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JOUR NAL O F E NVI R ONM E NT AL S C I E NC ES 87 (202 0) 1 – 9       5































                            Fig. 3 – The change of PM 2.5 ,PM 10 and total heavy metals concentrations with time.



           AGL) and wind directions of four different periods were  17% of the total concentration of all ions, and it also shows
           analyzed in Table 3. When air masses were transported from  that secondary pollution in the atmosphere of Beijing in
           the outside where far from Beijing, the trends in PM  winter was serious. The concentrations of these three kinds of
           concentrations were consistent with heavy metal concentra-  ions are related to the mass concentration of corresponding
           tions; however, when air masses accumulated locally or  gaseous precursors (SO 2 ,NH 3 ,NO x ), also affected by atmo-
           around Beijing, trends in PM concentrations and heavy metals  spheric humidity and temperature. The average concentra-
                                                                                                     −   2−  +
           were opposite. Air masses from near north-west and near  tions of several important water-soluble ions NO 3 ,SO 4 ,NH 4 ,
                                                                       −
                                                                    +
                                                                 +
           west of mainland brought PM with abundant heavy metals.  Na ,K ,Cl ,Ca 2+  and Mg 2+  were 12.39, 10.24, 11.76, 1.35, 1.51,
                                                                                  3
                                                               7.51, 6.06 and 0.74 μg/m . As shown in Fig. 4, the ratio of
           2.2. Water-soluble inorganic ions                   concentrations of hazy days and non-hazy days were 1.81,
                                                               1.60, 2.01, 1.42, 1.14, 1.21, 1.44 and 1.31 times, which are higher
           Water-soluble inorganic ions are an important component of  than the concentration ratio of hazy days and non-hazy days
                               2−   −      +
           atmospheric particles. SO 4 ,NO 3 and NH 4 account for 77.6% ±  of PM 10 . The ratio of water soluble ions in hazy and non-hazy
                                                               days accounted for 40.2% ± 13% and 38.93% ± 9% of concen-
             Table 3 – Comparison of characteristics of pollutions and  tration of PM 10 , respectively, and accounted for 54% ± 9% and
             wind directions in different periods.             50% ± 16%, respectively. Huang et al. (2013) once pointed out
                                                                                         2−   −      +
                                                               that the concentration ratio of SO 4 ,NO 3 and NH 4 in summer
             Period  Trends in    Characteristics of  Wind
                    concentration   pollutions    direction    was 3.0, 1.6 and 3.1 times, respectively, and was mainly
                                                               distributed in fine particles.
             12.29–  PM ↓ Heavy Metals 29: External  29: None                                          −      2
                                                                 In atmospheric aerosols, the mass ratio of NO 3 and SO 4
             12.30  ↑            transportation                − can be used to compare the contribution of stationary and
                                 30: Local       30:
                                                               mobile sources to NO 2 and SO 2 in the atmosphere (Xiao and
                                 accumulation    Northwest                                −   2−
             1.14–  PM ↑ Heavy Metals 14: Local  14:           Liu, 2003). The greater the NO 3 /SO 4 , the greater the
             1.15  ↓             accumulation    Northwest/    contribution of mobile sources such as automobile to NO 2
                                                                                         2−
                                                                                     −
                                                 West          and SO 2 , and the smaller NO 3 /SO 4 , the greater the contribu-
                                 15: External    15: None      tion of stationary source such as coal burning to NO 2 and SO 2 .
                                                                                     −
                                                                                        2−
                                 transportation                In developed countries, NO 3 /SO 4 is between 1.33 and 2.20.
             1.16–  PM ↑ Heavy Metals 16: External  16: None   Since coal accounts for more than 70% of total energy in
             1.17  ↑             transportation
                                                               China, and the vehicle ownership is lower than that of
                                 17: External    17: None                         −   2−
                                 transportation                developed countries, NO 3 /SO 4 is often less than 1, mainly
             1.4–1.5  PM ↓ Heavy Metals 4: External  4: None   0.13–0.67 (Yang et al., 2007). During the experiment period, the
                                                                             −
                                                                                 2−
                   ↓             transportation                proportion for NO 3 /SO 4 in Beijing was 1.31 and 1.16 in hazy
                                 5: External     5: North      days and non-hazy days, respectively. The value greater than
                                 transportation                        −   2−
                                                               1.0 for NO 3 /SO 4  in winter in Beijing was consistent with
             None: No sustained wind direction.                previous studies by Huang et al. (2016). Both the rapidly
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