The effects of synoptical weather pattern and complex terrain on the formation of aerosol events in the Greater Taipei area

Effects of boundary layer variations on physicochemical characteristics of aerosols in mid-low-altitude regions

Variations in the height of the boundary layer have a critical impact on the vertical transport of near-surface aerosols. Variations can affect the interactions between aerosols and clouds/fog by altering the scattering and absorption of solar radiation, significantly changing radiative forcing, convective precipitation, and regional climate. In this study, we simultaneously monitored air pollution and meteorological factors in a flat urban area (YunTech site, 50 m asl) and its peripheral mountainous region (MeiShan site, 980 m asl), analyzed the characteristics of pollutants under different atmospheric conditions, and explored the differences in the chemical reaction mechanisms of aerosols at various altitudes, aiming to clarify the evolution of the boundary layer in urban and suburban areas and its impact on the transport of pollutants. The results show that even without anthropogenic emissions, urban ground-level pollutants could be transported to peripheral mountainous areas through boundary layer height variations and local circulations, such as mountain-valley breezes. The PM2.5 concentration was higher at the urban site (average 31.14 ± 14.82μgm-3) and could be transported aloft by valley winds, leading to the gradual accumulation of daytime PM2.5 with an afternoon peak at the mountain site. Moreover, the nitrogen oxidation rate (NOR = [NO3-]/[NO3-] + [NO2]) exhibited clear site variations, the mountain site (average 0.41 ± 0.20) was higher than the urban site (average 0.19 ± 0.07), likely due to the atmospheric environment with thick clouds/fog and strong oxidation capacity in the mountain area. Our study has verified that aerosol characteristics, origins, formation pathways and transport mechanisms at the two measurement sites are significantly different under different conditions, which provides a theoretical basis for future air pollution prevention and regional climate research.

Spatiotemporal characteristic analysis of PM2.5 in central China and modeling of driving factors based on MGWR: a case study of Henan Province

Since the start of the twenty-first century, China's economy has grown at a high or moderate rate, and air pollution has become increasingly severe. The study was conducted using data from remote sensing observations between 1998 and 2019, employing the standard deviation ellipse model and spatial autocorrelation analysis, to explore the spatiotemporal distribution characteristics of PM2.5 in Henan Province. Additionally, a multiscale geographically weighted regression model (MGWR) was applied to explore the impact of 12 driving factors (e.g., mean surface temperature and CO2 emissions) on PM2.5 concentration. The research revealed that (1) Over a period of 22 years, the yearly mean PM2.5 concentrations in Henan Province demonstrated a trend resembling the shape of the letter "M", and the general trend observed in Henan Province demonstrated that the spatial center of gravity of PM2.5 concentrations shifted toward the north. (2) Distinct spatial clustering patterns of PM2.5 were observed in Henan Province, with the northern region showing a primary concentration of spatial hot spots, while the western and southern areas were predominantly characterized as cold spots. (3) MGWR is more effective than GWR in unveiling the spatial heterogeneity of influencing factors at various scales, thereby making it a more appropriate approach for investigating the driving mechanisms behind PM2.5 concentration. (4) The results acquired from the MGWR model indicate that there are varying degrees of spatial heterogeneity in the effects of various factors on PM2.5 concentration. To summarize the above conclusions, the management of the atmospheric environment in Henan Province still has a long way to go, and the formulation of relevant policies should be adapted to local conditions, taking into account the spatial scale effect of the impact of different influencing factors on PM2.5.

The influence of COVID-19 pandemic on PM2.5 air quality in Northern Taiwan from Q1 2020 to Q2 2021

The study aimed to investigate the PM_2.5 variations in different periods of COVID-19 control measures in Northern Taiwan from Quarter 1 (Q1) 2020 to Quarter 2 (Q2) 2021. PM_2.5 sources were classified based on long-range transport (LRT) or local pollution (LP) in three study periods: one China lockdown (P1), and two restrictions in Taiwan (P2 and P3). During P1 the average PM_2.5 concentrations from LRT (LRT-PM_2.5-P1) were higher at Fuguei background station by 27.9% and in the range of 4.9-24.3% at other inland stations compared to before P1. The PM_2.5 from LRT/LP mix or pure LP (Mix/LP-PM_2.5-P1) was also higher by 14.2-39.9%. This increase was due to higher secondary particle formation represented by the increase in secondary ions (SI) and organic matter in PM_2.5-P1 with the largest proportion of 42.17% in PM_2.5 from positive matrix factorization (PMF) analysis. A similar increasing trend of Mix/LP-PM_2.5 was found in P2 when China was still locked down and Taiwan was under an early control period but the rapidly increasing infected cases were confirmed. The shift of transportation patterns from public to private to avoid virus infection explicated the high correlation of the increasing infected cases with the increasing PM_2.5. In contrast, the decreasing trend of LP-PM_2.5-P3 was observed in P3 with the PM_2.5 biases of ∼45% at all the stations when China was not locked down but Taiwan implemented a semi-lockdown. The contribution of gasoline vehicle sources in PM_2.5 was reduced from 20.3% before P3 to 10% in P3 by chemical signatures and source identification using PMF implying the strong impact of strict control measures on vehicle emissions. In summary, PM_2.5 concentrations in Northern Taiwan were either increased (P1 and P2) or decreased (P3) during the COVID-19 pandemic depending on control measures, source patterns and meteorological conditions.

Air quality changes in Taiwan over the past decades and during the COVID-19 crisis

Over the past decades, Taiwan has achieved remarkable goals in air pollution reduction with the concentrations of several common air pollutants such as CO, NOx, PM10, PM2.5, and SO2 going down. In contrast to these achievements, the mitigation of O3 remains extremely tough due to the complexity of its formation process involving synergistic effects of precursor reductions and meteorological influences. During the local COVID-19 crises in Taiwan and the Level 3 alert in 2021, air pollutants directly emitted from the traffic such as CO and NOx present clear relationships with the drop of the recorded freeway traffic volume due to the alert, while PM10 and PM2.5 which are also relevant to the traffic do not show indications of being greatly influenced by the decrease of the traffic flow. Although road traffic is not regarded as a main source of SO2 by current understanding, the unusual SO2 variation patterns found in this study suggest a prolonged impact for months from the changes of travel behavior during the epidemic. In contrast, the epidemic did not exert influences on industrial SO2 concentration which accounts for a large portion of total SO2 in Taiwan, and a similar scenario is also seen in each type of O3 monitoring. Although some results discussed in this study are not in line with current consensuses and understandings in terms of the nation of certain air pollutants, these findings may disclose new perspectives which could be a potential benefit to air quality improvement projects in the future.

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.

Chemically and temporally resolved oxidative potential of urban fine particulate matter

Vehicle emissions are an important source of particulate matter (PM) in urban areas and have well-known adverse health effects on human health. Oxidative potential (OP) is used as a quantification metric for indexing PM toxicity. In this study, by using a liquid spot sampler (LSS) and the dithiothreitol (DTT) assay, the diurnal OP variation was assessed at a ground-level urban monitoring station. Besides, since the monitoring station was adjacent to the main road, the correlation between OP and traffic volume was also evaluated. PM components, including metals, water-soluble inorganic aerosols (WSIAs), black carbon (BC), and polycyclic aromatic hydrocarbons (PAHs), were also simultaneously monitored. The daytime and evening mean ± std volume-normalized OP (OPv) were 0.46 ± 0.27 and 0.48 ± 0.26 nmol/min/m³, and exhibited good correlations with PM_1.0 and BC; however, these concentrations were only weakly correlated with mass-normalized OP (OPm). The mean ± std OPm was higher in the daytime (41.3 ± 13.8 pmol/min/μg) than in the evening (36.1 ± 11.5 pmol/min/μg). According to the PMF analysis, traffic emissions dominated the diurnal OP contribution. Organic matter and individual metals associated with non-exhaust traffic emissions, such as Mn, Fe, and Cu, contributed substantially to OP. Diurnal variations of PAH concentrations suggest that photochemical reactions could enhance OP, highlighting the importance of atmospheric aging on PM toxicity.

Quantifying PM2.5 from long-range transport and local pollution in Taiwan during winter monsoon: An efficient estimation method

From autumn to the following spring, annually, the northeast monsoon transports PM_2.5 (particles less than 2.5 μm in aerodynamic diameter) from the Asian continent to downstream areas. Naturally, this triggered a question: What are the contributions of PM_2.5 from long-range transport (LRT) and local pollution (LP) at any downstream location? To answer that question, the present study developed an economical and efficient method that can easily estimate the contribution of PM_2.5 from LRT (LRT-PM_2.5) and PM_2.5 from LP (LP-PM_2.5). The method used PM_2.5 and meteorological observation data in Taiwan from 2006 to 2015 and a short-term simulation from January to May in 2010. The analysis classified the data into three types of PM_2.5 source patterns: LRT-Event (high concentration plume at the front edge of southward moving anticyclones/strong northeast wind), LRT-Ordinary (less concentration in common strong northeast wind), and LRT/LP Mix or Pure LP (PM_2.5 was from both LRT and LP or from only LP under weak northeast wind). During the ten-year period, the average LRT-PM_2.5 values at the northern tip of Taiwan were 31-39 μg m^-3, 12-16 μg m^-3, and 4-13 μg m^-3 for the LRT-Event, LRT-Ordinary, and LRT/LP Mix or Pure LP patterns, respectively. The 10-year average LRT-PM_2.5 and LP-PM_2.5 contributions were approximately 70:30 in northern Taiwan, 50:50 in central Taiwan, and 30:70 in southern Taiwan for the LRT-Event pattern; 60:40 in northern and 40:60 in central and southern Taiwan for the LRT-Ordinary pattern; and 30:70 in northern and 25:75 in central and southern Taiwan for the LRT/LP Mix or Pure LP pattern. Interestingly, LRT-PM_2.5 peaked in 2013 but has decreased annually since then, whereas LP-PM_2.5 has roughly decreased in the past ten years.

Chinese province-scale source apportionments for sulfate aerosol in 2005 evaluated by the tagged tracer method

Appropriate policies to improve air quality by reducing anthropogenic emissions are urgently needed. This is typified by the particulate matter (PM) problem and it is well known that one type of PM, sulfate aerosol (SO₄^2-), has a large-scale impact due to long range transport. In this study we evaluate the source-receptor relationships of SO₄^2- over East Asia for 2005, when anthropogenic sulfur dioxide (SO₂) emissions from China peaked. SO₂ emissions from China have been declining since 2005-2006, so the possible maximum impact of Chinese contributions of SO₄^2- is evaluated. This kind of information provides a foundation for policy making and the estimation of control effects. The tagged tracer method was applied to estimate the source apportionment of SO₄^2- for 31 Chinese province-scale regions. In addition, overall one-year source apportionments were evaluated to clarify the seasonal dependency. Model performance was confirmed by comparing with ground-based observations over mainland China, Taiwan, Korea, and Japan, and the model results fully satisfied the performance goal for PM. We found the following results. Shandong and Hebei provinces, which were the largest and second largest SO₂ sources in China, had the greatest impact over the whole of East Asia with apportionments of around 10-30% locally and around 5-15% in downwind receptor regions during the year. Despite large SO₂ emissions, the impact of south China (e.g., Guizhou, Guangdong, and Sichuan provinces) was limited to local impact. These results suggest that the reduction policy in south China contributes to improving the local air quality, whereas policies in north and central China are beneficial for both the whole of China and downwind regions. Over Taiwan, Korea, and Japan, the impact of China was dominant; however, local contributions were important during summer.

Effect of typhoon on atmospheric aerosol particle pollutants accumulation over Xiamen, China

Great influence of typhoon on air quality has been confirmed, however, rare data especially high time resolved aerosol particle data could be used to establish the behavior of typhoon on air pollution. A single particle aerosol spectrometer (SPAMS) was employed to characterize the particles with particle number count in high time resolution for two typhoons of Soulik (2013) and Soudelor (2015) with similar tracks. Three periods with five events were classified during the whole observation time, including pre - typhoon (event 1 and event 2), typhoon (event 3 and event 4) and post - typhoon (event 5) based on the meteorological parameters and particle pollutant properties. First pollutant group appeared during pre-typhoon (event 2) with high relative contributions of V - Ni rich particles. Pollution from the ship emissions and accumulated by local processes with stagnant meteorological atmosphere dominated the formation of the pollutant group before typhoon. The second pollutant group was present during typhoon (event 3), while typhoon began to change the local wind direction and increase wind speed. Particle number count reached up to the maximum value. High relative contributions of V - Ni rich and dust particles with low value of NO3(-)/SO4(2-) was observed during this period, indicating that the pollutant group was governed by the combined effect of local pollutant emissions and long-term transports. The analysis of this study sheds a deep insight into understand the relationship between the air pollution and typhoon.

A new conceptual model for quantifying transboundary contribution of atmospheric pollutants in the East Asian Pacific rim region

Transboundary transport of air pollution is a serious environmental concern as pollutant affects both human health and the environment. Many numerical approaches have been utilized to quantify the amounts of pollutants transported to receptor regions, based on emission inventories from possible source regions. However, sparse temporal-spatial observational data and uncertainty in emission inventories might make the transboundary transport contribution difficult to estimate. This study presents a conceptual quantitative approach that uses transport pathway classification in combination with curve fitting models to simulate an air pollutant concentration baseline for pollution background concentrations. This approach is used to investigate the transboundary transport contribution of atmospheric pollutants to a metropolitan area in the East Asian Pacific rim region. Trajectory analysis categorized pollution sources for the study area into three regions: East Asia, Southeast Asia, and Taiwan cities. The occurrence frequency and transboundary contribution results suggest the predominant source region is the East Asian continent. This study also presents an application to evaluate heavy pollution cases for health concerns. This new baseline construction model provides a useful tool for the study of the contribution of transboundary pollution delivered to receptors, especially for areas deficient in emission inventories and regulatory monitoring data for harmful air pollutants.

A time series analysis of multiple ambient pollutants to investigate the underlying air pollution dynamics and interactions

Understanding the temporal dynamics and interactions of particulate matter (PM) concentration and composition is important for air quality control. This paper applied a dynamic factor analysis method (DFA) to reveal the underlying mechanisms of nonstationary variations in twelve ambient concentrations of aerosols and gaseous pollutants, and the associations with meteorological factors. This approach can consider the uncertainties and temporal dependences of time series data. The common trends of the yearlong and three selected diurnal variations were obtained to characterize the dominant processes occurring in general and specific scenarios in Taipei during 2009 (i.e., during Asian dust storm (ADS) events, rainfall, and under normal conditions). The results revealed the two distinct yearlong NOx transformation processes, and demonstrated that traffic emissions and photochemical reactions both critically influence diurnal variation, depending upon meteorological conditions. During an ADS event, transboundary transport and distinct weather conditions both influenced the temporal pattern of identified common trends. This study shows the DFA method can effectively extract meaningful latent processes of time series data and provide insights of the dominant associations and interactions in the complex air pollution processes.

Relationship between fine particulate matter events with respect to synoptic weather patterns and the implications for circulatory and respiratory disease in Taipei, Taiwan

The aim of the study is to assess the relationship between PM2.5, synoptic weather patterns, and admissions for circulatory and respiratory disease. A PM2.5 event is defined as a day when the daily mean PM2.5 concentration exceeds 65 μg/m(3). PM2.5 events that coincided with the occurrence of PM attributed to Asian dust storm (ADS) and photochemical smog (PCS) were removed from the study in order to focus solely on the health effects from PM2.5. A one-tailed z-test and a relative risk (RR) estimate were performed. Hospital admissions for respiratory diseases were greater than those for circulatory diseases, and asthma-related diseases had a higher impact in the Adults group, and the maximum RR was 1.94 [1.37 2.77] on the first day after the event. It is evident that PM2.5 episodes connected to particular synoptic weather patterns pose a risk to health as large as ADS and PCS events.

Effect of photochemical smog associated with synoptic weather patterns on cardiovascular and respiratory hospital admissions in metropolitan Taipei

This study focuses on the relationship between photochemical smog (PS) (hourly ozone conc. >100 ppb), PS-related diseases, and the synoptic weather patterns during 2005-2009 in metropolitan Taipei. The results show that compared to respiratory ailments (ICD code 460-519) and asthma (ICD code 493), cardiovascular ailments (ICD code 390-459) were more clearly influenced by PS events. On the PS event day, the number of admissions of babies, children, and adults increased by 0.04 [95% CI 0.01-0.06], 0.03 [95% CI 0.00-0.07], and 1.12 [95% CI 0.36-1.89] (P < 0.05), respectively. The investigation showed that more than 70% of the PS events were associated primarily with the peripheral circulation of typhoons, Pacific pressure, and discrete Pacific pressure. PS events are a threat to public health. To avoid the ill effects of air pollution, residents should be informed about the daily weather patterns and air quality.

Simulation of long-range transport aerosols from the Asian Continent to Taiwan by a southward Asian high-pressure system

Aerosol is frequently transported by a southward high-pressure system from the Asian Continent to Taiwan and had been recorded a 100% increase in mass level compared to non-event days from 2002 to 2005. During this time period, PM2.5 sulfate was found to increase as high as 155% on event days as compared to non-event days. In this study, Asian emission estimations, Taiwan Emission Database System (TEDS), and meteorological simulation results from the fifth-generation Mesoscale Model (MM5) were used as inputs for the Community Multiscale Air Quality (CMAQ) model to simulate a long-range transport of PM2.5 event in a southward high-pressure system from the Asian Continent to Taiwan. The simulation on aerosol mass level and the associated aerosol components were found within a reasonable accuracy. During the transport process, the percentage of semi-volatile PM2.5 organic carbon in PM2.5 plume only slightly decreased from 22-24% in Shanghai to 21% near Taiwan. However, the percentage of PM2.5 nitrate in PM2.5 decreased from 16-25% to 1%. In contrast, the percentage of PM2.5 sulfate in PM2.5 increased from 16-19% to 35%. It is interesting to note that the percentage of PM2.5 ammonium and PM2.5 elemental carbon in PM2.5 remained nearly constant. Simulation results revealed that transported pollutants dominate the air quality in Taipei when the southward high-pressure system moved to Taiwan. Such condition demonstrates the dynamic chemical transformation of pollutants during the transport process from continental origin over the sea area and to the downwind land.