Source apportionment of VOCs and their impacts on surface ozone in an industry city of Baoji, Northwestern China

Level of surface ozone (O3) has been increasing continuously in China in recent years, while its contributors and formation pathways are less understood. In this study, distributions of volatile organic compounds (VOCs) and the roles on O3 pollution have been investigated in a typical industrial city of Baoji in Northwestern China by means of monitoring of their concentrations and other trace gases. The air samples have been collected at three sites according to urban function area. Concentration of VOCs in Weibin site, which near to industrial zone, was higher than most of other cities in China, and the ambient VOCs were dominated by aromatics and alkenes. The temporal variations of VOCs and O3 coincided with the surface wind, implying that the formation of O3 was impacted by both exports of plumes upwind and local photochemical reactions. Result of source apportionment indicated that industrial emission, vehicular exhaust, and solvent evaporation were three major pollution origins. Alkenes and aromatics contributed to the largest fractions of photochemical reactivity, suggesting the strong influences from industrial and traffic sectors. The study presents the characteristic VOCs and other factors in the contribution of O3 formation in China.

Ozone (O 3 ) is important constituent of the atmosphere, the concentration of ozone mostly decreases from stratosphere to surface ground downward 1 . Surface or tropospheric O 3 originated mainly from regular transportation from stratosphere 2 , direct emissions in source-dominated regions 3 , and photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NO x ) 4,5 . In between, photochemistry plays an important role on both sources and sinks of O 3 , subjected to the atmospheric NO x levels 4, 6 . Xue et al. 3 reported that the in-situ photochemical production is the main source for the surface O 3 in megacities in China based on calculations with observation-based model. In urban site, the mixing ratio of NO x is often high (up to 70 ppbv) due to large quantities of emissions from heavy industries and vehicular engines. The O 3 budget is thus sensitive to photochemical reactivity of VOCs and NO x 3, 7-9 . Under short term regulatory measures, reduction of anthropogenic VOCs emission is efficient in shrinking of O 3 peak. However, the approaches only lead the reaction mechanism between VOCs-NO x -O 3 transferred from NO x -dependent to VOCs-dependent while VOCs becomes a limiting factor, in the case of no decline of NO x emission. Instead, to further reduce surface O 3 level, regulations on both VOCs, biogenic VOCs (BVOCs) and NO x are needed in a long term run 9 .
Understanding of the sources of VOCs is a base for O 3 pollution control. Common anthropogenic activities include coal burning from industrial and residential uses, vehicle exhausts, gasoline volatilization, solvent use, petrochemical manufacturing and biomass burning [10][11][12][13][14][15][16][17] . In addition, biogenic source, in particular of vegetation emission, is a vital factor in VOCs budget [18][19][20] . Source apportionments of VOCs in ambient environments are always deployed with receptor models, like positive matrix factorization (PMF), chemical mass balance (CMB),

Results and Discussion
Characterization of VOCs. Table 1 summarizes the mixing ratios of different classes of VOCs PAMS quantified at the three sites (the mixing ratios of individual compound were listed in Table S1). Isoprene is classified separately from alkenes to evaluate its strong indication from biogenic sources. The average mixing ratios of total quantified VOCs PAMS (TVOCs PAMS ) were 48.03 ± 18.15, 17.00 ± 11.36, 17.27 ± 10.18 ppbv measured in the Weibin, Chencang and Miaogou sites, respectively. Comparing to the ambient levels in the Chinese megacities, our VOCs PAMS in Weibin site were higher than those in Beijing in summer of 2005 (36.4 ± 12.1 ppbv), but close to Southern regions such as Guangzhou (40.58 ± 0.89 ppbv) and Hong Kong (45.83 ppbv) where influenced by industrial and traffic emissions in major 7,8,23 .
In general, the abundances of most VOCs PAMS descended notably in the order of Weibin > Chencang > Miaogou. The composition of VOCs PAMS significantly varied among the sites as well. The highest levels of aromatics and alkenes were observed in the Weibin site, with the mixing ratios of 16.6 ± 8.0 and 7.7 ± 4.5 ppbv, respectively. The high abundances can be ascribed to the existence of fossil fuel combustion sources installed in the industries nearby the site 10,42 . Particularly, aromatics had a molar contribution of 51.0% of TVOCs PAMS in the Weibin site (Table 1). Benzene, the most abundant aromatic in the samples of Weibin site, is an well-known tracer for the emissions of coal, biomass burning, and automobile 42 . Propene was the next abundant compound, followed by undecane, 2-methylhexane, toluene, and iso-pentane. This distribution was consistent with the emission profiles of metal smelt, coke production and power plants, which were located at the upwind position of the Weibin site 27 . The result also implies that the strong impacts from combustion sources such as industrial coal burning and vehicular emission. In comparison with the Weibin site, lower mixing ratio of benzene was measured in the Chencang site, C 2 -C 5 alkenes, C 2 -C 5 alkanes, C 10 -C 12 alkanes and xylenes were higher than other VOCs PAMS , where the VOCs PAMS profile represents the more dominance of traffic-related sources 10,23 . This finding was relatable to the facts that the Chencang site was next to two highways (Lian-huo highway and Bao-han highway) and less factories nearby. At the Miaogou site, the mixing ratios of most VOCs PAMS were the lowest among the three sites, with exceptions of toluene, styrene, undecane, isoprene, and p-diethylbenzene. Toluene and styrene are universal components in solvents used in manufactures 43 , while C 9 -C 12 alkanes and isoprene are tracers for diesel exhaust and biogenic sources, respectively 10,18 . This VOCs PAMS distribution indicates that the air quality at Miaogou site was impacted by a mix of natural and anthropogenic pollution sources. Scattered small-scale paint factories presented around the site in the on-site survey, and the plants in the forests can contribute greatly on the biogenic emissions 10, 43 . One-week temporal variation of VOCs PAMS levels at the Weibin site is illustrated in Fig. 1. The mixing ratios of TVOCs PAMS in Weibin site varied with surface wind directions. Higher mixing ratios of VOCs PAMS were shown on the sampling days of June 16, June 17 and June 19, when the dominant wind was easterly ( Figure S1). Dense power plants, metal smelt, coke production and coal chemical industries were located in the eastern and northeastern regions of Baoji. The high-frequency easterly surface winds could thus bring up those discharged pollutants to the downwind locations. The consequent effects can be also reflected on the significant elevation of corresponding marker species such as benzene and propene. In addition, from the time series, the highest TVOCs PAMS was often seen in the afternoon when the easterly surface wind was dominated within a day. While the wind direction swiped from easterly to westerly gradually, the TVOCs PAMS had an obvious decline. This could also demonstrate the strong impact from the industrial emissions.
Correlation between VOCs species. Correlation analysis between individual VOCs PAMS compound was used to interpret the potential and dominant pollution sources 31 . Propane and n-butane are important VOCs from vehicular emissions, and their correlation acts a useful indicator for traffic contribution 10,31 . In the current study, propane was well-correlated with n-butane at both sites (0.82 < R 2 < 0.87), with slopes of 0.51, 0.37, and 0.49 for the Weibin site, Chencang and Miaogou, respectively (Fig. 2a). The ratios of n-butane/propane were close to that reported in a tunnel study (~0.5) 10 , demonstrating the inalienable input from vehicular emission in Baoji. The regressions of aromatics and long-chain alkanes (e.g., C 8 -C 12 ) varied significantly among the sampling sites, appointing to a wide variety of source contributions (Fig. 2c, Table S2). In the Weibin site, toluene was highly correlated (R 2 = 0.50-0.95) with other aromatics (e.g., ethylbenzene, xylenes, and styrene) and n-decane (R 2 = 0.65) but fairly to poorly with undecane and dodecane (R 2 = 0.29 and 0.47, respectively). Reversibly, good correlations (R 2 > 0.65) between benzene and the long-chain alkanes (except n-decane with an R 2 = 0.22) were found. Benzene was also correlated poorly with ethylbenzene, xylenes, and styrene (R 2 = 0.24-0.29) (Table S2). Benzene is a typical VOCs PAMS emitted from diesel-fueled engine, coal combustion, biomass burning, and natural gas combustion 10,42,[44][45][46] , while gasoline-fueled engine emission, paint production, printing, and other solvent involved activities release more toluene, ethylbenzene, xylene and styrene in composition 10,43 . Those correlations not only prove the high contribution of combustion sources in the Weibin site, and also suggest that the ambient levels of toluene, ethylbenzene, xylenes, styrene, 1,3,5-trimethylbenzene and n-decane might be additionally elevated by solvent-related industries 10,47 . Figure 2b presents the correlations between toluene and benzene varied from Weibin site to Miaogou site. In the Weibin site, the mixing ratio of benzene was far higher than toluene (slope of toluene verse benzene = 0.05), ascribed to the strong benzene emissions from the local factories and transportation from the industrial zone with the easterly wind. In contrast to the Weibin site, the mixing ratios of toluene, styrene and dodecane were unexpectedly high in the Miaogou site. This also indicates that the solvent involved manufacture processes in the scattered paint and printing factories had a large contribution to the emissions of toluene and other aromatics 43 .
The molar ratio of xylenes (m-/p-isomers) and ethylbenzene is often used to access the air mass aging, considering that the differences in degradation rates [i.e., hydroxyl (OH • ) reaction coefficients (K OH )], in which xylenes have higher K OH of 1.36-2.30 × 10 -11 in comparison of lower K OH of 7. 0 × 10 −12 for ethylbenzene 31,48,49 . In this study, the ratios of xylenes to ethylbenzene were both high at the Weibin site and Chencang sites (1.84 and 2.03, respectively, Fig. 3d), attribute to the fresh emissions from the pollution sources. However, a much low average ratio was found in the Miaogou site (0.42), additionally with a poor correlation between the two chemicals (R 2 = 0.10). It is reasonable that relatively more aged air mass in Miaogou region due to its geographical position 48 , even though its average VOCs PAMS levels were close to that at Chencang site.
Source apportionment of VOCs PAMS . Source apportionment was conducted with PMF receptor model.
The mixing ratios and uncertainties for the VOCs PAMS from those valid samples collected in the three sites were used. Calibration was run for 3-7 factors and with random seeds. The seven factors solution produced mathematical [(Q values (both robust and true) close to the theoretical Q value] with reasonable explanation. The results are illustrated in Fig. 3. In Factor 1, it has high loadings of benzene, propylene, 1-butene and 1-pentene in a descending order in mixing ratios, that was close to the profile of coal combustion emissions 10 . Factor 2 is filled with propane, n-butane, n-pentane, iso-pentane and toluene. Most of these VOC PAMS were relevant to the fuel evaporation, in particular of gasoline and CNG. Factor 3 is characterized by n-decane, undecane, xylenes, ethylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, p-diethylbenzene, 3-methylpentane, ethylene, 1-pentene. The composite was corresponding to and dominated by the diesel-fuel combustion 10 . However, it must be noted that 3-methylpentane, a common marker for gasoline exhaust, was unexpectedly high in this factor. Even though the major contribution of Factor 3 could be characterized by the diesel combustion, the gasoline emission might still contribute a few. Factor 4 is consistent with a typical gasoline exhaust profile. Factor 5 is singly filled with high abundance of iso-butane but no representative source could be identified, thus is marked as others. Factor 6 is dominated by those VOCs PAMS from solvent evaporation (i.e., toluene, styrene and long-chain alkanes) 43 . Isoprene is the major component in vegetation emission, even it is always detected in the vehicle exhaust 10 . Considering that high contribution of vegetation, isoprene is identified as a biogenic source marker. Factor 7 can be identified as biogenic source since isoprene had the highest contribution 10 .
Taking all of the samples in accounting, vehicle exhaust (44.5%), industrial discharges (20.1%), and solvent usage (19.7%) are the top three major pollution sources contributed to ambient VOCs PAMS in Baoji (Fig. 4). As mentioned above, Baoji is a typical industrial city, and the main industries include power plants, coal chemical industry, metal smelting, and coke productions 36 . Most of the related industries use coal as fuel or feed, this could cause high level of benzene in ambient air. In addition, heavy industries induce dense heavy truck usage for transport in the city Baoji. These together makes high contribution of industrial and diesel vehicle exhaust. With normalized to the mass of TVOCs PAMS , the contributions of the potential sources on each sample were obtained, and their strengths were compared by taking the mean values at the different sites (Table S3). The loading of TVOCs PAMS in the Weibin site was ~2.8 times on average of those in the Chencang and Miaogou. The industry emission and gasoline-related sources were the two largest contributors (37.9% and 29.0%, respectively) in the Weibin site. In the Chencang, diesel exhaust was the most important source, contributing to 59.4% of the TVOCs PAMS . Solvent-related emission contributed 36.7% of TVOCs PAMS in the Miaogou due to trace commercial painting activities. Covering with a high density of forest area, the mixing ratio of isoprene at the Miaogou was high. Therefore, a stronger strength of biogenic emission of 13.3% was found, in comparison with only 5.1 and 7.1% at the Weibin site and Chencang, respectively.
Chemical reactivity of VOCs. The chemical reactivity of VOCs PAMS was studied to investigate their potential oxidation ability with OH • , which can consequently lead the formation of surface O 3 7 . The sum of OH • loss rate (L OH ) was calculated on basis of reaction rate constant between an individual VOCs PAMS and OH • and its mixing ratio. Table 2 lists the L OH accounted at the three sites. As much higher TVOCs PAMS mixing ratios at the Weibin site, higher L OH (27.20 ± 12.55 s −1 ) was obtained in comparison of those at other two sampling sites. The L OH in Weibin site was mainly dominated by the groups of alkenes and aromatics due to the strong influences of industrial and traffic emissions. In addition to their high chemical reactivity, the two pollution sources are thus considered to play key roles on hydroxyl radical-driven oxidations in Weibin site Baoji. The air quality at the Chencang was less impacted by the direct industrial activities, and the L OH was thus less than half of that in the Weibin site. The group of alkanes, which has a lower chemical reactivity, was the major contributor (35.6%) to L OH . At the remote site, even though the mixing ratios of most VOCs PAMS were relatively low, the high abundance of extremely-reactive isoprene dominated 32.5% to L OH , resulting of an equivalent level of oxidation potential as that accounted in the Chencang.
Considering that alkenes play important roles on the chemical reactivity, the contributions of the most significant individuals at different concentration levels were displaced in Fig. 5. At a low mixing ratio region (0-35 ppbv), mostly in samples collected in the Chencang and Miaogou, the biogenic-emitted compounds (i.e., isoprene) showed their dominance and strong strengths to drive the L OH , while the impacts from anthropogenic sources shrined as a long distance far from the two sites. At a higher mixing ratio of TVOCs PAMS (>35ppbv, for the samples collected in the Weibin site), propene and 1-butene were the two most important contributors to L OH , representing that the industrial and traffic emissions not only can elevate the mixing ratios of VOCs PAMS and amounts of OH· oxidation, and also lead further surface O 3 formation.  Miaogou sites, showing maxima between noon and early evening ( Figure S2). In typical, double concentration peaks could be identified during this time interval. The first peak appeared around 13:00, and the next one was recorded between 16:00-17:00. The surface O 3 concentrations decreased gradually after 18:00 and the lowest values at the Weibin site and Miaogou sites were often observed at 07:00 and 08:00, respectively. It should be noted that the surface O 3 levels were significantly lower at the Weibin site than Miaogou site during the nighttime. The cases have been detailly explained in following sections. Influences from meteorological conditions. The correlations between surface O 3 and important meteorological parameters were fully studied. High O 3 concentrations were often detected under high ambient temperatures and low RHs, which offer favor conditions for either formation or reservation of O 3 50, 51 . In the current study, the O 3 values were positively correlated to the temperature in both of the Weibin site (R 2 = 0.59) and Miaogou (R 2 = 0.61) during daytime, when associated with higher light intensity and radiations. Besides, water vapor could scavenge O 3 and its precursors in atmosphere by wet deposition 52 . As expected, the surface O 3 levels was found to be negatively correlated with RHs at Weibin site (R 2 = 0.46) and Miaogou (R 2 = 0.51) sites.
The O 3 levels were also coincided with the surface zonal winds within our observation period (Fig. 6) 3, 53 . In the Weibin site, higher surface O 3 concentrations were often seen in accordance of strong easterly wind, consistent with higher mixing ratios of TVOCs PAMS measured simultaneously (Fig. 2). More obvious impacts from the direction of surface winds could be seen at the Miaogou site.    Ozone formation potential. While L OH was used to assess the VOCs activities, ozone formation potential (OFP) could be more direct method to measure the contributions of VOCs PAMS in the O 3 formation 8, 54 . Ozone formation potential (OFP) were calculated based on the average mixing ratios and the maximum incremental reactivity coefficients (MIR) of VOCs PAMS quantified in the present study 54 : where C i , represents the mixing ratio (ppbv) for species i. The MIRs were obtained from Carter 54 . The estimated OFP for the top 20 contributors were given in Table 3. The highest overall OFP (185.36 ppbv on average) was accounted in the Weibin site, in comparison of 42.89 and 56.74 ppbv at the Miaogou and Chencang sites, respectively. The contributions of each VOCs PAMS to overall OFP varied from the sites, that might be driven by their degrees of degradation and dilution in transportation processes 48 . In the Weibin site, alkenes and aromatics were dominated and totally contributed >90% of the overall OFP loading. The marker species of propene and m-diethylbenzene were the two most important VOCs PAMS which contributed to more than half of overall OFPs, indicating that vehicle and industrial emissions greatly affected the surface O 3 formation. At the Miaogou site, isoprene and solvent-characterized VOCs such as toluene, styrene, xylenes, and long-chain alkanes contributed most of OFPs loading. In another aspect, air mass aging can also contribute the variation of OFPs 28 . Even though the overall estimated OFPs was low at the Miaogou site, the actual O 3 levels were not the lowest as expected (Fig. 6). This can be ascribed to the transport of O 3 from other upwind districts 3 . The results from the current study concluded that the fresh plume from industrial and traffic sources at the upwind location could highly impact on the air quality in Baoji, and lead the occurrence of O 3 episodes in residential areas in summer.

Conclusions
Ozone pollution control is a challenging task in China, partly due to a huge number of VOCs emissions. In this study, the high levels and compositions of alkenes and aromatics in the the ambient of Baoji suggest the large contributions from the combustion sources in surrounding industries. The temporal variations of VOCs PAMS were driven by the directions of surface winds, in which the pollutants were transported from upwind regions. Aging of air mass was much obviously at the Miaogou site owing to its location distanced from the dense pollution origins.
The O 3 formation in Baoji was influenced by both export of well-processed industry plume and photochemical reactions. More long-term VOCs monitoring and environmental assessment are thus needed to interpret the characteristic roles of VOCs in the production of surface O 3 in different regions in China. Development of mathematical models is vital to conclude the data in assistance to solve the related environmental issue.   O 3 were 0.5 ppbv. The meteorological data (i.e., temperature, wind speed/direction and Relative Humidity) were recorded with an multi-parameter automatic weather stations (WXT520, Vaisala).

Chemical Analysis.
A total of 72 valid sorbent tube samples were collected. They were all analyzed using a thermal desorption (TD) unit (Series 2 UNITY-xr system, Markes International Ltd.) coupled with a gas chromatograph/mass spectrometric detector (GC/MSD, Models 7890 A/5977B, Agilent, Santa Clara, CA, USA) within one week. The chemical analysis procedure can be found in our previous work 55 . A tube was connected into the TD unit at room temperature (~25 • C) and purged with ultra-high purity (UHP) helium (He) gas at a flow rate of 40 mL min−1 for 60 s to eliminate air and oxygen intrusion. For the primary desorption stage, the analytes were desorbed at 330 • C for 5 min and refocused onto a cryogenic-trap (U-T1703P-2S, Markes International Ltd.) to capture high volatility target compounds at −15 °C. For the secondary desorption stage, the trap was dry-purged for 6 s and rapidly heated from −15 °C to 320 • C and maintained for 5 min. The analytes were passed via a heated transfer line at 160 °C, and re-refocused onto a cold GC capillary column head (Rtx ® -1, 105 m × 0.25mm × 1μm film thickness, Restek Corporation, Bellefonte, PA, USA) at −45 °C with an aid of liquid nitrogen (N 2 ) in GC oven. Once the second desorption is completed, the oven temperature program started at an initial temperature of −45 °C for 4 min, ramped to 230 °C at a rate of 6 °C min −1 , and maintained at 230 °C for 5 min. The constant flow rate of He carrier gas was 1.0 mL min −1 throughout the GC analysis. The MSD was operated in selective ion monitoring (SIM) mode at 230 °C and 70 eV for electron ionization. Identification was achieved by comparing the mass spectra and retention times of the chromatographic peaks with those authentic standards.