To investigate the impacts of relative humidity (RH) on secondary organic aerosol (SOA) concentrations and chemical reactions, the carbonaceous aerosol components [i.e., organic carbon (OC) and element carbon (EC)] were quantified in daily PM2.5 samples collected at a background site in East China during summer 2015. Based on the method of EC-tracer, the concentration of secondary organic carbon (SOC) demonstrated an obvious negative relationship with RH higher than 60%. Moreover, the ratio of SOC/EC also exhibited obvious decreasing trends with increasing RH, indicating negative effects for chemical production of SOA under high RH conditions. Due to high RH, photochemistry was weakened, gaseous oxidant concentrations was lowered (e.g., significantly decreased O3 levels), and the production rates of SOA were relatively low. On the other hand, because of more water uptake under higher RH conditions, the aerosol droplet acidity was reduced and enhancement of SOA formation by acidity was accordingly absent. In addition, high RH also plays an important role in changing viscosity of pre-existing aerosol coatings, which can affect reactive uptake yield of SOA. Overall, the results from this study imply that SOA production may be more associated with photochemical processes, while aqueous-phase chemistry is not very important for some SOA formation in a moist ambient environment. In the ambient atmosphere, oxidant concentrations, reaction rates, airborne species, etc., are highly variable. How do these factors affect SOA yields under given ambient environment warrants further detailed investigations.
Financial support was also provided partly by the Ministry of Science and Technology (MOST) of Taiwan, China (MOST 103-2113-M-007-005). We are grateful to Professor Xiaobin Xu for providing the O3 and CO data.
Chen, C.-L., S. J. Chen, L. M. Russell, et al., 2018: Organic aerosol particle chemical properties associated with residential burning and fog in wintertime San Joaquin Valley (Fresno) and with vehicle and firework emissions in summertime South Coast Air Basin (Fontana). J. Geophys. Res. Atmos., 123, 10707–10731, doi: https://doi.org/10.1029/2018JD028374.CrossRefGoogle Scholar
Ding, X., M. Zheng, L. P. Yu, et al., 2008: Spatial and seasonal trends in biogenic secondary organic aerosol tracers and water-soluble organic carbon in the southeastern United States. Environ. Sci. Technol., 42, 5171–5176, doi: https://doi.org/10.1021/es7032636.CrossRefGoogle Scholar
Lim, H. J., and B. J. Turpin, 2002: Origins of primary and secondary organic aerosol in Atlanta: Results of time-resolved measurements during the Atlanta Supersite Experiment. Environ. Sci. Technol., 36, 4489–4496, doi: https://doi.org/10.1021/es0206487.CrossRefGoogle Scholar
Lin, Y. H., Z. F. Zhang, K. S. Docherty, et al., 2012: Isoprene epoxydiols as precursors to secondary organic aerosol formation: Acid-catalyzed reactive uptake studies with authentic compounds. Environ. Sci. Technol., 66, 250–258, doi: https://doi.org/10.1021/es202554c.CrossRefGoogle Scholar
Riedel, T. P., Y. H. Lin, S. H. Budisulistiorini, et al., 2015: Heterogeneous reactions of isoprene-derived epoxides: Reaction probabilities and molar secondary organic aerosol yield estimates. Environ. Sci. Technol. Lett., 2, 38–42, doi: https://doi.org/10.1021/ez500406f.CrossRefGoogle Scholar
Zhang, H., J. D. Surratt, Y. H. Lin, et al., 2011: Effect of relative humidity on SOA formation from isoprene/NO photooxidation: Enhancement of 2-methylglyceric acid and its corresponding oligoesters under dry conditions. Atmos. Chem. Phys., 11, 6411–6424, doi: https://doi.org/10.5194/acp-11-6411-2011.CrossRefGoogle Scholar
Zhang, H. F., Z. F. Zhang, T. Q. Cui, et al., 2014: Secondary organic aerosol formation via 2-methyl-3-buten-2-ol photooxidation: Evidence of acid-catalyzed reactive uptake of epoxides. Environ. Sci. Technol. Lett, 1, 242–247, doi: https://doi.org/10.1021/ez500055f.CrossRefGoogle Scholar
Zhang, Q., J. L. Jimenez, M. R. Canagaratna, et al., 2007: Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes. Geophys. Res. Lett., 34, L13801, doi: https://doi.org/10.1029/2007GL029979.Google Scholar
Zhang, X. L., J. M. Liu, E. T. Parker, et al., 2012: On the gasparticle partitioning of soluble organic aerosol in two urban atmospheres with contrasting emissions: 1. Bulk water-soluble organic carbon. J. Geophys. Res. Atmos., 117, D00V16, doi: https://doi.org/10.1029/2012JD017908.Google Scholar