Air quality in the middle and lower reaches of the Yangtze River channel: A cruise campaign

Air quality in the middle and lower reaches of the Yangtze River channel: A cruise campaign Zhong Li, Chunlin Li, Xingnan Ye, Hongbo Fu, Lin Wang, Xin Yang, Xinke Wang,Zhuohui Zhao, Haidong Kan, Abdelwahid Mellouki, Jianmin Chen 1 Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Fudan Tyndall Center, Department of Environmental Science & Engineering, Institute of Atmospheric Sciences, Fudan University, Shanghai 200433, China


Introduction 25
Yangtze River is the longest river in China, originating from the Qinghai-Tibetan Plateau and extending 26 to the East China Sea, and it drains an area of 18,08,500 square km basin, of which is China's great 27 granary, and feeds nearly one-third of the national population (Liu et al., 2007;Jiang et al., 2008). 28 Currently, three densely city agglomerations, including Wuhan, Nanjing, and Shanghai (WNS), which digested, and used to calculate the element recoveries ranging from 91%-102%. The detection limits of 148 the trace elements were derived from the standard deviation (3δ) of the blank values. Details relating to 149 ICP-MS have been described in elsewhere (Li et al., 2015b). 150 Organic carbon (OC) and elemental carbon (EC) in the aerosol samples were analyzed by a 151 Thermal/Optical Carbon Analyzer (DRI Model 2001). Each sample was identified as four OC fractions 152 (OC1, OC2, OC3, and OC4 at 120, 250, 450, and 550 ℃, respectively, in a helium-air) and three EC 153 fractions (EC1, EC2, and EC3 at 550,700, and 800 ℃, respectively, in the mixture air (98% helium and 154 2% oxygen) by an IMPROVE thermal/optical reflectance (TOR) protocol. Pyrolyzed organic carbon 155 (POC) was separately detected by transmittance. IMPROVE OC was defined as OC1 + OC2 + OC3 + 156 OC4 + POC, and EC was calculated by EC1 + EC2 + EC3 − POC. 157

Satellite data and ship traffic data 158
The satellite databases, including Moderate Resolution Imaging Spectroradiometer (MODIS) with a 159 resolution of 10 × 10 km, Measurement of Pollutants in the Troposphere (MOPITT) and Ozone 160 Monitoring Instrument (OMI) on the National Aeronautics reaching a spatial resolution of 13 × 24 km at 161 nadir, and Space Administration's Earth Observing System (NASA's EOS) Aura satellite, were applied 162 to provide spatial distribution of aerosol particles and trace gases (Xu et al., 2011;Huang et al., 2012a). 163 The column levels of CO, NO2, SO2, and aerosol optical depth (AOD) were retrieved over the MLYR 164 region. In this study, all of data from satellite datasets were interpolated and averaged into grid cells with 165 a 0.25° × 0.25° resolution. 166 Ship positions and numbers in the Yangtze River channel were decoded by Automatic Identification 167 System (AIS) databases which were obtained from the Marine Department. A 15-day AIS datasets along 168 the Yangtze River were selected with a high time resolution (about 15min). 169

Potential source contribution function 170
The potential source contribution function developed by Hopke et al. (1995) was applied to derive the 171 potential source regions and spatial distributions. In this study, 3 day back trajectories arriving at height 172 of 500 m was calculated using National Oceanic and Atmospheric Administration's (NOAA's) Hybrid 173 Single Particle Lagrangian Integrated Trajectory (HYSPLIT-4) model (http://www.arl.noaa.gov/ready/ 174 open/hysplit4.html) with global meteorological data from NCEP reanalysis data (ftp://arlftp.arlhq.noaa. 8 gov/pub/archives/rean-alysis) (Draxler and Hess, 1998). The contribution of the potential sources during 176 YRC was calculated by the PSCF analysis with TrajStat (Wang et al., 2009). The domain sources were 177 restricted to 25°N-45°N and 110°E-125°E, which were divided into grid cells with a 0.5°×0.5° resolution. 178 The PSCF value for the ij th grid cell was defined as: 179  Table 1). The detailed meteorological information over the MLYR region was also summarized in the 193 supporting information. As shown in Figure S1, the first episode (EP-1), starting from 22 to 23 November,194 was characterized by the sampled air masses which came from the East China Sea, and were typically 195 influenced by the local industry and Shanghai harbor pollution. The ratio of T/B ranged from 0.6 to 2 196 with an average of 1.3, suggesting fresh air masses mixed by the aged ones. The air masses in the 197 secondary episode (EP-2), with B/T<1, originated from the rural areas (Anhui and Henan), carrying 198 agriculture emission ( Figure S2). Sampled air masses stagnated around Jiujiang to Wuhan from the third 199 episode (EP-3) to the fifth episode (EP-5). However, the fourth episode (EP-4) (Wuhan region) with the 200 low average T/B ratio of 0.97 undergone well atmospheric aging. The local air during EP-4 was in low 201 pressure system with low wind speeds that didn't favor the diffusion of the local pollution ( Figure S3). 202 Air mass with T/B ratio ≥ 2 were identified from fresh emissions. Both EP-3 and EP-5 (nearly Jiujiang) 203 were characterized by high T/B value ( Figure S2), suggesting that these two pollution episodes were 204 contributed mainly from regional fresh emissions. For the sixth episode (EP-6), the wind direction shifted 205 from southwest to northwest, and the vessel was again traveling through the rural area of middle reach 206 of Yangtze River, suggesting that air masses may originate from agricultural activities. Then in the 207 seventh episode (EP-7), a cold front arrived, and wind speeds increased significantly from average 3.84 208 m/s to 5.38 m/s (Table 2) with air masses transported from northern inland regions, which was further 209 confirmed by wind fields ( Figure S3) and the sharply decreases of RH ( Table 2). The last episode (EP-210 8) was in the YRD region where highly intensive anthropogenic activities released a large amount of the 211 pollutants. Air masses in EP-8, with the average T/B value of 1.73, were expected to mixture of aged 212 masses sources with regional fresh emissions. Overall, EP-1 and EP-8 (the YRD region) were mainly 213 influenced by fresh local emissions mixed with aged air masses, while agriculture emissions contributed 214 significantly during EP-2 and EP-6 episodes. Both EP-3 and EP-5 were characterized by fresh emissions, 215 even though the megapolis was not available in this region. The cruise started on November 22, but the 216 offline PM2.5 samples were collected after November 25. Thus, EP-1 description was ignored in the 217 present study. 218

Variability of air pollutants observed in the vessel 220
The PM2.5 and PM1.0 was sampled from 25 November to 5 December in 2015. The detail information is 221 also summarized in Table 1. The average mass concentrations of PM1.0 and PM2.5 during YRC were 96.69 222 ±  g m -3 and 119.29 ±  g m -3 , respectively. The average ratio of PM1.0/PM2.5 was 0.8 ± 0.085, 223 implying that PM2.5 mainly dominated by fine particles with the size of < 1.0 m. The detailed 224 meteorological information, including T, RH, pressure, wind direction and wind speed, and trace gaseous 225 in different episodes are also summarized in Table 2. The peak concentrations of PM2.5 were observed in 226 EP-4 and EP-7 (Table 1). However, there were obvious differences between EP-4 and EP-7 in the 227 meteorological parameters and trace gases levels, indicating that these two pollution events were 228 completely different. As mentioned in 3.1, sampled air masses in EP-4 mainly originated from local 229 emissions, whereas EP-7 was influenced by a long-transport of air pollution. Table 2, the average concentrations of CO, SO2, and NOX varied dramatically in the 231 different pollution episodes. Average concentrations of CO, and SO2 (993.96 ± 387.34, and 9.32 ± 4.33 232 ppbv, respectively) were slightly lower than those in the cities in winter, including Wuhan (1024.00, and 233 13.30 ppbv) ( represented typical urban NOX level. The NOX concentration peaked in EP-3, which was identified to 247 mainly come from local emissions. The high NOX level along YRC was identified to come from strong 248 regional emission. It could be derived that multiple sources of air pollution distributed on both banks of 249 the Yangtze River. 250

Regional distribution of air pollutants identified by remote sensing observation 251
The MLYR region is one of the most polluted areas in China, and the spatial distribution of various 252 pollutants were apparently different from coastal to inland region. As shown in Figure 2a were dominated. However, there was much missing data of AOD in central China due to heavy clouds. 256 As presented in Figure S4   indicating that BB was also a major contributor to PM2.5 during YRC. 322 17 elements of PM1.0 and PM2.5 were measured, and the average concentrations are summarized in 323 Table 3. For comparison, the data reported previously in the megacities (in winter) and the cruises are 324 also outlined in Table 4. Ca show the highest concentration among all elements (Table 3) at all locations 325 (Table 4), and shared 2.16% on average in PM2.5, partly due to cold front with floating dust in this 326 campaign. The secondary highest concentration among all elements was Fe (Table 3). This concentration 327 (1.64 g m -3 ) in the campaign was higher than those at many urban sites, such as Beijing The enrichment factors (EFs) were applied to distinguish crustal elements from the anthropogenic 338 sources. The formula to evaluate EFs was: 339 of which EFi is the enrichment factor of element i; Xi and XR are the concentrations of element i and 341 reference element of R in aerosol, respectively; X'i and X'R are the background content of elements in 342 the MLYR soil (Wei et al., 1991 ). Al was determined to originate from soil. Hence, it was selected as 343 the reference element for the calculation. The elements of EFs < 10 included: Al, K, Mg, and Na, all of 344 which were regarded from crustal or re-suspension local soil. The species with higher EFs (10 < EFs < 345 100) were thought to be the mixture of the crustal and anthropogenic sources, including Cr, Cu, Co, Ni, 346 and V. Trace elements of EFs > 100, including Ca, Zn, Se, Pb, As, Mo, Fe, and Cd, were attributed to 347 distributions, PCA was used to classify the main source of trace elements of PM2.5 using the rotate 349 component matrix and PSCF for individual element was performed to infer the potential source and/or 350 pathway regions. As shown in Figure 5a, trace elements were classified into four categories (PCA), which 351 could explain 86.73% of the variance, indicating that the major sources of elements of PM2.5 could be 352 considered and explained. More specifically, the first component (component 1) could account for 38.48% 353 of the variance, which was derived from coal combustion, including the high loadings of Cd, As, Pb, Tl, 354 and Se. Particularly, Se was generally considered as a tracer for coal combustion, due to its formation in 355 the high-temperature environment. Se produced by the rapid gas-to-particle conversion could undergo 356 long-range transport (Nriagu, 1989;Wen and Carignan, 2007). As shown in Figure S6b also showed the distribution of K and Mg, for which the coal combustion in this region could be primarily 403 responsible. Furthermore, Ca showed the high EFs (EFs > 100), suggesting that the crustal element may 404 not derive from natural source, but from anthropogenic re-suspension of road and/or construction 405 activities along the Yangtze River. To further evaluate the impact of anthropogenic Ca, the equation 406 below was applied: 407 (Ca/Al) crust is the ratio of Ca to Al in the crust, and its value is 0.5. According to this method, the average 409 Ca anthropogenic concentration was 2.15 g m -3 , and the peak level reached to 3.42 g m -3 on December 3. 410 IF all of Ca anthropogenic in the samples of other cities and cruises (Table 4) were calculated according to 411 the same method, its level in this cruise was much higher than those in other samples, suggesting that 412 anthropogenic dust was dominated and distributed in the YRD region during the period. 413 Resembling Ca 2+ distribution pattern, the maximum concentration and mass fraction of Na + and K + in 414 PM2.5 were also measured during EP-7. Significantly correlation between Ca 2+ and K + suggested that 415 dust could be the major source of K + in PM2.5 sampled during YRC (Figure 3  Shanghai, Jiangsu, and the east of Anhui were identified as the major potential source regions and/or 435 pathways, owing to ship emissions, nonferrous metal mining, and smelting industries. The Mongolian plateau also was also the source region, indicating that nature dust may be another possible source for Cr 437 and Ni. However, the high PSCF values of fine particle V were only derived from the YRD region and 438 Mongolian plateau (Figure 6j). It's partly that V was considered to originate from heavy oil combustion, 439 while Ni and Cr probably has other sources (Table S1)  levels of CO and low-concentrations of SO2 and NOX also confirmed BB in EP6 and EP7 (Table 2). 455 However, fire points couldn't be apparently observed in the satellite-detected fire maps 456 (http://firefly.geog.umd.edu/firemap/), due to heavy cloud cover on 27 November and 1 December. 457 During the whole observation periods, there was only one sample (#12, Figure S8) collected during BB 458 event. It was verified by MODIS fire points, due to a cold front blowing heavy clouds away ( Figure S4). 459 The slightly higher lev concentration was observed in the night that was attributed to the lower boundary 460 layer at night and BB for heating and cooking in the rural regions. 461 The levoglucosan concentration and ratio of OC to lev (OC/lev) were also widely applied to estimate 462 the contribution of BB to OC in PM2.5. An empirical model was utilized as proposed by Wan  ) ⁄ (5) 464 taken into account. So, the average (lev/OC)BB ratio of 8.14 % was selected to calculate the contribution 466 of BB to OC (Wan et al., 2017). Figure S9 presents the variation of lev/OC ratio along the Yangtze River. 467 The ratio of lev/OC during this cruise ranged from 0.03 % to 0.91 % with an average of 0.35 ± 0.24 %, 468 which was comparable to that of Lin'an in YRD region (Liang et al., 2017). However, the ratio of lev/OC 469 during YRC was near an order of magnitude of lower than its value in New Delhi (3.1 ± 0.8 %) ( OC to PM2.5 was slightly higher than 20%. The peak contribution of OC deriving from BB to total OC 474 of PM2.5 nearly accounted for 11% in EP-6, which was approached that of the Pearl River region sites 475 (13%) (Ho et al., 2014). Here, it's emphasized that our method based on empirical formula and value is 476 just rough estimation. Hence, the radiocarbon measurement ( 14 C) of carbonaceous aerosol and air quality 477 model simulation should need to confirm this result in the future. 478

Primary of ship emission 480
Over the past few decades, China's rapid economic development leads to the increasingly busy shipping 481 transportation in the Yangtze River. However, there is lack of data related to ship emission along the 482 stream of Yangtze River, especially in inland area. The ratio of V to Ni was used to judge whether ship 483 emission could influence air quality (Isakson et al., 2001). The average ratio of V/Ni in the cruise is 1.27, 484 which was in good agreement with the previous studies (Pandolfi et al., 2011;Zhang et al., 2014). 485 Emission factors of heavy metals from different types of fuel oil were also analyzed in our group (Table  486   S1). Only heavy oil contained V, while the V levels emitted from other diesel and petrol were under the 487 detector limits. In this study, only V was regarded as the tracer for heavy oil combustion. However, it 488 was still difficult to distinguish V from refinery and ship emission. Hence, the high-resolution back-489 trajectory and high-resolution of the ship position from the AIS data were applied to investigate ship 490 plume during this cruise. As plotted in Figure 7

Ship emission contribution to SO 2-4 , NO -3 , and OC 509
To in-depth characterize the contribution of the ship emissions to secondary fine particles, a lower limit 510 of the SO 2-4 /V, NO -3 /V, EC/V, and OC/V ratios (equal to the average minus one standard deviation) was 511 applied to estimate the particulate from heavy oil combustion in the course of the Yangtze River (Becagli 512 et al., 2017). As presented in Figure S11a-b, the mass ratio of SO 2-4 /V and NO -3 /V decreased rapidly with 513 increasing V concentration. According to ship traffic numbers, weather condition, and the emission 514 factors of different type oils (Table S1), the samples with V > 15 ng m -3 were mainly considered to come 515 from ship emissions. 516 The limit ratio of SO 2-4 /V, NO -3 /V and OC/V, and the estimation of ship emissions contributions to SO 517 2-4 , NO -3 , OC, and PM2.5 are summarized in Table S2 in supporting information. The minimum ratio of 518 NO -3 /V in this cruise was nearly twice greater than the limit ratio for SO 2-4 /V, which was contrary to the 519 previous results with higher SO 2- EC and OC were also estimated by the same methods for SO 2-4 and NO -3 , and the lower limit for OC/V 533 and EC/V ratio are also presented in Figure S11c (Table S1). Oceangoing ship emissions were probably the major air pollution 546 sources in the Shanghai port. Hence, it is urgent to establish emission control areas (ECAs) in Shanghai 547 ports. However, it is worth noting that our estimation based on empirical values was also limited by  Table   Table 1. The detailed information of PM2.5 and PM1.0 in the ambient during YRC.