The Vertical Distribution of VOCs and Their Impact on the Environment: A Review

: Volatile organic compounds (VOCs) play an important role in atmospheric chemistry. Primary VOCs take part in chemical and photochemical reactions, contributing to ozone (O 3 ) and secondary organic aerosol (SOA) formation, which may cause air pollution problems. High VOC concentrations might lead to dizziness, nausea, headaches, genotoxicity, reproductive weakness, and other diseases harmful to human health. Several studies have been performed to analyze the components, variations, or sources of VOCs at the ground level. In contrast, studies of the vertical distribution characteristics of VOCs are scarce, and the VOC potential for O 3 formation in the boundary layer is not yet well understood. To better understand the VOC vertical variation regularities and related reasons in temporal and spatial dimensions, thus to deepen the understanding of their effects on O 3 and SOA formation in the vertical direction and to identify the existing gaps in VOC vertical distributions, this study reviewed VOC sampling techniques, VOC vertical distribution characteristics, VOC diffusion models, and effects caused by VOCs. This work can be a valuable reference for decision making regarding environmental and health problems.


Introduction
Volatile organic compounds (VOCs) in the atmosphere are complex, and not only they can be emitted directly from the source to the ambient atmosphere, but they can also be generated through photochemical reactions. Some VOCs indirectly affect the natural environment by altering the Earth's atmosphere, resulting in climate change, acid deposition, and other effects. Several VOCs could also lead to chronic lesions of the skin, central nervous system, liver, and kidneys, which seriously threaten human health (Haick et al. [1]). VOCs also are the precursors of ozone (O 3 ) and secondary organic aerosols (SOAs) [2]. O 3 is the main component of photochemical smog and one of the leading indicators to measure its intensity. Tropospheric O 3 has a non-negligible effect on global warming as an important greenhouse gas. High near-ground O 3 concentrations can irritate the human eye and the respiratory system, cause air quality deterioration, and harm plant growth [3]. SOAs seriously affect lung ventilation function, causing asthma, bronchitis, and other diseases [4]. Many studies have investigated ground-level VOC components, variation, and source apportionment [5]. However, research on VOC distributions in the vertical direction is still scarce [6].
The tropospheric distribution of VOCs depends on many factors, including the emission rates, dispersion, and physicochemical properties of VOCs [7]. VOCs

VOC Vertical Distribution Samplers
VOC samplers have gone through newer methods, such as gas sampling bags, canister samplers, sorbent-filled tubes, and solid-phase microextraction (SPME) ( Table 1). Gas sampling bags have apparent economic advantages over sampling instruments and are easy to operate [28][29][30]. Tedlar, Kynar, FEP (Teflon), and Fluoride gas sampling bags are often used to collect samples of ambient VOCs (AVOCs) [31][32][33][34] (Table 2). However, they have low recovery of non-polar species and easily leak [35,36]. Compared with air sampling bags, the canister-based sampler could increase the sample holding time (up to 30 days) [37,38]. This method is more reliable than others because of the distinct advantages listed in Table 2 [39]. Adsorbent sampling is widely used; these samplers are attached to tethered balloons, towers, and high-rise buildings to collect air samples. Two adsorbents are used to sample VOCs in ambient air: inorganic adsorbents and organic porous polymer adsorbents [40]. Adsorbent sampling is characterized by good adsorption capability and ease of transportation. However, adsorbent tubes are sensitive to moisture content and have low recovery of non-polar species [36]. Solid-phase microextraction (SPME) is typically utilized in aircrafts and consists of fibers (of various kinds) covered with an extracting phase (which can be liquid, solid, or both) to absorb VOCs [41]. SPME is easy to perform, able to be applied in high-risk industrial areas, and friendly towards the environment and human health, and it also costs less in terms of analyses [42,43]. However, sunlight, wind, temperature, humidity, and other prevailing meteorological conditions influence the absorption of VOCs, and the method is unsuitable for sampling high-concentration pollutants [7]. Further details about these four sampling methods are shown in Table 2.  Buildings/towers are commonly built for active sampling with instruments that operate continuously. Flight mass spectrometers (PTR-TOF-MS) are often applied to towerbased measurements [49][50][51]. For example, Kaser et al. [49] collected air samples on a tower and investigated the interannual variability of terpenoid emissions using PTR-TOF-MS. They found a higher interannual isoprene flux variability than that of other terpenes [51]. Zhang et al. [51] observed the vertical profiles of isoprene and monoterpenes using the proton-transfer-reaction time of PTR-TOF-MS at heights of 3, 15, 32, 64, and 102 m above the ground on a tower in central Beijing and found that the isoprene mixing ratios were more significant in summer due to much stronger local emissions. In contrast, monoterpenes were lower in summer mainly due to their consumption of much higher levels of O 3 [51].
Other instruments, such as sorbent-filled tubes and canisters, can also be applied to sample VOCs at different heights and levels of a building/tower [18,52]. Mao et al. [18] collected VOCs at 8 m, 32 m, 140 m, and 280 m based on a 325 m meteorological tower in urban Beijing using specially designed stainless-steel flasks. Most VOC compound profiles showed a descending trend with the increase in height, except for the thermal inversion layer [18]. Vertical sampling using buildings/towers has the advantage of being easy to operate. Nevertheless, the flexibility of this VOC sampling method is limited because it depends on the height and location of the tower/building [36]. Additional details are shown in Table 3.

Tethered Balloons (Airships)
Tethered balloon measurement mainly refers to the vertical sampling of VOCs by fixing gas sampling bags, canisters, and other instruments to a tethered balloon. This technology has been the most widely used for vertical AVOC sampling [5,53,54]. Wohrnschimmel et al. [55] and Tsai et al. [54] collected VOC samples in the vertical direction using this method and found that propane, butane, and toluene concentrations decreased with the increase in height. Airships provide higher sampling heights than building-/tower-based sampling techniques, with a maximum sampling height of 1600 m being documented in the literature [56]. However, this technique also has some limitations, such as slower speed and short flight distances [36]. Other details about this technique are shown in Table 3.

Aircrafts
Aircraft sampling technology is mainly for continuous measurements. PTR-TOF ONLINE sampling from aircrafts has been developed in several studies [57][58][59]. For example, Veres et al. [59] measured the anthropogenic sources of VOCs in a football stadium with PTR-TOF-MS and found that human respiration/breath, ozonolysis of skin oils, and cigarette smoke/combustion were the dominant sources [59]. Koss et al. [58] observed VOC emissions and photochemical products over US oil-and gas-producing regions using high-resolution PTR-TOF-MS. They found that aromatics, cyclic alkanes, and alkanes were the typical compounds [58]. Additionally, canisters and SPME can also Atmosphere 2022, 13,1940 6 of 25 be installed on an aircraft to collect air samples [60,61]. Furthermore, aircrafts can also carry sensors to collect meteorological parameters and other air quality data [36]. Using aircrafts for vertical VOC sampling can overcome the limitations of building-, tower-, and balloon-based measurements in terms of sampling heights and locations [62,63]. Currently, the maximum altitude of aircraft sampling is 12 km. Nevertheless, this technique also has some challenges, as shown in Table 3 (e.g., the air samples may be contaminated because of aircraft exhaust emissions; there is a high risk when flying near the ground level; and its economic cost is higher than that of other techniques) [36].

Remote Sensing and Satellites
Remote sensing detects a trace of a specific substance by measuring the absorbance of light at various wavelengths. It has been used to provide reliable data required for air quality modeling and forecasting, emission estimation, monitoring long-occurring air pollutants, etc. [28,64]. Kumar and Viden [28] monitored VOCs at the ground level using a DOAS (differential optical absorption system). Different remote sensing methods are used to examine satellite-taken images to quantify the concentrations of various air pollutants. Many satellite platforms have been set up to perform VOC monitoring, such as ERS-2, ENVISAT, AURA, and METOP-A [65][66][67][68]. Jin et al. [69] combined 20 years of HCHO/NO 2 data from multiple satellites with ground-based observations to study the chemical changes in ground-level O 3 -NO X VOCs in urban areas of the United States. This technique is advantageous for filling in the gaps left by other monitoring strategies since it can deliver large-scale, high-resolution, real-time data at the regional, national, and even continental levels [65]. However, this technique has more limitations, as listed in Table 3. One of the primary limitations is that satellites only provide columnar data for the entire atmosphere and cannot extract vertical VOC profiles [36].

Unmanned Aerial Vehicles (UAVs)
UAVs equipped with AVOC samplers and sensors have been successfully used for air quality studies, including VOC sampling [70], particulate matter sampling [71,72], ozone observation, etc. [73]. Vo et al. used a UAV equipped with a battery-powered rotor, canisters, and sensors to collect air samples and meteorological data. Samples were collected up to 1000 m, and VOC concentrations decreased with the increase in height, except for the inversion layer. UAVs offer several advantages over aircrafts, such as flexibility in flight schedules and significantly lower costs [74]. Compared with other sampling techniques, UAVs require less space for taking off and landing [75]. The highest sampling height recorded in the literature is 1000 m [71]. Although UAVs have some disadvantages, as summarized in Table 3, they still have the highest potential to be more widely used in future AVOC research due to their advantages over other methods [36]. For instance, according to the USEPA TO-15 Method, canisters can be readily retrofitted with UAVs that can measure more than 100 VOC species at low concentrations (ppb). UAVs can be integrated with reliable, high-quality sensors to provide high-resolution air pollutant data and meteorological parameters. These 3D monitoring data can be sent to a cloud server for post-processing, and Global Information System (GIS) mapping techniques can be used to generate high-resolution temporal-spatial air quality maps for supporting the decision-making process [73,76].

VOC Analytical Techniques
Gas chromatography-flame ionization detection (GC-FID) and gas chromatographymass spectrometry (GC-MS) are two crucial gas chromatography (GC) techniques used for the analysis of VOCs. Especially, GC-MS is a powerful analytical technique in the study of VOCs owing to the vital identification function of mass spectrometry (MS). To date, many researchers have investigated the components and concentrations of VOCs in the atmosphere with promising results [77,78]. However, this method has many limitations. For example, this method is applied mostly offline, with a significant time lag. Moreover, Atmosphere 2022, 13,1940 7 of 25 the pretreatment of complex samples is time consuming, laborious, and economically costly [79,80].
In recent years, proton-transfer-reaction mass spectrometry (PTR-MS) has emerged as a valuable tool for studying VOCs in the atmosphere [80]. As mentioned in 2.2, PTR-TOF-MS is commonly used with tower and aircraft sampling techniques to study VOC concentration characteristics in the atmosphere at different altitudes [51,[57][58][59]. This online method could overcome the disadvantages of offline methods and has the advantages of high resolution, high sensitivity, and short monitoring time [80,81]. However, PTR-MS can only distinguish ions by their nuclear-to-cytoplasmic ratio, so it is challenging to distinguish isomers [82].

. Diurnal Variations in the Vertical Distribution of VOCs
The diurnal variation characteristics of the vertical distribution of VOCs are different on clear days and haze days. In general, the concentrations of total volatile organic compounds (TVOCs) and most components tend to decrease with the increase in height (except for the inversion layer) on both clear and haze days. However, Mao et al. [18] observed a particular case in which aromatic (mainly toluene) concentrations exhibited an increasing trend with the increase in height from 8 to 140 m at 10:00 p.m. and then a decreasing trend from 140 to 280 m ( Figure 1c). This phenomenon may be related to the high concentration of OH radicals at 140-280 m, which promoted the removal process of aromatics.

Seasonal Vertical Distribution Characteristics of VOCs
TVOC concentrations are generally higher in winter than in summer [93,94]. The concentrations of TVOCs in spring and autumn are between those in winter and summer. It is because the concentration of the hydroxyl (OH) radical is relatively high in summer, which increases the rate of NMHC removal from the atmosphere. Since the reactivity of VOCs generally decreases with the decrease in the temperature, VOCs react with more difficulty under winter conditions [42,95,96].
TVOC concentrations generally decrease with the increase in height in the four seasons due to anthropogenic emission sources being primarily accumulated at the ground level [97]. Nevertheless, the vertical distribution of specific VOC components is more complex between seasons ( Figure 2). For example, Zhang et al. [98] observed the vertical distributions of VOCs within 1000 m during late spring using a tethered balloon and found that alkanes accounted for 44.83% of TVOCs at the ground level but up to 67.90% at 50-350 m. In comparison, aromatics are active and easily converted to other compounds. Thus, the contribution of TVOCs tends to decrease at higher altitudes ( Figure 2a). Cabañas et al. [99] found that the proportion of aromatics decreased at higher altitudes on clear days, while it increased at higher altitudes on haze days (Figure 2b,c). One possible reason for the increased proportion of aromatics at high altitudes on haze days is as follows: The only necessary removal process for aromatics is their reaction with OH radicals during The morning rush hours from 7:00 to 9:00 a.m. are typically when VOC concentrations are at their highest. These are followed by the evening rush hours from 5:00 to 7:00 p.m., the hours between 12:00 a.m. and 2:00 p.m., and the hours between 11:00 p.m. and 1:00 a.m. (Figure 1), indicating that most VOC compounds in urban air originate from traffic and transportation emissions. The low atmospheric mixing height in the early morning may lead to insufficient vertical mixing, making VOC vertical dilution less possible than at other times of the day [18,92].
The vertical variation in TVOC concentrations is more significant on haze days, while the variation in VOC concentration is more significant at different times on clear days ( Figure 1). It is due to differences in the altitude of the thermal inversion layer on clear days, and the reaction activities of different components vary with the temperature and solar radiation. These factors eventually lead to more detailed vertical profiles at various times on clear days. For example, Mao et al. [18] found that the alkenes (mainly 1-butene) vertical profiles at 10:00 a.m. and 6:00 p.m. were consistent and showed a decreasing tendency with the increase in height from 8 to 32 m and an opposite tendency from 140 m to 280 m due to the occurrence of the inversion layer. At 2:00 p.m. and 10:00 p.m., VOC concentrations decreased with the increase in altitude in general (Figure 1e). For alkane (mainly isopentane) and aromatic (mainly toluene) concentrations, they decreased with the increase in height, except at 10:00 p.m., and the concentrations of alkanes reached their highest at 10:00 p.m. when the altitude was up to 140 m (Figure 1d). Additionally, alkane chemical properties are more stable than those of alkenes. They do not efficiently react with other compounds throughout the atmospheric transport process, resulting in higher alkane concentrations at 140 m [18]. Lin et al. [83] discovered that TVOCs and most components showed a decreasing trend with the increase in height in almost all periods during the day (Figure 1g-i). However, between 7:00 to 9:00 a.m., alkenes and aromatics concentrations increased with the increase in height from 13 to 32 m (Figure 1h,i). Temperature reversion can explain this phenomenon, which restricted the vertical dispersion of VOCs between 13 and 32 m. Furthermore, a significant number of aromatics emitted from industrial sources near the sample site during this time was another crucial cause. During the hours from 5:00 to 7:00 p.m., alkenes showed an increasing trend with the increase in height from the surface to 13 m (Figure 1 h). Although the causes for this discovery were unclear, it seemed likely that these particular VOCs were caused by local industrial sources rather than by emissions from vehicles and traffic [83].

Seasonal Vertical Distribution Characteristics of VOCs
TVOC concentrations are generally higher in winter than in summer [93,94]. The concentrations of TVOCs in spring and autumn are between those in winter and summer. It is because the concentration of the hydroxyl (OH) radical is relatively high in summer, which increases the rate of NMHC removal from the atmosphere. Since the reactivity of VOCs generally decreases with the decrease in the temperature, VOCs react with more difficulty under winter conditions [42,95,96].
TVOC concentrations generally decrease with the increase in height in the four seasons due to anthropogenic emission sources being primarily accumulated at the ground level [97]. Nevertheless, the vertical distribution of specific VOC components is more complex between seasons ( Figure 2). For example, Zhang et al. [98] observed the vertical distributions of VOCs within 1000 m during late spring using a tethered balloon and found that alkanes accounted for 44.83% of TVOCs at the ground level but up to 67.90% at 50-350 m. In comparison, aromatics are active and easily converted to other compounds. Thus, the contribution of TVOCs tends to decrease at higher altitudes ( Figure 2a). Cabañas et al. [99] found that the proportion of aromatics decreased at higher altitudes on clear days, while it increased at higher altitudes on haze days (Figure 2b,c). One possible reason for the increased proportion of aromatics at high altitudes on haze days is as follows: The only necessary removal process for aromatics is their reaction with OH radicals during the day [97], and this process is weakened, as the OH radical level is lower at higher altitudes due to the elevated NO levels close to the ground, which leads to the accelerated conversion of HO 2 into OH [18,98]. This tendency for aromatics contrasts with alkanes, which react with OH radicals and another significant atmospheric oxidant, the Cl atom [99]. The concentration of VOC components changes more significantly in winter than in summer (Figure 2d,e). That is, the atmospheric stratification is more stable in winter, which is not conducive to the dilution and diffusion of pollutants, allowing pollutants to accumulate [42,95,96]. The intense atmospheric convection and the higher planetary boundary layer (PBL) height in summer lead to a more uniform distribution of VOC concentrations at different altitudes [2]. The concentration of halohydrocarbon in winter decreases significantly with the increase in altitude, while alkanes show an opposite trend (Figure 2d). It is mainly related to the higher proportions of vehicular gasoline emissions, emissions and industrial sources, and coal burning in winter [2]. boundary layer (PBL) height in summer lead to a more uniform distribution of VOC centrations at different altitudes [2]. The concentration of halohydrocarbon in winter creases significantly with the increase in altitude, while alkanes show an opposite tr (Figure 2d). It is mainly related to the higher proportions of vehicular gasoline emissi emissions and industrial sources, and coal burning in winter [2].

Urban and Suburban
The predominant components of suburban VOCs are isoprene, aldehydes, and tone, mainly emitted from terrestrial vegetation [13]. Most distribution profiles of su ban biogenic VOCs (BVOCs) show a descending trend with the increase in height. H ever, the profiles of specific components are different. Ieda et al. [52] investigated the tical distributions of VOCs in a larch forest and found that the concentration of isop decreased with the increase in height from the surface to 22 m; then, it increased a above the canopy level (about 22 m), due to the forest floor and the upper canopy b mainly sinks of isoprene [52]. Lower AVOC concentrations can also be observed in urban areas due to long-distance transportation from urban areas. Liu et al. [100] inv gated the vertical distribution of VOCs in the suburbs of Shanghai using a UAV equip with summa canisters and found that alkanes, aromatics, and alkenes dominated

Urban and Suburban
The predominant components of suburban VOCs are isoprene, aldehydes, and ketone, mainly emitted from terrestrial vegetation [13]. Most distribution profiles of suburban biogenic VOCs (BVOCs) show a descending trend with the increase in height. However, the profiles of specific components are different. Ieda et al. [52] investigated the vertical distributions of VOCs in a larch forest and found that the concentration of isoprene decreased with the increase in height from the surface to 22 m; then, it increased again above the canopy level (about 22 m), due to the forest floor and the upper canopy being mainly sinks of isoprene [52]. Lower AVOC concentrations can also be observed in suburban areas due to long-distance transportation from urban areas. Liu et al. [100] investigated the vertical distribution of VOCs in the suburbs of Shanghai using a UAV equipped with summa canisters and found that alkanes, aromatics, and alkenes dominated the TVOCs in this area. Both of these three dominant species mainly originated from the nearby factory. The contribution of alkanes and alkenes to TVOCs shows a tendency to decrease with the increase in altitude. At the same time, aromatics tend to increase, mainly related to the chemical reaction of aromatics with OH mentioned above [100]. Das et al. [101] also found similar characteristics in suburban North Carolina (Figure 3).
samples up to 1000 m in Renwu and Chaozhou areas. They found that TVOC concentrations reached the highest (75 ppb) at 400 m in Chaozhou and 300 m (327 ppb) in Renwu [89]. The contribution of each component to TVOC concentrations can be seen in Figure  3. They noted that other than local vehicular exhaust, long-distance transport upwind sources might also have influenced the measured TVOC concentrations. Lin et al. [83] performed similar research in Kaohsiung and found that the VOC concentrations decreased with the increase in height from the surface to 110 m. This observation was mainly due to the short distance between the surface and traffic emissions, which led the pollutants to accumulate near the ground level [83].

Coastal and Inland
The VOC concentrations across the relatively slight economic growth gap between inland and coastal regions also reveal significant differences. Alkanes and aromatics contribute more to TVOCs in inland regions, while alkenes and halocarbons account for a smaller proportion. Using tethered balloons, Geng et al. [6] measured the vertical distribution of VOCs in the Beijing-Tianjin-Hebei region. They found alkanes, alkenes, and aromatics, and oxygenated VOCs (OVOCs) were the dominant components. Additionally, the TVOC concentration at 50-100 m (46.9 ppb) was 4.9 times higher than at 900- AVOC concentrations in urban areas are significantly higher than those in suburban areas due to the more concentrated anthropogenic emission sources (e.g., traffic emissions, industrial emissions, fossil fuel combustion, etc.). Vo et al. [89] collected vertical air samples up to 1000 m in Renwu and Chaozhou areas. They found that TVOC concentrations reached the highest (75 ppb) at 400 m in Chaozhou and 300 m (327 ppb) in Renwu [89]. The contribution of each component to TVOC concentrations can be seen in Figure 3. They noted that other than local vehicular exhaust, long-distance transport upwind sources might also have influenced the measured TVOC concentrations. Lin et al. [83] performed similar research in Kaohsiung and found that the VOC concentrations decreased with the increase in height from the surface to 110 m. This observation was mainly due to the short distance between the surface and traffic emissions, which led the pollutants to accumulate near the ground level [83].

Coastal and Inland
The VOC concentrations across the relatively slight economic growth gap between inland and coastal regions also reveal significant differences. Alkanes and aromatics contribute more to TVOCs in inland regions, while alkenes and halocarbons account for a smaller proportion. Using tethered balloons, Geng et al. [6] measured the vertical distribution of VOCs in the Beijing-Tianjin-Hebei region. They found alkanes, alkenes, and aromatics, and oxygenated VOCs (OVOCs) were the dominant components. Additionally, the TVOC concentration at 50-100 m (46.9 ppb) was 4.9 times higher than at 900-1000 m (8.0 ppb). Moreover, the concentration changed rapidly from the altitudes of 50-100 to 401-500 m, dropping to less than 30% of the original concentration. Among them, aromatic concentrations dropped the fastest (below 20%) from 50 to 500 m, followed by alkanes and OVOCs (below 30%), alkenes (below 40%), and hydrocarbons (below 50%). Additionally, the concentration decreasing rate for each component between 500-1000 m and 50-500 m was different, with aromatics reducing to less than 50%; alkenes, OVOCs, and hydrocarbons all reducing to less than 60%; and alkanes reducing to less than 70% [6].
VOC compositions and concentrations in coastal and inland areas are similar. Nevertheless, the changes in coastal areas are not noticeable in terms of vertical distribution, mainly because of the land-sea breeze. The sea breeze is a thermally induced local circulation that forms due to the differential heating of land and water [102]. Numerous studies have shown that air pollution in coastal areas can readily result from land-sea breeze circulation [103]. It is mainly due to sea breeze, which transports the polluted air masses from the ocean to the inland area [104]. From a dynamic perspective, due to shearing between the seaward synoptic wind and the landward gravity current, local contaminants may move back to the sea above the gravity current at a relatively slow rate. Such dynamic conditions may enhance an accumulation of pollutants above coastal industrial areas [105]. This results in high pollutant concentration levels in coastal areas and in the high-altitude atmosphere. Flocas et al. [60] observed elevated VOC concentrations (150-350 ppb) from 300 to 400 m above the ground level. When the altitude reached 1400-1600 m, VOC concentrations remained relatively high (100-200 ppb). The proportion of alkanes, aromatics, and alkenes declined with the increase in height in the lower altitude range. When the altitude exceeded 1108 m, the aromatic concentrations tended to increase. When the altitude exceeded 1271 m, alkanes also tended to increase. However, the concentration of each component did not change rapidly in the vertical direction due to the sea breeze circulation in this area [60].

VOC Vertical Distribution Models
Systematic and reliable VOC monitoring data play a primary role in applying air quality models for simulating O 3 dispersion in the atmosphere. System simulation results provide a valuable reference for air quality management [36]. Models such as AERMOD, ADMS, Models-3 CMAQ, CALPUFF, and WER-Chem are used to simulate VOC dispersion (Table 4).
AERMOD (AMS/US EPA) is a near-field, steady-state, Gaussian plume model based on planetary boundary layer turbulence structure and scaling concepts [9]. AERMOD consists of a meteorological preprocessor (AERMET) and a terrain preprocessor (AERMAP). The AERMOD algorithms have been streamlined, e.g., the treatment of terrain, so the model runs relatively quickly compared with ADMS. Hanna et al. [106] compared the result of AERMOD with five sets of fields and found that the model performed well overall. However, it also had the limitation of underpredicting the mean and maximum concentrations. To ensure that the results are reasonable, Holmes and Morawska [9] noted that modeling with the AERMOD model requires basic assumptions of the Gaussian plume model, such as the steady-state emission source, moderate wind speed, and moderate wind direction changes that are continuous.
ADMS is an advanced steady-state, Gaussian-like dispersion model. The model uses conventional meteorological elements to define the structure of the boundary layer and can better describe the atmospheric dispersion process. ADMS is especially appropriate for simulating atmospheric dispersion from elevated point sources. Although ADMS is similar to AERMOD in many aspects, there are still significant discrepancies in the criteria for input meteorology since AERMOD allows vertical wind and temperature profiles to be obtained. At the same time, ADMS requires one level of near-ground data. Additionally, the ADMS performance is slightly better than the AERMOD performance [107].
The Models-3 CMAQ modeling system has evolved from several models operated at EPA, such as RADM, SAQM, UAM, and ROM [108]. This modeling system comprises three modeling components (Models-3): a meteorological modeling system for describing atmospheric states and motions, emission models for anthropogenic and natural emissions injected into the atmosphere, and a chemistry-transport modeling system for simulating chemical transformation and fate [109]. Compared with other systems, it has the distinct advantages of simulating on a larger spatial scale and simulating more types of pollutants (up to more than 80 species, as recorded in the literature). However, due to the complexity of the model's algorithm, the calculation speed is relatively slow [109,110].
CALPUFF is a multi-layer, non-steady-state, puff dispersion model designed to model the dispersion of gases and particles [9]. The model can predict pollutants from a few kilometers to several hundred kilometers. Furthermore, CALPUFF can perform better than AERMOD in dealing with the wind direction. Kwak et al. [111] used this modeling system to calculate VOC concentrations. In their study, topography data were mainly acquired from a GIS application. In contrast, meteorological data were derived from 70 surface stations located throughout South Korea and 1 upper-air station. The modeling results not only clearly described the spatial concentration ranges of benzene (0.001-8.27 µg m −3 ), ethylbenzene (0.001-3.19 µg m −3 ), styrene (0.001-7.83 µg m −3 ), toluene (0.01-83.8 µg m −3 ), and xylene (0.005-66.8 µg m −3 ) but also found that the VOC concentrations were higher in urban regions with high population densities than in rural regions in almost all cases. However, the model is inadequate for simulating pollutant dispersion when the timescale is less than one hour or when the diffusion is substantially influenced by turbulence, as in an urban setting. Furthermore, it has a low temporal agreement and underpredicts pollutant concentrations [9].
WRF-Chem is a regional air quality model developed by NOAA/ESRL (National Oceanic and Atmospheric Administration/Earth System Research Laboratory). The model has the same grid structure on the air quality scale and meteorological scale, with no temporal interpolation, and is suitable for simulating meteorological-chemical feedbacks (climate change) from local to global scales and air quality conditions from the cloud scale to various weather scales [112,113]. Unlike the Models3-CMAQ model, the meteorological and chemical processes are carried out separately in WRF-Chem. WRF can simulate high-resolution numerical simulations in the 1-10 km range. Chem can simulate pollutant transport and dispersion, wet and dry deposition, gas-phase chemical reactions, source emissions, photo-decomposition, aerosol dynamics, and aerosol chemistry [107]. Chutia et al. [114] compared the WRF-Chem-simulated values of VOC dispersion with the ground-based observations over India. They ultimately found that the WRF-Chem simulations for acetone, acetaldehyde, and toluene were reasonable. However, the model overestimated propane over high-altitude sites and urban stations. This bias was mainly due to the inadequate description of the regional-scale topographic resolution (including forests, hills, and mountains) and the uncertainty of anthropogenic emissions [114]. Long calculation time and poor flexibility are the other limitations of WRF-Chem [112].
In summary, different VOC vertical dispersion models exhibit various performances in practical scenarios. Critical challenges to applying these models are the uncertainties of the model-predicted concentrations due to the lack of systematic and reliable VOC monitoring data. Comparing these models from the view of the complexity of the environment in the study area, the dimensions of the model, the character of the particle, the accuracy calculations, and the time scales, the Models-3 CMAQ model performs relatively better, and it can simulate primary pollutants on a larger spatial scale and under more complex weather conditions. Additionally, it combines more abundant functions with tropospheric ozone and acid deposition to provide technical support for air quality forecasting, assessment, and decision making. Therefore, this model has promising application.  O 2 and HO 2 , as crucial intermediates in the reaction chain, play an essential role in producing other compounds (Figure 4). R . O 2 radicals and HO 2 radicals in the atmosphere react rapidly with NO, converting NO to NO 2 , as shown in reactions (1) and (2). R . O 2 radicals react more rapidly, with a reaction rate of about (7−10) × 10 −12 cm 3 molecule −1 s −1 [118,119].
On the one hand, O( 3 P) atoms further react with O2 in air to form O3, as shown in reaction (4).
On the other hand, O3 generates NO2 and O2 again with the participation of NO, as shown in reaction (5).
This mechanism leads to a photochemical balance among NO, NO2, and O3, with no net formation or loss of O3, as seen in Figure 5a. However, when reactions (1) and (2) occur in the presence of VOCs in the atmosphere, it increases NO2 concentrations and breaks the photochemical balance of O3, as seen in Figure 5a, making NO2 convert to the direction of O3 production; thus, net O3 formation occurs [121]. The processes are shown in Figure  5b.  O 2 also reacts with HO 2 to produce organic hydroperoxides (ROOH) and oxygen at a reaction rate of about 10 −11 cm 3 molecule −1 s −1 . The reaction has a significant negative temperature effect [119,120]. O 3 is formed photochemically from the photolysis of NO 2 ; then, NO 2 photolyzes to form NO and O( 3 P) atoms, as shown in reaction (3).
On the one hand, O( 3 P) atoms further react with O 2 in air to form O 3 , as shown in reaction (4).
On the other hand, O 3 generates NO 2 and O 2 again with the participation of NO, as shown in reaction (5).
This mechanism leads to a photochemical balance among NO, NO 2 , and O 3 , with no net formation or loss of O 3 , as seen in Figure 5a. However, when reactions (1) and (2) occur in the presence of VOCs in the atmosphere, it increases NO 2 concentrations and breaks the photochemical balance of O 3 , as seen in Figure 5a, making NO 2 convert to the direction of O 3 production; thus, net O 3 formation occurs [121]. The processes are shown in Figure 5b Furthermore, NO and NO2 can also react with O3 to form N2O5 [121], as shown in reactions (6)- (8).
Among them, the wet and dry deposition of N2O5 and the uptake of N2O5 by aerosols are vital nighttime loss processes for NOX. N2O5 is toxic and reacts readily with H2O in the air, leading to nitric acid formation. Thus, this process also affects the environment and human health.
The reaction of OH radicals with NO and NO2 is also essential in the troposphere, and the processes produce acids such as HONO and HNO3, as shown in (9) and (10).
HONO + hv → OH + NO (11) Due to the heterogeneous hydrolysis of NO2 on the surface of aerosols and particulate matter, HONO formation can be observed during nighttime in urban areas. The HONO produced at night is rapidly photolyzed again after sunrise, leading to a rapid rise in OH radicals in the early morning, acting as an urban photochemical initiator [121]. Figure 6 shows the major NOX reaction processes under "clean" tropospheric conditions. Furthermore, NO and NO 2 can also react with O 3 to form N 2 O 5 [121], as shown in reactions (6)- (8).
Among them, the wet and dry deposition of N 2 O 5 and the uptake of N 2 O 5 by aerosols are vital nighttime loss processes for NO X . N 2 O 5 is toxic and reacts readily with H 2 O in the air, leading to nitric acid formation. Thus, this process also affects the environment and human health.
The reaction of OH radicals with NO and NO 2 is also essential in the troposphere, and the processes produce acids such as HONO and HNO 3 , as shown in (9) and (10).
HONO + hv → OH + NO (11) Due to the heterogeneous hydrolysis of NO 2 on the surface of aerosols and particulate matter, HONO formation can be observed during nighttime in urban areas. The HONO produced at night is rapidly photolyzed again after sunrise, leading to a rapid rise in OH radicals in the early morning, acting as an urban photochemical initiator [121]. Figure 6 shows the major NO X reaction processes under "clean" tropospheric conditions. In summary, there is a complicated relationship between the photochemical reactions of ozone and its precursors, VOCs and NOX, which slightly complicated the reduction in ozone pollution. Additionally, some secondary oxidation products have lower vapor pressure and can attach to the pre-existing primary particulate matter through homogeneous nucleation or condensation, leading to SOA formation, thus leading to adverse en- In summary, there is a complicated relationship between the photochemical reactions of ozone and its precursors, VOCs and NO X , which slightly complicated the reduction in ozone pollution. Additionally, some secondary oxidation products have lower vapor pressure and can attach to the pre-existing primary particulate matter through homogeneous nucleation or condensation, leading to SOA formation, thus leading to adverse environmental problems [121,122]. Therefore, understanding the chemical reaction mechanism of VOCs-NO X -O 3 is one of the significant scientific challenges related to urban air pollution. The vertical distribution characteristics of VOCs and NO X show an overall negative trend with respect to the vertical distribution of O 3 . Namely, VOCs and NO X concentrations decrease with the increase in height, while O 3 concentrations increase [53,54,98,123]. It is mainly because VOCs and NO X can generate O 3 in the atmosphere through a series of photochemical reactions (as mentioned in 4.1) after being emitted at the ground level. For example, Zhang et al. [98] used a tethered balloon to observe the vertical distribution of O 3 and its precursor VOCs in the lower 1000 m of the troposphere, at 50-350 m, 350-700 m, and 700-1000 m, in the suburbs of Shanghai, China. The results showed that the average hourly O 3 concentration was negatively correlated with the TVOC concentrations, with a correlation coefficient of −0.29 (p < 0.05) [98]. Wohrnschimmel et al. [55] measured ozone profiles between 0 and 1000 m above the ground. The result showed that the concentration of O 3 reached 60-70 ppb at the top of the surface layer and maintained these concentrations throughout the residual layer [55]. Tsai et al. [54] found that the O 3 concentration at 100-500 m was higher than the ground O 3 level, with a core up to 85 ppb at 300-500 m. The concentration tended to decrease with the increase in height when the altitude was up to 500 m [54]; this is because the concentration of VOC, as precursors of O 3 , reduce at this altitude, resulting in weak O 3 production [123].
However, with a further increase in altitude, VOC and NO X concentrations in the upper atmosphere gradually decrease, which leads to a net decrease in O 3 production. Therefore, when the altitude exceeds a specific range (generally above the PBL), the O 3 concentration also tends to decrease [123,124]. Yang et al. [125] divided the vertical distribution of O 3 into three layers based on the vertical distribution of NO X . The first layer was from the surface to 200 m above the ground, with a high NO concentration, which prompts the conversion between O 3 and NO 2 (as shown in reaction (5)), making O 3 and NO 2 concentrations exhibit a relatively strong negative correlation [125]. The concentration of NO 2 gradually decreases with the increase in altitude, while the O 3 concentration increases. Especially at night and in the early morning, considering an altitude of up to 220 m, the concentration of O 3 is significantly higher than that at the ground level [126]. It should be noted that NO X concentrations decrease faster at this altitude [123]. The second layer considered in the above-mentioned study was 200-1000 m above the ground, and the range of d[O 3 ]/dt (daytime chemical production of O 3 ) was 10-40 µg m −3 h −1 . The maximum chemical production of O 3 occurred at 300 m. The third layer was outside the PBL height, where NO X decreases rapidly. Additionally, the chemical production of O 3 is weak above the PBL [126], making the O 3 concentration tend to drop at this height. In addition, other factors such as weather conditions, prevailing wind direction, and pollution sources among different regions are also important factors that can affect the vertical distribution of VOCs, NO X , and O 3 [90,127].

Effect of VOCs
The impact of VOCs on the atmospheric environment is mainly caused by tropospheric chemical processes leading to the production of O 3 and SOAs. The O 3 formation potential (OFP) and the SOA potential (SOAP) can describe the ability of O 3 and SOA formation of VOCs, thus providing a more direct indication of the extent of the VOCs' impact on the atmosphere.
The primary contributions to the OFP are aromatics, alkanes, and alkenes generated from the petrochemical industry and vehicle exhaust [89,98,126]. The OFP contributions of alkenes and aromatics change more significantly in the vertical direction. Alkenes increase with the increase in height, while aromatics show the opposite tendency. The concentration variation of alkanes is not significant in the vertical direction, mainly due to their relatively stable chemical properties [98,128]. The relatively small concentration changes in the vertical distribution of VOCs can lead to significant changes in the OFP. For example, Zhang et al. [98] found that the alkene contributions to TVOC concentrations were 8.41% and 12.12% at the surface and at 50-350 m, respectively, with less than 4% variation. In contrast, the contribution of alkenes to the OFP varied significantly, contributing 19% at the surface and then increasing rapidly to 49% at 50-350 m.
The contribution of the SOAP is mainly dominated by aromatics (up to 90%) [129,130]. Furthermore, this phenomenon exists at all altitudes [131]. One possible reason for these characteristics is that the high-carbon-content structures of aromatics can enter the particle phase more simply. In contrast, alkanes and alkenes are mostly low-carbon-content structure components whose oxidation products have higher saturation and vapor pressure and are less likely to enter the particulate phase to form SOAs [132]. However, the correlation among VOCs and the OFP, VOCs, and the SOAP may be insignificant. High TVOC concentrations may not necessarily contribute to a high OFP or SOAP [89,100].
As mentioned above, the primary contributions to the OFP and the SOAP are aromatics, alkanes, and alkenes. Among them, the contribution of aromatics to the SOAP is up to 90%. Aromatics, alkanes, and alkenes are mainly emitted by the petrochemical industry and through vehicle exhaust. Mastering these characteristics and emission sources of the OFP and the SOAP could help governments and other institutions to control O 3 and SOA pollution more effectively.

Research Prospects
The discussion above clearly shows that techniques based on towers/buildings, tethered balloons, aircrafts, and UAVs are widely applied in VOC sampling. Additionally, some studies of VOC photochemical reactions and VOC vertical distribution characteristics have also been conducted in recent years, and research in these directions will likely continue in the future. However, some details still need to be explored in the following aspects: (1) Vertical sampling techniques for VOCs: Remote sensing and satellite technologies can monitor VOCs on the continental scale. However, it is not easy to obtain VOC vertical profiles. Thus, advanced sampling techniques must be explored to overcome the high cost, high risk, insufficient spatial scale, and other limitations of the current techniques. (2) Vertical distribution characteristics of VOCs: Comparisons between VOC vertical distribution characteristics in different seasons and regions are incredibly lacking, and the explanations for the VOC vertical distribution characteristics are insufficient in current studies. (3) Vertical distribution models of VOCs: Current VOCs dispersion models are mainly comprehensive models for simulating various pollutants, such as SO 2 and NO X , with result accuracy limitations when facing different environments. Moreover, most of them are focused on ground-level simulations, while a specialized VOC vertical distribution model is highly lacking. In addition, the atmospheric environmental problems (e.g., OFP and SOAP) caused by VOC emissions should also be explored deeply. (4) Effects caused by VOCs: Although research on VOC atmospheric chemistry has developed, the reaction rate constants and reaction mechanism of organic peroxides R . O 2 with NO and NO 2 radicals need to be further investigated.