Difference In Chemical Composition Of Pm2.5 And Investigation Of Its Causing Factors Between 2013 And 2015 In Air Pollution Intensive Monitoring Stations

In this study, difference in chemical composition of PM2.5 observed between the year 2013 and 2015 at six air quality intensive monitoring stations (Bangryenogdo (BR), Seoul (SL), Daejeon (DJ), Gwangju (GJ), Ulsan (US), and Jeju(JJ)) was investigated and the possible factors causing their difference were also discussed. PM2.5, organic and elemental carbon (OC and EC), and water-soluble ionic species concentrations were observed on a hourly basis in the six stations. The difference in chemical composition by regions was examined based on emissions of gaseous criteria pollutants (CO, SO2, and NO2), meteorological parameters (wind speed, temperature, and relative humidity), and origins and transport pathways of air masses.For the years 2013 and 2014, annual average PM2.5 was in the order of SL(congruent to DJ)>GJ>BR>US>JJ, but the highest concentration in 2015 was found at DJ, following by GJ(congruent to SL)>BR>US>JJ. Similar patterns were found in SO42-, NO3-, and NH4+. Lower PM2.5 at SL than at DJ and GJ was resulted from low concentrations of secondary ionic species. Annual average concentrations of OC and EC by regions had no big difference among the years, but their patterns were distinct from the PM2.5, SO42-, NO3-, and NH4+ concentrations by regions.4-day air mass backward trajectory calculations indicated that in the event of daily average PM2.5 exceeding the monthly average values, >70% of the air masses reaching the all stations were coming from northeastern Chinese polluted regions, indicating the long-range transportation (LTP) was an important contributor to PM2.5 and its chemical composition at the stations. Lower concentrations of secondary ionic species and PM2.5 at SL in 2015 than those at DJ and GJ sites were due to the decrease in impact by LTP from polluted Chinese regions, rather than the difference in local emissions of criteria gas pollutants (SO2, NO2, and NH3) among the SL, DJ, and GJ sites. The difference in annual average SO42- by regions was resulted from combination of the difference in local SO (2) emissions and chemical conversion of SO2 to SO42-, and LTP from China. However, the SO42- at the sites were more influenced by LTP than the formation by chemical transformation of locally emitted SO2. The NO3- increase was closely associated with the increase in local emissions of nitrogen oxides at four urban sites except for the BR and JJ, as well as the LTP with a small contribution. Among the meterological parameters (wind speed, temperature, and relative humidity), the ambient temperature was most important factor to control the variation of PM2.5 and its major chemical components concentrations. In other words, as the average temperature increases, the PM2.5, OC, EC, and NO3- concentrations showed a decreasing tendency, especially with a prominent feature in NO3-.Results from a case study that examined the PM2.5 and its major chemical data observed between February 19 and March 2, 2014 at the all stations suggest that ambient SO42- and NO3- concentrations are not necessarily proportional to the concentrations of their precursor emissions because the rates at which they form and their gas/particle partitioning may be controlled by factors (e.g., long range transportation) other than the concentration of the precursor gases.