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

The Korea - United States Air Quality Study (May - June 2016) 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, PM2.5) and conditions leading to violations of South Korean air quality standards (24-hr mean PM2.5 < 35 µg m-3). PM2.5 variability from AirKorea monitors across South Korea is evaluated. Detailed data from the Seoul vicinity are used to interpret factors that contribute to elevated PM2.5. 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 PM2.5. 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 PM2.5 accumulation in a shallow boundary layer. The combination of factors contributing to enhanced PM2.5 is challenging to model, complicating quantification of contributions to PM2.5 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 PM2.5 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 PM2.5 air quality.

Factors controlling surface ozone in the Seoul Metropolitan Area during the KORUS-AQ campaign.

To understand the characteristics of air quality in the Seoul Metropolitan Area, intensive measurements were conducted under the Korea-United States Air Quality (KORUS-AQ) campaign. Trace gases such as O3, NOx, NOy, SO2, CO, and volatile organic compounds (VOCs), photochemical byproducts such as H2O2 and HCHO, aerosol species, and meteorological variables including planetary boundary layer height were simultaneously measured at Olympic Park in Seoul. During the measurement period, high O3 episodes that exceeded the 90 ppbv hourly maximum occurred on 14 days under four distinct synoptic meteorological conditions. Furthermore, local circulation such as land-sea breeze and diurnal evolution of the boundary layer were crucial in determining the concentrations of precursor gases, including NOx and VOC as well as O3. During such episodes, the nighttime NOx and VOC and daytime UV levels were higher compared to non-episode days. The overall precursor levels and photochemical activity were represented fairly well by variations in the HCHO, which peaked in the morning during the high O3 episodes. This study revealed that toluene was the most abundant VOC in Seoul, and its concentration increased greatly with NOx due to the large local influence under stagnant conditions. When O3 was highly elevated concurrently with PM2.5 under dominant westerlies, NOx and VOCs were relatively lower and CO was noticeably higher than in other episodes. Additionally, the O3 production efficiency was the highest due to a low NOx with the highest NOz/NOy ratio among the four episodes. When westerlies were dominant in transport-south episode, the nighttime concentration of O 3 remained as high as 40~50 ppbv due to the minimum level of NOx titration. Overall, the Seoul Metropolitan Area is at NOx-saturated and VOC-limited conditions, which was diagnosed by indicator species and VOC/NOx ratios.

Seasonal variations of monoterpene emissions from coniferous trees of different ages in Korea.

Seasonal variations of emission rates and compositions from coniferous species were measured under controlled conditions using a vegetation enclosure method. Total emission rates and compositions of monoterpene compounds from young and adult trees in different seasons were compared. It was found that the total emission rates and the components of monoterpene varied significantly with tree species, age, and season. Total emissions from C. japonica and P. koraiensis were higher for older trees than for younger trees; however, significantly higher emissions were found from younger trees for C. obtusa. Higher monoterpene emission rates from each plant were found in spring and summer compared with autumn and winter emissions.

Ozone and other air quality-related variables affecting visibility in the southeast United States.

An analysis of ozone (O3) concentrations and several other air quality-related variables was performed to elucidate their relationship with visibility at five urban and semi-urban locations in the southeast United States during the summer seasons of 1980-1996. The role and impact of O3 on aerosols was investigated to ascertain a relationship with visibility. Regional trend analysis over the 1980s reveals an increase in maximum O3 concentration coupled with a decrease in visibility. However, a similar analysis for the 1990s shows a leveling-off of both O3 and visibility; in both cases, the results were not statistically significant at the 5% level. A case study of site-specific trends at Nashville, TN, followed similar trends. To better understand the relationships between O3 concentration and visibility, the analysis was varied from yearly through daily to hourly averaged values. This increased temporal resolution showed a statistically significant inverse relationship between visibility and O3. Site-specific hourly r2 values ranged from 0.02 to 0.43. Additionally, by performing back-trajectory analysis, it was found that the visibility degraded by air mass migration over polluted areas.

Surface flux measurements of CO2 and N2O from a dried rice paddy in Japan during a fallow winter season.

The CO2 and N2O soil emissions at a rice paddy in Mase, Japan, were measured by enclosures during a fallow winter season. The Mase site, one of the AsiaFlux Network sites in Japan, has been monitored for moisture, heat, and CO2 fluxes since August 1999. The paddy soil was found to be a source of both CO2 and N2O flux from this experiment. The CO2 and N2O fluxes ranged from -27.6 to 160.4 microg CO2/m2/sec (average of 49.1 +/- 42.7 microg CO2/m2/sec) and from -4.4 to 129.5 ng N2O/m2/sec (average of 40.3 +/- 35.6 ng N2O/m2/ sec), respectively. A bimodal trend, which has a sub-peak in the morning around 10:00 a.m. and a primary peak between 2:00 and 3:00 p.m., was observed. Gas fluxes increased with soil temperature, but this temperature dependency seemed to occur only on the calm days. Average CO2 and N2O fluxes were 27.7 microg CO2/m2/sec and 13.4 ng N2O/m2/sec, with relatively small fluctuation during windy days, while averages of 69.3 microg CO2/m2/sec and 65.8 ng N2O/m2/sec were measured during calm days. This relationship was thought to be a result of strong surface winds, which enhance gas exchange between the soil surface and the atmosphere, thus reducing the gas emissions from soil surfaces.

Peroxyacetyl Nitrate in Atlanta, Georgia: Comparison and Analysis of Ambient Data for Suburban and Downtown Locations.

Peroxyacetyl nitrate (PAN) concentrations were measured at downtown and suburban locations in Atlanta, GA, in July and August 1992 as part of the SOS-SORP/ONA (Southern Oxidants Study-Southern Oxidants Research Program on Ozone Non-Attainment). PAN concentrations were generally higher at the downtown location than at the suburban location, but on days when the O3 concentration exceeded 80 ppbv, PAN concentrations were similar at both locations. On days when O3 did not exceed 80 ppbv, suburban PAN concentrations were much lower than downtown concentrations and resembled those reported for rural areas in the eastern United States. Regression analysis of PAN and O3 on NOx and total non-methane hydrocarbons (TNMHC) showed PAN to be most strongly dependent on morning NOx concentrations, while O3 was most dependent on morning TNMHC concentrations. NOx, PAN, and meteorological data from the suburban site were used in a one-dimensional transport model to estimate the accumulation rate of PAN to be ~1.5 x 106 molecules cm-3 sec-1. A simple kinetic model estimated peroxyacetyl radical concentrations to be ~0.5 pptv at the suburban location.