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Photochemical ozone pollution is a serious air quality problem under weak synoptic conditions in many areas worldwide. Volatile organic compounds (VOCs) are largely responsible for ozone production in urban areas where NOx mixing ratios are high while usually not a limiting precursor to ozone. In this study, the Community Multiscale Air Quality model – Higher-order Direct Decoupled Method (CMAQ-HDDM) at an urban-scale resolution (1.0 km x 1.0 km) in conjunction with positive matrix factorization (PMF) was used to identify the dominant sources of highly sensitive VOC species to ozone formation in southern Taiwan, a complex region of coastal urban and industrial parks and inland mountainous areas. First-order, second-order and cross sensitivities of ozone concentrations to domain-wide (i.e. urban, suburban and rural) NOx and VOC emissions were determined for the study area. Negative (positive) first-order sensitivities to NOx emissions are dominant over urban (inland) areas, confirming ozone production sensitivity favors the VOC-limited regime (NOx-limited regime) in southern Taiwan. Furthermore, most of the urban areas also exhibited negative second-order sensitivity to NOx emissions, indicating a negative O3 convex response where the linear increase of O3 from decreasing NOx emissions was largely attenuated by the non-linear effects. Due to the solidly VOC-limited regime and the relative insensitivity of O3 production to increases or decreases of NOx emissions, this study pursued the VOC species that contributed the most to ozone formation. PMF analysis driven by VOCs resolved 8 factors including mixed industry (21 %), vehicle emissions (22 %), solvent usage (17 %), biogenic (12 %), plastic industry (10 %), aged air mass (7 %), motorcycle exhausts (7 %), and manufacturing industry (5 %). Furthermore, a composite index that quantitatively combined the CMAQ-HDDM sensitivity coefficient and PMF resolved factor contribution, was developed to identify the key VOC species that should be targeted for effective ozone abatement. Our results indicate that VOC control measures should target on (1) solvent usage for painting, coating and the printing industry, which emits abundant toluene and xylene, (2) gasoline fuel vehicle emissions of n-butane, isopentane, isobutane and n-pentane, and (3) ethylene and propylene emissions from the petrochemical industry.

Since the 24-hr PM

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, CO

A custom-built steam-based aerosol collector (SBAC) was developed to improve the detection limits in aerosol chemical analysis. The SBAC was coupled with wet chemical analytical instruments to characterize the chemical composition of ambient particulate matter with high-time resolution. The particulate matter was mixed with a jet of steam and formed large droplets because of the supersaturation condition in the cooling chamber. The enlarged droplets were subsequently collected using an inertial aerosol separator and delivered to selected instruments for chemical analysis. Experiments were subsequently conducted to evaluate our custom-built SBAC under various sampling flow rates. In particular, particle growth and particle collection efficiency experiments were conducted, and the SBAC was compared with a filter-based sampler. Sample solutions were collected, extracted, and analyzed to determine the influence of sampling flow rate, particle size, and aerosol chemical concentration on the SBAC's collection efficiency. The SBAC was found to provide data with a higher time resolution, to exhibit an improved detection limit, and to effectively perform aerosol chemical analysis compared with the filter-based samplers, although the effect of flow rate on SBAC performance is complicated and further experiments on this aspect are warranted. This paper contributes to the literature by providing guidelines for designing a SBAC for aerosol chemical analysis.

The aim of this study is to assess the offshore wind and solar power and to determine whether the future power supply in Taiwan will be stable. The estimated annual offshore wind and solar power generation for 2030 are 11343 GWh and 11367 GWh, respectively. Based on these results, it appears that the annual power supply can easily help balance the total power demand. However, the power demand is high during the summer peak months, and power generation may be insufficient during peak summer hours by 2030. Specifically, in 2024, the peak hourly percent reserve margin (PRM) in summer will be negative (-0.9%). If the installation of offshore wind turbines and solar panels is delayed, then the problem of insufficiency will be even more severe. However, if the offshore wind and solar photovoltaic projects are completed on schedule, and the first, second, and third nuclear power plants (NPPs) extend their service to 2030, then the hourly PRM could reach 15% during the summer peak hours from 2025 to 2030 and 5–11% in the other years. Moreover, if the fourth NPP opens, then the estimated summer peak hourly PRM would increase by 6–7%.

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 PM

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 μ g m−3, 12–16 μ g m−3, and 4–13 μ g 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.

The boreal spring biomass burning (BB) in the northern peninsular Southeast Asia (nPSEA) are lifted into the subtropical jet stream, get transported and deposited across nPSEA, South China, Taiwan, and even the western North Pacific Ocean. This paper as part of the 7-Southeast Asian Studies (7-SEAS) project effort attempts to improve the prediction capability of the chemical transport model (WRF-CMAQ) over a vast region including the mountainous near-source burning sites at nPSEA to its downwind region. Several sensitivity analyses of plume rise are compared in the paper and it discovers that the initial vertical allocation profile of BB plume and plume rise module (PLMRIM) are the main reasons causing the inaccuracies of the WRF-CMAQ simulations. The smoldering emission from the Western Regional Air Partnership (WRAP) empirical algorithm included has improve the accuracies of PM10, O3 and CO at the source. The best performance at the downwind sites is achieved with the inline PLMRIM that accounts for the atmospheric stratification at the mountainous source region with the high-resolution FINN burning emission dataset. The calibrated model greatly improves not only the BB emission prediction over near-source and receptor ground-based measurement sites but also the aerosol vertical distribution (MPLNET, CALIPSO) and column aerosol optical depth (MODIS AOD) of the BB aerosol along the transport route. Three distinct transport mechanisms from nPSEA to the western North Pacific are then identified while a particular mechanism which involves Asian cold surge is able to mix the BB smoke plumes into the boundary layer and affects the ground surface over the western Taiwan.

The performance of air quality modeling (AQM) depends largely on the uncertainty of emission inventory. Since the emission data is an important input for AQM, this study tried to validate the controversial emission inventory. The Taiwan EPA (TEPA) has released the latest TEDS10.0 (Taiwan Emission Database System, version 10.0) based on 2016. This emission has attracted high arguments among governments and academics. This study applied the SMOKE v4.6 (Sparse Matrix Operator Kerner Emissions) to process the TEDS. The study used the CEMS (Continuous Emission Monitoring System) data and replaced temporalized large point source which accounts for 70% of all point source emissions, updated the biogenic emission calculation, improved the temporal profile of NH3, several area sources, and all mobile sources. Then we utilized the CMAQ (Community Modeling and Analysis System) model to simulate a PM2.5 event. However, the performance of the abovementioned improvement for emission processing is still not satisfactory. Therefore, this study tried to adjust the emission inventory according to the comparison of simulations and observations. The performance of air quality modeling has been improved after adjustment. Meanwhile, this study provided suggestions of several aspects to be improved to the TEPA.

The long-term downward trend of NOX concentrations does not reflect the reduction of nitrate (NO3−) in Taiwan. Instead, the proportion of NO3− in PM2.5 increased in recent years. To probe the increasing importance of NO3− in PM2.5, this study applied the WRF/CMAQ modeling system to implement a simulation from 16 March 2017 to 30 April 2017, in which 5 p.m.2.5 events with daily average concentrations ≥35 μg m−3 and their corresponding correlation coefficients (R) of simulated and observed PM2.5 above 0.6 were selected for analysis. During the daytime, the reaction of NO2 and OH contributed more than 90% of the total HNO3. After sunset, the high concentrations of NO3 and N2O5 peaked, followed soon by the simultaneous rise of NO3−, aerosol water content, and HNO3 concentrations around midnight, which indicated that the heterogeneous reaction was the main formation mechanism of NO3−, accounting for approximately 30%–90% of total HNO3. At nighttime, the daytime-formed gaseous phase NO3− condensed, and low wind and low boundary layer height favored accumulation; therefore, PM2.5 peaked around the midnight period to the early morning. The sensitivity test showed that doubling and halving the NOX and NH3 emissions could directly lead to the highest production and reduction of NO3−, respectively, followed by doubling and halving NMHC emissions, which caused the highest and lowest O3 concentrations. The correlation analysis of the simulation results showed that the daytime maximum O3 and HNO3 were highly correlated. The relationships between daytime maximum O3, nighttime maximum NO3, N2O5, and HNO3 in pairs were also moderately to highly correlated. This study implies that in addition to direct reduction of NOX or NH3 emissions, controlling O3 is possibly another useful strategy to reduce NO3−. Because NOX emission reduction could conflict with controlling O3, this study suggests to re-examine the determination of NOX-limited and VOCS-limited regions in order to develop strategies for reducing NOX emission and O3 simultaneously.

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.

Real-time measurement of PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) water-soluble inorganic ions (WSIIs) was conducted at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level) in the western North Pacific (WNP) during the autumn 2015 and spring 2016. During autumn, the mountain-valley (M-V) circulation is effective in increasing the mass concentrations of PM2.5, WSIIs, and carbon monoxide (CO) at LABS from 12:00 to 18:00 local time every day. In sharp contrast to autumn, five high aerosol-loading events were recorded during the spring, with each event persisting for a few days. These events were harmonized with the long-range transport of biomass-burning (BB) smoke emissions from northern peninsular Southeast Asia (PSEA), as disclosed from the fire count map and backward trajectories. The plumes appear to mask their characteristic diurnal features driven by the local M-V circulation. During both seasons, the prominent pathway of SO2 to SO42− conversion was aqueous phase oxidation as revealed from the relationship between sulfur oxidation ratio (SOR) and aerosol liquid-water content. Furthermore, NO3− was found to be produced primarily via homogeneous reactions during the spring season. The optical measurements were also made along with the semi-continuous measurements of WSIIs in this study. The magnitude of aerosol scattering and absorption coefficients exhibited high values when LABS was either under the influence of M-V circulation or long-range BB transport, although the latter event was apparently greater. Large surface cooling (up to −30.8 W m−2) and atmospheric warming (up to +21.3 W m−2) at LABS attributed to absorption enhancement by BB-induced aerosols can potentially influence the regional radiation budget.

East Asian Dust (EAD) has had a measurable impact on global climate and air quality, including visibility and human health, in numerous locations around the globe over the past decade. The accuracy of an air quality modeling system to simulate dust events is vital for early warning systems. The most significant dust event observed in 6 years for northern Taiwan occurred during 4th–9th April 2018, and was characterized by high wind speeds (9–13 m s−1) upon arrival to the region. We assessed the windblown dust emission treatment across various modifications and found better model performance by decreasing the soil moisture factor and updating the aerosol speciation profile over East Asia. In the optimized CMAQ simulation, uplifted dust particles reached a maximum height of 500–700 hPa, which is crucial for effective transport of the dust plume to the downwind Taiwan region. However, by tracking the vertical distribution profile, we found the model was insufficient to estimate dust aerosol after exiting from over the continent and entering the marine boundary layer. Our simulation indicated the dust event consisted of two plumes, where the first one was significantly impacted by wet deposition (−70.65%) from a rainfall belt stretching across Japan, Korea and the East China Sea. Then, low wind speed during a period of no precipitation over the marine area led to even greater dust deposition (−89.11%) from the second plume, resulting in a consistent negative bias for the simulation. This modeling study highlights the importance of improving the dust emission treatment for a better simulation of dust aerosol transport over the marine boundary layer. To reduce the uncertainty in the dust outflow region, the deposition mechanisms for the CMAQ dust treatment should be revised.

The Lulin Atmospheric Background Station (LABS, 23.47 deg. N 120.87 deg. E, 2862 m ASL) in Central Taiwan was constructed in 2006 and is the only high-altitude background station in the western Pacific region for studying the influence of continental outflow. In this study, extensive optical properties of aerosols, including the aerosol light scattering coefficient [Sigma(sub s)] and light absorption coefficient [Sigma(sub a)], were collected from 2013 to 2014. The intensive optical properties, including mass scattering efficiency [Sigma(sub s)], mass absorption efficiency [Sigma(sub a)] single scattering albedo (Omega), scattering Angstrom exponent (A), and backscattering fraction (b), were determined and investigated, and the distinct seasonal cycle was observed. The value of [Alpha(sub a)] began to increase in January and reached a maximum in April; the mean in spring was 5.89 m(exp. 2) g(exp. -1) with a standard deviation (SD) of 4.54 m(exp. 2) g(exp. -1) and a 4.48 m(exp. 2) g(exp. -1) interquartile range (IQR: 2.95-7.43 m(exp. 2) g(exp. -1). The trend was similar in [Sigma(sub a)], with a maximum in March and a monthly mean of 0.84 m(exp. 2) g(exp. -1). The peak values of Omega (Mean = 0.92, SD = 0.03, IQR: 0.90 - 0.93) and A (Mean = 2.22, SD = 0.61, IQR: 2.12 = 2.47) occurred in autumn. These annual patterns of optical properties were associated with different long-range transport patterns of air pollutants such as biomass burning (BB) aerosol in spring and potential anthropogenic emissions in autumn. The optical measurements performed at LABS during spring in 2013 were compared with those simultaneously performed at the Doi Ang Kang Meteorology Station, Chiang Mai Province, Thailand (DAK, 19.93 deg. N, 99.05 deg. E, 1536 m a.s.l.), which is located in the Southeast Asia BB source region. Furthermore, the relationships among [Sigma(sub s)], [Sigma(sub a)], and (b) were used to characterize the potential aerosol types transported to LABS during different seasons, and the data were inspected according to the HYSPLIT 5-day backward trajectories, which differentiate between different regions of air mass origin.

The present study utilizes air quality modeling to probe the sources and characteristics of PM2.5 (particles less than 2.5 micrometers in aerodynamic diameter) at the northern tip of Taiwan (CAFE station) in the early stage of the Asian haze period. Since CAFE is the first place that is influenced by the Asian haze coming from the north, this study focused on the wind field, PM2.5 concentration, and PM2.5 composition at CAFE. During the research period (Oct. 16, 2015, to Nov. 15, 2015), four PM2.5 episodes occurred at CAFE. This study classified these four episodes into three types, according to their PM2.5 sources: the long-range transport (LRT) type, the local pollution (LP) type, and the LRT/LP mix type. For the LRT type, Asian outflows prevailed in a north to northeast wind at the north of Taiwan. The proportion of NO3– in the PM2.5 resolvable compositions was very small at CAFE due to evaporation during transport, whereas the relative proportion of sea salt increased due to strong winds. For the LP type, an east wind prevailed and formed a cyclone/lee vortex in northwest Taiwan. Although the background PM2.5 concentrations were low (4–20 µg m–3), the cyclone transported local anthropogenic emissions northward and elevated the PM2.5 levels at CAFE. For the LRT/LP mix type, an east wind also prevailed, but the background PM2.5 concentrations were at an intermediate level (20–30 µg m–3) because the Asian outflows had already transported haze to the West Pacific. The combined LRT and LP increased PM2.5 at CAFE. In addition, the proportions of NO3– (nitrate) for the LP and LRT/LP episodes were obviously higher than those on the days before and after. This suggests a considerable contribution on PM2.5 from LP.

The outflow of East Asian haze (EAH) has gathered much attention in recent years. For downstream areas, it is meaningful to understand the impact of crucial upstream sources and the process analysis during transport. This study evaluated the impact of PM2.5 from the three biggest industrial regions in Asian continent: Bohai Rim industrial region (BRIR), Yangtze River Delta industrial region (YRDIR), and Pearl River Delta industrial region (PRDIR) on Taiwan and discussed the processes during transport with the help of air quality modeling. The simulation results revealed the contributions of monthly average PM2.5 from BRIR and YRDIR were 0.7∼1.1 µg m−3 and 1.2∼1.9 µg m−3 (∼5 % and 7.5 % of total concentration) on Taiwan, respectively in January 2017. When the Asian anticyclone moved from Asian continent to the West Pacific, e.g. on Jan 9th 2017, the contributions from BRIR and YRDIR to northern Taiwan could reach 6∼8 and 9∼12 µg m−3. The transport of EAH from BRIR and YRDIR to low latitude regions was horizontal advection (HADV), vertical advection (ZADV), and vertical diffusion (VDIF) over Bohai Sea and East China Sea. Over Taiwan Strait and northern South China Sea, cloud processes (CLDS) was the major production process of PM2.5 due to high relative humidity environment. Along the transport from high latitude regions to low latitude regions, Aerosol chemistry (AERO) and Dry deposition (DDEP) were the major removal processes. When the EAH intruded northern Taiwan, the major production processes of PM2.5 at northen Taiwan were HADV and AERO. The stronger the EAH was the easier the EAH could influence central and southern Taiwan. Although PRDIR was located at the downstream of Taiwan under northeast wind, the PM2.5 from PRDIR could transport upward above boundary layer and moved eastwards. When the PM2.5 plume moved overhead Taiwan, PM2.5 could transport downward via boundary layer mixing (VDIF) and further enhanced by the passing cold surge. In contrast, for the simulation of July 2017, the influence from three industrial regions was almost negligible unless there was special weather system like thermal lows, which may carried pollutants from PRDIR to Taiwan, but the occurrence was rare.

Long-range pollution transport (LRT) events have a wide impact across East Asia, but are often difficult to track due to imprecise emission inventories and changing domain scales as the plume moves from source to receptor locations. This study adjusts a bottom-up emission inventory based on changes in remotely sensed NO2 column densities for a source region of East Asia, then with CMAQv5.2.1 simulates transport of LRT plumes to Taiwan. Adjustment of an emissions inventory based on satellite measurements during the COVID-19 lockdown in China led to a ~59% reduction in emissions over the relevant source area in China compared to base emissions. As a result, PM2.5 mass concentrations were reproduced to match observations (mean fractional bias, MFB of –13.9% and 18.5% at a remote and urban station) as the plume passed through northern Taiwan. Furthermore, the OMI-adjusted emissions simulation brought all of the major PM2.5 components to within ~50% of the measured values. Another LRT event from 2018 with more subtle OMI-adjustments to the emissions was also simulated and with improved overall PM2.5 mass concentration at the northern tip of Taiwan (MFB: –91.5%) compared to the base model (MFB: –102.1%), and an acceptable index of agreement (0.78). For the 2018 event, non sea-salt sulfate concentrations were consistently underpredicted (0.2–0.4), while nitrate concentrations were overpredicted by up to factor of 11. Copernicus Atmosphere Monitoring Service (CAMS) reanalysis of the PM2.5 concentrations shows high sulfate concentrations in eastern China in the areas associated with 72-h back-trajectories from northern Taiwan during both events, lending support for future model investigations of sulfate source area production and transport to Taiwan. In order to better track these LRT events out of East Asia and optimize OMI-adjustment methodology, it is recommended to explore other satellite-based products to map unaccounted for SO2 sources upstream of Taiwan.

Receptor modeling provides valuable information to help develop effective control strategies. Additionally, incorporating parametric variables into expanded receptor modeling improves the understanding of formation mechanisms and potential sources of secondary aerosol. This study was conducted in a rural township in central Taiwan, where the air pollution level was comparable with that in the urban area. Bihourly measurements were applied into an enhanced receptor modeling approach using positive matrix factorization (PMF). Eight potential sources, including oil combustion, coal combustion, secondary aerosol related, nitrate‐rich secondary aerosol, biomass burning, industry/vehicle, road dust, and SOM‐rich (dominated by secondary organic matter) secondary aerosol, were identified. SOM‐rich secondary aerosol (24%) contributed the most to PM2.5 mass, followed by biomass burning (19%) and nitrate‐rich secondary aerosol (18%). Contributions from three factors involving secondary formation features accounted for 55% of PM2.5 mass. Through the enhanced modeling approach, photo-oxidation formation, condensation and aqueous phase oxidation of volatile organic compounds, and transport of secondary nitrates from upwind urban area could be potential formation process and sources of secondary aerosol.

The transport of biomass burning (BB) aerosol from Indochina may cause a potential effect on climate change in Southeast Asia, East Asia, and the Western Pacific. Up to now, the understanding of BB aerosol composition modification during long-range transport (LRT) is still very limited due to the lack of observational data. In this study, atmospheric aerosols were collected at the Suthep/Doi Ang Khang (DAK) mountain sites in Chiang Mai, Thailand and the Lulin Atmospheric Background Station (Mt. Lulin) in central Taiwan from March to April 2010 and from February to April 2013, respectively. During the study period, an upwind and downwind relationship between the Suthep/DAK and Lulin sites (2400 km apart) was validated by backward trajectories. Comprehensive aerosol properties were resolved for PM 2.5 water-soluble inorganic ions, carbonaceous content, water-soluble/insoluble organic carbon (WSOC/WIOC), dicarboxylic acids and their salts (DCAS), and anhydrosugars. A Modification Factor (MF) is proposed by employing non-sea-salt potassium ion (nss-K + ) or fractionalized elemental carbon evolved at 580 °C after pyrolized OC correction (EC1-OP) as a BB aerosol tracer to evaluate the mass fraction changes of aerosol components from source to receptor regions during LRT. The MF values of nss-SO 4 2− , NH 4 + , NO 3 − , OC1 (fractionalized organic carbon evolved from room temperature to 140 °C), OP (pyrolized OC fraction), DCAS, and WSOC were above unity, which indicated that these aerosol components were enhanced during LRT as compared with those in the near-source region. In contrast, the MF values of anhydrosugars ranged from 0.1 to 0.3, indicating anhydrosugars have degraded during LRT.

The aerosol radiative effect can be modulated by the vertical distribution and optical properties of aerosols, particularly when aerosol layers are decoupled. Direct aerosol radiative effects over the northern South China Sea (SCS) were assessed by incorporating an observed data set of aerosol optical properties obtained from the Seven South East Asian Studies (7-SEAS)/Dongsha Experiment into a radiative transfer model. Aerosol optical properties for a two-layer structure of aerosol transport were estimated. In the radiative transfer calculations, aerosol variability (i.e., diversity of source region, aerosol type, and vertical distribution) for the complex aerosol environment was also carefully quantified. The column-integrated aerosol optical depth (AOD) at 500nm was 0.1-0.3 for near-surface aerosols and increased 1-5 times in presence of upper layer biomass-burning aerosols. A case study showed the strong aerosol absorption (single-scattering albedo (omega) approx. = 0.92 at 440nm wavelength) exhibited by the upper layer when associated with predominantly biomass-burning aerosols, and the omega (approx. = 0.95) of near-surface aerosols was greater than that of the upper layer aerosols because of the presence of mixed type aerosols. The presence of upper level aerosol transport could enhance the radiative efficiency at the surface (i.e., cooling) and lower atmosphere (i.e., heating) by up to -13.7 and +9.6W/sq m2 per AOD, respectively. Such enhancement could potentially modify atmospheric stability, can influence atmospheric circulation, as well as the hydrological cycle over the tropical and low-latitude marginal northern SCS.

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.

The direct aerosol radiative effects of biomass-burning (BB) aerosols over northern Indochina were estimated by using aerosol properties (physical, chemical, and optical) along with the vertical profile measurements from ground-based measurements with integration of an optical and a radiative transfer model during the Seven South East Asian Studies Biomass-Burning Aerosols Stratocumulus Environment: Lifecycles Interactions Experiment (7-SEASBASELInE) conducted in spring 2013. Cluster analysis of backward trajectories showed the air masses arriving at mountainous background site (Doi Ang Khang; 19.93degN, 99.05degE, 1536 m above mean sea level) in northern Indochina, mainly from near-source inland BB activities and being confined in the planetary boundary layer. The PM(sub10) and black carbon (BC)mass were 87 +/- 28 and 7 +/- 2 micrograms m(exp -3), respectively. The aerosol optical depth (AOD (sub 500) was found to be 0.26--1.13 (0.71 +/- 0.24). Finer (fine mode fraction is approximately or equal to 0.95, angstrom-exponent at 440-870 nm is approximately or equal to 1.77) and significantly absorbing aerosols(single scattering albedo is approximately or equal to 0.89, asymmetry-parameter is approximately or equal to 0.67, and absorption AOD 0.1 at 440 nm) dominated over this region. BB aerosols (water soluble and BC) were the main contributor to the aerosol radiative forcing (ARF), while others (water insoluble, sea salt and mineral dust) were negligible mainly due to their low extinction efficiency. BC contributed only 6 to the surface aerosol mass but its contribution to AOD was 12 (2 times higher). The overall mean ARF was 8.0 and -31.4 W m(exp -2) at top-of-atmosphere (TOA) and at the surface (SFC), respectively. Likely, ARF due to BC was +10.7 and -18.1 W m(exp -2) at TOA and SFC, respectively. BC imposed the heating rate of +1.4 K d(exp -1) within the atmosphere and highlighting its pivotal role in modifying the radiation budget. We propose that to upgrade our knowledge on BB aerosol radiative effects in BB source region, the long-term and extensive field measurements are needed.

This study presents measurements of aerosol chemical properties at Sonla, northern Vietnam (675 m a.s.l.) during spring time, when biomass burning (BB) was very active in the northern Indochina Peninsula, as part of the 7-SEAS (Seven South East Asian Studies) campaign in 2013. The gas chromatography-mass spectrometry (GC-MS) analysis of BB markers in 14 selected samples indicated that mixed softwood, hardwood, grass, and non-woody vegetation were burned. More than 50 organic compounds including levoglucosan, lignin and resin products, sugar and sugar alcohol compounds, fatty acids, phthalate esters, aromatic acids, poly-acids, and biogenic oxidation products (e.g., 2-methyltetrols, alkene triols, 3-hydroxyglutaric acid) were measured in PM_(2.5). Levoglucosan, a BB tracer, was the predominant species among aerosol sugars, with an average concentration of 1.62 ± 0.89 μg m^(-3), comprising 2.23 ± 0.5% of PM_(2.5) mass. For the collection period of the selected samples, backward air mass trajectories were classified into the source regions of Indochina (BBIC), southern China (BBSC), and the South China Sea (BBSS). All resolved molecular compounds show their dominance on the trajectory from BBIC, verifying the BB smoke origin of that region. Trajectory classification provides additional information, such as higher level of diethyl phthalate associated with BBSC trajectory, revealing urban or industrial influence, and more low-molecular-weight than high-molecular-weight fatty acids, indicating distributions with more microbial and lesser plant wax/vegetation burning contributions along the BBSC trajectory. In addition, we report, for the first time, stable carbon isotopic data (δ^(13)C) for PM_(2.5) aerosols in northern Vietnam, which ranged from -26.6 to -25.4 per mille in PM_(2.5), indicating contributions from burning of C_3 plants and fossil fuel combustion.

The Community Multi-scale Air Quality Model (CMAQ) is used to simulate the long-range transport of biomass burning (BB) pollutants from Southeast Asia (SEA) towards the Taiwan Central Mountain Range (CMR) in March and April 2010. The results show that a proportion of the BB plume was blocked and compressed at the windward side of CMR. High-altitude BB plume is shown to influence air quality on the ground via three mechanisms: (1) the subsidence in the anticyclone, (2) the downward motion in the cold surge, and (3) the vertical mixing of the boundary layer over land. Two case studies are further investigated to probe the chemical evolution of the air parcel approaching Mt. Lulin. The first case shows that the third mechanism also explained the increase in the concentrations of peroxyacyl nitrate (PAN), higher peroxyacyl nitrate (PANX), NH3, SO2, and volatile organic compounds in the BB plume when entering the land over western Taiwan. Meanwhile, the percentage of NO3– in the plume is also significantly increased. The second case reveals that valley wind transported air pollutants from the ground to the mountains. The air parcel, accompanied with considerable concentrations of PAN, PANX, SULF, and anthropogenic secondary organic aerosol, moved up Mt. Lulin. The pollutant concentrations, except for elemental carbon, in the air parcel decreased on approach to Mt. Lulin because the air parcel was mixed with a clean air.

Biomass-burning (BB) aerosols, acting as cloud condensation nuclei (CCN), can influence cloud microphysical and radiative properties. In this study, we present CCN measured near the BB source regions over northern Southeast Asia (Doi Ang Khang, Thailand) and at downwind receptor areas (Lulin Atmospheric Background Station, Taiwan), focusing exclusively on 13-20 March 2013 as part of 2013 spring campaign of the Seven SouthEast Asian Studies (7-SEAS) intensive observation. One of the campaigns objectives is to characterize BB aerosols serving as CCN in SouthEast Asia (SEA). CCN concentrations were measured by a CCN counter at 5 supersaturation (SS) levels: 0.15%, 0.30%, 0.45%, 0.60%, and 0.75%. In addition, PM2.5 and black carbon mass concentrations were analyzed by using a tapered element oscillating microbalance and an aethalometer. It was found the number-size distributions and the characteristics of hygroscopicity (e.g., activation ratio and k) of BB aerosols in SEA have a strong diurnal pattern, and different behaviors of patterns were characterized under two distinct weather systems. The overall average value was low (0.05-0.1) but comparable with previous CCN studies in other BB source regions. Furthermore, a large fraction of UV-absorbing organic material (UVBC) and high Delta-C among BB aerosols were also observed, which suggest the existence of substantial particulate organic matter in fresh BB aerosols. These data provide the most extensive characterization of BB aerosols in SEA until now.

The vertical distribution of aerosols is important for accurate surface PM2.5 retrieval and initial modeling forecasts of air pollution, but the observation of aerosol profiles on the regional scale is usually limited. Therefore, in this study, an approach to aerosol extinction profile fitting is proposed to improve surface PM2.5 retrieval from satellite observations. Owing to the high similarity of the single-peak extinction profile in the distribution pattern, the log-normal distribution is explored for the fitting model based on a decadal dataset (3248 in total) from Micro Pulse LiDAR (MPL) measurements. The logarithmic mean, standard deviation, and the height of peak extinction near-surface (Mode) are manually derived as the references for model construction. Considering the seasonal impacts on the planetary boundary layer height (PBLH), Mode, and the height of the surface layer, the extinction profile is then constructed in terms of the planetary boundary layer height (PBLH) and the total column aerosol optical depth (AOD). A comparison between fitted profiles and in situ measurements showed a high level of consistency in terms of the correlation coefficient (0.8973) and root-mean-square error (0.0415). The satellite AOD is subsequently applied for three-dimensional aerosol extinction, and the good agreement of the extinction coefficient with the PM2.5 within the surface layer indicates the good performance of the proposed fitting approach and the potential of satellite observations for providing accurate PM2.5 data on a regional scale.

This study aimed to simulate the transport of biomass burning (BB) aerosol originating from Southeast Asia (SEA) during the Dongsha Experiment conducted from March 2010 to April 2010. Transport pathways were reanalyzed and steering flow in the mid-latitude areas and anticyclones in low-latitude areas were found to control the transport of BB plume after it was injected to a high atmosphere. For the 12 simulated and observed events at Mt. Lulin (2862 m MSL; 23°28′07″ N, 120°52′25″ E), the 72 h backward trajectories were all tracked back to southern China and northern Indochina, which were the locations of the largest BB fire activities in SEA. Chemical evolutions of BB pollutants along the moving trajectories showed that organic matter was always the dominant component in PM2.5, consistent with the observations at both near-source regions and Mt. Lulin. For nitrogen species, nearly all NOx molecules oxidized into HNO3, NO3−, PAN, and PANX in fires or near fires. The synchronic consumption of NOx, SO2, and NH3 explained the production of the major components of inorganic salts. In the moving BB plume, sulfate concentration increased with decreased nitrate concentration. Ratios of ammonium to PM2.5 and elemental carbon to PM2.5 remained nearly constant because additional sources were lacking.

Eight carbonaceous fractions from aerosols were resolved using the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol (Chow et al., 1993). The aerosols were collected at the Mountain Lulin Atmospheric Background Station (Mt. Lulin, 2862 m a.s.l.) in Central Taiwan from April 2003 to April 2012. The monthly and yearly levels of organic carbon (OC) and elemental carbon (EC) varied consistently with PM 2.5 mass concentrations during biomass burning (BB) period. The highest monthly carbonaceous content was observed in March and the highest yearly carbonaceous concentration was observed in 2007. This finding is consistent with the BB activity in Indochina and indicates that carbonaceous content is a major component of BB aerosols. Lee et al. (2011) classified four trajectory groups from the air masses transported to Mt. Lulin during the aerosol collection period. For the air masses transported from the BB area (the BB group) in Indochina, the carbonaceous content was greater than the water-soluble ions in PM 2.5 , and the OC/EC ratio (4.8 ± 1.5) was high. With EC as the indicator of primary emission sources, the air masses of the BB group were found to contain more primary than secondary OC. The Anthropogenic group (from the local and free troposphere below the 700-hPa pressure level over the Asian continent) probably contained more secondary than primary OC or the sources of OC and EC could be quite diverse. The average char-EC/soot-EC (low-temperature EC/high-temperature EC) ratios were 3.9 ± 3.5, 0.4 ± 0.4, 0.9 ± 0.8, and 0.3 ± 0.4 for the trajectory groups BB , SNBB (from BB source areas during the non-BB period), Anthropogenic , and FT (from the oceanic area and the free troposphere above the 700-hPa pressure level over the Asian continent), respectively. The presence of a high char-EC/soot-EC ratio confirmed the correct classification of the BB group, whereas the low ratios from the other groups indicated the strong influence of vehicle exhaust. It is noted that higher OC and EC levels were obtained at Mt. Lulin as compared with those obtained at other high-elevation sites. This difference suggested that the Indochina BB plume exhibited a more serious climatic impact on the background air in East Asia than in other places in Asia and Europe. On the basis of the carbonaceous levels of the SNBB and FT groups, the background OC and EC levels of approximately 3000 m in the West Pacific are around 1.33 μg m −3 and 0.35 μg m −3 , respectively.

By modulating the Earth-atmosphere energy, hydrological and biogeochemical cycles, and affecting regional-to-global weather and climate, biomass burning is recognized as one of the major factors affecting the global carbon cycle. However, few comprehensive and wide-ranging experiments have been conducted to characterize biomass-burning pollutants in Southeast Asia (SEA) or assess their regional impact on meteorology, the hydrological cycle, the radiative budget, or climate change. Recently, BASE-ASIA (Biomass-burning Aerosols in South-East Asia: Smoke Impact Assessment) and the 7-SEAS (7-South-East Asian Studies)/Dongsha Experiment were conducted during the spring seasons of 2006 and 2010 in northern SEA, respectively, to characterize the chemical, physical, and radiative properties of biomass-burning emissions near the source regions, and assess their effects. This paper provides an overview of results from these two campaigns and related studies collected in this special issue, entitled “Observation, modeling and impact studies of biomass burning and pollution in the SE Asian Environment”. This volume includes 28 papers, which provide a synopsis of the experiments, regional weather/climate, chemical characterization of biomass-burning aerosols and related pollutants in source and sink regions, the spatial distribution of air toxics (atmospheric mercury and dioxins) in source and remote areas, a characterization of aerosol physical, optical, and radiative properties, as well as modeling and impact studies. These studies, taken together, provide the first relatively complete dataset of aerosol chemistry and physical observations conducted in the source/sink region in the northern SEA, with particular emphasis on the marine boundary layer and lower free troposphere (LFT). The data, analysis and modeling included in these papers advance our present knowledge of source characterization of biomass-burning pollutants near the source regions as well as the physical and chemical processes along transport pathways. In addition, we raise key questions to be addressed by a coming deployment during springtime 2013 in northern SEA, named 7-SEAS/BASELInE (Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment). This campaign will include a synergistic approach for further exploring many key atmospheric processes (e.g., complex aerosol–cloud interactions) and impacts of biomass burning on the surface–atmosphere energy budgets during the lifecycles of biomass-burning emissions.

Northern South China Sea (SCS) is adjacent to major atmospheric mercury (Hg) emission source regions; however, studies concerning regional atmospheric Hg distribution and cycling are very limited. Accordingly, measurements of atmospheric Hg were conducted in March and April during the 2010 Dongsha Experiment to study its spatial and temporal distribution. Atmospheric Hg was measured at Hengchun and Dongsha Island (Taiwan), Da Nang (Vietnam), Chiang Mai (Thailand) and over the northern SCS. Atmospheric Hg concentrations ranged between 1.54 and 6.83 ng m −3 , mostly higher than the Northern Hemisphere background value. Regional wind fields and backward trajectories indicated that the atmospheric Hg concentrations over northern SCS should principally reflect the export of the East Asian Hg emissions by northeast monsoon. However, significantly elevated Hg concentrations were always observed at Da Nang, possibly due to the influence of local Hg emissions. Chiang Mai is located in the intense biomass burning region in northern Thailand. Therefore, atmospheric Hg concentrations at Chiang Mai reflected the influence of regional biomass burning Hg emissions. Two dust storms were encountered at Dongsha Island, one on March 16 and the other on March 21, with atmospheric Hg enhancements. Compared with the 2008 summer values, elevated Hg levels were observed at Dongsha Island in the spring of 2010. Summer air masses were mainly from the deep SCS, representing relatively clean marine air. On the other hand, air masses were from the north in spring, passing eastern China or Taiwan prior to reaching Dongsha Island. Results of this research thus demonstrated the transport of atmospheric Hg from the East Asian continent to northern SCS by regional monsoon activity in spring, but special events, such as biomass burning and dust storms, can also cause enhancements of ambient Hg levels.

Biomass burning (BB) in Indochina produces a great amount of aerosols, but related information on this phenomenon is scarce. In the present study, aerosol samples were collected from Suthep Mountain (98° 53′E, 18° 48′N, 1396 m) in Chiangmai, Northern Indochina, in March and April 2010. To our knowledge, this study is the first to reveal the chemical characteristics of BB aerosol near the burning sources in Indochina. The composition analyzed included water-soluble ions, carbonaceous fractions obtained using the thermal optical reflectance method, low-molecular-weight dicarboxylates, anhydrosugars, and water-soluble organic carbon (WSOC). Enriched tracers from the collected aerosols, such as potassium ion (K+) and levoglucosan, confirmed that the samples were influenced by BB activity. The percentage of K+ in PM2.5 (2.51% ± 0.31%) in the present study is higher than that in other studies. The analysis also showed that directly emitted organic carbon (OC) is the most abundant component in the collected particles. The percentage of WSOC in OC in the present study is greater than that in laboratory experiments. Moreover, the char-EC to soot-EC ratio (defined as the ratio of the subtraction of pyrolized OC from the measured EC1, EC1-OP, over the sum of EC2 and EC3) of 9.4 ± 3.8 in PM2.5 is similar to the value of BB in the literature. Using the recognized BB aerosol tracers K+ and levoglucosan, the present study inferred that PM2.5 EC1-OP and OC3 are also good BB tracers near sources. The collected BB aerosols may have been contributed by the smoldering state of softwood burning.

Aerosol observations were conducted at Dongsha Island in two batches from 19 to 23 March and 10 to 19 April 2010. Dongsha Island is located in a remote area over the northern South China Sea (SCS), distantly surrounded by southern China, Taiwan, the Philippines, and the Indochinese Peninsula. During the study period, the average PM10 and PM2.5 mass concentrations were 26.5 ± 19.4 and 12.6 ± 6.0 μg m−3, respectively. In particular, a daily PM10 concentration of 94.1 μg m−3 caused by a yellow-dust event originating from the Asian Continent was recorded on 21 March. Other than this event, the PM2.5 and PM10–2.5 daily levels were 7.1 ± 1.2 and 12.6 ± 5.0 μg m−3, respectively, on days without pollution from anthropogenic sources in the surrounding areas. Water-soluble ions (WSIs) were the predominant components that accounted for 58.7% ± 10.5% and 51.1% ± 7.2% of the PM10 and PM2.5 mass. The second most abundant component was carbonaceous content, which accounted for 9.5% ± 4.7% and 17.5% ± 5.3% of PM10 and PM2.5, respectively. SO 4 2 − was the most abundant PM2.5 WSI, whereas the Na+ and Cl− pair was the most abundant PM10–2.5 WSI. Based on the U.S. IMPROVE protocol, the resolved carbonaceous fractions were mainly distributed in PM2.5 and influenced by coal combustion, mobile vehicles, and biomass burning. Most of the resolved WSIs in particles were in the liquid phase due to the humid environment around the northern SCS.

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.

PM2.5 WRF/Chem-MADRID abstract A Real-Time Air Quality Forecast (RT-AQF) system that is based on a three-dimensional air quality model provides a powerful tool to forecast air quality and advise the public with proper preventive actions. In this work, a new RT-AQF system is developed based on the online-coupled Weather Research and Forecasting model with Chemistry (WRF/Chem) with the Model of Aerosol Dynamics, Reaction, Ioniza- tion, and Dissolution (MADRID) (referred to as WRF/Chem-MADRID) and deployed in the southeastern U.S. during MayeSeptember, 2009. Max 1-h and 8-h average ozone (O3) and 24-h average fine particulate matter (PM2.5) are evaluated against surface observations from the AIRNow database in terms of spatial distribution, temporal variation, and domain-wide and region-specific discrete and categorical perfor- mance statistics. WRF/Chem-MADRID demonstrates good forecasting skill that is consistent with current RT-AQF models. The overpredictions of O3 and underprediction of PM2.5 are likely due to uncertainties in emissions such as those of biogenic volatile organic compounds (BVOCs) and ammonia, inaccuracies in simulated meteorological variables such as 2-m temperature, 10-m wind speed, and precipitation, and uncertainties in the boundary conditions. Sensitivity simulations show that the use of the online BVOC emissions can improve PM2.5 forecast in areas with high BVOC emissions and adjusting lateral bound- aries can improve domain-wide O3 and PM2.5 predictions. Several limitations and uncertainties are identified to further improve the model's forecasting skill.

Biomass burning (BB) in Southeast Asia (Indochina and southern China) occurs frequently in March and April every year. The burning plume is ordinarily transported eastward by the prevailing westerly, further affecting downstream air quality in East Asia. In this study, atmospheric aerosols were collected at the downstream Lulin Atmospheric Background Station (LABS, 2862 m a.s.l., central Taiwan) from April 2003 to April 2009. Results show that monthly means of PM2.5 were highest during the BB period, especially in March. The PM2.5 mean for BB activity was 17.5 μg m−3, while the daily PM2.5 mean can sometimes be above 40 μg m−3. The background PM2.5 level in free troposphere of the West Pacific was at 3.7 ± 1.8 μg m−3. This mean is roughly the same regardless of the air masses moving from China, Pacific Ocean, and South China Sea toward LABS. In addition, the highest PM2.5 level occurred in 2004, making it the most active year of BB for the whole observation period. Greater amounts of nitrate and potassium ions were observed in the PM2.5 collected during the BB period compared to the non-BB (NBB) period. Linear regression analysis on PM2.5 water-soluble ions shows a moderate correlation (R2 = 0.59) between non-sea-salt potassium and nitrate ions during the BB period. Furthermore, for all trajectory source origins, ammonium ion had the best correlation (R2 = 0.84) with non-sea-salt sulfate when the air masses were influenced by anthropogenic sources during the NBB period. The enhancement ratios of nitrate ion during the BB period could reach 6.7 and 9.7 relative to air masses from the BB source region and from the pristine area during the NBB period, respectively. During the study period, ammonia gas was found to be insufficient to neutralize sulfuric and nitric gases. Therefore, most aerosols were more acidic than basic. Our long-term observation of atmospheric aerosols with inter-annual variability is valuable in providing data for verifying BB source inventory and model performance in East Asia.

The occurrence of Asian dust storms, and the subsequent transport of yellow dust (YD) greatly influences the air quality of lee-side countries such as Korea and Japan. The dust is also frequently transported in a southward direction by a strong cold high-pressure system that affects the air quality in Taiwan. This study reports the aerosol properties that were monitored continuously at the Taiwan aerosol supersite during YD events in 2002. Based on the observations of meteorology and aerosols, we divided the time interval of a YD event into a before period , during period , and after period . Among the seven observed YD events, the second event was marked with the maximum hourly PM 10 level at 502 μg m −3 , and with the longest during period for a total of 147 h. The averages of the hourly PM 10 and PM 2.5−10 were much higher in the during period as compared to those in the before period . It is interesting to note that the time lapse in the during period was well correlated with the maximum level of both PM 10 and PM 2.5−10 . It must be noted that the PM 2.5 levels were dramatically increased in the after period , which was due to the accumulation of particles influenced by the anticyclonic outflow. The aerosol size distribution in the third YD event verified that supermicron particles dominated in the during period , and that submicron particles were predominant in the before and after periods . For the chemical properties of the aerosols, time series results indicated that sulfates were mostly contributed by the dust transport, and the others were more related to vehicle exhausts. However, they all accumulated in the period of atmospheric stagnancy.