOH denotes the rate constant (unit: cm3/(molecule·s)) of [VOC]i reacting with ·OH radical at 298 K, with values from Atkinson’s work [1, 2]. MIRi represents the maximum incremental reaction coefficient of the carbonyl compound i, and the specific value was referred from the literature [37, 38], Table 5 lists the specific values.Table 5
MIR and KOH values of carbonyl compounds.
CompoundMIR (g O3/g VOCS)KOH∗ 1012 (cm3/(molecule·s))OFP (μg·m−3)LOH (s−1)2018202020182020Formaldehyde9.469.3745.6457.581.091.15Acetaldehyde6.5415.0020.0818.730.630.59Acetone0.360.172.220.70.010.003Butyraldehyde5.2824.009.625.460.370.21Crotonaldehyde——Isovaleraldehyde4.4227.002.02—0.13—Propionaldehyde7.0820.007.887.520.230.22o-Tolualdehyde—18.00——0.120.06m-Tolualdehyde—17.00——0.050.08p-Tolualdehyde——Valeraldehyde4.4228.002.942.550.130.11Hexaldehyde3.799.581.71.150.030.02Benzaldehyde−0.5712.00−0.61−0.250.070.03Acrolein7.45——0.57——Dimethylbenzaldehyde——————Figure7 shows the calculated LOH and OFP of atmospheric carbonyl compounds at the sampling site. The results showed that formaldehyde, acetaldehyde, butyraldehyde, and propionaldehyde all played a major role in ·OH consumption in 2018 and 2020, and the four species accounted for 89.6% and 95.7% of the total ·OH consumption of carbonyl compounds in 2018 and 2020, respectively. In 2020, the LOH of formaldehyde was 0.63 s−1, accounting for 42.9% of the total LOH. Therefore, formaldehyde is the main carbonyl compound affecting atmospheric oxidation in Jinan. In addition, the contributions of acetaldehyde, butyraldehyde, and propionaldehyde cannot be ignored. In terms of the potential of O3 formation, formaldehyde has a leading position, with generation rates of 45.69 μg m−3 and 31.96 μg m−3 in 2018 and 2020, respectively. O3 formation can be significantly limited by inhibiting secondary generation from formaldehyde through the control of emissions of formaldehyde and its precursors. Acetaldehyde, butyraldehyde, and propionaldehyde showed large contributions to OFP. The OFP values of the four species accounted for more than 90% of the total OFP of carbonyl compounds. As shown in Figures 2 and 3, clear time series variation patterns of OFP and carbonyl concentrations with nearly simultaneous peaks were observed, which demonstrates the contribution of carbonyl photochemical reactions to ozone formation.Figure 7
LOH and OFP of carbonyl compounds in Jinan in 2018 and 2020.


## 3.5. In Situ Formation Pathway of Carbonyl Compounds

To further explore the secondary generation mechanism of atmospheric carbonyl compounds in Jinan, the main formation and consumption pathways of formaldehyde were analyzed using the MCM photochemical box model based on the observations. The two days of July 29 and July 30, 2020, during which severe photochemical pollution (high concentrations of carbonyl compounds and O3) was observed, were selected as typical cases of photochemical pollution for analysis. The specific simulation analysis process has been described in detail by Yang et al. [22, 23].As shown in Figure8, the formation and consumption pathways of formaldehyde in Jinan on July 29 and July 30 were basically the same. The formation reactions of formaldehyde were mainly controlled by the reaction of RO + O2. Among the reactions of RO and O2, the oxidation reaction of CH3O + O2 significantly contributed to the secondary generation of formaldehyde, accounting for 86% of the total generation rate of formaldehyde. The oxidation reaction was followed by the RO + O2 (alkenes) reaction (7%), while other reactions, such as free radical chain transfer, OH + OVOCS, and OVOCS photolysis, contributed little to the formation of formaldehyde, accounting for 7% in total. The main route of formaldehyde consumption is OH + HCHO and formaldehyde photolysis reactions, which account for 99% of the total consumption rate.Figure 8
Main formation and consumption pathways of formaldehyde determined through model simulation.The production and consumption rates of formaldehyde both exhibited peaks around noon, and the production rate remained higher than the consumption rate, showing net production. The daily average of the net production rates was 0.59 ppb/h and 0.63 ppb/h, lower than the simulated results of Yang et al. for Hong Kong [23], but both findings indicate that in situ photochemical reactions promoted the accumulation of formaldehyde. On the whole, the OBM for investigating atmospheric oxidative capacity and photochemistry simulation highlights the important role of CH3O + O2 in the secondary formation of formaldehyde, which is consistent with the results of previous studies [23].


## 3.6. Effects of Carbonyl Compounds on Secondary Aerosols

Carbonyl compounds have strong water solubility and can be absorbed into clouds and fog to react with ·OH to form oligomers, which promote the formation of SOA [39]. We used the secondary organic aerosol potential (SOAP), which was developed by Grosjean et al. [40, 41] to reflect the propensity of individual organic compound to form SOA and evaluate the contribution of these compounds to SOA through their calculated emission inventory of toluene-based compounds. SOAP was calculated using :(3)SOAP=VOCi×FACi,where [VOC]i is the mass fraction of VOC species i (unit: μg·m−3) and FACi denotes the aerosol formation coefficient (%). The value of FAC was proposed by Grosjean et al. [40, 41], based on a large number of smog chamber experiments and atmospheric chemical dynamics data. Among them, formaldehyde, acetaldehyde, acetone, butyraldehyde, and benzaldehyde were found to be the major contributors to SOA. As shown in Figure 9, under the influence of different SOAP values of different carbonyls, the change trends of the contribution of different carbonyls to SOA were different from the trends of their concentration change. Among the SOAP values of different carbonyl compounds, benzaldehyde was found to contribute the most to SOA. Although with a low concentration, benzaldehyde contributed the most to SOA in 2018 and 2020, even reaching 54 ppbV in 2018. Formaldehyde was the second largest contributor, but the contribution was less than 3 ppbV in both years. The change trends of the contributions of carbonyl compounds to SOA followed the same pattern in both years: benzaldehyde > formaldehyde > acetaldehyde > acetone > propionaldehyde > butyraldehyde.Figure 9
Comparison of SOAP values of atmospheric carbonyls in Jinan between 2018 and 2020.


## 4. Conclusions

A total of 15 carbonyl compounds were determined in the atmosphere of Jinan in the summers of 2018 and 2020 by field measurement with high temporal resolution (2 h). The main carbonyl compounds were formaldehyde, acetone, and acetaldehyde, which collectively accounted for 78.5% and 90.14% of the total carbonyl compounds in 2018 and 2020, respectively. Compared with other cities in China and in other countries, the concentrations of atmospheric carbonyls in Jinan correspond to the upper-intermediate level.Diurnal variation and correlation analyses showed that carbonyl compounds in Jinan were influenced by anthropogenic emission and photochemical reactions, and exhaust emissions from motor vehicles significantly contributed to the carbonyl compounds in Jinan. Further, MCM analysis showed that the formation of formaldehyde in Jinan was controlled by RO + O2 reactions, among which the oxidation reaction of CH3O + O2 is the largest contributor. The main pathway of depletion for formaldehyde is through its photolysis and reactions with the OH radical. Formaldehyde showed net formation, indicating that in situ photochemical reactions promoted the accumulation of formaldehyde.Among all the carbonyl compounds measured, formaldehyde, acetaldehyde, butyraldehyde, and propionaldehyde not only play an important role in OH depletion reactions, but also contribute to the formation of O3. Finally, benzaldehyde was found to have the largest contribution to SOA although its concentration is relatively low, followed by formaldehyde, acetaldehyde, acetone, and propionaldehyde.



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*Source: 3292598-2022-01-24.xml*