A Study On Atmospheric Particle-Bound PAHs And Its Deposition In Shanghai

Atmospheric transport is the main pathway for polycyclic aromatic hydrocarbons (PAHs) emitted from cities and surrounding areas to enter urban systems. Study of this pathway is important not only for better understanding the environmental behaviors of PAHs in urban system, but also for describing multi-media cycling processes of the pollutants. Dry and wet deposition are the main ways to remove PAHs from the atmosphere and the key pathways for regionally emitted PAHs to enter the urban surface system. Under the framework of urban systems, deposition is also a connection between atmosphere and underlying surfaces. Study of PAHs in atmospheric particles and deposition can enrich theories of both urban physical geography and environmental geochemistry. In the view of urban surface system, this paper studied PAHs in 230 daily-sampled PM2.5 and long-duration deposition samples at urban and suburban sites in Shanghai from April 2014 to March 2015, as well as deposited PAHs at different depths in soil column of simulated experiment conducted from February 2015 to April 2016. The study focused on spatial and temporal characters, transport mechanisms, and the source identification of PM2.5-bound PAHs and deposited PAHs, as well as the vertical transport of deposited PAHs in soil columns.The annual sum concentration of USEPA 16 priority PAHs in PM2.5 was 6.9 ng/m3. The values were 7.2 and 6.5 ng/m3 at suburban and urban site, respectively. The difference of PAH concentrations among the four seasons was significant. The highest concentrations of total PAHs and high molecular weight (HMW) PAHs were found in winter. HMW PAHs dominated in winter and spring with percentages more than 80%, while in the rest of the year especially in summer, light molecular weight (LMW) PAHs could contribute up to 47% to total PAHs. Both mean wind speed and minimum relative humidity (RH) were found correlating negatively with the concentration of PM2.5 and PAHs in winter and spring as well as on the annual scale. The pollutants had dominantly negative correlations with the temperature. Wind direction also affected the pollutants concentrations.Four emission sources of PM2.5-bound PAHs were revealed by PMF model:coal combustion, traffic, volatilization/biomass combustion, and coking, with contributions of 34.9%,27.5%,21.1% and 16.5%, respectively. We found that weather conditions had effects on the sources of PAHs. When wind speed changed from>2m/s to<2m/s, the contribution of traffic increased from 17.4% to 28.7%. Traffic emission was a more likely local source because it was emitted from a low height. The result indicated that local sources could have larger contributions when weather condition was more stagnant. The local-indicative source contribution was also influenced by the level of pollution. The contribution by traffic emission was larger when the concentration of total PAHs exceeded the average level.The trajectory clustering, potential source contribution function (PSCF) and concentration weighted trajectory (CWT) models showed clearly that air parcels moved from west (the Economic Circle of the Yangtze River Delta) had the highest concentrations of PM2.5. As for PAHs, the potential source area aslo included the Economic Circle of Around Bohai Sea. Sector analyses determined that the regional transport source contributed 39.8%of annual PM2.5,52.5% of PAHs,25.1% of LMW PAHs and 61.9% of HMW PAHs. The largest transported contribution of annual PM2.5 and PAHs was from western sectors.Based on simulated experiments, this paper studied the characteristics and influencing factors of the vertical transport of deposited PAHs in soil columns. Naphthalene and phenanthrene were dominant PAHs components in columns placed on campus, in accordance with the features of deposition. High content of 4-6 ring PAHs was found in columns placed on roads which identified the source of traffic. The result indicated that the contribution of on-the-spot emitted sources to urban surface system could have been underestimated. PAHs concentration was found to be decreased dramaticly with the increase of depth. Exponential decay equation successfully fitted the observed concentration. LMW PAHs had stronger ability than HMW PAHs to transport vertically in soil column. The most important factor that controlled transport ability was water leaching rather than absorption by organic matters in our simulated columns.Health risk of PM2.5-bound PAHs to people of different ages and exposure ways were assessed. Dermal exposure was the most important way resulting in a potential cancer risk, based on Incremental Lifetime Cancer Risk model, while both inhalation and oral exposure had negligible risks.