Atmospheric carbonyls in a heavy ozone pollution episode at a metropolis in Southwest China: Characteristics, health risk assessment, sources analysis

Investigation of formaldehyde sources and its relative emission intensity in shipping channel environment

Formaldehyde (HCHO) is considered one of the most abundant gas-phase carbonyl compounds in the atmosphere, which can be directly emitted through transportation sources. Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) was used to observe HCHO in the river channel of Wusong Wharf in Shanghai, China for the whole year of 2019. Due to the impact of ship activity, the annual average HCHO level in the channel is about 2.5 times higher than that in the nearby campus environment. To explain the sources of HCHO under different meteorological conditions, the tracer-pair of CO and Ox (NO2+O3) was used on the clustered air masses. The results of the source appointment show that primary, secondary and background account for 24.14% (3.34 ± 1.19 ppbv), 44.78% (6.20 ± 2.04 ppbv) and 31.09% (4.31 ± 2.33 ppbv) of the HCHO in the channel when the air masses were from the mixed direction of the city and channel, respectively. By performing background station subtraction at times of high primary HCHO values and resolving the plume peaks, directly emitted HCHO/NO2 in the channel environment and plume were determined to be mainly distributed between 0.2 and 0.3. General cargo ships with higher sailing speeds or main engine powers tend to have higher HCHO/NO2 levels. With the knowledge of NO2 (or NOx) emission levels from ships, this study may provide data support for the establishment of HCHO emission factors.

Comprehensive measurement of carbonyls in Lhasa, Tibetan Plateau: Implications for strong atmospheric oxidation capacity

Carbonyls are ubiquitous in the troposphere and play a crucial role in atmospheric oxidation capacity (AOC), particularly in photochemistry-active regions such as the Tibetan Plateau (TP). However, the composition and evolution of carbonyls over the TP is still poorly understood due to a lack of comprehensive observations and modelling. Here, we conducted an intensive field measurement of 37 carbonyls and their precursors at a suburban site in Lhasa during summer 2022. Markedly higher levels of carbonyls (7.24 ± 3.83 ppbv) were found during ozone pollution episodes, with 36 % higher than those during non-episodes. Formaldehyde was the most abundant carbonyl (38 %), which primarily originating from photochemical secondary formations. Simulations using the Rapid adaptive Optimization Model for Atmospheric Chemistry (ROMAC) indicated strong AOC in Lhasa, with the daytime maximum of ·OH and ·HO2 of 9.8 × 106 and 4.2 × 108 molecules cm-3, respectively, which were even higher than that in most of the megacities in China. Notably, AOC significantly enhanced with the increasing carbonyls during the episodes, with the concentrations of ·OH and ·HO2 were boosted 21 % and 67 % than those during non-episodes, respectively. Budget analysis revealed that the ·HO2 + NO (88 %) and ·OH + VOC (74 %) pathways dominated the generation and loss of ·OH, respectively. And for ·HO2, they were ·RO2 + NO (67 %) and ·HO2 + NO (83 %). This study provides valuable insights into the strong AOC in the ecologically-fragile and climate-sensitive TP region, and highlighted the crucial role of anthropogenic-biogenic interactions in the active photochemistry of TP.

Quantitative kinetics reveal that reactions of HO2 are a significant sink for aldehydes in the atmosphere and may initiate the formation of highly oxygenated molecules via autoxidation

Large aldehydes are widespread in the atmosphere and their oxidation leads to secondary organic aerosols. The current understanding of their chemical transformation processes is limited to hydroxyl radical (OH) oxidation during daytime and nitrate radical (NO3) oxidation during nighttime. Here, we report quantitative kinetics calculations of the reactions of hexanal (C5H11CHO), pentanal (C4H9CHO), and butanal (C3H7CHO) with hydroperoxyl radical (HO2) at atmospheric temperatures and pressures. We find that neither tunneling nor multistructural torsion anharmonicity should be neglected in computing these rate constants; strong anharmonicity at the transition states is also important. We find rate constants for the three reactions in the range 3.2-7.7 × 10-14 cm3 molecule-1 s-1 at 298 K and 1 atm, showing that the HO2 reactions can be competitive with OH and NO3 oxidation under some conditions relevant to the atmosphere. Our findings reveal that HO2-initiated oxidation of large aldehydes may be responsible for the formation of highly oxygenated molecules via autoxidation. We augment the theoretic studies with laboratory flow-tube experiments using an iodide-adduct time-of-flight chemical ionization mass spectrometer to confirm the theoretical predictions of peroxy radicals and the autoxidation pathway. We find that the adduct from HO2 + C5H11CHO undergoes a fast unimolecular 1,7-hydrogen shift with a rate constant of 0.45 s-1. We suggest that the HO2 reactions make significant contributions to the sink of aldehydes.

Comparison of three source apportionment methods based on observed and initial HCHO in Taiyuan, China

Identifying the sources of formaldehyde (HCHO) is key to reducing the pollution of HCHO and ozone (O3) on the ground level. Using the same datasets applied to the positive matrix factorization (PMF) model by (Hua et al., 2023), the initial concentrations of HCHO were estimated using the photochemical age and the sources of observed and initial HCHO were apportioned based on multiple linear regression (MLR) and photochemical age-based parameterization (PCAP) methods. These results suggest that the source of the initial HCHO can better reflect its contribution. The secondary formation contributed to 49.3-69.1 % of initial HCHO at four sites in Taiyuan based on MLR, which was higher (7.4-36.2 %) than the contributions of secondary formation from observed HCHO. The HCHO was mainly affected by anthropogenic secondary (10.8-34.4 %) and background sources (17.4-78.7 %) based on the PCAP method. We compared the results of the HCHO sources from the MLR, PCAP, and PMF models under photochemical loss. There was good agreement among the emission ratios of acetylene-based HCHO obtained by the different methods at the four sites. The correlation analysis of different source apportionment methods illustrated that primary emissions from the PCAP and the MLR model had the greatest correlation (0.22-0.60). Secondary formations from the PMF and MLR models showed good correlations at all four sites, with R values ranging from 0.42 to 0.83. The HCHO peak of diurnal variation simulated by MLR appeared late compared to the other methods, and the difference in daily variation of HCHO from the PMF model was significantly higher than that of PCAP and MLR. The overlapping conclusions of different source apportionment methods should be considered and used to guide efforts to improve air quality.

Elaborations of the influencing factors on the formation of secondary inorganic aerosols in a heavily polluted urban area of China

In this study, online water-soluble inorganic ions were detected to deduce the formation mechanism of secondary inorganic aerosol in Xianyang, China during wintertime. The dominant inorganic ions of sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) (the sum of those is abbreviated as SNA) accounted for 17%, 21%, and 12% of PM2.5 mass, respectively. While the air quality deteriorated from excellent to poor grades, the precursor gas sulfur dioxide (SO2) of SO42- increased and then decreased with a fluctuation, while nitrogen dioxide (NO2) and ammonia (NH3), precursors of NO3- and NH4+, and SNA show increasing trends. Meteorological factors including boundary layer height (BLH), temperature, and wind speed also show decline trends, except relative humidity (RH). Meanwhile, the secondary conversion ratio shows a remarkable increasing trend, indicating that there was a strong secondary transformation. From the perspective of chemical mechanisms, RH is positively correlated with sulfur oxidation ratios (SOR), nitrogen oxidation ratios (NOR), and ammonia conversion ratios, representing that the increase of humidity could promote the generation of SNA. Notably, SOR and NOR were also positively related to the ammonia. On the one hand, the low wind speed and BLH led to the accumulation of pollutants. On the other hand, the increases of RH and ammonia promoted more formations of SNA and PM2.5. The results advance our identification of the contributors to the haze episodes and assist to establish more efficient emission controls in Xianyang, in addition to other cities with similar emission and geographical characteristics.

Pollution characteristics, source appointment and environmental effect of oxygenated volatile organic compounds in Guangdong-Hong Kong-Macao Greater Bay Area: Implication for air quality management

Same as other bay areas, the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) is also suffering atmospheric composite pollution. Even a series of atmospheric environment management policies have been conducted to win the "blue sky defense battle", the atmospheric secondary pollutants (e.g., O3) originated from oxygenated volatile organic compounds (OVOCs) still threaten the air quality in GBA. However, there lacks a systematic summary on the emission, formation, pollution and environmental effects of OVOCs in this region for further air quality management. This review focused on the researches related to OVOCs in GBA, including their pollution characteristics, detection methods, source distributions, secondary formations, and impacts on the atmosphere. Pollution profile of OVOCs in GBA revealed that the concentration percentage among total VOCs from Guangzhou and Dongguan cities exceeded 50 %, while methanol, formaldehyde, acetone, and acetaldehyde were the top four highest concentrated OVOCs. The detection technique on regional atmospheric OVOCs (e.g., oxygenated organic molecules (OOMs)) underwent an evolution of off-line derivatization method, on-line spectroscopic method and on-line mass spectrometry method. The OVOCs in GBA were mainly from primary emissions (up to 80 %), including vehicle emissions and biomass combustion. The anthropogenic alkenes and aromatics in urban area, and natural isoprene in rural area also made a significant contribution to the secondary emission (e.g., photochemical formation) of OVOCs. About 20 % in average of ROx radicals was produced from photolysis of formaldehyde in comparison with O3, nitrous acid and rest OVOCs, while the reaction between OVOCs and free radical accelerated the NOx-O3 cycle, contributing to 15 %-60 % cumulative formation of O3 in GBA. Besides, the heterogeneous reactions of dicarbonyls generated 21 %-53 % of SOA. This review also provided suggestions for future research on OVOCs in terms of regional observation, analytical method and mechanistic study to support the development of a control and management strategy on OVOCs in GBA and China.

Sources of atmospheric oxygenated volatile organic compounds in different air masses in Shenzhen, China

As precursors of photochemical secondary pollutants, oxygenated volatile organic compounds (OVOCs) play an important role in atmospheric photochemistry. In this study, 23 OVOCs were monitored using a commercial proton transfer reaction time-of-flight mass spectrometer at an urban site in Shenzhen, China. During the campaign, the mean total concentration of OVOCs was 23.3 ± 15.5 ppb (mean ± standard deviation), with a total ozone formation potential (TOFP) of 87.3 ± 58.7 ppb. Aldehydes contributed the most to the concentration and TOFP of OVOCs, followed by ketones, alcohols, and carboxylic acids. Formaldehyde, acetone, and acetaldehyde were the three most abundant atmospheric carbonyls. An optimized photochemical age-based parameterization method was locally applied for the source apportionment of OVOCs. OVOCs in Shenzhen primarily originated from biogenic sources during the summer. Secondary anthropogenic sources were also important contributors of most carbonyl compounds. The campaign was divided into four periods. Two periods were dominated by the east wind from the relatively clean coastal areas, with the mean concentration of anthropogenic OVOCs largely decreasing during the Chinese National Day holidays. The other two periods were dominated by northwest wind and northeast wind, respectively, with larger OVOC contributions from anthropogenic sources, suggesting that pollution transport from the inland was a main contributor to OVOCs. This study highlights the important contributions of both local and regional OVOC sources in urban atmospheres.

Pollution characteristics, sources, and photochemical roles of ambient carbonyl compounds in summer of Beijing, China

Ambient carbonyls are important precursors of radicals and ground-level ozone (O3). In this study, sources, precursors, and impacts on radicals and O3 of carbonyls were investigated based on online observations of volatile organic compounds (VOCs) at an urban site in Beijing during June 2021. Carbonyls accounted for 36% and 42% of mixing ratios and OH reactivity for total measured VOCs, respectively. Formaldehyde was the most abundant carbonyl, with the mean level of 4.13 ± 2.28 ppb. Source apportionment results based on the multi linear regression (MLR) method suggested that secondary production contributed 41%, 25%, 36%, and 30% of formaldehyde, acetaldehyde, propanal, and acetone, respectively. Key precursors of carbonyls were then identified based on the calculation of their production rates. It was found that alkenes contributed 59%-80% of aldehydes production. Impacts of carbonyls on HOx radicals (OH and HO2) and O3 production were explored using a box model based on observations (OBM). Photolysis of HONO, formaldehyde, and O3 were the dominant primary sources of HOx radicals during daytime of O3 pollution days, with average relative contributions of 52%, 28%, and 19% to the total primary production rate of HOx, respectively. Aldehydes accounted for 32% (20% from formaldehyde) of average HOx removal rates. The relative incremental reactivity (RIR) values of NOx determined by the OBM were negative, suggesting that the O3-VOCs-NOx sensitivity was in the VOCs-limited regime. Using the observed concentrations of carbonyls as constraints of OBM, the absolute values of RIR for NOx tended to increase but those for anthropogenic VOCs tended to decrease. Formaldehyde showed the largest RIR value for anthropogenic VOCs during O3 pollution days. These findings indicated the important impacts of carbonyls on O3 production and O3-VOCs-NOx sensitivity.

Assessment of atmospheric levels of carbonyls in an urban environment of Argentina

It is well-documented that carbonyl compounds have adverse effects on human health. On the other hand, these oxygenated volatile organic compounds (OVOCs) are precursors of secondary pollutants such as tropospheric ozone or peroxy acetyl nitrate (PAN). In particular, formaldehyde, the simplest carbonyl, is the most abundant carbonyl in the air generated from the degradation of most volatile organic compounds (VOCs). This work presents for the first time the characterization and determination of levels of carbonyl compounds by passive monitoring performed from April-December 2021 in the city of Córdoba, Argentina, the second most populated Mediterranean city located in the center of the country. Annual concentrations, considering the 11 carbonyls measured, were in the range of 0.13-8.75 μgm-3. Formaldehyde and acetaldehyde were the carbonyls detected in the highest annual average concentrations of 4.44 ± 1.75 μgm-3 and 3.85 ± 1.44 μgm-3, respectively. These carbonyls represent a contribution of around 40-57% on total carbonyls measured. Statistical analysis to determine significant differences and Pearson correlations with the meteorological parameters were performed. Spring and summer were found to be the seasons with the highest carbonyl concentration linked to forest fire episodes, especially in springtime. The values for the C1/C2 and C2/C3 ratios showed that sources of carbonyl formation are anthropogenic. In addition, the prop-Equiv concentration was determined, where formaldehyde and acetaldehyde were the main producers of tropospheric ozone. The ozone formation potential (OFP) showed that spring and summer are the seasons where carbonyls contribute to the formation of tropospheric ozone.This study represents a first approach of the carbonyl concentration in the city and of the influence of meteorological parameters on the behavior of carbonyls.

Development of an enhanced method for atmospheric carbonyls and characterizing their roles in photochemistry in subtropical Hong Kong

Carbonyl compounds are ubiquitous and play vital roles in tropospheric photochemistry and oxidation capacity, particularly affecting radical cycling and ozone formation. An enhanced method based on ultra-high-performance liquid chromatography and electrospray ionization tandem mass spectrometry was developed to simultaneously quantify 47 carbonyl compounds with carbon (C) numbers from 1 to 13. Applying this new method to samples collected at ground and ship-borne field campaigns, we investigated the abundance, characteristic distribution, and photochemical activity of carbonyl species in the coastal atmosphere of Hong Kong. The total concentration of detected carbonyls ranged from 9.1 to 32.7 ppbv and exhibited distinct spatial variability. In addition to the usual abundant carbonyl species (formaldehyde, acetaldehyde, and acetone), aliphatic saturated aldehydes with C ≥ 5 (particularly hexaldehyde and nonanaldehyde), and di‑carbonyls, exhibit significant abundance and photochemical reactivity in the coastal site and on the sea. The measured carbonyls could contribute to an estimated peroxyl radical formation rate of 1.88-8.43 ppb/h via OH oxidation and photolysis, greatly enhancing oxidation capacity and radical cycling. The ozone formation potential (OFP) estimated from the maximum incremental reactivity (MIR) was dominated (69 %-82 %) by formaldehyde and acetaldehyde, with significant contribution (4 %-13 %) from di‑carbonyls. Furthermore, another dozens of long-chain carbonyls without MIR values, which were typically below detection or not included in the conventional analytical method, would increase the ozone formation rate by an additional 2 %-33 %. Additionally, the glyoxal, methylglyoxal, benzaldehyde and other α, β-unsaturated aldehydes also exhibited considerable contribution to secondary organic aerosol (SOA) formation potential. This study highlights the importance of various reactive carbonyls in the atmospheric chemistry of urban and coastal regions. The newly developed method can effectively characterize more carbonyl compounds and advance our understanding of their roles in photochemical air pollution.

Spatial characterization of HCHO and reapportionment of its secondary sources considering photochemical loss in Taiyuan, China

Formaldehyde (HCHO) plays an important role in atmospheric ozone (O₃) formation. To accurately identify the sources of HCHO, carbonyls and volatile organic compounds (VOCs) were measured at three urban sites (Taoyuan, TY-U; Jinyuan, JY-U; Xiaodian, XD-U) and a suburban site (Shanglan, SL-B) in Taiyuan during a high O₃ period (from July 20 to August 3, 2020). The average mixing ratio of HCHO at XD-U (8.1 ± 2.8 ppbv) was comparable to those at TY-U (7.4 ± 2.1 ppbv) and JY-U (7.0 ± 2.3 ppbv) but higher (p < 0.01) than that at SL-B (4.9 ± 2.3 ppbv). HCHO contributed to 54.3-59.9 % of the total ozone formation potentials (OFPs) of non-methane hydrocarbons (NMHCs) at four sites. The diurnal variation of HCHO concentrations reached a peak value at 12:00-15:00, which may be attributed to the strong photochemical reaction. To obtain more accurate source results of HCHO under the condition of photochemical loss, the initial concentrations of NMHCs were estimated based on photochemical age parameterization and incorporated into the positive matrix factorization (PMF) model (termed IC-PMF). According to the IC-PMF results, secondary formation (SF) contributed the most to HCHO at XD-U (35.6 %) and SL-B (25.1 %), whereas solvent usage (SU) (40.9 %) and coking sources (CS) (36.0 %) were the major sources at TY-U and JY-U, respectively. Compared to the IC-PMF, the conventional PMF analysis based on the observed data underestimated the contributions of SU (100.5-154.2 %) and biogenic sources (BS) (28.5-324.7 %). Further reapportionment of secondary HCHO by multiple linear regression indicated that SU dominated the sources of HCHO at SL-B (28.3 %) and TY-U (41.7 %), while industrial emissions (IE) and CS contributed the most to XD-U (26.6 %) and JY-U (43.0 %) in Taiyuan from north to south, respectively.

Pollution characteristics and sources of carbonyl compounds in a typical city of Fenwei Plain, Linfen, in summer

Field measurements of atmospheric carbonyl compounds (carbonyls) and essential precursors of O₃ were carried out in the urban area of Linfen City (Linfen) where serious O₃ pollution has occurred in recent years due to its unique terrain. Carbonyls were sampled using an automatic carbonyl sampler in August 2019 to determine their pollution characteristics and sources. An average concentration of ten carbonyls was 27 ± 5.7 μg m^-3 detected using an HPLC-UV system. The concentrations of most detected carbonyls in August were significantly higher than those in the winter months in China. Acetone, formaldehyde and acetaldehyde were the most abundant species, accounting for 73% of all detected carbonyls. Formaldehyde, acetaldehyde, and methacrolein (MACR) were the most significant contributors to OH• reactivity and ozone generation, indicating that these three carbonyls were the key species influencing the production of O₃. The concentrations of formaldehyde, acetaldehyde, and MACR showed similar diurnal variations on most days, with high values during the daytime reaching a peak at 10:00. However, the concentrations of the latter two species varied less than that of formaldehyde during the day. The acetone concentration generally increased continuously from morning to night, with the maximum value around 22:00. The C1/C2 ratio in summer was higher than that in winter. These results indicated that the carbonyls in Linfen were not only affected by anthropogenic sources such as vehicle exhaust but also by secondary photochemical production. The results of formaldehyde source apportionment showed that the contributions of background, primary, and secondary sources to the observed formaldehyde concentration were 27.6%, 36.6%, and 35.8%, respectively. Additionally, this study revealed for the first time that the vertical transport of air masses containing high concentrations of O₃ and NO₃ radicals above the boundary layer could increase the secondary generation of formaldehyde at night in summer.

Pollution mechanisms and photochemical effects of atmospheric HCHO in a coastal city of southeast China

Formaldehyde (HCHO) is a vital reactive carbonyl compound, which plays an important role in the photochemical process and atmospheric oxidation capacity. However, the current studies on the quantification of HCHO impacts on atmospheric photochemistry in southeast coastal areas of China with an obvious upward trend of ozone remain scarce and unclear, thus limiting the full understanding of formation mechanism and control strategy of photochemical pollution. Here, systematic field campaigns were conducted at a typical coastal urban site with good air quality to reveal HCHO mechanism and effects on O₃ pollution mechanism during spring and autumn, when photochemical pollution events still frequently appeared. Positive Matrix Factorization model results showed that secondary photochemical formation made the largest contributions to HCHO (69 %) in this study. Based on the photochemical model, the HCHO loss rates in autumn were significantly higher than those in spring (P < 0.05), indicating that strong photochemical conditions constrain high HCHO levels in certain situations. HCHO mechanism increased the ROx concentrations by 36 %, and increased net O₃ production rates by 31 %, manifesting that the reduction of HCHO and its precursors' emissions would effectively mitigate O₃ pollution. Therefore, the pollution characteristics and photochemical effects of HCHO provided significant guidance for future photochemical pollution control in relatively clean areas.

Spatiotemporal patterns and ozone sensitivity of gaseous carbonyls at eleven urban sites in southeastern China

Gaseous carbonyls are essential trace gases for tropospheric chemistry and contribute significantly to the formation of ambient air ozone (O₃) in densely populated regions, especially in China. Pollution characterization and the analysis of O₃, nitrogen oxides, and volatile organic compounds (O₃-NO_X-VOCs) sensitivities of carbonyls were investigated from October 22 to 28, 2018 at eleven urban sites in nine cities in Fujian Province, southeastern China. The total mixing ratios of 15 kinds of gaseous carbonyls (Σ15OVOCs) was 12.15 ± 2.53 ppbv in Fujian Province. The concentrations in the eastern coastal regions were higher than those in the western mountainous regions. Formaldehyde, acetone, and acetaldehyde were the top three species of Σ15OVOCs concentration. Photochemical formation during the daytime and vehicle emission during the rush hours significantly contributed to formaldehyde and acetaldehyde. The shoe-making industry is well developed in Putian, where the acetone mixing ratio was significantly higher than in other cities. The O₃-NO_X-VOCs sensitivities at all urban sites were in VOC-limited or transitional regimes based on the ratios of formaldehyde to NO₂; from morning to afternoon, the VOC-limited sensitivity decreased, and the NO_X-limited sensitivity increased gradually. Formaldehyde contributed the most significant O₃ formation potential (OFP) proportion of the Σ15OVOCs. The OFP of carbonyl species accounted for half of the total VOCs in Fuzhou and Putian, suggesting that more attention needs to be given to gaseous carbonyls control. Overall, the links inferred by this study provide evidence and clues to mitigate the increasing ambient O₃ concentration on the west coast of the Taiwan Strait.

Risk Assessment and Prediction of Air Pollution Disasters in Four Chinese Regions

Evaluating the regional trends of air pollution disaster risk in areas of heavy industry and economically developed cities is vital for regional sustainable development. Until now, previous studies have mainly adopted a traditional weighted comprehensive evaluation method to analyze the air pollution disaster risk. This research has integrated principal component analysis (PCA), a genetic algorithm (GA) and a backpropagation (BP) neural network to evaluate the regional disaster risk. Hazard risk, hazard-laden environment sensitivity, hazard-bearing body vulnerability and disaster resilience were used to measure the degree of disaster risk. The main findings were: (1) the air pollution disaster risk index of Liaoning Province, Beijing, Shanghai and Guangdong Province increased year by year from 2010 to 2019; (2) the mean absolute error (MAE), root mean square error (RMSE) and mean absolute percentage error (MAPE) of each regional air pollution disaster risk index in 2019, as predicted by the PCA-GA-BP neural network, were 0.607, 0.317 and 20.3%, respectively; (3) the predicted results were more accurate than those using a PCA-BP neural network, GA-BP neural network, traditional BP neural network, support vector regression (SVR) or extreme gradient boosting (XGBoost), which verified that machine learning could be used as a method of air pollution disaster risk assessment to a considerable extent.