Oxidative potential of urban haze in a coastal area: chemical characterisation, source identification and lung deposition

Abstract

Ambient aerosols, one of the components to deteriorate air quality, have been associated with chronic and acute effects on human health. The oxidative potential (OP) of ambient particles has been applied as a new metric for aerosol toxicity rather than mass concentration. A comprehensive year-round field campaign was conducted in a coastal city of Ningbo, China to examine OP of PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) and size-fractioned PM, as well as their chemical characterisation, source contribution and respiratory deposition.
Using dithiothreitol (DTT) cell-free assay, OP of water-soluble component (OPws) and OP of methanol-soluble component (OPmeth) of PM2.5-based samples were measured. Extrinsic (volume-normalised) OPmeth (OPvmeth) exhibited stronger seasonal variability than that of OPws (OPvws). Different seasonal trends were observed for OPvws and OPvmeth, with highest averaged levels in autumn (4.09 ± 2.02 nmol min-1 m-3) and winter (11.51 ± 2.56 nmol min-1 m-3), respectively, and lowest in summer (OPvws: 3.24 ± 1.39 nmol min-1 m-3, OPvmeth: 0.79 ± 0.83 nmol min-1 m-3). In contrast, the intrinsic (mass normalized) OP, OPmws and OPmmeth exhibited highest in summer, with 0.21 ± 0.16 and 0.17 ± 0.21 nmol min-1 µg-1, respectively. The analysis of major chemical components of aerosol samples, including carbonaceous species (organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC)), water-soluble inorganic ions (WSIIs), trace elements, polycyclic aromatic hydrocarbons (PAHs) and quinones, were used for source apportionment analysis via chemical mass balance (CMB) model. Seven emission sources of PM2.5 mass were identified, including secondary aerosol (SA), coal combustion (CC), industry emission (IE), vehicle emission (VE), biomass burning (BB), fugitive dust (FD), and sea salts (SS). Using multiple linear regression (MLR), the dominant contributions from IE and SS to OPvws and IE, BB, and CC to OPvmeth were identified. We believe this is the first study to report DTT oxidation for SS and the reason for that is also explained. VE and FD-derived OPv exhibited non-linearity due to antagonistic and/or synergistic interactions with SS and CC sources. Based on spectra of chemical composition data obtained from CMB model, we further compared OP values among PM2.5 of similar mass concentrations with ranges of 20-30 and 30-40 µg m-3. For Ningbo itself, we observed that IE-emitted PM2.5 presents the highest intrinsic OP, while SS aerosol is redox-active in the coastal area. Our results showed that aerosols of similar mass concentrations exhibited remarkably different OP values, depending on their chemical compositions and source contributions.
For size-resolved particles, both concentrations of PM and its related OP (OPvws and OPvmeth) were substantially higher during haze period (29.76-40.97µg m-3 and 0.50-3.22 nmol min-1 m-3 of OPvws and 29.76-40.97µg m-3 and 0.40-2.43 nmol min-1 m-3 of OPvmeth) than non-haze period (9.15-22.20 µg m-3 and 0.19-0.95 nmol min-1 m-3 of OPvws and 9.15-22.20 µg m-3 and 0.21-0.88 nmol min-1 m-3) for all size ranges. The maximum of OPvws centred in ultrafine-mode PM while OPvmeth centred in coarse mode. Ultrafine particles contributed increasingly to the OPvws deposition when penetrating into deeper regions of the respiratory tract, from 1.22 nmol min-1 in head airway to 1.80 nmol min-1 in pulmonary region, however, the deposited doses of PM and their DTT oxidation in coarse- and accumulation-modes were highest in respective head and pulmonary regions during both periods. The OPvws resided in ultrafine particles was driven by SS and Cu, deposited mainly in pulmonary region in the two scenarios. However, SNA and soluble transition metals of Fe and Cr dominated the OPvws from coarse-mode particles, which had largest depositions in head airway during the two periods. The OPvmeth resided in coarse particles was driven by Acy and 1,2-NQ during haze period and Acy during non-haze period. Nap, BbF and Ind dominated the OPvmeth of accumulation-mode PM in polluted days while Acy and Flt in clear days. The deposited OPvws in TB region had a combined contribution from ultrafine (SS, Cu) and coarse particles (SNA, Fe, Cr) in polluted days, while deposited OPvmeth level in the TB region was highest in coarse mode during haze period and ultrafine mode during non-haze period.
In conclusion, this study has demonstrated the temporal variations and potential sources of OP induced from ambient PM. Investigations have revealed that the health effects of atmospheric aerosols are associated with PM OP and deposition efficiency of OP in different respiratory tracts. Overall, this study, based on a combination of field studies, laboratory experiments and statistical models, has provided essential insights into the mechanisms about how PM impact respiratory health from the perspective of OP in Ningbo, China.

Date of Award	Jul 2022
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Jun He (Supervisor), Bencan Tang (Supervisor) & Sarah E. Metcalfe (Supervisor)