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Haze pollution is affected by local air pollutants, regional transport of background particles and precursors, atmospheric chemistry related to secondary aerosol formation, and meteorological conditions conducive to physical, dynamical, and chemical processes. In the large, populated and industrialized areas like the Asian continental outflow region, the combination of regional transport and local stagnation often exacerbates urban haze pollution. However, the detailed chemical processes underlying the enhancement of urban haze induced by the combined effect of local emissions and transported remote pollutants are still unclear. Here, we demonstrate an important role of transported hygroscopic particles in increasing local inorganic aerosols, by studying the chemical composition of PM 2.5 collected between October 2012 and June 2014 in Seoul, a South Korean megacity in the Asian continental outflow region, using the ISORROPIA II thermodynamic model. PM 2.5 measured under the condition of regional transport from the upwind source areas in China was higher in mass concentration and richer in secondary inorganic aerosol (SIA) species (SO42- , NO3- , and NH4+) and aerosol liquid water (ALW) compared to that measured under non-transport conditions. The secondary inorganic species and ALW were both increased, particularly in cases with high PM 2.5 levels, and this indicates inorganic species as a major driver of hygroscopicity. We conclude that the urban haze pollution in a continental outflow region like Seoul, particularly during the cold season, can be exacerbated by ALW in the transported particles, which enhances the nitrate partitioning into the particle phase in NOx - and NH3 -rich urban areas. This study reveals the synergistic effect of remote and local sources on urban haze pollution in the downwind region and provides insight into the nonlinearity of domestic and foreign contributions to receptor PM 2.5 concentrations in numerical air quality models. [ABSTRACT FROM AUTHOR]

Together with emissions of air pollutants and precursors, meteorological conditions play important roles in local air quality through accumulation or ventilation, regional transport, and atmospheric chemistry. In this study, we extensively investigated multi-timescale meteorological effects on the urban air pollution using the long-term measurements data of PM10, SO2, NO2, CO, and O3 and meteorological variables over the period of 1999-2016 in Seoul, South Korea. The long-term air quality data were decomposed into trend-free short-term components and long-term trends by the Kolmogorov-Zurbenko filter, and the effects of meteorology and emissions were quantitatively isolated using a multiple linear regression with meteorological variables. In terms of short-term variability, intercorrelations among the pollutants and meteorological variables and composite analysis of synoptic meteorological fields exhibited that the warm and stagnant conditions in the migratory high-pressure system are related to the high PM10 and primary pollutant, while the strong irradiance and low NO2 by high winds at the rear of a cyclone are related to the high O3. In terms of long-term trends, decrease in PM10 (-1.75µgm-3yr-1) and increase in O3 (+0.88ppbyr-1) in Seoul were largely contributed by the meteorology-related trends (-0.94µgm-3yr-1 for PM10 and +0.47ppbyr-1 for O3), which were attributable to the subregional-scale wind speed increase. Comparisons with estimated local emissions and socioeconomic indices like gross domestic product (GDP) growth and fuel consumptions indicate probable influences of the 2008 global economic recession as well as the enforced regulations from the mid-2000s on the emission-related trends of PM10 and other primary pollutants. Change rates of local emissions and the transport term of long-term components calculated by the tracer continuity equation revealed a decrease in contributions of local emissions to the primary pollutants including PM10 and an increase in contributions of local secondary productions to O3. The present results not only reveal an important role of synoptic meteorological conditions on the episodic air pollution events but also give insights into the practical effects of environmental policies and regulations on the long-term air pollution trends. As a complementary approach to the chemical transport modeling, this study will provide a scientific background for developing and improving effective air quality management strategy in Seoul and its metropolitan area. [ABSTRACT FROM AUTHOR]

Abstract: We examine the impacts of aerosols on regional meteorology due to intense Siberian forest fires occurred in May 2003 using both reanalysis data and global model simulations. Our analysis of the NCEP-DOE reanalysis data shows 99% statistical significant changes in meteorological variables over East Asia in May 2003 relative to the 30 years climatology. In particular a significant surface cooling was observed up to −3.5 K over Siberia and extended to the North Pacific region with the surface pressure increases up to 5.6 hPa. Whereas, smoke aerosols affected the large-scale circulations and resulted in the increases in rainfall rates of 2.9 mm day−1 averaged over the NW Pacific (10–35°N, 130–170°E). We use the climate model simulations with and without biomass burning emissions over Siberia to examine the effects of smoke aerosols on the regional meteorology. The simulated results show consistent changes in meteorological variables including surface temperature, surface pressure and precipitation rates with the observations over East Asia and the NW Pacific, which support that the observed changes are likely due to smoke aerosols from the Siberian forest fires. The implication is that smoke aerosols from the forest fires should be properly considered to correctly simulate both regional climate and synoptic scale weather patterns. [Copyright &y& Elsevier]

In this study, we investigated the daily variability of PM 10 concentrations in January in Korea during the past 19 years (2001–2019), as well as the associated atmospheric circulation patterns. The daily PM 10 concentrations were classified into three cases: low (L; < 50 μg/m3), high (H; 50–100 μg/m3), and extremely high (EH; ≥ 100 μg/m3). We found that the strength of the East Asian winter monsoon influenced the PM 10 variability in the L and H cases. However, the EH cases were strongly influenced by the rapid growth of barotropic warming (anticyclonic anomaly) over the eastern North Atlantic and Northern Europe (ENE), and the stationary Rossby waves grew rapidly over Eurasia within only four days. Analysis of the quasi-geostrophic geopotential tendency budget revealed that the anticyclonic anomaly over the ENE was enhanced by vorticity advection. Linear baroclinic model experiments confirmed that vorticity forcing over the ENE induces favorable atmospheric conditions for the occurrence of EH PM 10 events in East Asia. As a result, the PM 10 concentration sharply increased sharply by approximately three times over four days. This study suggests that understanding atmospheric teleconnections between the ENE and East Asia can effectively predict the occurrence of EH PM 10 events in Korea, helping to reduce the human health risks from atmospheric pollution. [Display omitted] • Extremely high PM 10 (EH) cases are associated with atmospheric stagnant pattern. • In EH cases, the PM 10 concentration increased about 3 time in 4 days. • Stagnant patterns with EH cases are affected by the North Atlantic forcing. • North Atlantic anticyclonic anomaly forcing is developed by a vorticity advection. • The EH cases are induced by stagnant pattern through the Rossby wave propagation. [ABSTRACT FROM AUTHOR]

Simulation of summertime U.S. surface ozone diurnal cycle is influenced by the model representation of planetary boundary layer (PBL) mixing, spatial resolution, and precursor emissions. These factors are investigated here for five major regions (Northeast, Midwest, Southeast, California, and Southwest) by using the Model for Ozone And Related chemical Tracers version 2.4 (MOZART-2.4), with important modifications, to conduct sensitivity experiments for summer 1999 with three PBL mixing schemes, two horizontal resolutions and two emissions datasets. Among these factors, the PBL mixing is dominant. The default non-local scheme well reproduces the observed ozone diurnal variation, where the timing for the afternoon maximum and the morning minimum is within 1h of the observed; biases for the minimum are less than 5ppb except over the Southeast; and the ozone maximum–minimum contrast (OMMC) is within 10ppb of observations except for the overprediction by 18.9ppb over the Northeast. In contrast, the local scheme significantly overestimates the OMMC by 10–34ppb over all regions as ozone and precursors are trapped too close to the ground. On the other hand, the full-mixing assumption underestimates the OMMC by 0–25ppb, except over the Northeast, as the nighttime ozone decline is greatly underpredicted. As compared to PBL mixing, the effects of horizontal resolutions and precursor emissions being used are smaller but non-negligible. Overall, with the non-local mixing scheme, relatively high horizontal resolution (∼1.1°) and updated emissions data, the modified MOZART is capable of simulating the main features of the observed ozone diurnal cycle. [Copyright &y& Elsevier]

In May 2003, intense forest fires occurred over Siberia, which were the largest fires in the past decade. In order to quantify the effects of these fires on regional air quality in East Asia, we used a global chemical transport model (CTM) with a biomass burning emission inventory constrained by satellite. Our focus was mainly on the enhancements of the ozone and aerosol concentrations due to these fires over East Asia. We first evaluated the model extensively by comparing the simulated and the observed ozone and aerosol concentrations at the EANET sites and found that the simulation reproduced the observed variability of those species. However, some discrepancies were found in the model when compared with the MODIS AOD observations. We tested the sensitivity of the model AOD to different injection heights of fire emissions and found that the model with an injection height of 4.5km was in better agreement with the observations. We then used our model results to quantify the influences of Siberian forest fires on ozone and aerosols concentrations which were computed using the differences between the simulations with and without Siberian forest fire emissions. The peak increases in the surface PM10 and ozone concentrations were up to 90μgm−3 and 33ppbv, respectively, over Siberia. In the downwind regions, the increases ranged from 5 to 30μgm−3 and from 3 to 20ppbv for PM10 and ozone concentrations, respectively, having an important implication for air quality over East Asia. Finally, we computed the radiative forcing of aerosols and ozone from the Siberian forest fires as a measure of climate impact. Siberian forest fires were found to act mainly as a cooling agent resulting in a negative radiative forcing of −5.8Wm−2 at the surface over East Asia. The value at the TOA was −1.5Wm−2, indicating that a considerable absorption of radiation occurred in the atmosphere. This result implies that the Siberian forest fires may affect the regional climate over East Asia by intensifying atmospheric stability. [Copyright &y& Elsevier]

Abstract: Brown carbon aerosols were recently found to be ubiquitous and effectively absorb solar radiation. We use a 3-D global chemical transport model (GEOS-Chem) together with aircraft and ground based observations from the TRACE-P and the ACE-Asia campaigns to examine the contribution of brown carbon aerosol to the aerosol light absorption and its climatic implication over East Asia in spring 2001. We estimated brown carbon aerosol concentrations in the model using the mass ratio of brown carbon to black carbon (BC) aerosols based on measurements in China and Europe. The comparison of simulated versus observed aerosol light absorption showed that the model accounting for brown carbon aerosol resulted in a better agreement with the observations in East Asian-Pacific outflow. We then used the model results to compute the radiative forcing of brown carbon, which amounts up to −2.4 W m−2 and 0.24 W m−2 at the surface and at the top of the atmosphere (TOA), respectively, over East Asia. Mean radiative forcing of brown carbon aerosol is −0.43 W m−2 and 0.05 W m−2 at the surface and at the TOA, accounting for about 15% of total radiative forcing (−2.2 W m−2 and 0.33 W m−2) by absorbing aerosols (BC + brown carbon aerosol), having a significant climatic implication in East Asia. [Copyright &y& Elsevier]