Ambient BTX Observation nearby Main Roads in Hefei during Summer Time

In order to study the sources, chemical evolutions and impacts on air quality of BTX (benzene, toluene, xylene, etc.) in a typical unban area, a field campaign was conducted with our homemade differential optical absorption spectroscopy (DOAS) instrument in May 2014 at the North part of First Ring Road of Hefei. During the entire measurement, BTX showed high levels in the morning and at night except toluene and phenol. The observed average of benzene is 5.11 μg m, toluene is 19.15 μg m, m-xylene is 14.74 μg m, p-xylene is 1.47 μg m, phenol is 0.67 μg m, which are at a low pollution level compared with other cities. The ratios of toluene to benzene range from 1.6 to 8 and the correlation coefficient R of benzene and CO is about 0.88, suggesting the main source of local BTX is traffic emissions. Phenol shows negative correlation with benzene (R = 0.87) in the afternoon, which means that the oxidization of benzene by OH radicals is an important source of phenol. The relations with meteorological conditions were also disscussed, indicating the contribution of solvent evaporation from nearby point sources to toluene in the afternoon. Wind speed is another important factor to the concentration distribution of BTX. The correlation coefficient R of benzene and PM2.5 is 0.83, inferring the influence of street traffic emission on roadside pollution levels.


INTRODUCTION
With the acceleration of industrialization and rapid vehicle population growth, large amounts of volatile organic compounds (VOCs) are emitted into the air.VOCs play a key role in the formation of O 3 and other photochemical oxidants which are closely related to the photochemical smog (Tang, 1990;Atkinson, 2000).VOCs also contributes to secondary organic aerosol (SOA) formation by homogeneous nucleation or adsorbed onto the surface of the particles (Yuan, 2012;Huang et al., 2014).In addition, some kinds of VOCs are harmful to human body.Benzene is a kind of human carcinogen (USEPA, 2012a) and toluene will stimulate human nervous system strongly (USEPA, 2012b).Because of the effects on atmospheric cleanliness and human health, BTX has aroused intensive concern in the populated urban areas.
Following the first measurements of BTX in 1968 (Lonneman et al., 1968), many previous observations of BTX were performed by DOAS (Volkamer et al., 1998;Xie et al., 2006), Gas Chromatography-Mass Spectrometer/ Flame Ionization Detection (GC-MS/FID) (Kelessis et al., 2006;Rattanajongjitrakorn and Prueksasit, 2014) and Proton Transfer Reaction Mass Spectrometer (PTR-MS) (Taipale et al., 2008;Liu et al., 2012) in metropolis.DOAS is a non-contact technique for measuring multiple trace gases simultaneously in real time.It can avoid the influences of sampling and the path-averaged results are representative for studying the BTX pollution level of an area (Platt and Stutz, 2008).
This study was designed to characterize the air pollution in unban area mainly related to traffic emissions.Ambient BTX is continuously monitored by our homemade DOAS instrument for half a month at roadside of an open street representing average traffic flow conditions in Hefei.Hefei is the capital city of Anhui Province and located in eastern China.With urban expansion, the industrial and GDP growth rates maintain in the forefront of all capital cities of China in recent years (Anhui News, 2013).The resident population is more than 7.7 million (ASB, 2015) and vehicle population exceeded 1.2 million at the end of 2014 (Hefei Daily, 2015).Besides that, Hefei is constructing the subway after 2009, which will continue until 2025 (Hefei Daily, 2014;NDRC, 2014).The pollution levels of BTX were evaluated and correlations between BTX, BTX and CO, meteorological conditions, PM 2.5 (particles with aerodynamic diameter not above 2.5 µm) were investigated to explore the sources, chemical evolutions and influence of BTX.

DOAS Setup and Measurement Site
The set-up of the DOAS system has been described in detail in our previous studies (Qin et al., 2006;Xie et al., 2006), so here we confine ourselves to a brief description.The sketch of instrument is shown in Fig. 1, which includes a Cassegrain telescope with 220 mm diameter acting as transmitting and receiving component, a 150 w Xenon lamp (Hamamatsu Photonics.)as the light source, an array of retro reflectors, a 600 µm diameter fiber, a UV band spectroscopy (Ocean Optics QE65000) and computer.The spectral detection range was 210-335 nm, with a resolution of 0.33 nm.The detector was cooled to -10°C, to reduce the effects of dark current.
The observation site is located at the North part of First Ring Road in Yaohai District of Hefei, and the traffic there is busy during rush hours (According to our statistical results, there are 3980 vehicles per hour while private cars, buses and motorcycles are about 3430, 480 and 70 per hour, respectively).As shown in Fig. 2, the telescope was placed on the 4th floor of an office building nearby the road, about 10 m high above the ground.The array of retro reflectors was placed outside a southwest direction building on the 5th floor, with a distance of 427 m from the telescope.The optical path (red line) passed above the main road.There are two subway lines (green lines) and four subway stations (blue dots) under construction within 1 km distance, one petrol station (blue triangles) with 600m distance in the southwest and other two petrol stations in the northeast with 400 m and 1 km distances, respectively.There is also an automobile 4S (Sale, Sparepart, Service and Survey) center (blue five-pointed star) with 1.7 km distance in the northeast.

Data Analysis
The measurement principle and retrieval procedure of DOAS have been described in detail in our previous studies (Xie et al., 2006;Lu et al., 2016).Spectra were analyzed in the wavelength range of 251-278 nm for BTX.Main interfering gases within this band are O 2 , O 3 and SO 2 .The high resolution absorption cross sections of benzene, toluene, m-xylene, p-xylene, phenol (Etzkorn et al., 1999), O 3 (Voigt et al., 2001) and SO 2 (Vaele et al., 1994) were used in the spectra fitting.The saturated absorption of O 2 does not follow Lambert-Beer's law to be the major confounding factor (Volkamer et al., 1998).Two oxygen absorption spectra with optical path of 697 m and 1133 m are used as reference spectrum to eliminate interference absorption of O 2 (Xie et al., 2006).The detection limits with 854m optical path are as follows: benzene is 1.7 µg m -3 , toluene is 1.6 µg m -3 , mxylene is 1.4 µg m -3 , p-xylene is 0.4 µg m -3 , phenol is 0.08 µg m -3 .The time resolution of BTX is about 12-33 s.The uncertainty caused by instrument like dark current, offset and stray light is about 2%.Combining the uncertainty of least-squares fitting (10%) and the absorption cross section (5%) yields the overall uncertainty of 12% for measuring BTX with this instrument.The temporal resolutions of meteorological data obtained from Yaohai meteorological site and CO, O 3 and PM 2.5 from Hefei Environmental monitoring station are 1 hour.

RESULTS AND DISCUSSION
The Observation Result
The diurnal variations of BTX, O 3 and CO during the entire measurement period are shown in Fig. 4. The values of benzene, m-xylene, p-xylene and CO are high in the morning and night, which shows the effect of traffic rush hours and pollutant accumulation.While toluene, phenol and O 3 have high values in the afternoon.Normally, the major source of urban benzene is vehicle emissions, whereas toluene is also associated with industrial emissions, solvent and fuel storage (Liu et al., 2008).In the daytime, the petrol stations are often busy refueling, considerable waterproof paint (Yao et al., 2008) is used in the construction of nearby subways and subway stations and many cars are being painted in the automobile 4S center.The average temperature in the afternoon is high (close to 36°C) and will accelerate the volatilization of paints and solvents.All these may be able to explain why benzene and toluene show different variation characteristics in the afternoon.

Pollution Levels of BTX
Fig. 5 shows the frequency distribution histogram of benzene and toluene (hourly average).Both frequency distribution profiles for the two species are single-peaked.The occurrence frequency of concentration values for Benzene between 3 µg m -3 and 6 µg m -3 is about 64%, and that for toluene between 15 µg m -3 and 25 µg m -3 is about 72%.European standard (the European Union) of benzene is 5 µg m -3 as annual average (EPA, 2008).In this campaign 48.6% data exceeded the standard.The WHO standard of toluene is less than 7.5 µg m -3 as daily average (Xie, 2003).66.9% data have exceeded the standard during this observation.
In order to figure out the BTX pollution level nearby major road of Hefei, the averaged BTX observations of this campaign are compared with early reports of various urban main roads and shown in Table 2. Hefei is at a relatively low pollution level of BTX in the 12 listed cities.Compared to our study in 2002, benzene and toluene increased by 36.6% and 38%, respectively.

BTX Pollution Source Analysis The Correlations Analysis during the Aromatics Event
Urban ambient BTX is mainly from human activities,   the correlation between them is a significant indicator for studying the pollution source (Xie, 2003).Fig. 6 illustrates the correlations between BTX in the Aromatics Event.It can be inferred that BTX have the same main source in the Aromatics Event.Another indicator of BTX pollution source is the ratio between aromatic hydrocarbons.Especially, the ratio of toluene and benzene is often set as the identification of transport emissions when it falls within a particular range.The ratio of toluene and benzene for urban traffic source is usually 1.5-3.0,for fuel evaporation process is 2.7-4.0 (Xie, 2003) and for solvent evaporation is more than 20 in Yao et al., 2008.The ratios of toluene and benzene in Fig. 6(b) are mainly between 1.6 and 5 and have a linear fitting slope of 2.65.In Fig. 6(d), the ratios are mainly between 3 and 8 and the fitting slope is 2.96.It indicates that ambient toluene and benzene are mainly from traffic emissions and the effect from solvent or fuel evaporation is greater in the daytime.
Ambient CO of main roads is primarily from vehicle emissions (Tang, 1990).Fig. 7 shows the correlation diagram of observed benzene and CO in the Aromatics Event and the correlation coefficient R is 0.88 with a slope of 0.2.Considering the good correlation of BTX in Fig. 6, it can be proved that the main source of BTX in the Aromatics Event is transport emissions.

The Analysis during the Photochemistry Event
Aromatics react with OH radicals in the troposphere and it is important to clear aromatics.The main reaction mechanism of BTX and OH radicals is addition reaction (Atkinson, 2000).Ambient benzene is oxidized by OH radicals and produce phenol and other products (Tang, 1990).The correlation of benzene and phenol with correlation coefficient R of 0.87 and slope of -0.18 in the Photochemistry Event are shown in Fig. 8(a).It is obvious that the increasing of phenol was accompanied by the decreasing of benzene.Fig. 8(b) shows the diurnal variations of phenol: benzene ratio and ozone.The ratio increases significantly from 7:00, peaks (0.2) at 14:00, and declines to 0.08 at 22:00.The high O 3 concentration in the afternoon indicates the occurrence of photochemical reaction.We also analysis the correlation between phenol and other BTX compounds we measured and the correlation coefficients R are less than 0.4.It can be inferred that the reaction of benzene and OH radicals is an important source of phenol in the Photochemistry Event.
To calculate the photochemical age, two species with large difference in k OH values are often used.Here, we choose m-xylene and benzene to calculate the photochemical age as follows (Yuan, 1990): where k M (18.9 × 10 -12 cm 3 molecule -1 s -1 ) and k B (1.22 × 10 -12 cm 3 molecule -1 s -1 ) are the reaction rate constants of OH radicals with m-xylene and benzene, respectively (Atkinson, 2006).

Mxy
Ben is the ratio of ambient m-xylene to benzene.Fig. 9 shows the diurnal variation of m-xylene to benzene and the calculated photochemical age which is also called OH exposure.The ratio is high in the morning and early night reflecting the traffic emission in rush hours and low in the afternoon since the consumption of m-xylene is greater than benzene during the photochemical reaction.It has a slight drop in the midnight which is likely caused by the reaction with NO 3 radicals.The highest ratio (3.35) at 8:00 is set as the initial emission ratio.The diurnal variation of OH exposure has a negative correlation with the ratio and peaks at 13:00 with a value of 3.7 × 10 10 molecule s cm -3 .
The Relationships with Meteorological Conditions Fig. 10 shows the time series of temperature, wind speed   and wind direction.During the entire observation, the temperature of the site ranges from 23 to 36°C with an average of 29°C.Low wind speed prevails over the measurement period, and ranges up to 4 m s -1 on May 22, 24 and 29.Fig. 11 is the relative frequency distrubution plot of wind speed against wind direction.We can see clearly that south is the dominant direction and wind direction from N-NW to W-SW is absent.High speed wind is mainly from the south and northeast.Fig. 12(a) shows the relative frequency distrubution plot of benzene against wind direction.Benzene of low concentration (< 4 µg m -3 ) is mainly from the south (12.2%) and S-SW (6.3%).For high concentration (>10 µg m -3 ), which mostly appeared in the Aromatics Event, the main directions are northeast (0.79%), south (0.79%) and S-SW (0.79%) where located busy roads near railway station and one main road (Changjiang East Road).Fig. 12(b) shows the relationship of benzene and wind speed.We can see the higher the wind speed, the lower the concentration of benzene, which is due to the faster diffusion of pollutants.Benzene of high concentration mainly occurs in low wind or calm conditions.

The Analysis of Afternoon Toluene
In foregoing paragraphs, we mention toluene appeared unusual high values in the afternoon (see Fig. 4) and suggest the sources may be the fuel and solvent evaporation from nearby gas stations, subway stations under construction and the automobile 4S center.Here we study the distribution of toluene with high value in the afternoon against the wind direction and show the result in Fig. 13(a).Toluene was mainly from S-SW where located the petrol gas station and subway stations and northeast where located the two petrol gas stations and the automobile 4S center.This is consistent with our suggestion well.We also estimate the diurnal variations of toluene related to the fuel and solvent evaporation based on the tracer mathod (Yuan, 1990).This estimation presume the proprtion of BTX to be unchanged during the diffusion process and the reaction of phenol with OH radicals to be negligible.The reaction rate of OH radicals with toluene is 4.8 times of the reaction with benzene (Ye et al., 2012).The ratios of benzene to toluene and phenol during the night are used to eliminate the impact of traffic sources.Toluene from fuel and solvent evaporation (Tol * ) is calculated as follows: and shown in Fig. 13(b).The values of Tol * are close to 0 in the night and up to 12.95 µg m -3 at 14:00, accounting for about 38% of the whole toluene pollution in the daytime.

The Relationship with Roadside Pollution Levels
The concentration of PM 2.5 is an index of air pollution levels, especially of the atmospheric visibility.Vehicle emissions is generally regarded as a significant source of air pollution in cities (Huang et al., 2014).In the end of May 2014, the air pollution near the site was serious and the concentration of PM 2.5 was closed to 250 µg m -3 .We chose the period from May 26 to 31 to study the correlation of benzene and PM 2.5 and the result is shown in Fig. 14.The correlation coefficient R of benzene and PM 2.5 is 0.83 with a slope of 16.13.The results indicate a noticeable influence of street traffic emissions on measured roadside pollution levels.14.74 µg m -3 , 1.47 µg m -3 and 0.67 µg m -3 , respectively.Nearly 49% data of benzene exceeded the European standard and 67% data of toluene exceeded the WHO standard.The pollution of BTX in Hefei is not serious compared to other cities and there are 36.6%and 38% increase for benzene and toluene in summer since 2002, respectively.Ambient BTX of the site is mainly from local traffic emissions based on the correlation analysis.Phenol in the afternoon is mainly from the reaction of benzene and OH radicals.The calculated photochemical age peaks at 13:00 with a value of 3.7 × 10 10 molecule s cm -3 .Volatilization of paints and solvents accounts for about 38% of the whole toluene pollution in the daytime and the source could be nearby gas stations, subways under construction and an automobile 4S center.Wind speed is another important factor affecting the concentration of BTX.The street traffic emissions visibly effects the roadside pollution levels.
Figs. 6(a) and 6(b) are during the night of May 30 and Figs.6(c) and 6(d) are from the night of May 22 to the same time of May 24.From Figs. 6(a) and 6(c) we can see similar variations of benzene and toluene and the correlation coefficients R in Figs.6(b) and 6(d) are 0.81 and 0.73, respectively.As shown in Figs.6(e), 6(f) and 6(g), The correlation coefficients R of benzene and phenol, p-xylene and m-xylene in the Aromatics Event are 0.91, 0.91 and 0.81, respectively.

Fig. 6 .
Fig. 6.The time series and correlation of Ben and Tol (a, b) in the night of May 30; (c, d) from the night of May 22 to May 24; The correlation between Ben and (e) Phe; (f) Pxy; (g) Mxy in the Aromatics Event.

Fig. 7 .
Fig. 7.The correlation between Ben and CO in the Aromatics Event.

Fig. 8 .Fig. 9 .
Fig. 8. (a) The correlation between Ben and Phe in the Photochemistry Event; (b) Diurnal variation of Phe/Ben ratios comparing with O 3 .

Fig. 11 .
Fig. 11.Relative frequency of wind speed against wind direction.

Fig. 12 .
Fig. 12.(a) Relative frequency of Ben against wind direction; (b) Relation of wind speed and Ben.

Table 1 .
The statistical results of BTX*, O 3 and CO.

Table 2 .
Comparison of BTX in various urban main roads.