Fine particulate matter (PM) release is regulated by environmental policies in most countries. This study investigated long–term trends in the mass extinction efficiency (Q_e) of aerosols in Northeast Asia. For this purpose, the Q_e was calculated using visibility, PM_2.5 recorded between 2015 and 2020, and PM₁₀ recorded between 2001 and 2020 at eight Korean sites. The Q_e of PM₁₀ (Q_e,10) showed an increasing trend with 0.06~0.22 (m²/g)/yr in seven cities except for Jeju. The Q_e of PM_2.5 (Q_e,2.5) also showed an increasing trend with 0.28–2.47 (m²/g)/yr in all cities. In this study, PM₁₀ and PM_2.5, were divided into low, moderate, and high concentrations, and the Q_e value change by year was examined. Q_e,10 showed a tendency to decrease at low concentrations (19–21 μg/m³). However, at moderate (69–71 μg/m³) and high concentrations (139–141 μg/m³), Q_e,10 increased in most regions. Q_e,2.5 showed an increasing trend at low concentration (9–11 μg/m³), moderate concentration (29–31 μg/m³), and high concentration (69–71 μg/m³), except for Suwon and Pohang, where data were insufficient for analysis. Both Q_e,10 and Q_e,2.5 showed an increasing trend. The increase in Q_e indicated that the visibility-impairing effect of PM can increase even if the same concentration of PM is present. The visibility-impairing effects of PM vary based on the composition, size and other characteristics of the particles in the atmosphere at a given point in time and not simply the quantity of particles. This means that reducing the quantity of particles does not reliably produce a proportionate improvement in visibility. Air quality policies must take the variable nature of PM particles and their effect on visibility into account so that more consistent improvements in air quality can be achieved.

To identify the long-term trend of particle size variation, we analyzed aerosol optical depth (AOD, τ) separated as dust (${\mathsf{\tau }}_{\mathrm{D}}$) and coarse-(${\mathsf{\tau }}_{\mathrm{PC}}$) and fine-pollution particles (${\mathsf{\tau }}_{\mathrm{PF}}$) depending on emission sources and size. Ångström exponent values are also identified separately as total and fine-mode particles (${\mathsf{\alpha }}_{\mathrm{T}}$ and ${\mathsf{\alpha }}_{\mathrm{PF}}$). We checked these trends in various ways; (1) first-order linear regression analysis of the annual average values, (2) percent variation using the slope of linear regression method, and (3) a reliability analysis using the Mann–Kendall (MK) test. We selected 17 AERONET sun/sky radiometer sites classified into six regions, i.e., Europe, North Africa, the Middle East, India, Southeast Asia, and Northeast Asia. Although there were regional differences, τ decreased in Europe and Asian regions and increased in the Middle East, India, and North Africa. Values of ${\mathsf{\tau }}_{\mathrm{PC}}$ and ${\mathsf{\tau }}_{\mathrm{PF}}$, show that aerosol loading caused by non-dust aerosols decreased in Europe and Asia and increased in India. In particular, ${\mathsf{\tau }}_{\mathrm{PF}}$ considerably decreased in Europe and Northeast Asia (95% confidential levels in MK-test), and ${\mathsf{\tau }}_{\mathrm{PC}}$ decreased in Northeast Asia (Z-values for Seoul and Osaka are −2.955 and −2.306, respectively, statistically significant if |z| ≥ 1.96). The decrease in ${\mathsf{\tau }}_{\mathrm{PC}}$ seems to be because of the reduction of primary and anthropogenic emissions from regulation by air quality policies. The meaningful result in this paper is that the particle size became smaller, as seen by values of ${\mathsf{\alpha }}_{\mathrm{T}}$ that decreased by −3.30 to −30.47% in Europe, North Africa, and the Middle East because ${\mathsf{\alpha }}_{\mathrm{T}}$ provides information on the particle size. Particle size on average became smaller over India and Asian regions considered in our study due to the decrease in coarse particles. In particular, an increase of ${\mathsf{\alpha }}_{\mathrm{PF}}$ in most areas shows the probability that the average particle size of fine-mode aerosols became smaller in recent years. We presumed the cause of the increase in ${\mathsf{\alpha }}_{\mathrm{T}}$ is because relatively large-sized fine-mode particles were eliminated due to air quality policies.

Black carbon (BC) absorption aerosol optical depth (AAOD_BC) defines the contribution of BC in light absorption and is retrievable using sun/sky radiometer measurements provided by Aerosol Robotic Network (AERONET) inversion products. In this study, we utilized AERONET-retrieved depolarization ratio (DPR, ${\mathsf{\delta }}^{\mathrm{p}}$), single scattering albedo (SSA, ω), and Ångström Exponent (AE, å) of version 3 level 2.0 products as indicators to estimate the contribution of BC to the absorbing fractions of AOD. We applied our methodology to the AERONET sites, including North and South America, Europe, East Asia, Africa, India, and the Middle East, during 2000–2018. The long-term AAOD_BC showed a downward tendency over Sao Paulo (−0.001 year⁻¹), Thessaloniki (−0.0004 year⁻¹), Beijing (−0.001 year⁻¹), Seoul (−0.0015 year⁻¹), and Cape Verde (−0.0009 year⁻¹) with the highest values over the populous sites. This declining tendency in AAOD_BC can be attributable to the successful emission control policies over these sites, particularly in Europe, America, and China. The AAOD_BC at the Beijing, Sao Paulo, Mexico City, and the Indian sites showed a clear seasonality indicating the notable role of residential heating in BC emissions over these sites during winter. We found a higher correlation between AAOD_BC and fine mode AOD at 440 nm at all sites except for Beijing. High pollution episodes, BC emission from different sources, and aggregation properties seem to be the main drivers of higher AAOD_BC correlation with coarse particles over Beijing.

Absorption aerosol optical depth induced by black carbon (AAOD_BC) was retrieved using the depolarization ratio and single scattering albedo provided by the Aerosol Robotic Network (AERONET) inversion products over East Asia. Our analysis considered AERONET data from six sites in East Asia that are mostly affected by anthropogenic pollution, black carbon (BC) emissions, and natural mineral dust, during the period 2001–2018. We identified a rapid reduction in total aerosol optical depth (AOD_T) of −0.0106 yr⁻¹ over Beijing, whereas no considerable trend was observed at the Korean and Japanese sites. The long-term data for AAOD_BC showed decreasing trends at all sites. We conclude that successful emission control policies were the major underlying driver of AOD_T and AAOD_BC reductions over East Asia, particularly in China, during the study period. Values of the AAOD_BC/AOD_T ratio revealed that, although these policies were successful, the Chinese government needs to undertake stricter measures toward reducing BC emissions. We found that AAOD_BC follows seasonal trends, peaking in the colder months. This suggests that in East Asia, particularly in China, domestic coal burning is still of concern.

Rapid industrialization and the influx of human resources have led to the establishment of industrial complexes near urban areas, exposing residents to various air pollutants. This has led to a decline in air quality, impacting neighboring residential areas adversely, which highlights the urgent need to monitor air pollution in these areas. Recent advancements in technology, such as Solar Occultation Flux (SOF) and Sky Differential Optical Absorption Spectroscopy (SkyDOAS) used as remote sensing techniques and mobile extraction Fourier Transform Infrared Spectrometry (MeFTIR) used as an in situ technique, now offer enhanced precision in estimating the pollutant emission flux and identifying primary sources. In a comprehensive study conducted in 2020 in the Sinpyeong Jangrim Industrial Complex in Busan City, South Korea, a mobile laboratory equipped with SOF, SkyDOAS, and MeFTIR technologies was employed to approximate the emission flux of total alkanes, sulfur dioxide (SO₂), nitrogen dioxide (NO₂), formaldehyde (HCHO), and methane (CH₄). Using the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) diffusion model, pollutant dispersion to residential areas was simulated. The highest average daily emission flux was observed for total alkanes, with values of 69.9 ± 71.6 kg/h and 84.1 ± 85.8 kg/h in zones S₁ and S₂ of the Sinpyeong Jangrim Industrial Complex, respectively. This is primarily due to the prevalence of metal manufacturing and mechanical equipment industries in the area. The HYSPLIT diffusion model confirmed elevated pollution levels in residential areas located southeast of the industrial complex, underscoring the influence of the dominant northwesterly wind direction and wind speed on pollutant dispersion. This highlights the urgent need for targeted interventions to address and mitigate air pollution in downwind residential areas. The total annual emission fluxes were estimated at 399,984 kg/yr and 398,944 kg/yr for zones S₁ and S₂, respectively. A comparison with the Pollutant Release and Transfer Registers (PRTRs) survey system revealed that the total annual emission fluxes in this study were approximately 24.3 and 4.9 times higher than those reported by PRTRs. This indicates a significant underestimation of the impact of small businesses on local air quality, which was not accounted for in the PRTR survey system.