Long-term (2003-2018) trends in aerosol chemical components at a high-altitude background station in the western North Pacific: Impact of long-range transport from continental Asia

Applying the total carbon-black carbon approach method to investigate the characteristics of primary and secondary carbonaceous aerosols in ambient PM2.5 in northern Taiwan

Ambient fine particulate matter (PM2.5) comprises a diverse array of carbonaceous species, and the impact of carbonaceous aerosols (CA) extends to both long-term and short-term effects on human health and the environment. Understanding the distinctive composition of CA is crucial for gaining insights into the origins of airborne particulate matter. Due to their diverse physicochemical properties and intricate heterogeneous reactions, CA often exhibits temporal and spatial variations. Ground-based and highly time-resolved apportionment methods play a vital role in discerning CA emissions. This study utilized high-time resolution data of total carbon (TC) and black carbon (BC) for CA apportionment in northern Taiwan. The advanced numerical model (TC-BC(λ)), coupled with continuous measurement data, facilitated CA allocation based on optical absorption characteristics, organic or elemental carbon composition, and the distinction between primary and secondary origins. Primary carbonaceous aerosols dominated the monitoring site, accounting for 67.5 % compared to the 32.5 % contribution from secondary forms of CA. The summer season exhibited a maximum increase in secondary organic aerosols (SOA) at 41.5 %. Diurnal variations for primary emissions, such as BCc and primary organic aerosols (POA), showed marked peaks for BCff and POAnon-abs during morning rush hours. In contrast, BCbb and POABrC displayed bimodal peaks with increased concentrations during evening hours. Conversely, SOA exhibited significantly different diurnal trends, with SOABrC peaking late at night due to aqueous phased reactions and a noontime peak of SOAnon-abs observed due to photo-oxidation processes. Furthermore, the study employed backward trajectory analysis and concentration-weighted trajectories (CWTs) to examine the long-range transport of CA, identifying potential sources, origins, and transport patterns of CA components to the receptor site in Taiwan during different seasons.

Deseasonalized trend of ground-level ozone and its precursors in an industrial city Kaohsiung, Taiwan

Mitigating ground-level ozone (GLO) remains challenging due to its highly nonlinear formation process. Thus, understanding GLO pollution trends is crucial for developing effective control strategies, especially Kaohsiung industrial city, Taiwan. Based on the long-term monitoring data set of 2011-2022, temporal analysis reveals that monthly mean GLO peaks in autumn (40.66 ± 5.10 ppb), carbon monoxide (CO) and major precursors such as nitrogen oxides (NOx), nonmethane hydrocarbons (NMHC) reach their highest levels in winter. The distinct seasonal variation of air pollutants in Kaohsiung is primarily influenced by the unique blocking effect of the mountainous area under the northeasterly wind, as the city is situated downwind, causing high GLO levels during autumn due to the accumulation of stagnant air hindering the dispersion of pollutants. Over the 12 years (2011-2022), the deseasonalized trend analysis was conducted with p < 0.001, revealing a stabilization trend of GLO (+0.04 ppb/yr) from a previous sharp increase. The observed improvement is credited to a drastic decrease in total oxidants (Ox) at -0.63 ppb/yr due to significantly reducing their precursors. Furthermore, the effectiveness of precursor reduction is also supported by GLO daily maximum profile changes. While high GLO events (>120 ppb) decrease, days within midrange (60-80 ppb) rise from 24.4% to 33.3%. A notable difference emerges when comparing daytime and nighttime GLO. While daytime GLO decreased at -0.22 ppb/yr, nighttime GLO increased at +0.34 ppb/yr. Weakened nocturnal titration effects accounted for the nighttime increase. The distinct spatial variations in GLO trends on a citywide scale underscore that areas with complicated industrial activities may not benefit from a continuing reduction of precursors compared to less-polluted areas. The findings of this study hold significant implications for improving GLO control strategies in heavily industrialized city and provide valuable information to the general public about the current state of GLO pollution.

Integrating satellite and model data to explore spatial-temporal changes in aerosol optical properties and their meteorological relationships in northwest India

This study aims to analyze the temporal and spatial distribution of Aerosol Optical Properties across Northwest India using aerosol data from MODIS (Moderate Resolution Imaging Spectroradiometer) and OMI (Ozone Monitoring Instrument) sensors from 2003 to 2022. Therefore, this study investigated the decadal, interannual, and seasonal changes in aerosol optical properties, vegetation index, and meteorological parameters in the northwest Indian region (8 boxes). Using GIOVANNI (Goddard Earth Sciences Data and Information Services Center (GES DISC) Online Visualization and Analysis Infrastructure), we retrieved daily and monthly Aqua and Terra MODIS products of aerosol optical depth (AOD), Angstrom exponent (AE), normalized difference vegetation index (NDVI), and OMI aerosol index (AI) to examine the spatiotemporal variations by using statistical approaches. The results demonstrated that the decadal averages of aerosol properties showed values of AOD 0.35 (Aqua) and 0.34 (Terra) and AE 1.20 (Aqua) and 1.10 (Terra) with the highest levels during the post-monsoon. Notably, the mean interannual concentrations of AOD and NDVI consistently surpass 0.3, and AE and AI exceed 1 in most locations, underscoring the persistence of high aerosol loading. Also, the study revealed a negative decadal change in AOD of about -8.24 %, while AE, AI, and NDVI showed positive decadal changes of about 9.24 %, 15.09 %, and 12.67 %, respectively. In addition, aerosol optical properties and local meteorology strongly correlated (-0.8 to +0.8). Principal Component Analysis (PCA) identifies meteorological parameters as significant drivers, with the first three components explaining over 70 % of the variation in aerosol optical properties. The NOAA HYSPLIT trajectory model suggests that the long-distance dust transport from the Arabian Peninsula frequently penetrates Gujarat province and then to northwest India. The results contributed to air quality management strategies and provided valuable insights into regional climate and air quality with the influence of meteorology.

A numerical study of reducing the concentration of O3 and PM2.5 simultaneously in Taiwan

Since the 24-hr PM_2.5 (particle aerodynamic diameter less than 2.5 μm) concentration standard was regulated in Taiwan in 2012, the PM_2.5 concentration has been decreasing year by year, but the ozone (O₃) concentration remains almost the same. In particular, the daily maximum 8-hr average O₃ (MDA8 O₃) concentration frequently exceeds the standard. The goal of this study is to find a solution for reducing PM_2.5 and O₃ simultaneously by numerical modeling. After the Volatile Organic Compounds (VOC_S)-limited and nitrogen oxides (NO_X)-limited areas were defined in Taiwan, then, in total, 50 scenarios are simulated in this study. In terms of the average in Taiwan, the effect of VOC_S emission reduction is better than that of NO_X on the decrease in PM_2.5 concentration, when the same reduction proportion (20%, 40%) is implemented. While the effect of further NO_X emission reduction (60%) will exceed that of VOC_S. The decrease in PM_2.5 is proportional to the reduction in precursor emissions such as NO_X, VOC_S, sulfur dioxides (SO₂), and ammonia (NH₃). The lower reduction of NO_X emission for whole Taiwan caused O₃ increases on average but higher reduction can ease the increase, which suggests the implement of NO_X emission reductions must be cautious. When comparing administrative jurisdictions in terms of grids, districts/towns, and cities/counties, it was found that controlling NO_X and VOC_S at a finer spatial resolution of control units did not benefit the decrease in PM_2.5 but did benefit the decrease in O₃. The enhanced O₃ control strategies obviously cause a higher decrease of O₃ throughout Taiwan due to NO_X and VOC_S emission changes when they are implemented in the right places. Finally, three sets of short-term and long-term goals of controlling PM_2.5 and O₃ simultaneously are drawn from the comprehensive rankings for all simulated scenarios, depending on whether PM_2.5 or O₃ control is more urgent. In principle, the short-term scenarios could be ordinary or enhanced version of O₃ decrease with lower NO_X/VOC_S emissions, while the long-term scenario is enhanced version of O₃ decrease plus high emission reductions for all precursors.

Insights into aerosol chemical composition and optical properties at Lulin Atmospheric Background Station (2862 m asl) during two contrasting seasons

Continental outflows from peninsular Southeast Asia and East Asia dominate the widespread dispersal of air pollutants over subtropical western North Pacific during spring and autumn, respectively. This study analyses the chemical composition and optical properties of PM₁₀ aerosols during autumn and spring at a representative high-altitude site, viz., Lulin Atmospheric Background Station (23.47°N, 120.87°E; 2862 m a.s.l.), Taiwan. PM₁₀ mass was reconstructed and the contributions of major chemical components were also delineated. Aerosol scattering (σ_sp) and absorption (σ_ap) coefficients were regressed on mass densities of major chemical components by assuming external mixing between them, and the site-specific mass scattering efficiency (MSE) and mass absorption efficiency (MAE) of individual components for dry conditions were determined. NH₄NO₃ exhibited the highest MSE among all components during both seasons (8.40 and 12.58 m² g^-1 at 550 nm in autumn and spring, respectively). (NH₄)₂SO₄ and organic matter (OM) accounted for the highest σ_sp during autumn (51%) and spring (50%), respectively. Mean MAE (mean contribution to σ_ap) of elemental carbon (EC) at 550 nm was 2.51 m² g^-1 (36%) and 7.30 m² g^-1 (61%) in autumn and spring, respectively. Likewise, the mean MAE (mean contribution to σ_ap) of organic carbon (OC) at 550 nm was 0.84 m² g^-1 (64%) and 0.83 m² g^-1 (39%) in autumn and spring, respectively. However, a classification matrix, based on scattering Ångström exponent, absorption Ångström exponent, and single scattering albedo (ω), demonstrated that the composite absorbing aerosols were EC-dominated (with weak absorption; ω = 0.91-0.95) in autumn and a combination of EC-dominated and EC/OC mixture (with moderate absorption; ω = 0.85-0.92) in spring. This study demonstrates a strong link between chemical composition and optical properties of aerosol and provides essential information for model simulations to assess the imbalance in regional radiation budget with better accuracy over the western North Pacific.

Humic-like substances (HULIS) in springtime aerosols at a high-altitude background station in the western North Pacific: Source attribution, abundance, and light-absorption

Atmospheric humic-like substances (HULIS) are important components of biomass-burning (BB) emissions and highly associated with light-absorbing organic aerosols (often referred to as brown carbon). This study highlights the importance of BB-emitted HULIS aerosols in peninsular Southeast Asian outflow to the subtropical western North Pacific. We determined various key light-absorbing characteristics of HULIS i.e. mass absorption cross-section (MAC_HULIS), absorbing component of the refractive index (k_HULIS), and absorption Ångström exponent (AAE_HULIS) based on ground-based aerosol light absorption measurements along with HULIS concentrations in springtime aerosols at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level), which is a representative high-altitude remote site in the western North Pacific. Daily variations of HULIS (0.58-12.92 μg m^-3) at LABS were mostly linked with the influence from incoming air-masses, while correlations with BB tracers and secondary aerosols indicated the attribution of primary and secondary sources. Stronger light absorption capability of HULIS was clearly evident from MAC_HULIS and k_HULIS values at 370 nm, which were about ~1.5 times higher during BB-dominated days (1.16 ± 0.75 m² g^-1 and 0.05 ± 0.03, respectively) than that during non-BB days (0.77 ± 0.89 m² g^-1 and 0.03 ± 0.04, respectively). Estimates from a simple radiative transfer model showed that HULIS absorption can add as much as 15.13 W g^-1 to atmospheric warming, and ~46% more during BB-dominated than non-BB period, highlighting that HULIS light absorption may significantly affect the Earth-atmosphere system and tropospheric photochemistry over the western North Pacific.