Photochemical Pollution Research Articles

Characterization of peroxyacetyl nitrate (PAN) under different PM2.5 concentration in wintertime at a North China rural site

As a secondary pollutant of photochemical pollution, peroxyacetyl nitrate (PAN) has attracted a close attention. A four-month campaign was conducted at a rural site in North China Plain (NCP) including the measurement of PAN, O3, NOx, PM2.5, oxygenated volatile organic compounds (OVOCs), photolysis rate constants of NO2 and O3 and meteorological parameters to investigate the wintertime characterization of photochemistry from November 2018 to February 2019. The results showed that the maximum and mean values of PAN were 4.38 and 0.93 ± 0.67 ppbv during the campaign, respectively. The PAN under different PM2.5 concentrations from below 75 μg/m3 up to 250 μg/m3, showed different diurnal variation and formation rate. In the PM2.5 concentration range of above 250 μg/m3, PAN had the largest daily mean value of 0.64 ppbv and the fastest production rate of 0.33 ppbv/hr. From the perspective of PAN's production mechanism, the light intensity and precursors concentrations under different PM2.5 pollution levels indicated that there were sufficient light intensity and high volatile organic compounds (VOCs) and NOx precursors concentration even under severe pollution level to generate a large amount of PAN. Moreover, the bimodal staggering phenomenon of PAN and PM2.5 provided a basis that PAN might aggravate haze through secondary organic aerosols (SOA) formation.

Rapid increase in atmospheric glyoxal and methylglyoxal concentrations in Lhasa, Tibetan Plateau: Potential sources and implications

Glyoxal (Gly) and methylglyoxal (Mgly) are the intermediate products of several volatile organic compounds (VOCs) as well as the precursors of brown carbon and may play key roles in photochemical pollution and regional climate change in the Tibetan Plateau (TP). However, their sources and atmospheric behaviors in the TP remain unclear. During the second Tibetan Plateau Scientific Expedition and Research in the summer of 2020, the concentrations of Gly (0.40 ± 0.30 ppbv) and Mgly (0.57 ± 0.16 ppbv) observed in Lhasa, the most densely populated city in the TP, had increased by 20 and 15 times, respectively, compared to those measured a decade previously. Owing to the strong solar radiation, secondary formations are the dominant sources of both Gly (71%) and Mgly (62%) in Lhasa. In addition, primary anthropogenic sources also play important roles by emitting Gly and Mgly directly and providing abundant precursors (e.g., aromatics). During ozone pollution episodes, local anthropogenic sources (industries, vehicles, solvent usage, and combustion activities) contributed up to 41% and 45% in Gly and Mgly levels, respectively. During non-episode periods, anthropogenic emissions originating from the south of Himalayas also have non-negligible contributions. Our results suggest that in the previous decade, anthropogenic emissions have elevated the levels of Gly and Mgly in the TP dramatically. This study has important implications for understanding the impact of human activities on air quality and climate change in this ecologically fragile area.

Atmospheric oxidation capacity and ozone pollution mechanism in a coastal city of southeastern China: analysis of a typical photochemical episode by an observation-based model

Abstract. A typical multi-day ozone (O3) pollution event was chosen to explore the atmospheric oxidation capacity (AOC), OH reactivity, radical chemistry, and O3 pollution mechanism in a coastal city of southeastern China, with an observation-based model coupled to the Master Chemical Mechanism (OBM-MCM). The hydroxyl radical (OH) was the predominant oxidant (90 ± 25 %) for daytime AOC, while the NO3 radical played an important role in AOC during the nighttime (72 ± 9 %). Oxygenated volatile organic compounds (OVOCs; 30 ± 8 %), NO2 (29 ± 8 %), and CO (25 ± 5 %) were the dominant contributors to OH reactivity, accelerating the production of O3 and recycling of ROx radicals (ROx = OH + HO2 + RO2). Photolysis of nitrous acid (HONO, 33 ± 14 %), O3 (25 ± 13 %), formaldehyde (HCHO, 20 ± 5 %), and other OVOCs (17 ± 2 %) was a major ROx source, which played an initiation role in atmospheric oxidation processes. Combined with regional transport analysis, the reasons for this O3 episode were the accumulation of local photochemical production and regional transport. The results of sensitivity analysis showed that volatile organic compounds (VOCs) were the limiting factor of radical recycling and O3 formation, and the 5 % reduction of O3 would be achieved by decreasing 20 % anthropogenic VOCs. Controlling emissions of aromatics, alkenes, and alkanes with ≥4 carbons was beneficial for ozone pollution mitigation. The findings of this study provide significant guidance for emission reduction and regional collaboration for future photochemical pollution control in the relatively clean coastal cities of China and similar countries.

Effects of Meteorological Factors on the Vertical Distribution of Peroxyacetyl Nitrate in Autumn in Tianjin

With the decrease of PM2.5 concentration, atmospheric photochemical pollution in North China Plain has attracted more and more attention. Peroxyacetyl nitrate (PAN) is an important product and reliable indicator of atmospheric photochemical reaction. In this study, the measured concentrations of PAN near the surface and at 220 m were obtained from the Tianjin Meteorological Tower in October 2018. The results showed that the average concentrations of PAN at ground and 220m platform were 1.65 ± 1.34 ppb and 1.76 ± 1.41 ppb respectively during the day, and the mixture was relatively uniform. The average concentrations of PAN were 0.99 ± 0.94 ppb and 1.42 ± 1.28 ppb during the nighttime, respectively, indicating that the vertical difference was significantly increased. The standard deviation of PAN concentration was close to the average value, indicating that the concentration fluctuated greatly. The correlation coefficient between PAN and PM2.5 was higher than that of O3, indicating that stable weather was an important factor affecting PAN concentration. Based on the analysis of the vertical wind speed by the wind profile radar and ozone concentration vertical distribution data, we believe that there is a downdraft in the upper atmosphere of the boundary layer. The higher concentration of PAN at 220 m at night came from the high-altitude pollution air mass. The analysis of 40 m and 200 m turbulence data showed that the weak turbulence hindered the vertical exchange between the surface and the upper atmosphere, resulting in the high concentration of PAN unable to reach the surface. The boundary layer structure is one of the important reasons for the vertical difference of PAN concentration at night in autumn in NCP.

A novel dual-scale ensemble learning paradigm with error correction for predicting daily ozone concentration based on multi-decomposition process and intelligent algorithm optimization, and its application in heavily polluted regions of China

Accurate prediction of daily ozone concentration is imperative to prevent and control photochemical pollution in China. A novel ensemble learning paradigm is proposed in this paper to perform high-precision ozone concentrations prediction, including model preprocess, optimized dual-scale prediction, and error correction. Firstly, the original ozone series is decomposed into two detailed sequences and two approximate sequences by wavelet packet decomposition (WPD). The detailed sequences are reconstructed into several new subsequences by complementary ensemble empirical mode decomposition (CEEMD) and fuzzy entropy (FE). Secondly, the extreme learning machine (ELM) and the support vector machine (SVM) are both optimized by the bald eagle search (BES) algorithm to carry out dual-scale parallel prediction. Finally, the error series is decomposed and predicted by the variational mode decomposition (VMD) and the optimized ELM to correct the previously predicted ozone and obtain the final ozone prediction. The actual ozone data from two typical cities in China are collected as the inputs of the model for empirical analysis under two scenarios, and one-day ahead prediction results show that: the RMSE values of the proposed model are 2.5319 and 3.2069 at Taiyuan and Shanghai sites when predicting low levels of ozone, respectively, while the RMSE values of the proposed model are 2.8451 and 3.8702 at two sites when predicting high levels of ozone, respectively; the proposed model outperforms the comparison models and four existing models under each scenario, demonstrating its robustness and serviceability. The results of this study can provide a valuable reference for analyzing the tendency of pollution.

Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during haze events in autumn

Abstract. Photochemical pollution over the North China Plain (NCP) is attracting much concern. We usually view peroxyacetyl nitrate (PAN) as the second most important photochemical pollutant featuring high mixing ratios during warm seasons. Our observations at a background site in the NCP identified high PAN concentrations, even during haze events in autumn. The substantial increasing ratios of PAN, by 244 % and 178 %, over the morning hours (08:00–12:00 local time) on 20 and 25 October 2020 were 10.6 and 7.7 times larger than those on clean days. Polluted days are characterized by higher temperature, higher humidity, and anomalous southerly winds compared with clean days. Enhanced local photochemistry has been identified as being the dominant factor that controls the PAN increase in the morning at the rural site, as the time when prevailing wind turns to a southerly wind is too late to promote direct transport of PAN from the polluted urban region. By removing the effect of direct transport of PAN, we provide a quantitative assessment of net PAN chemical production rate of 0.45 ppb h−1 for the mornings of polluted days, also demonstrating the strong local photochemistry. Using observations and calculated photolysis rates, we find that acetaldehyde oxidation by hydroxyl radical (OH) is the primary pathway of peroxyacetyl radical formation at the rural site. Acetaldehyde concentrations and production rates of HOx (HOx= OH + HO2) on polluted days are 2.8 and 2 times as large as those on clean days, leading to a remarkable increase in PAN in the morning. Formaldehyde (HCHO) photolysis dominates the daytime HOx production, thus contributing to fast photochemistry of PAN. Our observational results suggest the cause of a rapid increase in PAN during haze events in autumn at a rural site of the NCP and provide evidence of important role of HCHO photolysis in secondary pollutants at lower nitrogen oxide emissions. This highlights the urgency of carrying out strict volatile organic compound controls over the NCP during the cold season and not just in summer.

Concentration and Reactivity of Carbonyl Compounds in the Atmosphere of North China

To reveal the characteristics of photochemical pollution in North China, adsorbing columns with 2, 4-dinitrophenylhydrazine(DNPH) were used to sample carbonyl compounds in Shijiazhuang and Xinglong between May 2018 and April 2019. The samples were analyzed by high-performance liquid chromatography to understand the composition, volume fraction, source, ·OH loss rate, and ozone formation potential of the carbonyl compounds. A total of 13 carbonyl compounds containing carbonyl groups were determined, of which acetone, formaldehyde, and acetaldehyde were highest at(6.46±5.25)×10-9, (3.76±2.29×10-9), and(2.65±1.74)×10-9 in Shijiazhuang compared to(1.85±1.27)×10-9, (1.29±1.02)×10-9, and(0.72±0.48)×10-9 in Xinglong, respectively. The estimated maximum ozone formation potential(OFP) of formaldehyde was much higher than that of acetaldehyde; the C1/C2 and C2/C3 ozone formation potential(OFP) of formaldehyde was much higher than that of acetaldehyde; and the C1/C2 and C2/C3 values showed that vehicle exhaust and fossil fuel combustion were the main sources in Shijiazhuang in association with the higher level of industrialization. In Xinglong, the carbonyl compounds mainly originated from natural sources. Acetaldehyde(1.77 s-1), formaldehyde(1.57 s-1), and butyraldehyde(0.42 s-1) contributed most to L·OH in Shijiazhuang, and formaldehyde(0.53 s-1), acetaldehyde(0.47 s-1), and butyraldehyde(0.12 s-1) were the three main contributors to L·OH in Xinglong. The carbonyl compounds contributing most to O3 production were formaldehyde and acetaldehyde at(34.61×10-9 O3) and (16.73×10-9 O3) in Shijiazhuang, compared to (11.77×10-9 O3) and (4.47×10-9 O3) in Xinglong, respectively.

Multiplexed Analysis of Photochemical Oxidants Using a Nanoparticle-Based Optoelectronic Nose.

Photochemical pollutants pose a substantial threat to human health in both outdoor and indoor environments. Herein, we prepare a class of gold nanoparticle-based colorimetric sensor arrays on optimized hydrophobic substrates using a simple pin-printing method for accurate identification and quantification of various gas-phase oxidants, as these microdetectors are low cost, sensitive, and easy to fabricate. For an array of AuNP sensors modified with various thiol-terminated ligands, a unique and distinguishable change in color (i.e., red, green, and blue response patterns) was obtained for each specific pollutant for molecular fingerprinting. Remarkable discrimination among 15 gases at a fairly low vapor concentration (i.e., 500 ppb) was illustrated using standard chemometric methods. Using digital imaging, the AuNP colorimetric sensor array offers ultrasensitive dosimetric identification of gas-phase oxidants relevant to outdoor and indoor air pollution, with limits of detection generally at sub-ppb levels for 2 h measurement. As a practical application, the sensor array is able to predict the overall air quality in indoor office environments over 24 h. Such sensor array based on chemically induced sintering of nanoparticles has significant implications for the development of nanosensors used in continuous monitoring of potential airborne pollutants at low concentrations from a large number of locations in a cost-effective manner.

Update on volatile organic compound (VOC) source profiles and ozone formation potential in synthetic resins industry in China

The synthetic resin industry plays an important role in Volatile organic compounds (VOCs) emissions from industrial sources. However, owing to various products and their different emission characteristics, it is extremely difficult to study the source profiles of synthetic resins. In this study, the product-based pollution characteristics of VOCs from eight synthetic resin enterprises were investigated in Shanghai, China. Up to 133 VOCs were identified, including 106 based on the Photochemical Assessment Monitoring Stations (PAMS) and the Toxic Organics (TO-15) methods, and the remaining 27 were identified based on the new mass spectrometry analysis method. Aromatics (39.7%) and oxygenated VOCs (29.9%) accounted for a relatively high proportion in the synthetic resin industry. The product-based source profiles of each process unit are compiled. Generally, 1,4-dioxane, methyl isobutyl ketone, toluene, benzene, styrene, propane, and dichloromethane are the most abundant species in synthetic resin. Furthermore, the product-based ozone formation potentials (OFPs) and sources reactivity (SR) were calculated, the synthetic resin industry SR range from 0.3 g g−1 to 4.6 g g−1. Results suggest that toluene, benzene, styrene, propylene, ethylene, and oxygenated VOCs (including 1,4-dioxane, methyl isobutyl ketone, and aldehyde) should be preferentially controlled to reduce the OFPs. A three-level classification was established to evaluate the degree of photochemical pollution in different industries. Emission factors were calculated and ranked for eight synthetic resins. A VOC emission inventory of Chinese synthetic resin from 2005 to 2018 was compiled. It is estimated that the Chinese synthetic resin emitted 23.96 Gg of VOCs in 2018. In this study, a product-based VOC source profile and emission inventory of the synthetic resin industry were established for the first time. Finally, combined with product types, processes, and processing equipment, feasible recommendations for reducing VOC emissions in the synthetic resin industry are proposed.

Analysis of the photochemical characteristics and sensitivity of ozone pollution in Xi’an

<p indent="0mm">The Fenwei Plain is an important agricultural production base and transportation hub in China with a residential population of over 55 million. With rapid industrialization and urbanization, this area has experienced serious photochemical pollution in the summer season. In this study, a field campaign was conducted in summer 2019 with measurements of ozone and photochemical precursors in Xi’an, a megacity in the Fenwei Plain. An explicit case was performed to explore O₃-NO_{<italic>x</italic>}-VOCs sensitivity and the local ozone budget using an observation-based model. Results revealed that precursors significantly promoted ozone production. The positive matrix factorization (PMF) model combined with the relative incremental reactivity (RIR) method was applied to identify the key VOC sources that affect ozone formation. An ozone pollution episode lasting <sc>11 d</sc> was selected and divided into three periods: Initial (July 8–11), polluted (July 12–15), and sweep (July 16–18) periods. It was found that strong solar radiation intensity, high temperature, low relative humidity, and low pressure prevailed during the polluted period, which favored ozone production. The maximum O₃ concentrations at the upwind CB site and urban DHS site were found to be 125 ppbv during the polluted period, whereas at the QL site, the maximum O₃ concentrations were much lower (98 ppbv). The maximum local ozone production rate <sc>(<italic>P</italic>(O_{<italic>x</italic>}))</sc> of the three sites during the polluted period was significantly higher than that in the initial and sweep periods. The <italic>P</italic>(O_{<italic>x</italic>}) was 17.7, 33.9, and 14.9 ppbv/h for the CB, DHS, and QL sites, respectively. The <italic>in situ</italic> O_{<italic>x</italic>}<italic> </italic>budget demonstrated that ozone pollution at the CB and DHS sites was mainly attributed to local photochemical production, whereas the regional transportation effect could not be ignored at the QL site. The O₃-NO_{<italic>x</italic>}-VOCs sensitivity analysis revealed that ozone production regimes were transitional, VOCs-limited, and NO_{<italic>x</italic>}-limited at the CB, DHS, and QL sites, respectively, and the regimes stayed unchanged for all the three sites during the entire ozone pollution episode. However, during the initial period at the DHS site, NO_{<italic>x</italic>} reduction led to a negative effect on ozone control. The ozone formation regime obtained from the emission rate box model was consistent with that of the RIR method. From the proportion of grouped VOCs, the alkane and OVOCs were the dominant species in the mixing ratio (78%–86%), whereas the contribution of aromatics to OFP (16%–24%) was second only to that of OVOCs. Finally, by combining the PMF model and RIR method, ozone production was found to be more sensitive to precursors reduction in the initial period than in the polluted period at the three sites, which indicated that pollutants control measures for ozone pollution mitigation will be more effective in the initial than in polluted periods. Therefore, during an ozone pollution episode, the time to take action for ozone pollution control is not during the polluted period when the ozone concentrations peak, but should be brought forward, when controlling the concentration of precursors has the highest efficiency in reducing ozone production.

NOx emission and energy conversion efficiency studies on ammonia-powered micro-combustor with ring-shaped ribs in fuel-rich combustion

Ammonia, as an alternative option of conventional hydrocarbon energy source, can achieve carbon free emission. However, the production of NOx is responsible for photochemical pollution and human health. In order to reduce NOx emission at the premise of guaranteeing energy conversion efficiency, three micro-combustors with different ring-shaped ribs in a cross-sectional view of 1) rectangular, 2) Ո-shaped and 3) Ս-shaped are proposed and fuelled by NH3. Extensive numerical comparisons are made with 3D models built by structured mesh. The effects of 1) inlet velocity, 2) equivalence ratio, 3) rib shape and 4) key parameters of these ribs are evaluated. In addition, the normalized sensitivity of NO formation and related reaction pathways are analyzed. It is found that the double-rib structure is more conducive to achieving better thermal performance. Besides, HNO is found to contribute the most of NO, while NH and NH2 are beneficial to the consumption of NO. When the dimensionless distance x between the rib and the combustor inlet is 3/9, comparing the condition of Φ being 1.1 and 1.0, NO emission is reduced by 47.0%, while the radiation efficiency is only decreased by 1.76%. This work has proven that a high energy conversion efficiency and a low NO emission can be achieved at the same time by properly optimizing the internal structure of the combustor in fuel-rich condition.

Concurrent heatwaves and extreme Ozone (O3) episodes: combined atmospheric patterns and impact on human health

BACKGROUND AND AIM: In the context of the global environmental change, more recurrent and/or intense heat waves and extreme ozone episodes are likely to occur during the next decades. The main purpose of this research is to assess how extreme temperature may trigger the appearance of high ozone levels and how the concurrence of these two extremes could be linked to the synoptic general circulation and impacts human health and wellbeing. METHODS: We have used surface maximum temperature and ozone observations during extended summers (April–September) in two cities from Morocco: Casablanca and Marrakech, between 2010 and 2019. A percentile-thresholding method was applied to identify heatwaves and ozone episodes and a statistical approach was used to assess the connection between the observed parameters and climate indexes (North Atlantic Oscillation (NAO), Mediterranean Oscillation (MO), and Saharan Oscillation (SaOI)). The weather types behind the concurrence of extremes were identified and Heat and Air Quality Indexes were assessed to determine their combined health risks. RESULTS:Our findings show that the concurrence of heatwaves and ozone episodes depends both on the specific city and the large-scale atmospheric circulation. The likely synoptic pattern behind the concurrence of these events occurs when the country is under the combined influence of an anticyclonic area in the north and the Saharan trough extending the depression centered in the south. This pattern generates a warm southern flow and may foster photochemical pollution and then impact human health. CONCLUSIONS:Our study is an attempt to assess extreme temperature and ozone episodes combined features inducing large-scale atmospheric circulation and health effects. It explores the hypothesis that particular weather patterns increase the vulnerability of individuals especially those sensitive to air pollution. This work is a first step to the establishment of an alert system and will help to provide recommendations for coping with concurrent heatwaves and air pollution episodes. KEYWORDS: Ozone, Temperature extremes, North Atlantic Oscillation, Mediterranean Oscillation, Saharan Oscillation

Spatiotemporal estimation of hourly 2-km ground-level ozone over China based on Himawari-8 using a self-adaptive geospatially local model

Ground-level ozone (O3) is a primary air pollutant, which can greatly harm human health and ecosystems. At present, data fusion frameworks only provided ground-level O3 concentrations at coarse spatial (e.g., 10 km) or temporal (e.g., daily) resolutions. As photochemical pollution continues increasing over China in the last few years, a high-spatial–temporal-resolution product is required to enhance the comprehension of ground-level O3 formation mechanisms. To address this issue, our study creatively explores a brand-new framework for estimating hourly 2-km ground-level O3 concentrations across China (except Xinjiang and Tibet) using the brightness temperature at multiple thermal infrared bands. Considering the spatial heterogeneity of ground-level O3, a novel Self-adaptive Geospatially Local scheme based on Categorical boosting (SGLboost) is developed to train the estimation models. Validation results show that SGLboost performs well in the study area, with the R2s/RMSEs of 0.85/19.041 μg/m3 and 0.72/25.112 μg/m3 for the space-based cross-validation (CV) (2017–2019) and historical space-based CV (2019), respectively. Meanwhile, SGLboost achieves distinctly better metrics than those of some widely used machine learning methods, such as eXtreme Gradient boosting and Random Forest. Compared to recent related works over China, the performance of SGLboost is also more desired. Regarding the spatial distribution, the estimated results present continuous spatial patterns without a significantly partitioned boundary effect. In addition, accurate hourly and seasonal variations of ground-level O3 concentrations can be observed in the estimated results over the study area. It is believed that the hourly 2-km results estimated by SGLboost will help further understand the formation mechanisms of ground-level O3 in China.

Estimation of atmospheric emissions from maritime activity in the Veracruz port, Mexico

ABSTRACT The port of Veracruz is one of the most important ports in Mexico and is currently in the process of a major expansion. The new port area, “Bahía Norte”, will be three times larger than the current port, “Bahía Sur”. Atmospheric emissions from the Veracruz port system, specifically from the engines of ships undergoing maneuvering and hotelling operations, were determined on a daily basis from 2018 to 2019 for sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), non-methane volatile organic compounds (NMVOC), particles (PM), particles smaller than 10 micrometers (PM10), particles smaller than 2.5 micrometers (PM2.5) and carbon dioxide (CO2). A bottom-up method was used to estimate the atmospheric emissions, based on official data from the European Environment Agency. The method utilizes on technical information on the type of ship, the power of the main engine (ME) and auxiliary engine (AE), load factor, specific fuel consumption, and spent time in the maneuvering and hotelling phases. The highest atmospheric emissions occurred in the hotelling phase. For the “Bahía Sur” emissions (Mg/year) were 328.6, 993.3, 122.3, 30.6, 22.9, 19.1, 18.5 and 52,723.4 for SO2, NOx, CO, NMVOC, PM, PM10, PM2.5 and CO2, respectively. For “Bahía Norte” the corresponding emissions were 43.9, 132.7, 16.3, 4.1, 3.1, 2.5, 2.3 and 7,040.9, also in the hotelling phase. The average combined of the atmospheric emissions from maneuvering and hotelling phases (Mg/year) were 1.18, 3.49, 0.44, 0.13, 0.09, 0.08, 0.07, and 189.23 for SO2, NOx, CO, NMVOC, PM, PM10, PM2.5 and CO2, respectively. During the 2018–2019 period of the study the “Bahía Sur” contributed ~87% and the “Bahía Norte” ~13% of the atmospheric emissions. Implications: The atmospheric emissions reported in this study can be used for the simulation of air quality considering environmental or photochemical pollution models, since atmospheric emissions are reported on a daily basis and compliance with air quality can be monitored considering the reference-concentration established by the Official Mexican Standards in order to propose prevention, minimization and control measures. In addition, the analysis of the information that was carried out in this study may be applied to other port systems in Mexico that are located on the Pacific coast and the Gulf-Caribbean.

Verrucomicrobial methanotrophs grow on diverse C3 compounds and use a homolog of particulate methane monooxygenase to oxidize acetone

Short-chain alkanes (SCA; C2-C4) emitted from geological sources contribute to photochemical pollution and ozone production in the atmosphere. Microorganisms that oxidize SCA and thereby mitigate their release from geothermal environments have rarely been studied. In this study, propane-oxidizing cultures could not be grown from acidic geothermal samples by enrichment on propane alone, but instead required methane addition, indicating that propane was co-oxidized by methanotrophs. “Methylacidiphilum” isolates from these enrichments did not grow on propane as a sole energy source but unexpectedly did grow on C3 compounds such as 2-propanol, acetone, and acetol. A gene cluster encoding the pathway of 2-propanol oxidation to pyruvate via acetol was upregulated during growth on 2-propanol. Surprisingly, this cluster included one of three genomic operons (pmoCAB3) encoding particulate methane monooxygenase (PMO), and several physiological tests indicated that the encoded PMO3 enzyme mediates the oxidation of acetone to acetol. Acetone-grown resting cells oxidized acetone and butanone but not methane or propane, implicating a strict substrate specificity of PMO3 to ketones instead of alkanes. Another PMO-encoding operon, pmoCAB2, was induced only in methane-grown cells, and the encoded PMO2 could be responsible for co-metabolic oxidation of propane to 2-propanol. In nature, propane probably serves primarily as a supplemental growth substrate for these bacteria when growing on methane.

Spatial distribution and source apportionment of peroxyacetyl nitrate (PAN) in a coastal region in southern China

Peroxyacetyl nitrate (PAN) is a typical secondary product of photochemical pollution and has attracted considerable attention because of its important role in atmospheric photochemical reactions and biotoxicity. Although existing studies have determined the dominant species of volatile organic compounds (VOCs) that can generate PAN, there are few studies including quantitative source analysis of PAN and precursor VOCs. In this study, PAN and precursor VOCs were measured at three sites in Shenzhen, a megacity on the southeast coast of China. The mean daytime (8:00–18:00) concentrations of PAN at the three sites during the sampling period were 0.70 ppbv (YMK, rural), 1.31 ppbv (DXC, residential) and 1.52 ppbv (FC, industrial), which had a different spatial distribution pattern compared to those of O3 and better reflect the degree of local anthropogenic emissions. Based on the steady-state model, the calculated PA radical formation rates cumulative contribution by six major carbonyl compounds at YMK and DXC were 4.4 × 106 molecules*cm−3*s−1 and 6.8 × 106 molecules*cm−3*s−1, respectively, and acetaldehyde contributed the highest percentage (44.4–54.3%) followed by acetone (15.1–25.6%), diacetyl (10.0–10.9%) and methylglyoxal (12.2–13.4%). Using an improved photochemical age-based parameterization method to analyze the daytime sources of PA radicals, the results show that biogenic sources were the most important source of PA radicals, accounting for 27.1% and 29.8% at YMK and DXC, respectively, and the contribution of anthropogenic secondary sources at DXC (25.8%) was significantly higher than at YMK (17.0%). The contribution of biomass burning at YMK reached 24.2%, emphasizing a critical role of biomass burning in atmospheric photochemical pollution in rural areas.

Biomass-burning emissions could significantly enhance the atmospheric oxidizing capacity in continental air pollution

Volatile organic compounds (VOCs) are important precursors of photochemical pollution. However, a substantial fraction of VOCs, namely, oxygenated VOCs (OVOCs), have not been sufficiently characterized to evaluate their sources in air pollution in China. In this study, a total of 119 VOCs, including 60 OVOCs in particular, were monitored to provide a more comprehensive picture based on different online measurement techniques, proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) and online gas chromatography/mass spectrometry (GC/MS), at a receptor site in southeastern China during a photochemically active period. Positive matrix factorization (PMF) and photochemical age-based parameterization were combined to identify and quantify different sources of major VOCs during daytime hours, with the advantage of including VOC decay processes. The results revealed the unexpected role of biomass burning (21%) in terms of ozone (O3) formation potential (OFP) when including the contributions of OVOCs and large contributions (30–32%) of biomass burning to aldehydes, as more OVOCs were measured in this study. We argue that biomass burning could significantly enhance the continental atmospheric oxidizing capacity, in addition to the well-recognized contributions of primary pollutants, which should be seriously considered in photochemical models and air pollution control strategies.

Atmospheric formaldehyde, glyoxal and their relations to ozone pollution under low- and high-NOx regimes in summertime Shanghai, China

With the increasing concerns on summertime atmospheric photochemical pollution, the diagnosis and prevention of ozone pollution have been paid close attention. Both formaldehyde (HCHO) and glyoxal (CHOCHO) are ubiquitous oxidation intermediates of volatile organic compounds (VOCs). The ratio of glyoxal to formaldehyde (RGF) is used as a metric for VOCs emission sources. In this study, the mixing ratios of HCHO and CHOCHO have been measured by the active differential optical absorption spectroscopy (DOAS) method in the urban area of Shanghai during summertime in 2018, as well as other trace gases. The average levels of HCHO and CHOCHO are 3.31 ± 1.43 ppbv and 0.164 ± 0.073 ppbv, respectively. The similar diurnal patterns and high correlation between HCHO, CHOCHO and ozone levels implied that daytime photochemical processes are the dominant formation pathway for these trace gases. We find that with increased NOx levels, HCHO shows higher ozone formation potential relative to glyoxal. The RGF ratio increases with temperature and decreases with NO2 levels. By investigating the coupling of typical VOCs species such as acetylene, toluene and isoprene with HCHO and CHOCHO, RGF is found to be strongly impacted by the ambient VOCs profiles, suggesting that RGF should be used with caution when linking it to a given VOC precursor source. Finally, the RGF variations with ozone pollution episodes and weather processes are also discussed.

Characteristics of peroxyacetyl nitrate (PAN) in the high-elevation background atmosphere of South-Central China: Implications for regional photochemical pollution

Over the past decade, secondary pollution characterized by active photochemistry has sharply increased and become the major contributor to air pollution in China, despite the great reduction of primary emissions. Peroxyacetyl nitrate (PAN) is an excellent indicator of photochemical pollution. Observation of PAN in high-elevation atmosphere offers a rare opportunity to understand the degree of photochemical activity in a wide region. In this study, we conducted a field campaign at a high-altitude (1690 m a.s.l.) national background site on the border of South and Central China during photochemically polluted seasons (Sep–Nov 2018). PAN showed significantly higher concentrations than other high-elevation sites worldwide, with hourly mean and maximum values of 0.52 ± 0.45 ppbv and 2.33 ppbv, respectively. Episodes with much higher PAN levels (1.03 ± 0.40 ppbv) than backgrounds (0.24 ± 0.13 ppbv) were captured. Nighttime peaks suggested mixed impacts of regional transport and VOCs oxidation by nitrate radical and O3. High background O3 (44.30 ± 0.44 ppbv) was derived from the significant positive correlation of O3 with PAN. Contributions of downward transported PAN from upper troposphere were found to be non-negligible. Further analyses revealed that regional transport from Central China contributed substantially to the high levels of PAN, with only a minor contribution coming from the south where the Pearl River Delta (PRD) metropolitan region is located. Our results might indicate the great achievement in air quality improvement in the PRD, and also suggest that a strategic shift of control focus from metropolitan to developing areas is urgently needed for mitigating the severe photochemical pollution in China.

Ambient volatile organic compounds at a receptor site in the Pearl River Delta region: Variations, source apportionment and effects on ozone formation

We present the continuously measurements of volatile organic compounds (VOCs) at a receptor site (Wan Qing Sha, WQS) in the Pearl River Delta (PRD) region from September to November of 2017. The average mixing ratios of total VOCs (TVOCs) was 36.3 ± 27.9 ppbv with the dominant contribution from alkanes (55.5%), followed by aromatics (33.3%). The diurnal variation of TVOCs showed a strong photochemical consumption during daytime, resulting in the formation of ozone (O3). Five VOC sources were resolved by the positive matrix factorization (PMF) model, including solvent usage (28.6%), liquid petroleum gas (LPG) usage (24.4%), vehicle exhaust (21.0%), industrial emissions (13.2%) and gasoline evaporation (12.9%). The regional transport air masses from the upwind cities of south China can result in the elevated concentrations of TVOCs. Low ratios of TVOCs/NOx (1.53 ± 0.88) suggested that the O3 formation regime at WQS site was VOC-limited, which also confirmed by a photochemical box model with the master chemical mechanism (PBM-MCM). Furthermore, the observation on high-O3 episode days revealed that frequent O3 outbreaks at WQS were mainly caused by the regional transport of anthropogenic VOCs especially for aromatics and the subsequent photochemical reactions. This study provides valuable information for policymakers to propose the effective control strategies on photochemical pollution in a regional perspective.