Size Distribution , Seasonal Variations and Sources of Carbonaceous Aerosol in a Typical Industrial City Nanjing in Yangtze River Delta , China

In order to investigate the size distributions and seasonal variations of carbonaceous aerosols (OC and EC), the carbonaceous species were collected and then analyzed by using a 9-stage Anderson-type aerosol sampler and DRI Model 2001A Thermal/Optical Carbon Analyzer on the typical industrial city Nanjing in Yangtze River Delta, China in the summer, autumn and winter of 2013 and spring of 2014. OC, EC, SOC and POC exhibited obvious seasonal variations, with the highest level in winter (39.1±14.0, 5.7±2.1, 23.6±11.7 and 14.1±5.7 μg·m-3) and the lowest level in summer (20.6±6.7, 3.3±2.0, 12.2±3.8 and 8.4±4.1 μg·m-3), and were mainly centralized in PM1.1 in four seasons. The concentrations of OC in PM1.1 varied in the order of winter > autumn > spring > summer, while EC ranked in the order of autumn > winter > summer > spring. In the PM1.1-2.1 and PM2.1-10, the concentrations of OC and EC decreased in the sequence of winter > spring > autumn > summer. The size spectra of OC, EC and SOC had bimodal distributions in four seasons, except for EC with four peaks in summer. The size spectra of POC varied greatly with seasons, exhibiting bimodal distribution in winter, trimodal distribution in spring and summer, and four peaks in autumn. The OC/EC ratios were 7.0, 6.3, 7.6 and 6.9 in spring, summer, autumn and winter, respectively, which demonstrated the abundance of secondary organic aerosols in Nanjing. The sources of carbonaceous aerosol varied significantly with seasons, and were dominated by vehicle exhaust, coal and biomass burning in PM2.1, and dominant by dust, coal and biomass burning in PM2.1-10. Keyword: size distribution; OC; EC; seasonal variation; Yangtze River Delta Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 1 March 2017 doi:10.20944/preprints201703.0010.v1 Peer-reviewed version available at Atmosphere 2017, 8, , 73; doi:10.3390/atmos8040073

With the rapid growth of China's economy and urbanization, the carbonaceous aerosols are proved to be one of the major pollutants in China (Lu et al., 2011;Zhang et al., 2012).As one of the three major regions in eastern China (Yangtze River Delta, Beijing-Tianjin-Hebei and Pearl River Delta), the YRD has been suffering the highest aerosol concentrations and the longest pollution episodes (Cao et al., 2003;Duan et al., 2007;Li et al., 2015;Wang et al., 2013Wang et al., , 2014)).
The organic matter (OM) and black carbon (BC) in PM1 was estimated to be 32.1 and 9.1 % in summer, and 30.3 and 16.9 % in winter, respectively in the YRD (Huang et al., 2013).Cao et al. (2009a) reported the annual sum concentration of OC and EC was 25.5 μg•m -3 and constituted about 28.5 % in PM10 in Hangzhou.Li et al. (2015) found the mass fraction of the total carbonaceous aerosol (TCA) in PM2.5 was estimated to be 23 % in Nanjing.Feng et al. (2009) calculated the total amount of OC and EC was in the range of 17.5-20.4μg•m -3 and TCA contributed about 30 % of PM2.5 in Shanghai.In 2010, emissions of three major PM2.5 species in the YRD, namely OC, EC and sulfate, are 136.9kt, 75.0 kt and 76.2 kt, respectively (Fu et al., 2013).Hou et al. (2011) demonstrated that concentrations of OC and EC on haze days were 1.5-3.9times higher than those on clean days in Shanghai.However, these studies were mainly centralized in the characteristics of carbonaceous aerosols in PM2.5 or PM10 with few concerns about their size distributions in different seasons.An understanding of the size distribution characteristics of EC and OC is essential when studying the aerosol sources, light extinction, and their effects on regional and global climate and human health (Lan et al., 2011;Park et al., 2011;Wan et al., 2015).In this study, the size distributions of carbonaceous aerosols (OC and EC) over different seasons were collected and further analyzed by using a 9-stage Anderson-type aerosol sampler and DRI Model 2001A Thermal/Optical Carbon Analyzer on the typical industrial city Nanjing in Yangtze River Delta, China.We also calculated the concentrations of secondary organic carbon (SOC) and primary organic carbon (POC).This paper reported the concentration levels and the seasonal variations of these measured species.Furthermore, we discussed the carbonaceous aerosol sources in different size segment in four seasons.It is important to analyze the size distribution of OC, EC, OC/EC ratios and SOC for better understanding the sources, formation mechanisms, and control strategy of carbonaceous aerosols in the typical polluted city.

Observation stations and experiment descriptions
The observation site was located on the meteorology building (32.21ºN, 118.72ºE) of the Nanjing University of Information Science & Technology campus, which is 40 m above the ground.The information of the station and its' surroundings is shown in Figure 1.The Nanjing Chemical Industry Park (NCIP) is located at approximately 3 km of the measurement site.In addition, there are some iron and steel plants and cogeneration power plants in the range of 1 km from the site.This region represents a combination of traffic, urban, industry and croplands sources.
The measurements were carried out during Apr 17 to May 22, 2014 (spring) 1).Table 1 also shows that the RH in four seasons were higher than 50%, the wind speed were large in spring and summer, and small in autumn and winter..43, 0.43-0.65, 0.65-1.1, 1.1-2.1, 2.1-3.3, 3.3-4.7, 4.7-5.85.8-9.0 and 9.0-10.0μm for carbonaceous aerosols.The flow rate required by the Anderson-type aerosol sampler is 28.3 L/min.The sampler was operated with a 81 mm Quartz fiber filters (Whatman, Clifton, England), and the filters were baked at 800 °C for 4 h before sampling in order to minimize the background of organic carbon.
OC and EC collected by Quartz filter were then analyzed with a DRI Model 2001A Thermal/Optical Carbon Analyzer, using the IMPROVE_A (Interagency Monitoring of Protected Visual Environments) protocol (Chow et al., 2001)  selected at random from every 10 samples to carry out a duplicate sample analysis.The errors in the measurement presented here were less than 10% for TC.Field blanks were used to determine any possible background contamination; the OC and EC concentrations in the field blanks were less than 1 % of the sample levels.
For calibration and quality control, measurement with prefired filter blank, standard sucrose solution and replicate analysis were performed.Field blank samples were collected in each season and their concentration levels of OC and EC were 0.5-0.7 and 0-0.03 μg•cm -2 , less than 3 % and 1 % of the samples, respectively.The samples were corrected by each field blank samples.The detect limit for OC and EC is 0.45 μg•cm -2 and 0.06 μg•cm -2 , respectively.More information about the instruments (e.g., precision and calibration) and the methods of data quality control were reported by Cao et al. (2003), Chow et al. (2001Chow et al. ( , 2002a)), Han et al. (2007) and Li et al. (2012Li et al. ( , 2015)).Peer-reviewed version available at Atmosphere 2017, 8, , 73; doi:10.3390/atmos8040073

Seasonal variations of the carbonaceous aerosol
Fig. 2a exhibits that the seasonal variations of OC ranked in the order of winter (39.1 μg•m -3 ) > autumn (30.5 μg•m -3 ) > spring (25.2 μg•m -3 ) >summer (20.6 μg•m -3 ) in Nanjing, which was similar to that of EC, who had the average concentrations of 3.6, 3.3, 4.0 and 5.7 μg•m -3 in spring, summer, autumn and winter according to Fig. 2b.This phenomenon might be attributed to the monsoon effects, the air masses in summer were clean due to winds majorly from the southeast ocean area.In winter, the air masses were mainly from inland area and carried amounts of pollutants attributed to the dominant northwest wind.
The concentrations of OC and EC in four seasons were mainly centralized in fine particles (Fig. 2), the proportions of which in PM2.1 to the total mass concentration were 59.1-69.2% and 51.2-71.9%, and were 43.6-54.3% and 34.5-58.7 % for the proportion in PM1.1.The clear enrichment of carbonaceous composition in smaller particle size, particularly for EC, indicates the important roles of certain combustion sources of anthropogenic origin, from which OC and EC were confirmed to be concentrated in fine particles (Li et al., 2007(Li et al., , 2015)).
The maximum values of OC were located at 0.65-1.1 μm in spring, autumn and winter, accounting for 18.9 %, 19.9 % and 21.6 % of the total concentration, respectively.In summer, the maximum value of OC was located at 0-0.43 μm, accounting for 19.8 % of the total concentration.
The maximum value of EC was located at 1.1-2.1 μm in spring and winter and at 0-0.43 μm in summer and autumn, accounting for 20.3 %, 16.8 %, 32.7 % and 23.0 % of the total concentration, respectively.The concentrations of OC in PM1.1-2.1 were almost unanimous in spring and autumn, as a result of frequent biomass burning processes in the two seasons.It was observed that the biomass burning mainly affected aerosol chemical composition at 1.1-2.1 μm (Pósfai et al., 2003;Wang et al, 2012).The concentrations of OC in PM1.1-2.1 was 7.4 μg•m -3 in winter and was 3.1 times larger than summer.This phenomenon was mainly because of stable boundary layer and few precipitations in winter, besides, haze events occurred frequently as well.

Size distributions of carbonaceous aerosol
Fig. 3a shows that the size spectra of OC had bimodal distributions in four seasons.The highest peak appears in the size range of 0.43-0.65 μm except for spring and the second peak is located at 4.7-5.8μm.It should be pointed out that the highest peak in spring was observed to shift to a larger size (i.e.0.65-1.1 μm).
The peak at 0.43-0.65 μm for OC was normally formed by the condensation and coagulation growth process of fine particles (gas-to-particle) (Jaffrezo et al., 2005；Miguel et al., 2004;Seinfeld and Pandis, 2012).The larger peak at 4.7-5.8μm appeared to be related to the dust particles, which provide surfaces for the uptake of gas species and serves as a carrier for carbonaceous constituents (Seinfeld et al., 2004).In addition, the biogenic aerosols, such as algae, pollen, vegetation debris, virus, and microorganisms may be responsible for the second peak as well (Wan et al., 2015).emissions and coal and biomass combustion process (Cao et al., 2005;Kupiainen et al., 2007;Wang et al., 2015).The EC peak at 4.7-5.8μm may be related to the re-suspension of EC-containing soil/dust particles (Lan et al., 2011).However, the size spectra of EC had four peaks in summer (Fig. 3b) locating at 0.5, 1.6, 4.0 and 7.4 μm, respectively.The reason is not clear, and may be associated with subway construction at Ningliu road during observation period.The roles of secondary organic aerosol (SOC) in haze, visibility, climate, and heath has been recognized for decades (Cao et al., 2003;Chow et al., 1995;Gentner et al., 2012;Li et al., 2012Li et al., , 2015)).As shown in Table 3, most ratios of the minimum valve of OC/EC concentrations in four seasons are above 2.0, indicating that the SOC were considered to be formed in the observation period.For the control of particulate pollution, it is very crucial to quantify the contributions of the primary and secondary organic carbon to carbonaceous aerosol.However, there is no simple direct analytical technique to attain the evaluation of SOC formation in ambient aerosols, several indirect methodologies have been applied (Cao et al., 2003;Castro et al., 1999;Huang et al., 2012;Pandis et al., 1993;Turpin et al., 1991).According to Castro et al. (1999), the production of SOC can be calculated from the following equation:  The concentrations of SOC in PM1.1 and PM2.1-10 varied identically in the order of winter > autumn > spring > summer, and varied in the order of winter > autumn > summer > spring in PM1.1-2.1 (table 2).The proportions of SOC in PM1.1, PM1.1-2.1 and PM2.1-10 to the total concentration were 44.1-57.1 %, 10.4-17.7 % and 28.9-43.6%, respectively.The formation mechanism of SOC is not clear at present, which is generally considered to be from gas phase reactions followed by gas-to-particle conversion of products, i.e., through nucleation, condensation, and/or sorption, perhaps followed by aerosol phase reactions (Castro et al., 1999;Gentner et al., 2012).

Preprints
The concentrations of POC in PM1.1 ranked in the order of winter > autumn > summer > spring, and in the order of winter > spring > autumn> summer in PM1.1-2.1 and PM2.1-10 (table 2).
POC in PM1.1 was the highest in winter (6.4 μg•m -3 ) and were unanimous in the rest three seasons (4.1-4.9 μg•m -3 ).POC in PM1.1-2.1 varied greatly with seasons, and had the highest level in winter (3.0 μg•m -3 ) and lowest level in summer (0.9 μg•m -3 ).The concentrations of POC in PM2.1-10 were almost unanimous in all seasons, the value of which in winter (4.7 μg•m -3 ) was slightly higher than other seasons (3.0-3.5 μg•m -3 ).POC are mainly from direct emissions of combustion process, and mostly centralized in fine particles.POC in coarse particles are observed to be mainly from the emission of plant spores and pollen, vegetation debris, tire rubber and soil organic matter wind erosion and some non-combustion industrial activities (Chow et al., 1995;Schauer et al., 1996).

Relationships between OC and EC
The relationships between OC and EC can provide valuable information for the source origins of carbonaceous aerosols (Turpin et al., 1990(Turpin et al., , 1995)).Strong correlation indicates common source origins and transport process.Fig. 4a shows that the correlation coefficients of OC with EC were the highest in summer (0.73), followed by spring (0.54) and autumn (0.52), and the lowest in winter (0.25).The strong correlations between OC and EC in spring, summer and autumn samples suggested that they were derived largely from the similar sources, opposite to those in winter.

Implications of the OC/EC ratios
The ratio of OC/EC is an important diagnostic index that has been used to reflect the type and source strength of carbonaceous aerosols (Turpin and Lim, 2001).The ratios can be mainly affected by emission sources, secondary organic aerosol (SOA) formation, and different removal rates by deposition (Cachier et al., 1996).The OC/EC ratios exceeding 2.0 have been used to indicate the presence of secondary organic aerosols (Cao et al., 2005;Chow et al., 2006;Turpin and Huntzicker, 1995); 1.0-4.2indicate diesel and gasoline vehicle exhaust emissions (Schauer et al., 1999(Schauer et al., , 2002)), 2.4-14.5 indicate biomass burning emissions (Ram et al., 2010), 2.5-10.5 indicate coal combustion emissions (Chen et al., 2006), 32.9-81.6 indicate cooking emissions (He et al., 2004).Watson et al. (1994) reported the OC/EC ratio of 13.1 for dust emissions.suggesting their sources from vehicle exhaust, coal and biomass burning emissions.The value of OC/EC in the coarse particles varied greatly with sizes in summer, with the highest peak value of 26.9 at 4.7-5.8μm, the lowest value of 8.8 at 9-10 μm, suggesting that the sources were from dust, coal and biomass burning.
The spectra of OC/EC had trimodal distributions in autumn and winter (Fig. 5).The peak at 0.43-1.1 μm had the highest value of 26.9-33.1 in autumn, suggesting cooking emission sources.

Figure
Figure 1 Location of sampling site in Nanjing

Figure 2
Figure 2 Size distributions of OC and EC mass concentration in four seasons

Figure 3
Figure 3 Spectral distributions of OC, EC, SOC and POC mass concentration in four seasons

Figure 4
Figure4The relationships between OC and EC in four season at Nanjing

Fig. 5
Fig.5reveals that the ratio of OC/EC exceeded 2 in all seasons, which demonstrated the abundance of SOC in Nanjing.The spectra of OC/EC had bimodal distributions in spring and summer.The two peaks were located at 0.43-1.1 μm and 2.1-5.8μm in spring, with values of 18.0 and 14.0, respectively.Therefore, the sources of carbonaceous species in fine particles may originate from coal and biomass burning, and from coal and biomass burning and dust emissions in coarse particles.Besides, long-range transport from northern area was another important source of carbonaceous aerosols in Nanjing.The values of OC/EC in < 2.1 μm were 4.2-9.4 in summer,

Figure 5
Figure 5 Spectral distributions of OC/EC in four seasons

Table 1
Observation period and major meteorological elements in Nanjing