Investigation of factors controlling PM2.5 variability across the South Korean Peninsula during KORUS-AQ

Abstract

deployed instrumented aircraft and ground-based measurements to elucidate causes of poor air quality related to high ozone and aerosol concentrations in South Korea. This work synthesizes data pertaining to aerosols (specifically, particulate matter with aerodynamic diameters <2.5 micrometers, PM 2.5) and conditions leading to violations of South Korean air quality standards (24-hr mean PM 2.5 < 35 µg m-3). PM 2.5 variability from AirKorea monitors across South Korea is evaluated. Detailed data from the Seoul vicinity are used to interpret factors that contribute to elevated PM 2.5. The interplay between meteorology and surface aerosols, contrasting synoptic-scale behavior vs. local influences, is presented. Transboundary transport from upwind sources, vertical mixing and containment of aerosols, and local production of secondary aerosols are discussed. Two meteorological periods are probed for drivers of elevated PM 2.5. Clear, dry conditions, with limited transport (Stagnant period), promoted photochemical production of secondary organic aerosol from locally emitted precursors. Cloudy humid conditions fostered rapid heterogeneous secondary inorganic aerosol production from local and transported emissions (Transport/Haze period), likely driven by a positive feedback mechanism where water uptake by aerosols increased gas-to-particle partitioning that increased water uptake. Further, clouds reduced solar insolation, suppressing mixing, exacerbating PM 2.5 accumulation in a shallow boundary layer. The combination of factors contributing to enhanced PM 2.5 is challenging to model, complicating quantification of contributions to PM 2.5 from local versus upwind precursors and production. We recommend co-locating additional continuous measurements at a few AirKorea sites across South Korea to help resolve this and other outstanding questions: carbon monoxide/carbon dioxide (transboundary transport tracer), boundary layer height (surface PM 2.5 mixing depth), and aerosol composition with aerosol liquid water (meteorologically-dependent secondary production). These data would aid future research to refine emissions targets to further improve South Korean PM 2.5 air quality.

New in situ aerosol hyperspectral optical measurements over 300–700 nm – Part 2: Extinction, total absorption, water- and methanol-soluble absorption observed during the KORUS-OC cruise

“…In Part 1 (Jordan et al, 2021) of this two-part work, hyperspectral aerosol extinction (σ ext ) measurements from a field campaign in South Korea were used to examine the usefulness of characterizing the spectra with second-order polynomial rather than traditional linear fits (i.e., power laws with negative slopes known as Ångström exponents) to the logarithmically transformed spectra. It was found that secondorder polynomials provided better fits to the data and that using the two-coefficient mapping approach introduced by Schuster et al (2006) holds promise for providing more detailed information on ambient aerosol size distributions than can be obtained from single Ångström exponents alone.…”

Physical and chemical constraints on transformation and mass-increase of fine aerosols in northeast Asia

“…Therefore, the vertical profiles of nanoparticles and their evolution during the EOF2 case suggest that the entrainment of nanoparticles from the upper layer into the shallow and humid boundary layer and the increase in their size via hygroscopic growth, consequently contributing to increase in PM2.5 mass concentration. In fact, the PM2.5 mass concentration increased at Aewol, up to 63 g m -3 (Table 2), as well as Seoul during the EOF2 case (Jordan et al, 2020). Recently, it was suggested that pollutants were carried from the upstream areas in the upper layer by westerly winds, remained above the nocturnal boundary layer, and entrained into the PBL during the day; this led to a high PM10 concentration at the surface (Lee et al, 2019).…”

“…We considered the deliquescence relative humidity (DRH) of inorganic salts, such as NH4NO3 and (NH4)2SO4, as a crucial factor for the rapid particle growth, because the SIA was the predominant constituent of PM2.5 during the KORUS-AQ period (Jordan et al, 2020). In the morning, the burst of particle>3.5 nm above 4 was found near the surface and just above the inversion layer with RH below the DRHs of both (NH4)2SO4 and NH4NO3 (Fig.…”

“…In contrast, the number of particle>3.5 nm tended to be backed up below 1000 m. The mixing through the inversion layer can cause gas-to-particle partitioning and further hygroscopic growth, leading to an increase in the number of accumulation-mode particles. Considering the fact that the synoptic circulation is the main factor determining the level of atmospheric constituents over the Korean peninsula (Jordan et al, 2020;Peterson et al, 2019), this process would have been active throughout the entire EOF2 period.…”

“…When the PM2.5 mass concentration was highly elevated over the Korean peninsula during EOF2 case (May 25-27) (Jordan et al, 2020), the coating of rBC was noticeably thick (48  39 nm) relative to other period of the campaign (Fig. 7a).…”