A reduction in gaseous pollutants is an important method for mitigating PM_2.5 concentration in the atmosphere, and the reduction in SO₂/NH₃/NO_x is beneficial to control secondary inorganic aerosols in PM_2.5. In this study, the Weather Research and Forecasting model with Chemistry model (WRF-Chem) was applied to study the impact on the PM_2.5 and its secondary inorganic aerosols using the scenario simulation method in the Beijing–Tianjin–Hebei (BTH) region. The results showed that the BTH region is characterized by being NH₃-rich and having a higher [NH₄⁺]/[SO₄²⁻] ratio in southern BTH, with a ratio of more than 6.0. Source contribution to PM_2.5 was highest in the 30%_SO₂_40%_NH₃_40%_NO_x scenario, with a contribution ratio of 6.8%, followed by 3.8% contribution in the 30%_SO₂_40%_NH₃ scenario, and a 3.4% contribution in the 30%_SO₂_60%_NH₃_60%_NO_x scenario. These results indicate that synergistic reduction measures may be suitable for controlling PM_2.5 concentrations. A lower sensitivity factor, β value between PM_2.5 and NH₃ suggests that solely reducing NH₃ emissions is not beneficial for the BTH region. However, this study indicates that the sensitivity of NO₃⁻ would improve significantly if NH₃ emissions are reduced sharply. A slight reduction in NH₃ was found to be beneficial for controlling NO₃⁻ in medium and small cities, while a significant decrease in NH₃ would be more suitable for mega-cities. The study also observed that SO₄²⁻ and its constituents continued to decrease with a consistent β value of approximately 0.14 in the 30%_SO₂_%_NH₃ scenario and between 10.5 and 12.8 in the 30%_SO₂_%_NH₃_%_NO_x scenario. These findings suggest that a synergistic reduction in SO₂-NH₃-NO_x emissions may be more effective in reducing PM_2.5 concentrations and its secondary inorganic aerosols (SIAs). However, it is important to ensure that the reduction in NH₃ and NO_x exceeds 60% in low SO₂ concentration conditions.

Atmospheric aerosols in Southeast Asia, influenced by climate and seasonal circulation, are examined here. This study analyzes the impact of the 2015 El Niño and monsoonal variability on aerosol properties over Penang, Malaysia, from 2015–2019. Aerosol Optical Depth (AOD), Ångström Exponent (AE), Fine Mode Fraction (FMF), and Single Scattering Albedo (SSA) were analyzed using AERONET observations, complemented by satellite-derived fire data and NOAA HYSPLIT back-trajectory analysis. Pronounced seasonal variability was observed, with elevated AOD during the Southwest Monsoon (0.72 ± 0.15) associated with biomass burning and mixed urban aerosols, and lower AOD during the Northeast Monsoon (0.47 ± 0.12) due to cleaner maritime air masses. The inter-monsoon period exhibited the lowest AOD (0.28 ± 0.10), reflecting enhanced wet scavenging and mixed aerosol sources. Interannually, the 2015 El Niño recorded substantially higher aerosol loading, including extreme AOD events (>1.75), driven by intensified regional fire activity under dry conditions. A statistically significant but weak correlation (R² = 0.12, p = 0.047) indicates biomass burning contributed to AOD, though transport processes were the dominant driver. Trajectory analysis confirmed that aerosols originated from fire-affected Sumatra during the Southwest Monsoon and from the South China Sea during the Northeast Monsoon. These results show that climate and winds drive aerosol changes, so regional monitoring and cross-border air management in Southeast Asia are needed.

Atmospheric aerosols strongly regulate surface solar irradiance in tropical coastal environments through scattering and absorption. This study examines aerosol–irradiance interactions over Penang, Malaysia, using Aerosol Robotic Network (AERONET) observations of aerosol optical depth (AOD), single scattering albedo (SSA), and extinction Ångström exponent (AE); NASA’s Prediction of Worldwide Energy Resource (POWER) irradiance data; and Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) reanalysis for aerosol compositional context. Bottom-of-atmosphere radiative forcing efficiency (BOA RFE) was quantified for global, direct and diffuse irradiance (GHI, DNI and DHI) under clear- and all-sky conditions during 2014–2018. Results show persistent aerosol-induced attenuation of surface radiation, with GHI and DNI RFE predominantly negative, while DHI RFE remains consistently positive, indicating redistribution of solar energy from direct to diffuse components. Time resolved analysis reveals daily GHI RFE typically ranging from approximately −0.5 to −3.5 W m⁻² per unit AOD, with episodic excursions below −4 W m⁻² per AOD during high-aerosol events, whereas DNI RFE frequently reaches values below −0.8 W m⁻² per AOD, confirming its greater sensitivity to aerosol extinction. In contrast, DHI RFE commonly exceeds +5 W m⁻² per AOD and intermittently surpasses +10 W m⁻² per AOD, reflecting enhanced scattering and multiple-scattering effects. AOD-stratified analysis demonstrates a nonlinear weakening of forcing efficiency with increasing aerosol burden, with mean GHI RFE decreasing from approximately −1.6 to −0.4 W m⁻² per AOD between low- and high-AOD regimes, accompanied by corresponding reductions in DNI (−0.35 to −0.1 W m⁻² per AOD) and DHI (+3.3 to +0.8 W m⁻² per AOD). Overall, aerosol loading is identified as the dominant control on BOA radiative forcing efficiency in this tropical coastal environment, while SSA and AE act as secondary modulators.

Atmospheric aerosols strongly regulate surface solar irradiance in tropical coastal environments through scattering and absorption. This study examines aerosol–irradiance interactions over Penang, Malaysia, using Aerosol Robotic Network (AERONET) observations of aerosol optical depth (AOD), single scattering albedo (SSA), and extinction Ångström exponent (AE); NASA’s Prediction of Worldwide Energy Resource (POWER) irradiance data; and MERRA-2 reanalysis for aerosol compositional context. Bottom-of-atmosphere radiative forcing efficiency (BOA RFE) was quantified for global, direct and diffuse irradiance (GHI, DNI and DHI) under clear and all-sky conditions during 2014–2018. Results show persistent aerosol-induced attenuation of surface radiation, with GHI and DNI RFE predominantly negative, while DHI RFE remains consistently positive, indicating redistribution of solar energy from direct to diffuse components. Time resolved analysis reveals daily GHI RFE typically ranging from approximately −0.5 to −3.5 W m⁻² per unit AOD, with episodic excursions below −4 W m⁻² per AOD during high-aerosol events, whereas DNI RFE frequently reaches values below −0.8 W m⁻² per AOD, confirming its greater sensitivity to aerosol extinction. In contrast, DHI RFE commonly exceeds +5 W m⁻² per AOD and intermittently surpasses +10 W m⁻² per AOD, reflecting enhanced scattering and multiple-scattering effects. AOD-stratified analysis demonstrates a nonlinear weakening of forcing efficiency with increasing aerosol burden, with mean GHI RFE decreasing from approximately −1.6 to −0.4 W m⁻² per AOD between low- and high-AOD regimes, accompanied by corresponding reductions in DNI (−0.35 to −0.1 W m⁻² per AOD) and DHI (+3.3 to +0.8 W m⁻² per AOD). Overall, aerosol loading is identified as the dominant control on BOA radiative forcing efficiency in this tropical coastal environment, while SSA and AE act as secondary modulators.

Downward shortwave solar energy is a very important parameter for the surface energy balance, which affects human life and other activities of the ecosystem. It is also one of the critical terms in the study of climate change that requires consistent contributions from different researchers. In this paper, we proposed the use of Discrete Ordinate Radiative Transfer method (DISORT) to estimate the downward Total Solar Flux (TSF) in Penang, Malaysia. The measured aerosols optical properties obtained from the ground-based Aerosol Robotic Network (AERONET) data were used as inputs to the DISORT codes. Data of the pyranometer ground-based measurements of TSF were also used for validation of the simulated results from this study. The results for two days, 17 January 2015 and 1 February 2015, have been used to test the performance and accuracy of the method. It is found that there is good agreement between the results from this study and the ground-based measurement results available from AERONET. The validation results show that for the first day (r = 0.9968), (R² = 0.9936), root-mean-square error (RMSE) of 36.66 and mean absolute percentage error (MAPE) is 16%. For the second day (r = 0.9986), (R² = 0.9971), (RMSE = 33.06), (MAPE = 12%). The small variation between the two results was due to error that might have arisen because of the model imperfection, and from the small inefficiency of the pyranometer and sunphotometer. Nevertheless, this study shows that the proposed integration of ground-based and model simulation is an excellent and a valid alternative, for the estimates of solar radiation and the study of the impact of aerosols on the TSF everywhere and at all times, particularly when the results from other sources are not available.