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

Since the 24-hr PM2.5 (particle aerodynamic diameter less than 2.5 µm) concentration standard was regulated in Taiwan in 2012, the PM2.5 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 PM2.5 and O3 simultaneously by numerical modeling. After the Volatile Organic Compounds (VOCS)-limited and nitrogen oxides (NOX)-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 VOCS emission reduction is better than that of NOX on the decrease in PM2.5 concentration, when the same reduction proportion (20%, 40%) is implemented. While the effect of further NOX emission reduction (60%) will exceed that of VOCS. The decrease in PM2.5 is proportional to the reduction in precursor emissions such as NOX, VOCS, sulfur dioxides (SO2), and ammonia (NH3). The lower reduction of NOX emission for whole Taiwan caused O3 increases on average but higher reduction can ease the increase, which suggests the implement of NOX emission reductions must be cautious. When comparing administrative jurisdictions in terms of grids, districts/towns, and cities/counties, it was found that controlling NOX and VOCS at a finer spatial resolution of control units did not benefit the decrease in PM2.5 but did benefit the decrease in O3. The enhanced O3 control strategies obviously cause a higher decrease of O3 throughout Taiwan due to NOX and VOCS emission changes when they are implemented in the right places. Finally, three sets of short-term and long-term goals of controlling PM2.5 and O3 simultaneously are drawn from the comprehensive rankings for all simulated scenarios, depending on whether PM2.5 or O3 control is more urgent. In principle, the short-term scenarios could be ordinary or enhanced version of O3 decrease with lower NOX/VOCS emissions, while the long-term scenario is enhanced version of O3 decrease plus high emission reductions for all precursors.

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 PM2.5 (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 PM2.5 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 PM2.5 from LRT (LRT-PM2.5) and PM2.5 from LP (LP-PM2.5). The method used PM2.5 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 PM2.5 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 (PM2.5 was from both LRT and LP or from only LP under weak northeast wind). During the ten-year period, the average LRT-PM2.5 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-PM2.5 and LP-PM2.5 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-PM2.5 peaked in 2013 but has decreased annually since then, whereas LP-PM2.5 has roughly decreased in the past ten years.

Construction of a cryogen-free thermal desorption gas chromatographic system with off-the-shelf components for monitoring ambient volatile organic compounds.

An automated gas chromatographic system aimed at performing unattended measurements of ambient volatile organic compounds was configured and tested. By exploiting various off-the-shelf components, the thermal desorption unit was easily assembled and can be connected with any existing commercial gas chromatograph in the laboratory to minimize cost. The performance of the complete thermal desorption gas chromatographic system was assessed by analyzing a standard mixture containing 56 target nonmethane hydrocarbons from C2 -C12 at sub-ppb levels. Particular attention was given to the enrichment efficiency of the C2 compounds, such as ethane (b.p. = -88.6°C) and ethylene (b.p. = -104.2°C), due to their extremely high volatilities. Quality assurance was performed in terms of the linearity, precision and limits of detection of the target compounds. To further validate the system, field measurements of target compounds in ambient air were compared with those of a commercial total hydrocarbon analyzer and a carbon monoxide analyzer. Highly coherent results from the three instruments were observed during a two-month period of synchronized measurements. Moreover, the phenomenon of opposite diurnal variations between the biogenic isoprene and anthropogenic species was exploited to help support the field applicability of the thermal desorption gas chromatographic method.

Spatiotemporal characterization of PM2.5, O3, and trace gases associated with East Asian continental outflows via drone sounding.

East Asian continental outflows with PM2.5, O3, and other species may determine the baseline conditions and affect the air quality in downwind areas via long-range transport (LRT). To gain insight into the impact and spatiotemporal characteristics of airborne pollutants in East Asian continental outflows, a versatile multicopter drone sounding platform was used to simultaneously observe PM2.5, O3, CO2, and meteorological variables (temperature, specific humidity, pressure, and wind vector) above the northern tip of Taiwan, Cape Fuiguei, which often encounters continental outflows during winter monsoon periods. By coordinating hourly high-spatial-resolution profiles provided by drone soundings, WRF/CMAQ model air quality predictions, HYSPLIT-simulated backward trajectories, and MERRA-2 reanalysis data, we analyzed two prominent phenomena of airborne pollutants in continental outflows to better understand their physical/chemical characteristics. First, we found that pollutants were well mixed within a sounding height of 500 m when continental outflows passed through and completely enveloped Cape Fuiguei. Eddies induced by significant fluctuations in wind speeds coupled with minimal temperature inversion and LRT facilitated vertical mixing, possibly resulting in high homogeneity of pollutants within the outflow layer. Second, the drone soundings indicated exceptionally high O3 concentrations (70-100 ppbv) but relatively low concentrations of PM2.5 (10-20 µg/m3), CO2 (420-425 ppmv), and VOCs in some air masses. The low levels of PM2.5, CO2, and VOCs ruled out photochemistry as the cause of the formation of high-level O3. Further coordination of spatiotemporal data with air mass trajectories and O3 cross sections provided by MERRA-2 suggested that the high O3 concentrations could be attributed to stratospheric intrusion and advection via continental outflows. High-level O3 concentrations persisted in the lower troposphere, even reaching the surface, suggesting that stratospheric intrusion O3 may be involved in the rising trend in O3 concentrations in parts of East Asia in recent years in addition to surface photochemical factors.

Using drone soundings to study the impacts and compositions of plumes from a gigantic coal-fired power plant.

The immense impacts of coal-fired power plant plumes on the atmospheric environment have caused great concern linked to climate and health issues. However, studies on the field observations of aerial plumes are relatively limited, mainly due to the lack of suitable plume observation tools and techniques. In this study, we use a multicopter unmanned aerial vehicle (UAV) sounding technique to study the influences of the aerial plumes of the world's fourth-largest coal-fired power plant on the atmospheric physical/chemical conditions and air quality. A set of species, including 106 volatile organic compounds (VOCs), CO, CO2, CH4, PM2.5, and O3, and meteorological variables of temperature (T), specific humidity (SH), and wind data, are collected by the UAV sounding technique. The results reveal that the large-scale plumes of the coal-fired power plant cause local temperature inversion and humidity changes, and even affect the dispersion of pollutants below. The chemical compositions of coal-fired power plant plumes are significantly different from those of another ubiquitous vehicular source. High fractions of ethane, ethene, and benzene and low fractions of n-butane and isopentane found in plumes could serve as the key features to help distinguish the influences of coal-fired power plant plumes from other pollution sources in a particular area. By taking the ratios of pollutants (e.g., PM2.5, CO, CH4, and VOCs) to CO2 in plumes and the CO2 emission amounts of the power plant into calculation, we enable the easy quantification of the specific pollutant emissions released from power plant plumes to the atmosphere. In summary, observation by using drone soundings dissecting the aerial plumes provides a new methodology that allows aerial plumes to be readily detected and characterized. Furthermore, the influences of the plumes on the atmospheric physical/chemical conditions and air quality can be assessed rather straightforwardly, which was not easily achievable in the past.

Integrated ground and vertical measurement techniques to characterize overhead atmosphere: Case studies of local versus transboundary pollution.

Much attention has been found to the long-range transport (LRT) of air pollutants and their adverse effects on downwind air qualities resulting from the Chinese haze, which frequently occurs in association with winter monsoon. This study integrates ground-based measurements, unmanned aerial vehicles (UAVs), and model simulations to characterize the meteorological, chemical, and particulate matter (PM) properties comprehensively for the events that were LRT or local pollution (LP) dominated in northern Taiwan during the wintertime of 2017. During the two types of episodes, various approaches were made to investigate the vertical mixing conditions and PM properties with UAV flights. A confined and PM accumulated feature near ground level with a temperature inversion was found during the LP event. In contrast, a vertically homogeneous atmospheric structure with strong winds was suggested during the LRT event. Independent measurements of criteria air pollutants, meteorological variables, volatile organic compounds (VOCs), and micropulse lidar (MPL) made at the ground level were closely supported by the vertical measurements. When synchronizing all these observational and numerical tools in a three-dimensional manner, the characterization of air masses and possible origins of pollution, such as LP vs. LRT, has now become more versatile and capable of gaining a complete picture of atmospheric conditions that define air quality.

Distinct transport mechanisms of East Asian dust and the impact on downwind marine and atmospheric environments.

East Asian dust episodes have a multitude of impacts, including on human health, environment, and climate over near-source and receptor regions. However, the mechanistic understanding of the synoptic conditions of these outbreaks at different altitude layers, and their eventual environmental impacts are less studied. The present study analyzed the synoptic transport patterns of East Asian dust during multiple dust generation episodes that occurred over only a few days apart in northern China, and which eventually delivered high PM10 concentrations to surface level and high-altitude locations in Taiwan. Whether the dust plume was uplifted ahead of or behind the 700 hPa trough over East Asia determined its trajectory and eventual impact on the environment downwind. The total dust (iron) deposition over the ocean surface preceding arrival to Taiwan was 2.4 mg m-2 (0.95 µg m-2) for the episode impacting the surface level and 5.0 mg m-2 (4.6 µg m-2) for the episode impacting high-altitude Taiwan. Dust deposition in marine areas east of China was more intense for the higher altitude transport event that was uplifted behind the 700 hPa trough and resulted in twice higher marine Chl-a concentrations. Furthermore, we estimated a dust-induced direct radiative effect over a high mountainous region in Taiwan of -6.2 to -8.2 W m-2 at the surface, -1.9 to -2.9 W m-2 at the top of the atmosphere and +3.9 to +5.3 W m-2 in the atmosphere. This dust-induced atmospheric warming and surface cooling are non-negligible influences on the atmospheric thermal structure and biogeochemical cycle over the western North Pacific. Overall, this study highlights the significant impacts of dust particles on the marine ecosystem and atmospheric radiation budget over the downwind region, thus lays the foundation for linking these impacts to the initial synoptic conditions in the source area.

A study of the vertical homogeneity of trace gases in East Asian continental outflow.

East Asian continental outflows containing with pollutants may deteriorate air quality in the downwind region via long-range transport (LRT). In particular, cold fronts with high wind speeds generally promote the LRT of air pollutants to further downwind areas, including Taiwan. To gain an insightful understanding of the characteristics and vertical homogeneity of trace gases in East Asian continental outflows, as well as their relation with atmospheric meteorological conditions, whole air samples were collected above a cape at the northern tip of the island of Taiwan during frontal passages. Aerial samples were collected at multiple altitudes from the surface to a maximum height of 700 m with a multicopter sounding platform carrying a robotic whole air sampling device. Simultaneously, aerial meteorological variables of temperature and wind vector from near the surface to a maximum height of 1000 m were also measured during the whole air sampling periods. An array of 106 volatile organic compounds (VOCs) as well as CO, CO2, and CH4 were analyzed to characterize the air composition and vertical homogeneity of trace gases. The results revealed rather homogeneous vertical distributions of most VOCs, CO, CO2, and CH4 in the frontal passages, indicating well-mixed conditions of trace gases in the East Asian continental outflows. The strong wind shear and minimal temperature inversion associated with the frontal passage likely induced turbulence and increased vertical mixing. Furthermore, higher levels of species characteristic of the East Asian continent were observed from the surface up to hundreds of meters above the cape, revealing a strong inflow of polluted air masses from the East Asian continent brought by cold frontal passages.

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.

This study examined the long-term trends in chemical components in PM2.5 (particulate matter with aerodynamic diameter ≤2.5 µm) samples collected at Lulin Atmospheric Background Station (LABS) located on the summit of Mt. Lulin (2862 m above mean sea level) in Taiwan in the western North Pacific during 2003-2018. High ambient concentrations of PM2.5 and its chemical components were observed during March and April every year. This enhancement was primarily associated with the long-range transport of biomass burning (BB) smoke emissions from Indochina, as revealed from cluster analysis of backward air mass trajectories. The decreasing trends in ambient concentrations of organic carbon (-0.67% yr-1; p = 0.01), elemental carbon (-0.48% yr-1; p = 0.18), and non-sea-salt (nss) K+ (-0.71% yr-1; p = 0.04) during 2003-2018 indicated a declining effect of transported BB aerosol over the western North Pacific. These findings were supported by the decreasing trend in levoglucosan (-0.26% yr-1; p = 0.20) during the period affected by the long-range transport of BB aerosol. However, NO3- displayed an increasing trend (0.71% yr-1; p = 0.003) with considerable enhancement resulting from the air masses transported from the Asian continent. Given that the decreasing trends were for the majority of the chemical components, the columnar aerosol optical depth (AOD) also demonstrated a decreasing trend (-1.04% yr-1; p = 0.0001) during 2006-2018. Overall decreasing trends in ambient (carbonaceous aerosol and nss-K+) as well as columnar (e.g., AOD) aerosol loadings at the LABS may influence the regional climate, which warrants further investigations. This study provides an improved understanding of the long-term trends in PM2.5 chemical components over the western North Pacific, and the results would be highly useful in model simulations for evaluating the effects of BB transport on an area.

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.

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

The aerosol in the Taipei basin is difficult to transport outward under specific weather patterns owing to complex terrain blocking. In this study, seven weather patterns are identified from synoptic weather maps for aerosol events, which occurred from March 2002 to February 2005. Among the identified weather patterns, High Pressure Peripheral Circulation (HPPC), Warm area Ahead of a cold Front (WAF), TYPhoon (TYP), Pacific High Pressure system stretching westerly (PHP), Weak High Pressure system (WHP), and Weak Southern Wind (WSW) are related to terrain blocking. The remaining pattern is High Pressure system Pushing (HPP). The classification of the pollution origin of the air masses shows that 15% of event days were contributed by long-range transport (LRT), 20% by local pollution (LP), and 65% by LRT/LP mix. Terrain blocking causes aerosol accumulation from high atmospheric stability and weak winds occurring under HPPC, TYP, and PHP weather patterns when the Taipei basin is situated on the lee side of the Snow Mountains Chain (SMC). Terrain blocking also occurs when the Taipei basin is situated on the upwind of SMC and Mt. Da-Twen under WAF and WSW patterns. To study the variation of aerosol properties under the mixed influence of terrain and pollution origin, we conducted a field observation simultaneously at the urban, suburban, and background sites in the Greater Taipei area from April 14 to 23, 2004. Terrain blocking plays an important role in aerosol accumulation in the stagnant environment when the Taipei basin is on the lee side of SMC. On the other hand, the PM(2.5) sulfate level is stable with a fraction of 30% in PM(2.5) during the observation period at the urban (25%-33%) and background (25%-41%) sites. It indicates that background PM(2.5) sulfate is high on the West Pacific in winter.

Apportionment of the sources of high fine particulate matter concentration events in a developing aerotropolis in Taoyuan, Taiwan.

To investigate the characteristics and contributions of the sources of fine particulate matter with a size of up to 2.5 µm (PM2.5) during the period when pollution events could easily occur in Taoyuan aerotropolis, Taiwan, this study conducted sampling at three-day intervals from September 2014 to January 2015. Based on the mass concentration of PM2.5, the sampling days were classified into high PM2.5 concentration event days (PM2.5>35 µg m(-3)) and non-event days (PM2.5<35 µg m(-3)). In addition, the chemical species, including water-soluble inorganic ions, carbonaceous components, and metal elements, were analyzed. The sources of pollution and their contributions were estimated using the positive matrix factorization (PMF) model. Furthermore, the effect of the weather type on the measurement results was also explored based on wind field conditions. The mass fractions of Cl(-) and NO3(-) increased when a high PM2.5 concentration event occurred, and they were also higher under local emitted conditions than under long range transported conditions, indicating that secondary nitrate aerosols were the major increasing local species that caused high PM2.5 concentration events. Seven sources of pollution could be distinguished using the PMF model on the basis of the characteristics of the species. Industrial emissions, coal combustion/urban waste incineration, and local emissions from diesel/gasoline vehicles were the main sources that contributed to pollution on high PM2.5 concentration event days. In order to reduction of high PM2.5 concentration events, the control of diesel and gasoline vehicle emission is important and should be given priority.