Source Apportionment of Volatile Organic Compounds (VOCs) by Positive Matrix Factorization (PMF) supported by Model Simulation and Source Markers - Using Petrochemical Emissions as a Showcase

Volatile Organic Compounds on Rhodes Island, Greece: Implications for Outdoor and Indoor Human Exposure

Volatile organic compounds (VOC) are considered a class of pollutants with a significant presence in indoor and outdoor air and serious health effects. The aim of this study was to measure and evaluate the levels of outdoor and indoor VOCs at selected sites on Rhodes Island, Greece, during the cold and warm periods of 2023. Spatial and seasonal variations were evaluated; moreover, cancer and non-cancer inhalation risks were assessed. For this purpose, simultaneous indoor-outdoor air sampling was carried out on the island of Rhodes. VOCs were determined by Thermal Desorption-Gas Chromatography/Mass Spectroscopy (TD-GC/MS). Fifty-six VOCs with frequencies ≥ 50% were further considered. VOC concentrations (∑56VOCs) at all sites were found to be higher in the warm period. In the warm and cold sampling periods, the highest concentrations were found at the port of Rhodes City, while total VOC concentrations were dominated by alkanes. The Positive Matrix Factorization (PMF) model was applied to identify the VOC emission sources. Non-cancer and cancer risks for adults were within the safe levels.

Photochemical oxidation of VOCs and their source impact assessment on ozone under de-weather conditions in Western Taiwan

The application of statistical models has excellent potential to provide crucial information for mitigating the challenging issue of ozone (O3) pollution by capturing its associations with explanatory variables, including reactive precursors (VOCs and NOX) and meteorology. Considering the large contribution of O3 in degrading the air quality of western Taiwan, three-year (2019-2021) hourly concentration data of VOC, NOX and O3 from 4 monitoring stations of western Taiwan: Tucheng (TC), Zhongming (ZM), Taixi (TX) and Xiaogang (XG), was evaluated to identify the effect of anthropogenic emissions on O3 formation. Owing to the high-ambient reactivity of VOCs on the underestimation of sources, photochemical oxidation was assessed to calculate the consumed VOC (VOCcons) which was followed by the source identification of their initial concentrations. VOCcons was observed to be highest in the summer season (16.7 and 22.7 ppbC) at north (TC and ZM) and in the autumn season (17.8 and 11.4 ppbC) in southward-located stations (TX and XG, respectively). Results showed that VOCs from solvents (25-27%) were the major source at northward stations whereas VOCs-industrial emissions (30%) dominated in south. Furthermore, machine learning (ML): eXtreme Gradient Boost (XGBoost) model based de-weather analysis identified that meteorological factors favor to reduce ambient O3 levels at TC, ZM and XG stations (-67%, -47% and -21%, respectively) but they have a major role in accumulating the O3 (+38%) at the TX station which is primarily transported from the upwind region of south-central Taiwan. Crucial insights using ML outputs showed that the finding of the study can be utilized for region-specific data-driven control of emission from VOCs-sources and prioritized to limit the O3-pollution at the study location-ns as well as their accumulation in distant regions.

Source-apportionment and spatial distribution analysis of VOCs and their role in ozone formation using machine learning in central-west Taiwan

This study assessed the machine learning based sensitivity analysis coupled with source-apportionment of volatile organic carbons (VOCs) to look into new insights of O₃ pollution in Yunlin County located in central-west region of Taiwan. One-year (Jan 1 to Dec 31, 2021) hourly mass concentrations data of 54 VOCs, NO_X, and O₃ from 10 photochemical assessment monitoring stations (PAMs) in and around the Yunlin County were analyzed. The novelty of the study lies in the utilization of artificial neural network (ANN) to evaluate the contribution of VOCs sources in O₃ pollution in the region. Firstly, the station specific source-apportionment of VOCs were carried out using positive matrix factorization (PMF)-resolving six sources viz. AAM: aged air mass, CM: chemical manufacturing, IC: Industrial combustion, PP: petrochemical plants, SU: solvent use and VE: vehicular emissions. AAM, SU, and VE constituted cumulatively more than 65% of the total emission of VOCs across all 10 PAMs. Diurnal and spatial variability of source-segregated VOCs showed large variations across 10 PAMs, suggesting for distinctly different impact of contributing sources, photo-chemical reactivity, and/or dispersion due to land-sea breezes at the monitoring stations. Secondly, to understand the contribution of controllable factors governing the O₃ pollution, the output of VOCs source-contributions from PMF model along with mass concentrations of NO_X were standardized and first time used as input variables to ANN, a supervised machine learning algorithm. ANN analysis revealed following order of sensitivity in factors governing the O₃ pollution: VOCs from IC > AAM > VE ≈ CM ≈ SU > PP ≈ NO_X. The results indicated that VOCs associated with IC (VOCs-IC) being the most sensitive factor which need to be regulated more efficiently to quickly mitigate the O₃ pollution across the Yunlin County.

Impact of shipping emissions regulation on urban aerosol composition changes revealed by receptor and numerical modelling

Various shipping emissions controls have recently been implemented at both local and national scales. However, it is difficult to track the effect of these on PM2.5 levels, owing to the non-linear relationship that exists between changes in precursor emissions and PM components. Positive Matrix Factorisation (PMF) identifies that a switch to cleaner fuels since January 2020 results in considerable reductions in shipping-source-related PM2.5, especially sulphate aerosols and metals (V and Ni), not only at a port site but also at an urban background site. CMAQ sensitivity analysis reveals that the reduction of secondary inorganic aerosols (SIA) further extends to inland areas downwind from ports. In addition, mitigation of secondary organic aerosols (SOA) in coastal urban areas can be anticipated either from the results of receptor modelling or from CMAQ simulations. The results in this study show the possibility of obtaining human health benefits in coastal cities through shipping emission controls.

Differences between VOCs and NOx transport contributions, their impacts on O3, and implications for O3 pollution mitigation based on CMAQ simulation over the Yangtze River Delta, China

The relationship between O₃ and its precursors during urban polluted episodes remains unclear. In this study, the simultaneous source apportionment of VOCs, NO_x, and O₃ over the Yangtze River Delta (YRD) region during the O₃ polluted episode on July 24-30, 2018, was performed based on the Integrated Source Apportionment Method (ISAM) embedded in the Community Multiscale Air Quality Modeling System (CMAQ). The results of the ISAM were compared with those of the Brute Force Method (BFM) and Positive Matrix Factorization (PMF). Furthermore, the differences between the transport contributions of VOCs and NO_x, and their impacts on O₃ were analyzed. The results indicate that observations of VOCs species can be well captured by simulated VOCs, and the ISAM has a significant advantage in the source apportionment of VOCs, especially for sources emitting highly reactive species. In the clean and polluted periods, the local contribution percentages of VOCs in urban sites ranged from 60 % to 77 %, much higher than those of NO_x (31 %-43 %) and O₃ (16 %-33 %). NO_x and O₃ have strong transport abilities with high and close contribution percentages, which are highly correlated, mainly because oxygen atoms produced by the photolysis of NO₂ in the aged air mass combined rapidly with O₂ to form O₃ during transport. The VOCs chemical loss caused by the oxidation of OH radicals during transport makes the ability of VOCs for long-distance transport much weaker than that of NO_x. Furthermore, owing to the sufficient aging of VOCs, those contributed by long-distance transport have little effect on O₃. To a certain extent, controlling one's NO_x emissions can help other cities more, while controlling one's VOCs emissions can help itself more. Therefore, it is recommended to attach enough importance to joint prevention and control of NO_x among cities and even long-distance areas to alleviate regional O₃ pollution.

Spatial distribution, source apportionment and health risk assessment of inorganic pollutants of surface water and groundwater in the southern margin of Junggar Basin, Xinjiang, China

Surface water (SW) and groundwater (GW) are crucial water supply sources in the southern margin of Junggar Basin in Xinjiang. The sources of toxic components in SW and GW and their negative effects on human health are of great concern. A total of 40 SW and 596 GW samples were collected at the oasis belt to analyze distribution, sources and potential health risks of inorganic pollutants in SW and GW. Results revealed that SW quality was severely affected by Hg, 30.0% of which had Hg concentration greater than the national drinking water standard. High Hg SW was mainly distributed near Manas County and Urumqi City. GW quality was mostly affected by SO₄^2-, 24.7% of which had SO₄^2- concentration greater than the national drinking water standard. High SO₄^2- GW primarily occurred in the northwest and middle of the study area. Source apportionment of inorganic pollutants identified geological background, municipal wastewater disposal, water-rock interaction, geological environment, geological structure and industrial emission were the prominent potential sources of inorganic pollution in SW, with contribution rates of 1.2%, 10.0%, 43.6%, 35.1%, 6.3% and 3.8%, respectively. Five potential pollution sources in GW (including geological background, municipal wastewater disposal, water-rock interaction, geological environment and aquifer burial depth) were identified, with contribution rates of 0.7%, 9.6%, 77.6%, 11.1% and 1.0%, respectively. Probabilistic health risk assessment showed that Cl^- and As in SW and GW were the main inorganic pollutants threatening human health. Non-carcinogenic risks for adults and children were negligible, while carcinogenic risks cannot be negligible. Furthermore, the contribution of potential pollution sources to health risks was quantified using positive matrix factorization coupling with health risk assessment model. Based on which, we offered the suggestion that water quality improvement in contaminated areas should be implemented in combination with pollution monitoring systems.

Long-term trend in surface ozone in Houston-Galveston-Brazoria: Sectoral contributions based on changes in volatile organic compounds

This study investigated the long-term variations in ambient levels of surface ozone, volatile organic compounds (VOCs), and nitrogen oxides (NO_x) within the Houston-Galveston-Brazoria (HGB) region. Analysis of ozone levels revealed an overall reduction in the maximum daily 8-h average ozone (MDA8 O₃) from 2000 to 2019 (April-October) with an average rate of ∼ -0.48 ppb/yr across HGB. With a few exceptions, the MDA8 O₃ reduction rates were more pronounced for the monitoring sites closer to the Houston Ship Channel (HSC). Meanwhile, ambient levels of NO_x and most VOC species (across the three representative sites as Houston Bayland Park, Haden Road, and Lynchburg Ferry) decreased significantly within the same investigation period, reflecting the impact of emission reductions. The positive matrix factorization (PMF) model applied to the mentioned sites identified regional background ozone, petrochemical emissions, engine combustion, natural gas/fuel evaporation, and solvent/painting/rubber industries as the major sources of MDA8 O₃. The regional background ozone was the predominant source, accounting for 59-70% of MDA8 O₃ across the three sites. Regarding the local anthropogenic emissions, natural gas/fuel evaporation was the largest contributor (19.5 ± 6.1%) to MDA8 O₃ at Houston Bayland Park, whereas petrochemical facilities (10.9 ± 4.9%) and solvent/painting/rubber industries (18.1 ± 9.5%) were the largest factor at Haden Road and Lynchburg Ferry, respectively. Notable reductions were found in the contributions of petrochemical emissions, engine combustion, and natural gas/fuel evaporation to MDA8 O₃ within 2000-2019, but an increasing trend was revealed in the role of solvent/painting/rubber industries on MDA8 O₃ most probably due to the enhanced demand for their products. Results of this study corroborated the success of emission control policies in limiting ozone precursors and provided useful details for prioritizing emission reduction policies to further reduce ozone pollution in the HGB.

Global review of source apportionment of volatile organic compounds based on highly time-resolved data from 2015 to 2021

Highly time-resolved data for volatile organic compounds (VOCs) can now be monitored. Source analyses of such high time-resolved concentrations provides key information for controlling VOC emissions. This work reviewed the literature on VOCs source analyses published from 2015 to 2021, and assesses the state-of-the-art and the existing issues with these studies. Gas chromatography system and direct-inlet mass spectrometry are the main monitoring tools. Quality control (QC) of the monitoring process is critical prior to analysis. QC includes inspection and replacement of instrument consumables, calibration curve corrections, and reviewing the data. Approximately 54% published papers lacked details on the quantitative evaluation of the effectiveness of QC measures. Among the reviewed works, the number of monitored species varied from 5 to 119, and fraction of papers with more than 90 monitored species increased yearly. US EPA PMF v5.0 was the most commonly used (∼86%) for VOC source analyses. However, conventional source apportionment directly uses the measured VOCs and may be problematic given the impacts of dispersion and photochemical losses, uncertainty setting of VOCs data, factor resolution, and factor identification. Excluding species with high-reactivity or estimation of corrected concentrations were often applied to reduce the influence of photochemical reactions on the results. However, most reports did not specify the selection criteria or the specific error fraction values in the uncertainty estimation. Model diagnostic indexes were used in 99% of the reports for PMF analysis to determine the factor resolution. Due to lack of known local source profiles, factor identification was mainly achieved using marker species and characteristic species ratios. However, multiple sources had high-collinearity and the same species were often used to identify different sources. Vehicle emissions and fuel evaporation were the primary contributors to VOCs around the world. Contribution of coal combustion in China was substantially higher than in other countries.

Sources of ambient non-methane hydrocarbon compounds and their impacts on O3 formation during autumn, Beijing

The field observation of 54 non-methane hydrocarbon compounds (NMHCs) was conducted from September 1 to October 20 in 2020 during autumn in Haidian District, Beijing. The mean concentration of total NMHCs was 29.81 ± 11.39 ppbv during this period, and alkanes were the major components. There were typical festival effects of NMHCs with lower concentration during the National Day. Alkenes and aromatics were the dominant groups in ozone formation potential (OFP) and OH radical loss rate (L_OH). The positive matrix factorization (PMF) running results revealed that vehicular exhaust became the biggest source in urban areas, followed by liquefied petroleum gas (LPG) usage, solvent usage, and fuel evaporation. The box model coupled with master chemical mechanism (MCM) was applied to study the impacts of different NMHCs sources on ozone (O₃) formation in an O₃ episode. The simulation results indicated that reducing NMHCs concentration could effectively suppress O₃ formation. Moreover, reducing traffic-related emissions of NMHCs was an effective way to control O₃ pollution at an urban site in Beijing.

Tracing riverine sulfate source in an agricultural watershed: Constraints from stable isotopes

The sulfate pollution in water environment gains more and more concerns in recent years. The discharge of domestic, municipal, and industrial wastewaters increases the riverine sulfate concentrations, which may cause local health and ecological problems. To better understand the sources of sulfate, this study collected water samples in a typical agricultural watershed in East Thailand. The source apportionment of sulfide was conducted by using stable isotopes and receptor models. The δ³⁴S_SO4 value of river water varied from 1.2‰ to 16.4‰, with a median value of 8.9‰. The hydrochemical data indicated that the chemical compositions of Mun river water were affected by the anthropogenic inputs and natural processes such as halite dissolution, carbonate, and silicate weathering. The positive matrix factorization (PMF) model was not suitable to trace source of riverine sulfate, because the meaning of the extracted factors seems to be vague. Based on the elemental ratio and isotopic composition, the inverse model yielded the relative contribution of sulfide oxidation (approximately 46.5%), anthropogenic input (approximately 41.5%), and gypsum dissolution (approximately 12%) to sulfate in Mun river water. This study indicates that the selection of models for source apportionment should be careful. The large contribution of anthropogenic inputs calls an urgent concern of the Thai government to establish effective management strategies in the Mun River basin.

Multi-scale volatile organic compound (VOC) source apportionment in Tianjin, China, using a receptor model coupled with 1-hr resolution data

The multi-scale chemical characteristics and source apportionment of volatile organic compounds (VOCs) were analysed in Tianjin, China, using 1-hr resolution VOC-species data between November 1, 2018 and March 15, 2019. The average total VOC (TVOC) concentration was 30.6 ppbv during the heating season. The alkanes accounted for highest proportion of the TVOC, while the alkenes were the predominant species forming ozone, especially ethylene. Compared to the clean period, the concentration of acetylene during the haze events showed highest increase rate, followed by the ethane; and the concentrations and proportions of alkanes and alkenes were highest during the growth stage (GS) of haze events. The multi-scale apportionment results suggested petrochemical industry and solvent usage (PI/SU, 31.2%), vehicle emissions and liquefied petroleum gas (VE/LPG, 20.5%), and combustion emissions (CE, 19.1%) were the main VOC sources during the heating season. Compared to the clean period, the contributions of PI/SU, VE/LPG, CE, and refinery emissions notably increased during the haze events, while that of gasoline evaporation decreased. The contributions of PI/SU and RPI showed significantly increase during the GS of haze events, whereas most sources decreased during the dissipation stage of haze events. Diurnal-variations in source contributions during the haze events were clearer than the clean period, and the contributions of PI/SU, VE/LPG, and CE during the haze events were markedly higher at night. These findings provide valuable information to inform effective VOC control and prevention measures with specific relevance for the control of ozone pollution in Tianjin.