Volatile organic compounds (VOCs) are key precursors of secondary air pollutants such as ozone and secondary organic aerosols, which have widespread health and environmental impacts. In urban settings, the diversity of emission sources (both anthropogenic and biogenic) and the complexity of emission characteristics lead to substantial uncertainty in urban VOC emissions and their source attribution. Here, we present comprehensive eddy covariance measurements of VOC fluxes in a Chinese megacity. The measurements capture a wide range of VOC species spanning multiple functional groups and chemical reactivities, with low-carbon-number compounds contributing substantially to the total emission flux. Seasonally resolved observations reveal pronounced variability in VOC fluxes, driven by both environmental conditions (e.g., temperature) and changes in anthropogenic activities. Using these flux measurements, we compare positive matrix factorization (PMF) analyses based on flux and concentration datasets, demonstrating the advantages of flux-based source apportionment in resolving urban emission sources. Our findings highlight that flux measurements provide new insights into the sources and emission characteristics of urban atmospheric composition.

Mitigating ozone pollution remains a pressing challenge for megacities worldwide. Biogenic volatile organic compounds (VOCs) are proposed as pivotal precursors to urban ozone formation. However, direct observational evidence is scarce and the extent of this effect across cities is unclear. Here, by combining an unprecedented eddy covariance flux dataset from Beijing with modeling analysis, we find that urban vegetation contributes over half of organic reactivity essential for ozone production, predominantly driving ozone exceedances during hot periods. The unexpectedly high biogenic influences are attributed to the abundance of urban trees with strong emission potentials of VOCs within the city. Model extrapolations reveal that similar or even greater challenges exist for other cities across Asia and Oceania, where urban greening strategies often favor highly emitting trees. Our findings underscore the critical need for selective urban planning in greening programs to counteract a warming climate.

Volatile organic compounds (VOCs) are key precursors of secondary air pollutants such as ozone and PM2.5, which have significant health and environmental impacts. In urban areas, the complex emission patterns of VOCs─driven by diverse anthropogenic activities─present major challenges for accurate source characterization and quantification. In this study, we conducted eddy covariance flux measurements of VOC in a Chinese megacity to elucidate strengths and origins of VOC in urban region. The results revealed pronounced temporal and compositional differences between VOC fluxes and concentrations. By linking compositional differences between VOC fluxes (fresh emissions) and concentrations (chemically processed air), we provide direct evidence of the critical role of the chemical evolution in shaping atmospheric compositions of VOCs in urban air. We demonstrate the advantages of using flux data in source apportionment for emissions by comparing positive matrix factorization (PMF) analyses based on flux and concentration data sets. Flux-based source apportionment revealed that urban VOC tracers, such as toluene, are primarily associated with volatile chemical product (VCP) rather than traffic. The temperature-driven dynamics of biogenic and anthropogenic (average: 53% vs 47%) contributions to the total monoterpene fluxes are quantified, highlighting the shifting importance of anthropogenic and biogenic sources in urban environments. These findings underscore the value of integrating flux and concentration data sets to enhance understanding of urban VOC sources and their atmospheric evolution.

In urban atmosohere, volatile methyl siloxanes (VMSs) was treated as the important maker from pesonal care products (PCPs) of volatile chemical products (VCPs), yet the link between their realistic emission profiles and secondary organic aerosol (SOA) formation remains elusive. Moving beyond previous studies using pure standards, we systematically characterized the dynamic emissions and SOA evolution of four typical PCPs using an evaporation chamber coupled with an oxidation flow reactor and other online gas/aerosol mass spectrometry. VMS dominated the emissions (>95%) for most PCP products and exhibited prolonged evaporation time scales compared to coemitted solvent VOCs. Emission factors for VMS ranged from approximately 132 to 300 g kg-1. We quantified the PCP -derived SOA formation yield and saturated and unsaturated molecular composition for the first time and systematically discussed VMS oxidation pathways. Furthermore, molecular fingerprints revealed that while cross-reactions between different VMS precursors occurred readily, cross-reactions between VMS and other VOCs were kinetically inhibited due to mismatches in reactivity and evaporation timing. The evolution of SOA volatility was found to be highly pathway-dependent, with oligomerization driving a volatility reduction of 0.8 or up to several orders of magnitude. These findings provide a mechanistic basis for the atmospheric persistence of VMS and their potential for long-range transport to remote regions, with direct implications for improving source apportionment of VCPs and refining SOA formation in atmospheric models.

Tropospheric ozone (O3) is a major air pollutant that negatively affects human health and vegetation, and plays a central role in climate change and atmospheric chemistry. Current simulations of tropospheric O3 concentrations in climate and air-quality models are significantly limited by the inaccurate representation of O3 dry deposition rate-particularly in urban areas, where field measurements remain scarce. We hypothesize that O3 dry deposition in the urban environment is controlled by factors similar to those over vegetation, albeit via potentially different mechanisms. Accordingly, we performed O3 and reactive nitrogen oxide (NOx = [NO] + [NO2]) flux measurements using eddy covariance (EC) technique from a meteorological tower in an urban area of Beijing (IAP) during the spring and summer, complemented by EC flux measurements of volatile organic compounds (VOCs) using Vocus-PTR-TOF-MS. Our analyses indicated that the downward ozone velocity (Vdw,O3) at the IAP measurement site is controlled by both gas-phase chemical reactions and surface uptake, with comparable contributions. The reaction of O3 with NO emissions dominated the chemical contribution to Vdw,O3, with a significant contribution of NO near the ground, particularly in the morning. The effect of relative humidity (RH) on Vdw,O3, likely via surface wetness accumulation similar to its effect on non-stomatal O3 deposition over vegetation, showed a logarithmic and hyperbolic dependency of Vdw,O3 on RH, for 5 % < RH < 30 % and RH > 70 %, respectively. Both NO emissions from elevated sources and RH < 30 %-conditions associated with a higher frequency of positive O3 flux events-dramatically limited Vdw,O3.

The vertical distribution of reactive trace gases can greatly help understand the complex atmospheric evolution under the joint impacts of surface emission, chemical removal, and regional transport. Focusing on the core area of the North China Plain, aircraft-based observations were conducted in September 2017 and July 2019 to reveal the vertical distributions of volatile organic compounds (VOCs) measured by high-time resolution mass spectrometry. Generally decreasing trends of VOC concentrations with altitudes were captured, indicating strong surface source emissions and chemical removal within the planetary boundary layer (PBL). Ethanol exhibited the highest concentration within the PBL with an average of 46.7 ppbv and the largest ratio (16.5) between the average below and above the PBL heights. The vertical-averaged VOCs above Baoding were greater than those in Beijing by factors ranging from 1.2 to 3.5, suggesting richer precursors for secondary pollutant formation in Baoding. Increases of several VOC species, including styrene and acetonitrile, at high altitudes (>2500 m) were captured in Beijing. Correlation analysis further revealed the significant influences of industrial and biomass burning emissions. Our results highlight the critical role of both local emissions and regional transport in shaping the VOC vertical distributions, which may affect atmospheric organic chemistry across various atmospheric layers in the region.

Volatile organic compounds (VOCs) play crucial roles in regulating the formation of tropospheric ozone. However, limited knowledge on the interactions between vertical VOC variations and photochemical ozone formation in the planetary boundary layer (PBL) has hindered effective ozone control strategies, especially in large cities. In this study, we investigated the vertical changes in concentrations, compositions, and key driving factors of a large suite of VOCs using online gradient measurements taken from a 325 m tall tower in urban Beijing. The impacts of these vertical VOC variations on photochemical ozone formation were also analyzed using box model simulations. Our results indicate that VOCs exhibited distinct vertical variation patterns due to their differences in sources and chemical reactivities, along with the diurnal evolution of the PBL. During daytime, reactive VOCs (e.g., hydrocarbons) are rapidly oxidized as they mix upward, accompanied by the formation and accumulation of oxygenated VOCs (OVOCs) in the middle and upper layers. In addition, the photochemical formation of ozone responds positively to changes in both NOx and VOCs. As a result, the production rate of ozone declines with height due to the simultaneous decreases in concentrations of reactive VOCs and NOx but remains high in the middle and upper layers. The strong production of ozone aloft is primarily driven by the presence of high OVOC concentrations. Therefore, careful consideration should be given to the vertical variations in both photochemical ozone production rates and formation regimes in the whole PBL when developing regional ozone control strategies.

Recent observations have revealed unexpectedly high concentrations of monoterpenes (MT) in urban areas, frequently surpassing those in forested regions. These findings suggest significant anthropogenic contributions (MTANT), challenging the traditional view that MT emissions are predominantly natural (MTNAT) in current inventories. This oversight likely results in a substantial underestimation of MT's role in urban ozone (O3) production. Therefore, we developed a novel approach to generate a gridded emission inventory (EI) of MTANT, integrating flux measurements of MT and carbon monoxide (CO). Results show that MTANT emission rate in Beijing core areas exceeds MTNAT by a factor of 1.83, with household volatile chemical products (VCPs) contributing 56% of total MTANT emissions. Incorporating MTANT emissions into the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) model significantly improved the simulation of diurnal MT variations (correlation coefficient, r = 0.985) and reduced the normalized mean bias (NMB) in surface MT concentration predictions by 53%. Notably, the combined effects of anthropogenic and biogenic MT emissions increased summertime maximum daily 8-h average (MDA8) O3 levels by 12.8 ppb in Beijing core areas, with MT from household VCPs (MTVCP) accounting for 62% of the MTANT-driven O3 increase. This study provides a robust quantitative foundation for assessing the impact of anthropogenic MT emissions on urban air quality and highlights the urgent need for targeted regulatory measures to mitigate their growing contribution to O3 pollution.

Closure studies on the total OH reactivity (OHR) are essential for validating comprehensive measurements of reactive volatile organic compounds (VOCs) in various atmospheric environments. Nonetheless, discrepancies persist between measured OHR and calculated OHR in many field observations, primarily due to the presence of unknown VOCs and their reaction rate constants with OH radicals. This study optimized the method for calculating the rate constant (kOH) between isomers by categorization. The missing OH reactivity was reevaluated in an urban and a regional environment based on molecular formula. A notable missing OH reactivity was found during the rush hours in the morning and evening at the urban site, whereas the measured OHR agreed well with the calculated OHR at the regional site. Our analysis revealed that the oxidation products of VOCs significantly contributed to the OHR in highly-oxidizing atmospheres. After adding oxygenated VOCs, a negative correlation between missing OH reactivity and OH exposure was obtained. Finally, we determined the mean kOH values for unknown VOCs to be 9.0 x 10-11 cm3 molecule-1 s-1 at the urban site, respectively. These findings indicate that a comprehensive measurement of oxygenated VOCs can facilitate a comprehensive assessment of reactive VOCs under highly-oxidizing atmospheric conditions.

Oxygenated volatile organic compounds (OVOCs) play a crucial role in atmospheric chemistry, significantly influencing radical production and VOC degradation through photolysis. However, current research on OVOC photolysis is limited by insufficient species coverage in the mechanisms and incomplete understanding from a species-specific perspective. In this study, the photolysis frequencies of 109 OVOCs were compiled into a comprehensive photolysis dataset. Based on their molecular structures, a parameterization for the photolysis frequencies of carbon- and nitrogen-containing OVOCs was developed. By establishing a relationship between species structure and photolysis frequency, this approach avoids the limitation of insufficient quantum yield data, enabling the estimation of photolysis rate constants for compounds lacking experimental measurements. Photolysis frequencies for the dataset species were successfully reproduced with 21 reference values and 10 adjustment coefficients. Using an automated program based on this method, photolysis rate constants for 3039 OVOCs were predicted, and the Master Chemical Mechanism (MCM) v3.3.1 chemical mechanism was updated and expanded to include photolysis for 714 additional species. The introduction of the new photolysis mechanism has altered both the concentrations of photodegradable OVOCs and the relative proportions of their removal pathways. Non-HCHO OVOCs, particularly multifunctional species with carbonyl groups, contribute significantly to ROx radical production. At three different sites, non-HCHO OVOC photolysis accounts for 25 %–45 % of ROx production, surpassing HCHO photolysis. The importance of oxidation products from aromatics and alkenes is highlighted, offering new insights into OVOC photolysis from a species-specific perspective.

A limited understanding of urban methane (CH4) emissions in China challenges the evaluation of the coal-to-gas switch toward carbon neutrality by 2060. CH4 flux was measured in urban Beijing using the eddy covariance method during a summer campaign. With a mean of 152.6 +/- 107.9 nmol/m(2)/s, the CH4 flux was estimated to depend little on the intensity of human activities, with minimal influence from biogenic sources. Emission hotspots with large temporal variability were identified in the study fetch area, which increased the mean CH4 flux by 12.5%. Based on the lack of large, known biogenic sources in nonhotspot (background) areas, we attributed the CH4 flux in these areas (135.6 +/- 70.56 nmol/m(2)/s) mainly to natural gas. Thus, we estimate as an upper limit that natural gas contributes 88.9% to the total CH4 flux in urban Beijing. However, poor alignment between the dominant sources in the inventories and the characteristics of the measured CH4 flux were observed, suggesting substantial underestimation of emissions from natural gas sources in the inventories. A leakage ratio of 1.4% (0.7-2.1%) of consumed natural gas was determined in Beijing. Pinpointing emissions with more granular methods could improve our understanding of the urban CH4 source profile in Beijing.

Formic acid is the most abundant organic acid in the troposphere and has significant environmental and climatic impacts. Isocyanic acid poses severe threats to human health and could be formed through the degradation of formic acid. However, the lack of vertical observation information has strongly limited the understanding of their sources, particularly in urban regions with complex pollutant emissions. To address this issue, we assessed the impact of long tubes on the measurement uncertainties of formic and isocyanic acids and found that the tubing impact was negligible. Then, we conducted continuous (27 d) vertical gradient measurements (five heights between 5–320 m) of formic and isocyanic acids using long tubes based on a tall tower in Beijing, China, in the summer of 2021. Results show that the respective mean mixing ratios of formic and isocyanic acids were 1.3 ± 1.3 ppbv and 0.28 ± 0.16 ppbv at 5 m and were 2.1 ± 1.9 ppbv and 0.43 ± 0.21 ppbv at 320 m during the campaign. The mixing ratios of formic and isocyanic acids were substantially enhanced in the daytime and correlated with the diurnal change of ozone. Upon sunrise, the mixing ratios of formic and isocyanic acids at different heights simultaneously increased, even in the residual layer. In addition, positive vertical gradients were observed for formic and isocyanic acids throughout the day. The positive vertical gradients of formic and isocyanic acids in the daytime imply the enhancement of their secondary formation in urban regions aloft, predominantly due to the enhancements of oxygenated volatile organic compounds. Furthermore, the afternoon peaks and positive vertical gradients of formic and isocyanic acids in the nighttime also indicate their minor contributions from primary emissions from ground-level sources. The formation pathway of isocyanic acid through HCOOH–CH3NO–HNCO was enhanced with height but only accounted for a tiny fraction of its ambient abundance. The abundance and source contributions of formic and isocyanic acids in the atmospheric boundary layer may be highly underestimated when being derived from their ground-level measurements. With the aid of numerical modeling techniques, future studies could further identify key precursors that drive the rapid formation of formic and isocyanic acids and quantitatively assess the impacts of the enhanced formation of the two acids aloft on their budgets at ground level.

Volatile chemical products (VCPs) have become an important source of reactive organic gases (ROGs) in urban areas worldwide. Industrial activities can also utilize a large number of VCPs and emit many organic gases into the atmosphere. Due to multiple sampling and measurement challenges, only a subset of ROG species is usually measured for many industrial VCP sources. This study aims to investigate the emissions of ROGs from five industrial VCP sources in the Pearl River Delta (PRD) region of China, including the shoemaking, plastic surface coating, furniture coating, printing, and ship coating industries. A more comprehensive speciation of ROG emissions from these industrial VCP sources was developed by the combination of proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) and the gas chromatography–mass spectrometer/flame ionization detector (GC–MS/FID). Our study identified oxygenated ROG species (OVOCs) as representative ROGs emitted from these sources, which are highly related to specific chemicals used during industrial activities. Moreover, mass spectra similarity analysis revealed significant dissimilarities among the ROG emissions from industrial activities, indicating substantial variations between different industrial VCP sources. Except for the ship coating industry utilizing solvent-borne coatings, the proportions of OVOCs range from 67 % to 96 % in total ROG emissions and 72 % to 97 % in total OH reactivity (OHR) for different industrial sources, while the corresponding contributions of OVOCs in the ship coating industry are only 16 ± 3.5 % and 15 ± 3.6 %. The industrial VCP sources associated with solvent-borne coatings exhibited a higher ozone formation potential (OFP), reaching as high as 5.5 and 2.7 g O3 g−1 ROGs for the ship coating and furniture coating industries, primarily due to contributions from aromatics. We find that a few species can contribute the majority of the ROG emissions and also their OHR and OFP from various industrial VCP sources. Our results suggest that ROG treatment devices may have limited effectiveness for all ROGs, with treatment efficiencies ranging from −12 % to 68 %. Furthermore, we found that ambient measurements in industrial areas have been significantly impacted by industrial VCP sources, and ROG pairs (e.g., methyl ethyl ketone (MEK) / C8 aromatics ratio) can be utilized as reliable evidence by using high-time-resolution ROG measurements from PTR-ToF-MS. Our study demonstrated the importance of measuring a large number of ROGs using PTR-ToF-MS for characterizing ROG emissions from industrial VCP sources.

Higher alkanes are a major class of intermediate volatile organic compounds (IVOCs) emitted by vehicles, which have been considered as important precursors of secondary organic aerosol (SOA) in urban area. Dynamometer experiments were conducted to characterize emissions from gasoline and diesel vehicles in China. Three types of higher alkanes, namely acyclic, cyclic, and bicyclic alkanes, were explicitly quantified through the novel proton transfer reaction time-of-flight mass spectrometer with NO+ ionization (NO+ PTR-ToF-MS) with time response of 1second. We show that higher alkanes from gasoline vehicles are mainly associated with initial engine start period, whereas emissions of higher alkanes from diesel vehicles are persistent during driving. Therefore, total emissions from diesel vehicles are substantially larger than gasoline vehicles. Our results indicate that over half of organic emissions from diesel vehicles are higher alkanes (56.9%), with similar fractions for acyclic, cyclic, and bicyclic alkanes. The contributions of higher alkanes in OH reactivity (62.0%) and SOA formation potential (97.6%) for diesel emissions of organic compounds further emphasize the importance of higher alkanes. Our findings provide new insights on vehicular emissions of organic compounds, and the emission information should be included in future emission inventory and air quality models.

Understanding the near-ground vertical and temporal photochemical O3 formation mechanism is important to mitigate O3 pollution. Here, we measured the vertical profiles of O3 and its precursors at six different heights, ranging from 5 to 335 m, using a newly built vertical observation system in the Pearl River Delta (PRD) region of China. The net photochemical ozone production rate (P(O3)net) and O3 formation sensitivities at various heights were diagnosed using an observation-based model coupled with the Master Chemical Mechanism (MCM v3.3.1). Moreover, to assess model performance and identify the causative factors behind O3 pollution episodes, the P(O3)net was measured at 5 m above ground level with a custom-built detection system. In total, three O3 pollution episodes and two non-episodes were captured. The identified O3 pollution episodes were found to be jointly influenced by both photochemical production and physical transport, with local photochemical reactions playing a major role. The high index of agreement (IOA) calculated by comparing the modelled and measured P(O3)net values indicated the rationality of investigating the vertical and temporal variability in O3 formation mechanisms using model results. However, the measured P(O3)net values were generally higher than the modelled P(O3)net values, particularly under high-NOx conditions, which may indicate a potential underestimation of total RO2 by the model. Throughout the measurement period, the contribution of different reaction pathways to O3 production remained consistent across various heights, with HO2 + NO as the major O3 production pathway, followed by RO2 + NO. We observed that P(O3)net decreased with an increase in measurement height, which was primarily attributed to the reduction in O3 precursors, such as oxygenated volatile organic compounds (OVOCs) and non-methane hydrocarbons (NMHCs). The O3 formation regimes were similar at different heights during both episodes and non-episodes, either being located in the VOC-sensitive regime or in the transition regime that is more sensitive to VOCs. Diurnally, photochemical O3 formation typically remained in the VOC-sensitive regime during the morning and noon, but it transitioned to the transition regime and was more sensitive to VOCs in the afternoon at around 16:00 LT (local time). Vertical and temporal photochemical O3 formation is most sensitive to OVOCs, suggesting that targeting specific VOCs for control measures is more practical and feasible at the observation site. The vertical temporal analysis of O3 formation mechanisms near the ground surface in this study provides critical foundational knowledge that can be used to formulate effective short-term emergency and long-term control strategies to combat O3 pollution in the PRD region of China.

Abstract. Volatile chemical products (VCPs) have become an important source of reactive organic gases (ROGs) in urban areas worldwide. Industrial activities can also utilize a large amount of VCPs and emit many organic gases into the atmosphere. Due to multiple sampling and measurement challenges, only a subset of ROG species is usually measured for many industrial VCP sources. This study aimed to investigate the emissions of ROGs from five industrial VCP sources in China, including shoemaking, plastic surface coating, furniture coating, printing, and ship coating industries. More comprehensive speciation of ROG emissions from these industrial VCP sources was developed by the combination of the proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) along with gas chromatography-mass spectrometer/flame ionization detector (GC-MS/FID). Our study identified oxygenated ROG species (OVOCs) as representative ROGs emitted from these sources, which are highly related to specific chemicals used during the industrial activities. Moreover, mass spectra similarity analysis revealed significant dissimilarities among the ROG emission sources, indicating substantial variations between different industrial VCP sources. Except for the ship coating industry utilizing solvent-borne coatings, the proportions of OVOCs range from 67 % to 96 % in total ROG emissions and 72 % to 97 % in total OH reactivity (OHR) for different industrial sources. The industrial VCP sources associated with solvent-borne coatings exhibited a higher ozone formation potential (OFP), reaching as high as 5.5 and 2.7 g O3·g-1 ROGs for ship coating and furniture coating industries, primarily due to contributions from aromatics. The fractions of the ten most abundant species in total ROG emissions, OHR, and OFP indicated a highly centralized of ROG emissions from various industrial VCP sources. Our results suggest that ROG treatment devices may have limited effectiveness for all ROGs, with treatment efficiencies ranging from -12 % to 68 %. Furthermore, we found that ROG pairs (e.g., methyl ethyl ketone (MEK) /C8 aromatics ratio) could serve as effective indicators for distinguishing industrial VCP sources, particularly for measurements in industrial areas. Our study demonstrated the importance of measuring a large number of ROGs using PTR-ToF-MS for characterizing ROG emissions from industrial VCP sources.

The vertical variations and key drivers of ozone and its precursors, namely NOx and VOCs, in the atmospheric boundary layer, have vital impacts on surface ozone budgets but are poorly understood so far. Using online gradient measurements from a 356 m tower, we obtained continuous vertical profiles of ozone and its precursors, which exhibited strong gradients throughout the day. In the daytime, the vertical gradients of ozone precursors are significantly regulated by reactions with OH radicals. At night, our observations confirmed more intense VOC reactions with NO3 radicals in the residual layer than in the boundary layer. Additionally, we found that residual layer entrainment could contribute to over half of the boundary-layer ozone enhancements in the morning periods. Our results underscore the importance of considering vertical changes of ozone and its precursors in the atmospheric boundary layer when developing future ozone mitigation strategies.