1 Institute for Environmental and Climate Research, Jinan University,Guangzhou 511443, China
2 State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences,Beijing 100190, China
3 Guangdong⁃Hongkong⁃Macau Joint Laboratory of Collaborative Innovation for Environmental Quality,Guangzhou 511443, China
Nitrated mono⁃aromatic hydrocarbons(NMAHs)are a class of pollutants with a wide variety of homologous species and complex physical and chemical properties,which are getting increasingly concerned in the researches of atmospheric chemistry.With the advance of measurement technology,more and more types of NMAHs species can be identified in field observations,and the generation mechanisms of different NMAHs are revealed in laboratory research.Base on this, we collected the published literatures of domestic and foreign research in the past 15 years on NMAHs in this paper. Firstly, We summarized its characteristics of temporal and spacial distribution, main sources, gas⁃particle partitioning in Europe and China as well as other places. Secondly, combing with laboratory experiments and field observations, we summarized generation and loss mechanism of NMAHs,and discussed the research progress in optical absorption, environment and health effects, as well as measurement techniques. Finally, the important scientific problems that need to be solved and future potential research directions are summarized.
Zonghan Xue, Nan Ma, Weigang Wang.
Nitrated Mono⁃Aromatic Hydrocarbons in the Atmosphere.
Progress in Chemistry, 2022, 34(9): 2094-2107 DOI:10.7536/PC211215
2 Emission and distribution characteristics of NMAHs in the atmosphere
2.1 Temporal and spatial distribution
2.2 Main sources
2.3 Gas⁃particle partitioning
3 Mechanisms of generation and loss of NMAHs
3.1 Mechanisms of generation and loss in gas phase
3.2 Mechanisms of generation in particle phase
4 Optical absorption of NMAHs
5 Environment and health effects of NMAHs
6 Measurement technology of NMAHs
6.1 Outline measurement technology
6.2 Online measurement technology
7 Conlusions and outlook
1 Introduction
Nitrated monocyclic aromatic hydrocarbons (NMAHs) generally refer to a class of compounds containing at least one nitro group (—NO2) and one hydroxyl group (— OH) on a single benzene ring, which are important components of the atmosphere that have attracted increasing attention[1⇓⇓~4][1]. There are many kinds of monocyclic aromatic nitro compounds, which can be divided into the following categories according to their molecular structural formulas: nitrophenols (NPs) and their methyl derivatives (MNPs), nitrocatechols (NCs) and their methyl derivatives (MNCs), dinitrophenols (DNPs) and its methyl derivatives[5]. The terminology and classification of these substances are different in different literatures. In addition to monocyclic aromatic nitro compounds, other terms also classify these substances as aromatic nitro compounds.Nitrophenols, nitrophenols, aromatic nitro derivatives, aromatic nitro chromophores, or as components of brown carbon, humic-like substances, water-soluble organic carbon compounds, water-soluble organic nitrogen compounds[2,3][5⇓⇓⇓~9][10,11][1,11⇓⇓~14][15][16][17][4,18][4][19]. These terms are mainly used to describe a certain part of the same substance in the published literature.
NMAHs are widely distributed in the atmospheric environment, which may affect air quality and human health, and have ecological toxicity[20⇓~22]. It can be produced by direct emission into the atmosphere or by secondary reactions of precursors in the atmosphere[2,12,23⇓~25]. NMAHs in the atmosphere mainly exist in the form of gas in the atmosphere or in the form of solid or liquid in aerosol particles[12,26]. These species are considered to be an important component of water-soluble organic nitrogen because of their nitro groups, which have certain hygroscopicity, and may become cloud condensation nuclei without mixing with inorganic salts, which may promote aerosol activation and cloud formation, and may affect the indirect radiative forcing of aerosols[17,27][19]. At the same time, because of its light absorption characteristics, it is considered to be an important component of brown carbon, thus enhancing the direct radiative forcing of aerosols[17,18]. Therefore, NMAHs can affect the radiative balance of the Earth's climate system through direct and indirect radiative forcing. In this context, NMAHs have gradually received more and more attention.
Earlier, Harrison et al. Summarized the research status of atmospheric NMAHs[14]. This paper points out that they exist widely in the environment (including air, rain, cloud, fog, snow), but due to technical reasons, the understanding of them is not deep enough[14]. Over the next decade, advances in measurement technology have led to a more detailed understanding of the emission distribution and sources, generation and loss mechanisms, and biological toxicity of NMAHs in the atmosphere, and the discovery of the absorption characteristics and importance of NMAHs species.Therefore, people pay more attention to the effects of NMAHs on aerosol light absorption and atmospheric radiation balance on the basis of investigating the spatial and temporal distribution, generation mechanism and environmental health effects of NMAHs species[2,9,12,18,22][8,16,25,28][10,20,29][1,6,7,17,30][17,30]. In order to fully understand the properties of NMAHs, it is necessary to systematically review these research progress.
2 Emission distribution and source characteristics of monocyclic aromatic nitro compounds in the atmosphere
2.1 Temporal and spatial distribution
At present, researchers focus on NMAHs in field observation in Europe (including Germany, France, Greece, Belgium, Italy, Hungary, Austria, Czech Republic, Slovenia and other countries) and Asia (including Hong Kong, North China Plain, Shanghai, Nanjing, Jinan and other places)[18,21,26,31][32][21][9][33][18][3][3,4][27,34][5][6,8][35][36][12]. It is also concerned in North America (Canada), South America (Brazil) and the Arctic (Svalbard Islands, Norway) (Table 1)[37][18,21][3].
The average concentrations of NMAHs at different types of sites were different. The annual average concentrations of NMAHs sampled in rural, suburban, and urban areas of Strasbourg, France, were 21.1-3, 23.5-3, and 25.2ng·m-3[32], respectively. The observed concentrations of different types of stations in this area were not significantly different, but showed a trend of urban > suburban > rural. Clays focused on the difference of 4 NC (4 ∥ Nitrocatechol) concentration between the urban background point, rural background point in Hungary and the background observation point of tropical rain forest in Brazil, and found that the urban background point (0.5~8.5 ng·m-3) and rural background point (0.3 ng·m-3) were much smaller than the observed concentration of tropical rain forest (97~520ng·m-3), but were close to the concentration of 4 NP (4-Nitrophenol) in Ontario, Canada[18][37]. Teich et al. Compared the observed concentrations of NMAHs at rural background sites in China and Germany, and found no significant difference[6]. Higher concentrations of NMAHs were observed in the background points of observation towns in Slovenia, which were 1.16~150.59 ng·m-3[27].It is higher than the urban background point concentrations observed in Jinan, China and Czech Republic (13.5~105.4 ng·m-3), (8.8~102 ng·m-3) and much higher than the urban background point concentrations in other areas of China (Hong Kong area (0.28~19.73 ng·m-3), Beijing area (6.63 ng·m-3), northern China (2.5~48.4 ng·m-3))[12][4][5][8][38]. Shahpoury et al. compared the observed concentration differences of NMAHs species in urban and rural background sites in Germany, Austria, Czech Republic and other central European countries with those in polar background sites in Norway, a northern European country, and found that 4 NC, 4 NP, 3M4NP (3 Methy 4 nitrophenol) and 2M4 NP (2 Methy 4 nitrophenol) were the main species in urban and rural background sites, but 3NSA (3 Nitrosalic y[3]. In contrast, only a few NMAHs species were observed at the polar background point in Norway, and the concentration was very low, indicating that the polar region was still a relatively clean area without significant pollution by NMAHs[3].
The average observed concentrations of NMAHs in different regions showed seasonal variation. The observation results in Hong Kong, Shanghai and Nanjing showed that the seasonal average concentration of NMAHs was winter > autumn > spring > summer, that is, the highest in winter and the lowest in summer[5,35,36]. It is worth noting that the seasonal average concentrations in Shanghai and Nanjing are higher than those in Hong Kong (Table 1). The results of one year observation in Belgium also showed that the seasonal average concentration of NMAHs was the highest in winter, followed by autumn, spring and summer[9]. Although the results observed in Jinan, China are also similar in winter (105.4 ng·m-3)> spring (34.0 ng·m-3)> summer (13.5 ng·m-3), the average concentration differences between seasons are much larger than those in Hong Kong and Belgium[12]. This observation is close to one Czech observed concentration level – winter (93~102 ng·m-3)> summer (8.8 ng·m-3)[4]. The observation results in Slovenia also show that the average concentration of NMAHs in winter is higher than that in summer. It is worth noting that the observation results also show that the average concentration of some species such as 4NC and MNCs in winter can be nearly 500 times higher than that in summer[27]. It should be noted that not all the observed results are the highest in winter and the lowest in summer. The observed results in France showed that the seasonal average concentration of NMAHs in urban areas was (18.2~31.7 ng·m-3)> in spring and (26.1~28.4 ng·m-3)> in autumn and winter (21.6 ng·m-3), and the observed concentration in winter was the lowest[32]. Different from the seasonal variation of total NMAHs concentration, specific species of NMAHs did not show similar seasonal regularity[27].
The concentration distribution of NMAHs in coarse and fine particles is also affected by the season. Most NACs species tend to be distributed in coarse particles in spring and in fine particles in summer, which may be due to the suitable temperature in spring, a large part of gas-phase NMAHs are condensed on coarse particles after formation, and a large number of fine particles are produced due to the secondary formation of high temperature in summer[12]. The atmospheric lifetime of NMAHs species is significantly affected by their different distributions in coarse and fine particles. NMAHs in coarse particles are easily removed from the atmosphere by dry and wet deposition, while NMAHs in fine particles and gaseous phase may remain in the atmosphere for a longer time[12,39].
2.2 Primary source
NMAHs may come from natural sources or anthropogenic emission sources. The natural sources are mainly biomass combustion emissions from forest wildfires and secondary generation in the atmosphere[12,18,23,24][2,12,25]. Anthropogenic sources are mainly coal combustion, wood combustion and motor vehicle exhaust emissions[2,12,24,38,41][9,27][41⇓~43].
The main sources of NMAHs in different regions are quite different. Biomass burning and coal burning in winter are the main sources of NMAHs in Thessaloniki, Greece. In Mainz, Germany, the winter aging process dominates the formation of NMAHs[2]. In Shanghai, China, high concentrations of NMAHs were observed during severe urban haze, and showed that NMAHs were closely related to biomass burning, fungal spores, pollen, combustion emissions and other factors[24]. However, other studies have shown that NMAHs in Shanghai are also closely related to fossil fuel combustion and secondary formation, especially secondary formation contributes greatly to the formation of NSAs in summer in Shanghai[35]. Secondary generation, biomass combustion, coal combustion and vehicle exhaust are the four main sources of NMAHs in Jinan, China, but the contribution of the four sources to NMAHs is different in different seasons[12]. Biomass combustion and coal combustion in winter are the main sources of NMAHs, while automobile exhaust in spring and secondary production in summer are the main sources of NMAHs[12]. The three main sources of NMAHs in Nanjing suburbs are traffic source, coal-biomass mixed source and industrial emission source, which contribute 32%, 44% and 24% of NMAHs in atmospheric fine particles, respectively[36]. It is worth noting that the contribution of coal-fired biomass in autumn accounts for more than half[36].
Coal combustion is an important anthropogenic source of NMAHs in the atmosphere. NMAHs from fresh coal burning plume observed on the summit of Great Dane, UK[44]. Domestic peat and coal burning in Cork, Ireland, emits a large amount of NMAHs[45]. The direct emission from urban coal combustion in northern China and the plume after aging contribute greatly to NMAHs in fine particles[38]. Knobloch et al. Determined the existence of 2 NP in the gas phase of lignite combustion gas in the laboratory, and obtained its emission factor as 1.5 mg·kg-1[46]. Lu et al. Further studied the NMAHs emission characteristics of nine commonly used types of residential coal, including lignite, bituminous coal, and anthracite, and found that the higher the coal rank (anthracite > bituminous coal > lignite), the lower the NMAHs emission factor (0.2~10.1 mg·kg-1)[13,47]. They used the emission factor method to estimate that the total amount of NMAHs emitted from coal combustion by Chinese residents in 2016 was about 178 ± 42 mg.
Vehicle exhaust emissions can detect the presence of NMAHs, and have been identified as one of the main sources of NMAHs in atmospheric particulate matter in urban areas[48,49][50]. Without post-treatment, the concentration of NPs and MNPs emitted from vehicle exhaust is in the range of 9~36nmol·L-1[51]. However, after studying diesel vehicle exhaust, Inomata and others believe that vehicle exhaust after-treatment devices are also an important factor leading to vehicle NMAHs emissions[42,43][42]. Lu et al. investigated in detail the NMAHs emissions of 18 types of motor vehicles using different fuels such as diesel, gasoline, and compressed natural gas, and found that vehicle exhaust emissions were dominated by 4 NP and MNPs (accounting for about 38.4% ~ 68% of NMAHs by mass)[41]. According to their estimation, the vehicle exhaust emission NMAHs in China in 2017 was about 58.9 Mg ( (-86%, 85%), 95% confidence interval)[41]. Field observations in Rome, Italy, Mainz, Germany, Strasbourg, France and Jinan, China have analyzed the results of vehicle exhaust emissions as the main factor affecting the content of NMAHs in the atmosphere[31⇓~33,38].
Levoglucan is recognized as a tracer of biomass combustion, and the high correlation of some NMAHs species with levoglucan indicates that they have similar sources or similar atmospheric processes[52]. This fraction of NMAHs species can be used as potential biomass burning tracers. Chow et al. Observed in Hong Kong, China, and found that in winter and spring, NPs and some of their derivatives MNPs showed good correlation with DNPs, NCs and some of their derivatives MNCs, and the concentration of total NMAHs showed good correlation with the concentration of levoglucosan. As shown in Table 2, the R value ranged from 0.71 to 0.83[5]. The study points out that the lower R value (0.45 ~ 0.57) in summer is due to the influence of other special anthropogenic combustion activities at the sampling site[5]. This conclusion is close to the results of Iinuma et al., who observed a good correlation between MNCs and levoglucosan in the burning plume of aged biomass in remote rural areas of Germany, with an R value of 0.749[23]. Kahnt et al. Observed 4 NC, MNCs and DMNCs at a rural observation site in Belgium, and their correlations with levoglucosan were 0.71, 0.66 and 0.65, respectively[9]. Claeys et al. Observed high concentrations of 4NC, MNCs and DMNCs in biomass burning events in Brazil, but they did not analyze the correlation between NMAHs and levoglucosan[18]. Caumo et al. Used 4 NC, MNCs, DMNCs in NMAHs and levoglucosan as tracers of sugarcane burning to study the effect of sugarcane burning on winter PM10 in Sao Paulo, Brazil[21]. In addition, there was no significant correlation between 4NG and levoglucosan observed during urban haze, R = -0.12[24].
2.3 Distribution Characteristics of "Qi" and "Grain"
The refers to the ratio of particle phase NMAHs concentration to the total concentration of gas phase and particle phase, which is an important characteristic value reflecting the "gas-particle" distribution characteristics of NMAHs. It can be calculated by formula (1) based on actual observation, and can also be calculated by formula (2, 3) using equilibrium adsorption distribution theory[8,25]. At the same time, the gas phase concentration can also be calculated from the particle phase concentration of NMAHs in combination with equations (1 to 3), or the particle phase concentration can be calculated from the gas phase concentration[8,25,53,54].
Note: — — particle phase concentration of NMAHs;
— — gas phase concentration of NMAHs;
– effective saturation mass concentration, μg·m-3;
— mass concentration of organic aerosol, μg·m-3;
— relative molecular mass of NMAHs, g·mol-1;
— species activity factor (assumed to be 1);
— — saturated vapor pressure, Pa;
— gas constant, 8.314 J·mol-1·K-1;
— — temperature, K.
The saturated vapor pressure of NMAHs is the core factor affecting the distribution characteristics of "gas particles"[8,12,14,25,55⇓~57]. However, at present, the understanding of the saturated vapor pressure of NMAHs is still insufficient, and there is a lack of sufficient measurement data[12,25]. On the one hand, there are many kinds of NMAHs, and more importantly, most of them belong to semi-volatile organic compounds, so it is difficult to accurately measure their saturated vapor pressure[58][14,55,56,58]. Bannan et al. Used a Knudsen cell diffusion mass spectrometer (Knudsen effusion mass spectrometry, KEMS) to measure the vapor pressure of some NMAHs species in the temperature range of 298 – 318 K, and then used the saturated vapor pressure prediction method proposed by O 'Meara et al. To evaluate the saturated vapor pressure of NMAHs species in the solid state and supercooled liquid state, but the prediction results were significantly higher[58,59][58]. In the absence of NMAHs saturated vapor pressure data, values can be calculated using the UManSysProp model developed at the University of Manchester[60]. Wang et al. Used this method to predict that 4 NP, MNPs (2M4 NP, 3M4 NP) mainly existed in the gas phase ( < 10%), and DMNPs, 4 NC, MNCs (3M6 NC (3 ∥ Methy ∥ 6 ∥ nitrocatechol), 3M5NC (3 ∥ Methy ∥ 5 ∥ nitrocatechol), 4M5NC (4 ∥ Methy ≠ 5 ∥ nitrocatehol) mainly existed in the particle phase ( > 60%)[8]. However, there are differences between the predicted results and some observed results. Taking 4 NP as an example, the predicted value ( = 0.83%) is significantly lower than some observed values ( = 17%, = 82%)[33,61].
The "gas-particle" distribution characteristics of NMAHs may also be affected by ambient temperature[14]. Li et al. Measured that the seasonal average concentration of fine particle NMAHs in Jinan area of China was in the range of 13.5~105.4 ng·m-3, and the concentration in winter was significantly higher than that in spring and summer[12]. Due to the lack of NMAHs concentration data in the gas phase in winter, they only analyzed the distribution characteristics of NMAHs species concentration in the gas phase and particle phase in spring and summer (Fig. 1). In spring, 4NP and MNPs (2M4NP, 3M4NP) were almost equally distributed in the gas and grain phases, while in summer, they were distributed in the gas phase. In spring and summer, 4 NC, MNCs (4M5 NC, 3M5 NC) and NSAs (5NSA, 3NSA) were almost all distributed in the particulate phase. In spring and summer, 2,4DNP (2,4 dinitrophenol) and 4M2,6DNP (4 Methy 2,6 Dinitrophenol) tend to distribute in the gas phase[12]. In summer, 4 NP and DNPs were mainly in the form of gas phase in the mountains of Bavaria and Munich, Germany[26]. In winter-spring alternation in Rome, Italy, 4NP, 3M4NP and 2,6-DM4NP (2,6-Dimethy-4-nitrophenol) mainly existed in particulate phase ( > 75%), while 2NP, 4M2NP (4-Methy-2-nitrophenol) and 5M2NP (5-Methy-2-nitrophenol) mainly existed in gas phase ( < 30%). In the spring and autumn in Strasbourg, France, 4M2NP was mainly found in the gas phase, while 2,4DNP was mainly found in the particulate phase[57]. In the smoke chamber experiment, the products of nitration of guaiacol, such as 4NG and 4,6DNG (4,6 Dinitroguaiacol), were preferentially present in the gas phase, while 5NG (5 Nitroguaiacol) was highly concentrated in the particle phase[11]. Based on this, Wang et al pointed out that if only the concentration of NMAHs in the gas phase or particle phase is considered in the study, it is likely to lead to the underestimation or neglect of the relevant concentration results or correlation results[8].
3 Formation and loss mechanism of monocyclic aromatic nitro compounds
3.1 Gas phase formation and loss mechanism
The formation and loss mechanism of NMAHs has been a hot and difficult topic for a long time. Aromatic volatile organic compounds such as benzene, toluene, cresol, phenol and catechol are considered to be important precursors for the formation of NMAHs[14,23,65]. As shown in Fig. 2, taking phenol as an example, the directly discharged phenol (primary) or the phenol generated by the reaction of benzene and OH radicals (secondary) and OH/NO3 radicals undergo a series of oxidation reactions to generate intermediate products, which are further oxidized by NO2 to final products[8]. Xie et al.'s Laboratory results show that NCs are the main products of "benzene ⁃NOx” photooxidation system" and MNCs are the main products of "toluene ⁃NOx” photooxidation system"[7][7,66]. Wang et al. Summarized the atmospheric secondary formation pathways of NMAHs species such as NPs, NCs, MNPs and MNCs based on previous studies, and their research results further pointed out that NCs and MNCs were mainly formed at high ambient NOx concentrations at night, while NPs and MNPs were mainly formed at low ambient NOx concentrations in the daytime[8][16,28,62⇓~64]. This is consistent with many observations[2,3,8,12,24]. Yuan et al. Summarized the similar formation mechanism of NPs based on the results of smoke chamber experiment and field observation[25][8]. The difference is that they believe that the detected phenoperoxyl radical is the main contribution pathway of phenoxyl radical compared with the reaction of benzene and phenol with free radicals to form phenoxyl radicals (Fig. 2)[25].
Other studies have shown that wood burning produces large amounts of cresols (p-cresol, o-cresol, m-cresol), almost all of which exist in the gas phase, and they are important precursors for the formation of MNCs[67][23]. The kinetics of the reaction of cresol with OH radicals in the presence of NOx, as well as the gas-phase products, have been reported[68⇓⇓~71]. The main products of m-cresol are both 3MC (3-Methycatechol) and 4MC (4-Methycatechol), the main products of o-cresol are 3MC, and the main products of p-cresols are 4MC. 3MC and 4MC can be further nitrated to form 3M5NC, 3M 6NC and 4M5 NC[23]. Another possible pathway for the formation of MNCs is the substitution of cresol by nitro group to form MNPs and then react with OH radicals to form MNCs, but this pathway has been proved to have little contribution to the formation of MNCs[68].
In addition to the deposition loss of NPs generated in the atmosphere, as shown in Figure 2, there are two other loss pathways: photolysis and free radical reaction, among which photolysis is the most important loss pathway of NPs (73.59%)[8,25,72]. Nitrophenoxyl radical is the main product of NPs photolysis loss pathway, and the proportion of HONO or phenoxyl radical reduced is relatively small[25]. This was followed by the sedimentation depletion pathway (19.05%) and the free radical reaction depletion pathway (7.36%)[25]. Nitrophenoxyl radicals can be further oxidized to DNPs by NOx[25]. In the actual observation, it was found that the peak of DNPs generation appeared at night and regularly lagged behind the peak of NPs, which further proved this view[25].
3.2 Formation mechanism of particle phase
At present, the research on the formation mechanism of particle phase of NMAHs species in the laboratory mainly focuses on the exploration of MNCs and NGs. Frka et al. Predict that 3MC reacts with in the liquid phase of particulate matter to form 3M5 NC and 3M4 NC in a certain proportion (3M5NC:3M4NC=1∶2.4,R2=0.990) rather than 3m 6 NC by electrophilic substitution reaction mechanism, and this hypothesis has been verified by atmospheric observation in Ljubljana, Slovenia[28]. Vidovic et al. Further proposed three pathways for the formation of 3MNCs (3Methy nitrocatechols) from the precursor 3MC in the nocturnal particulate matter liquid phase reaction pathway, including the direct participation of HNO2 in the conversion of 3MC through continuous oxidation and conjugate addition reactions in a typical atmospheric environment, the electrophilic substitution of aromatic compounds under strong acidic conditions, and the recombination of free radical species under strong acidic conditions[16]. As suggested by other researchers, the liquid-phase oxidation reaction pathways (including photooxidation and nocturnal oxidation) of particulate matter are important for the formation of MNCs, and the latter two liquid-phase reaction pathways occurring in acidic particles at high NOx at night are of great significance for the formation of 3MNCs[16,28]. Under this condition, Wang et al. Found that 3M5NC and 4M5 NC were enhanced at night and showed a high degree of spatial and temporal consistency in their observations. They proposed that (3M5 NC + 4M5 NC)/4 NP could be used to measure the importance of the liquid phase reaction pathway of particulate matter relative to the gas phase reaction pathway to produce NMAHs[8].
Nitrating agents produced by photolysis of nitrate and nitrite can affect the photonitration of aromatic compounds[73]. Marussi et al. Summarized this and pointed out that the effect of particulate matter pH on nitrification products should be carefully studied[15][73]. Kroflic et al. Described the complex mechanism of NGs formation through HONO radical reaction pathway and HOONO non-radical reaction pathway in mild liquid phase environment (ambient temperature, pH = 4 ~ 5), and showed that this reaction pathway is very important for NGs formation in the nighttime atmosphere[74]. Subsequently, they further simulated the heterogeneous nitration process of guaiacol in atmospheric aerosol. It is worth noting that water can promote the formation of NGs and NCs in the reaction mediated by OH radicals[11].
In addition, the increase of relative humidity is considered to contribute to the liquid phase oxidation reaction in particulate matter, because the increase of relative humidity is conducive to the increase of aerosol water content and the decrease of viscosity.It can promote the absorption of organic precursors and gas molecules, the diffusion of reactants in particulate matter and the chemical reaction in particulate matter, but the specific impact and role remain to be explored[75,76][77][78][79].
4 Optical Absorption Properties of Monocyclic Aromatic Nitro Compounds
Brown carbon and humic-like substances have been shown to play an important role in changing the light absorption of atmospheric aerosols and affecting the radiation balance[17,18]. NMAHs are considered to be an important component of brown carbon, and their strong light absorption characteristics in the ultraviolet band have attracted close attention[6,7,18,41,80⇓⇓~83].
Mohr et al. First measured the radiation absorption of NMAHs in the near ultraviolet band in 2012. The maximum absorption wavelengths of 4 NP, 4M2 NP, 4 NC and 2,4D NP were 317, 319, 337 and 292 nm, respectively (Table 2), which contributed 4% ± 2% of the light absorption of brown carbon at 365 nm in winter wood burning in Detlin, UK[1]. Zhang et al. Measured that 8 NMAHs species contributed 4% to the light absorption of aerosol total solution extracts at 365 nm wavelength in the Los Angeles Basin[17]. On the basis of more extensive research on NMAHs species, Teich et al. Found that the light absorption efficiency was related to the pH of the solution, and the higher the pH value, the greater the absorption efficiency. The Abs370 of the light absorption efficiency of NMAHs solution under acidic and alkaline conditions (defined as the absorption efficiency of the solution at 370 nm) ranged from 0.21~21.8 Mm-1 to 0.63~27.2 Mm-1, respectively.The light absorption contribution ratio of the total aerosol extract is 0.1% ~ 1.25% and 0.13% ~ 3.71%, respectively, which is more than five times of its mass contribution[6]. Xie et al. Studied the optical absorption characteristics of NMAHs in a wider spectral range (365 ~ 550 nm), and found that the optical absorption contribution at 365 and 400 nm is more than 2 times higher than the mass contribution[7]. Dong et al. Found that the absorption efficiency per unit mass of 12 common species in NMAHs was significantly higher than that of brown carbon in the ambient atmosphere, ranging from 3.2~37.9 m2/gC[84]. Their observations at six stations in eastern China showed that the average absorption of NMAHs at 365 nm accounted for 1. 3% to 5. 9% of brown carbon in the atmosphere[84]. In addition to the photoabsorption effect, NMAHs also have the "photobleaching" effect, which generally refers to the process of light absorption reduction caused by aging during the atmospheric transport of brown carbon[85]. Wang et al. Introduced in their review article on brown carbon that NMAHs can rapidly "photobleach" during atmospheric aging, resulting in a decrease in the total light absorption of brown carbon[85].
Considering that the optical absorption contribution of some NMAHs is much greater than their mass contribution, a small amount of NMAHs may have a significant impact on the optical properties of aerosols, and it is suggested that more attention should be paid to their impact on the optical properties and radiation balance of atmospheric aerosols in future studies[6,7][30][6].
5 Environmental Health Effects of Monocyclic Aromatic Nitro Compounds
NMAHs may sink into the soil with snowfall in winter and enter the plant through the absorption of plant roots or leaf cuticle, thus affecting the absorption of plant nutrients, hindering plant development, hindering the normal growth and fruiting of plants, and causing the degradation of forest vegetation, so they have ecotoxicity[23][14][86][87][20].
After Na Nałezcz Jawecki et al. found that NPs had toxic effect on Vibrio fischeri, Pflieger et al. found that NGs had more obvious inhibitory effect on Vibrio fischeri, and the toxicity of a single substance from weak to strong was GUA (Guaiacol) < 6NG (6 Nitroguaiacol) < 4NG < 4,6 DNG. Although no obvious combined effect between substances was observed in the experiment, the toxicity of NGs species has been proved[107][20][88][20]. However, Su et al. Observed that the combined toxicity of some mixtures of NMAHs species and copper solution to Photobacterium phosphoreum was antagonistic under certain conditions, so they suggested developing a toxicity assessment model for complex mixtures to avoid using single substance toxicity data to assess the toxicity of mixtures and draw wrong conclusions[29]. Yang et al. Summarized and showed the progress and application prospects of many microbial degradation technologies for NMAHs, but there are still many shortcomings in practical application[89].
NMAHs can affect human and animal health, leading to respiratory diseases, skin diseases and cardiovascular and cerebrovascular diseases[14][90,91]. Many studies have proved the damage and effect of 3M-4NP on rat respiratory epithelial cells, spleen lymphocytes and chicken embryo spermatogonia, and found that 3M-4NP has reproductive toxicity to quail, rats, chickens and other animals[92][93][94][95,96][97,98][94]. Majewska et al. studied the biological toxicity of three NPs homologues on eukaryotic cell models of respiratory system, and found that the increase of NPs concentration would lead to changes in cell morphology and a significant decrease in cell activity.They proposed that NPs at low concentration (100μg·mL-1) may react with phospholipids by causing structural rearrangement of phospholipids on the cell membrane, and at high concentration (2 mg·mL-1), NPs may directly cause the destruction of phospholipids on the cell membrane and cause substantial damage to the cell membrane[10]. To some extent, this explains the cell damage caused by wood combustion smoke, and also suggests that direct exposure to high concentrations of biomass combustion atmosphere should be avoided to avoid damage to the respiratory tract[99]. Kitanovski et al. First used artificial lung fluid to evaluate and quantify the bioabsorbability of NMAHs in inhalable particulate matter PM3 after inhalation into the lung[4]. They found that the bioabsorbability of NMAHs was very high-about 55% to 75% of the mass of NMAHs leached from winter samples in acidic (pH = 4.5) artificial lung fluid was absorbed, while the NMAHs leached from summer samples was almost completely absorbed (94%)[4]. About 56% to 66% of the mass of NMAHs leached from winter and summer samples in neutral (pH = 7.4) artificial lung fluid was absorbed[4]. The high bioabsorbability of NMAHs indicates that NMAHs may be more likely to cause lung damage when inhaled into the lungs.
6 Measurement techniques for monocyclic aromatic nitro compounds
6.1 Off-line measurement technique
NMAHs are semi-volatile organic species, so instruments that measure VOCs species can be used to measure NMAHs. High performance liquid chromatography (HPLC) or Gas chromatography mass spectrometry (GC/MS) is the most widely used method to measure NPs[14,25,32,100]. In order to optimize the separation of phenol, NPs and DNPs by high performance liquid chromatography, Belloli et al. Tested the performance of seven chromatographic columns, and found that the silicon-based C18 material filled with 5 μm particles (100 100 Å) performed best, which could achieve the analysis of 4 H resolution in laboratory and field observation[101]. The disadvantage of this method is that it is time-consuming and laborious, and it can not respond quickly online[25,32].
6.2 On-line measurement technique
Chemical ionization mass spectrometer (CIMS) can measure NMAHs in particle phase on line, so it has been studied or applied in many works[1][25,102,103]. The m/Z signals of NPs, MNPs, NCs, MNCs, DNPs and MDNPs are 138.0197, 152.0353, 154.0140, 168.0300, 183.0047 and 166.0510, respectively[1,25]. Special attention should be paid to the fact that the peak value of the target ion peak should be the largest or much larger than the peak value of the adjacent peak when fitting the signal peak, otherwise the fitting accuracy of the target peak signal will be affected, resulting in inaccurate mass calibration[25][104,105][104]. In the work of Yuan et al., DNPs signal peak fitting encountered this problem, resulting in the error of mass correction as high as 81%[25].
Kitanovski et al. Used Liquid chromatography/mass spectrometry (LC/MS) to study NMAHs including NPs, MNPs, NGs, 4NC, and NSAs[27,34]. In order to separate and detect NMAHs in atmospheric aerosols, they initially established a new chromatographic method based on reversed-phase chromatography combined with ultraviolet/visible light detection, which uses Liquid chromatography/electrospray ionization linear ion trap mass spectrometry (LC/ESI LITMS) system to separate and detect target analytes in winter aerosols[34]. Subsequent studies have used this method to identify NMAHs with good results[106]. Ultra performance liquid chromatography/electrospray ionization quadrupole time of flight mass spectrometry (UPLC/ESI/Q/TOF/MS) can also be used for NMAHs detection. Based on this technique, Zhang et al. Analyzed several nitrogen-containing compounds that may absorb ultraviolet light at 365 nm[17].
7 Conclusion and prospect
The field observation experiments and laboratory experiments on atmospheric NMAHs species at home and abroad have deepened the understanding of these substances. At present, the understanding of the spatial and temporal distribution, source characteristics, "gas particle" distribution characteristics, generation and loss mechanisms, optical absorption characteristics, environmental health effects, and measurement techniques of NMAHs is mainly as follows.
(1) The average observed concentrations of NMAHs at urban and rural background sites in Europe and China were generally higher than those at urban and rural background sites in North America and South America. To some extent, the observation results of polar background observation stations show that the high latitude region is less affected by NMAHs than the middle and low latitude region. The average observed concentrations of NMAHs in different regions showed seasonal variation characteristics, and in most regions, the average concentrations of NMAHs were the highest in winter, followed by those in autumn, spring and summer, but there was no uniform trend in the seasonal average concentrations of specific species.
(2) The natural emission sources of atmospheric NMAHs species include biomass combustion and atmospheric secondary generation, and the anthropogenic sources include coal combustion, wood combustion, motor vehicle emissions and industrial emissions; Affected by the season, the contribution of different sources to the total concentration of NMAHs is different in different seasons. NCs and MNCs in NMAHs species can be used as tracers of biomass combustion.
(3) The "gas-particle" distribution characteristics of NMAHs species mainly depend on three factors: saturated vapor pressure, ambient temperature, and particle size. Saturated vapor pressure is the core factor. In the actual atmosphere in spring and summer, 4 NC and MNCs tend to partition in the particle phase, NSAs almost all partition in the particle phase, and DNPs tend to partition in the gas phase. In spring, the distribution ratio of MNPs in the two phases of gas and grain is close to the same, while in summer, it tends to distribute in the gas phase.
(4) Benzene, toluene, cresol, phenol and catechol are important precursors for the formation of NMAHs; Both gas phase and particulate matter liquid phase reactions are important reaction pathways for the formation of NMAHs, and the gas phase pathway is more important. NCs and MNCs were mainly formed at high NOx concentrations at night, while NPs and MNPs were mainly formed at low NOx concentrations in the daytime. Acidic particles in high NOx environment at night are important for the generation of MNCs. The formation process of NGs may be affected by temperature, relative humidity, pH, nitrating agent and other factors. Photolysis is the most efficient loss path for NMAHs in gas-phase reactions.
(5) The strong radiation absorption in the ultraviolet region makes NMAHs species an important component of brown carbon, and the light absorption contribution of some species is twice or even more than five times its mass contribution. The light absorption efficiency per unit mass of NMAHs is significantly higher than that of brown carbon in the environment, but the light absorption of brown carbon in the aged NMAHs can be reduced due to "photobleaching".
(6) NMAHs can hinder the growth, development, flowering and fruiting of plants, cause vegetation degradation, and have ecotoxicity. In addition, it can also affect human and animal health, causing respiratory diseases, skin diseases and cardiovascular and cerebrovascular diseases, and has reproductive toxicity. NPs, NGs and MNPs have been identified as species that can cause adverse effects. In addition, due to the pollutant properties of NMAHs, it will have an important impact on air quality and ecosystem.
(7) NMAHs measurement techniques include off-line measurement techniques and on-line measurement techniques. The off-line measurement technology represented by chromatographic technology has certain limitations. On-line measurement techniques, represented by chemical ionization mass spectrometers, make it possible to comprehensively characterize and quantify more NMAHs species.
In general, great progress has been made in the understanding of atmospheric NMAHs species, but there are still many shortcomings, which to some extent hinder our comprehensive understanding of atmospheric NMAHs species. The contents that need further study include the following aspects.
(1) Focus on China, establish NMAHs species emission inventories of major anthropogenic emission sources such as motor vehicle emissions and coal combustion, so as to facilitate source control.
(2) It is necessary to strengthen the research of observation technology and observation instruments, carry out field observation in more places, pay more attention to NGs species in observation, and increase the research on the formation mechanism of NSAs species in laboratory research.
(3) It is necessary to focus on the optical absorption properties of NMAHs species and understand the optical absorption properties of more NMAHs species in a wider spectral range, so as to comprehensively assess the impact of NMAHs on aerosol optical absorption properties and atmospheric radiation balance.
(4) The saturated vapor pressure is the core factor affecting the "gas-particle" distribution of NMAHs species, but the accurate determination of its saturated vapor pressure is still one of the problems that have not been completely solved, and new measurement methods need to be established and improved.
(5) The atmospheric hygroscopicity and activation characteristics of water-soluble species in NMAHs are concerned in laboratory research work to understand the atmospheric hygroscopic behavior and activation behavior of such species.
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