T.J. Wang

Flow patterns influencing the seasonal behavior of surface ozone and carbon monoxide at a coastal site near Hong Kong

Abstract Surface O 3 and CO were measured at Cape D’Aguilar, Hong Kong during the period of January 1994 to December1996 in order to understand the temporal variations of surface O 3 and CO in East Asia–West Pacific region. The isentropic backward trajectories were used to isolate different air masses reaching the site and to analyze the long-range transport and photochemical buildup of O 3 on a regional scale. The results show that the diurnal variation of surface O 3 was significant in all seasons with daily O 3 production being about 20xa0ppbv in fall and 10xa0ppbv in winter, indicating more active photochemical processes in the subtropical region. The distinct seasonal cycles of O 3 and CO were found with a summer minimum (16xa0ppbv)–fall maximum (41xa0ppbv) for O 3 and a summer minimum (116xa0ppbv)–winter maximum (489xa0ppbv) for CO. The isentropic backward trajectory cluster analyses suggest that the air masses (associated with regional characteristics) to the site can be categorized into five groups, which are governed by the movement of synoptic weather systems under the influence of the Asian monsoon. For marine-originated air masses (M-SW, M-SE and M-E, standing for marine-southwest, marine-southeast and marine-east, respectively) which always appear in summer and spring, the surface O 3 and CO have relatively lower mixing ratios (18, 16 and 30xa0ppbv for O 3 , 127, 134 and 213xa0ppbv for CO), while the continental air masses (C-E and C-N, standing for continent-east and continent-north, respectively) usually arrive at the site in winter and fall seasons with higher O 3 (43 and 48xa0ppbv) and CO (286 and 329xa0ppbv). The 43xa0ppbv O 3 and 286xa0ppbv CO are representative of the regionally polluted continental outflow air mass due to the anthropogenic activity in East Asia, while 17xa0ppbv O 3 and 131xa0ppbv CO can be considered as the signature of the approximately clean marine background of South China Sea. The very high CO values (461–508xa0ppbv) during winter indicate that the long-range transport of air pollutants from China continent is important at the monitoring site. The fall maximum (35–46xa0ppbv) of surface O 3 was believed to be caused by the effects of the weak slowly moving high-pressure systems which underlie favorable photochemical production conditions and the long-range transport of aged air masses with higher O 3 and its precursors.

An ozone episode in the Pearl River Delta: Field observation and model simulation

[1]xa0In the fall of 2007 concurrent air sampling field measurements were conducted for the first time in Guangzhou (at Wan Qing Sha (WQS)) and Hong Kong (at Tung Chung (TC)), two cities in the rapidly developing Pearl River Delta region of China that are only 62 km apart. This region is known to suffer from poor air quality, especially during the autumn and winter months, when the prevailing meteorological conditions bring an outflow of continental air to the region. An interesting multiday O3 pollution event (daily maximum O3 > 122 ppbv) was captured during 9–17 November at WQS, while only one O3 episode day (10 November) was observed at TC during this time. The mean O3 mixing ratios at TC and WQS during the episode were 38 ± 3 (mean ± 95% confidence interval) and 51 ± 7 ppbv, respectively, with a mean difference of 13 ppbv and a maximum hourly difference of 150 ppbv. We further divided this event into two periods: 9–11 November as Period 1 and 12–17 November as Period 2. The mixing ratios of O3 and its precursors (NOx and CO) showed significant differences between the two periods at TC. By contrast, no obvious difference was found at WQS, indicating that different air masses arrived at TC for the two periods, as opposed to similar air masses at WQS for both periods. The analysis of VOC ratios and their relationship with O3 revealed strong O3 production at WQS during Period 2, in contrast to relatively weak photochemical O3 formation at TC. The weather conditions implied regional transport of O3 pollution during Period 1 at both sites. Furthermore, a comprehensive air quality model system (Weather Research and Forecasting–Community Multiscale Air Quality model (WRF-CMAQ)) was used to simulate this O3 pollution event. The model system generally reproduced the variations of weather conditions, simulated well the continuous high O3 episode event at WQS, and captured fairly well the elevated O3 mixing ratios in Period 1 and low O3 levels in Period 2 at TC. The modeled surface O3 distributions and flow structures clearly illustrated the occurrence of O3 formation and the impact of regional transport on O3 levels in Period 1 in the Pearl River Delta. Further analysis of O3 formation indicated that horizontal transport was the main contributor to the O3 increase at TC during Period 1, while at WQS O3 levels were dominated by photochemical production during both periods. The low O3 levels at TC during Period 2 were attributable to lower temperatures and the arrival of fresh maritime air masses brought in by strong easterly winds. This study highlights how contrasting precursor concentrations and photochemical conditions can occur over a very small distance, and it provides a rare opportunity to better understand ozone production and precursor source origins on a finer scale in this region.

A modeling study on acid rain and recommended emission control strategies in China

This paper presents a brief description of the sources and characteristics of air pollution in China, documenting acid rain aggravation and its regional distribution in the past years. Simulation of SO2 ground-level concentration and sulfur deposition in 1995 was performed with the Nanjing University developed acid deposition model system (NJUADMS) and compared with the national observations and the model output of the RAINS-ASIA. Furthermore, the acid rain control policy and its countermeasures adopted for the country are presented.

The interactions between anthropogenic aerosols and the East Asian summer monsoon using RegCCMS

An online coupled regional climate-chemistry model called RegCCMS is used to investigate the interactions between anthropogenic aerosols and the East Asian summer monsoon (EASM) over East Asia. The simulation results show that the mean aerosol loading and optical depth over the region are 17.87u2009mg/m2 and 0.25, respectively. Sulfate and black carbon (BC) account for approximately 61.2% and 7.8% of the total aerosols, respectively. The regional mean radiative forcing (RF) is approximately −3.64, −0.55, and +0.88u2009W/m2 at the top of the atmosphere for the total aerosol effect, the total aerosol direct effect, and the BC direct effect, respectively. The surface direct RF of BC accounts for approximately 31% of the total RF of all aerosols. Because of the total aerosol effect, both the energy budgets and air temperature are considerably reduced in the region with high aerosol loadings, leading to decreases in the land-ocean air temperature gradient in summer. The total column-absorbed solar radiation and surface air temperature decrease by 8.4u2009W/m2 and 0.31u2009K, respectively. This cooling effect weakens horizontal and vertical atmospheric circulations over East Asia. The wind speed at 850u2009hPa decreases by 0.18u2009m/s, and the precipitation decreases by 0.29u2009mm/d. The small responses of solar radiation, air temperature, and atmospheric circulations to the BC warming effect are opposite to those of the total aerosol effect. The BC-induced enhancement of atmospheric circulation can increase local floods in south China, while droughts in north China may worsen in response to the BC semidirect effect. The total aerosol effect is much more significant than the BC direct effect. The East Asian summer monsoon becomes weaker due to the total aerosol effect. However, this weakness could be partially offset by the BC warming effect. Sensitivity analyses further indicate that the influence of aerosols on the EASM might be more substantial in years when the southerlies or southwesterlies at 850u2009hPa are weak compared with years when the winds are strong. Changes in the EASM can induce variations in the distribution and magnitude of aerosols. Aerosols in the lower troposphere over the region can increase by 3.07 and 1.04u2009µg/m3 due to the total aerosol effect and the BC warming effect, respectively.

Characteristics of atmospheric mercury deposition and size-fractionated particulate mercury in urban Nanjing, China

A comprehensive measurement study of mercury wet deposition and size-fractionated particulate mercury (HgP) concurrent with meteorological variables was conducted from June 2011 to February 2012 to evaluate the characteristics of mercury deposition and particulate mercury in urban Nanjing, China. The volume-weighted mean (VWM) concentration of mercury in rainwater was 52.9 ng L −1 with a range of 46.3–63.6 ng L −1. The wet deposition per unit area was averaged 56.5 μg m −2 over 9 months, which was lower than that in most Chinese cities, but much higher than annual deposition in urban North America and Japan. The wet deposition flux exhibited obvious seasonal variation strongly linked with the amount of precipitation. Wet deposition in summer contributed more than 80 % to the total amount. A part of contribution to wet deposition of mercury from anthropogenic sources was evidenced by the association between wet deposition and sulfates, as well as nitrates in rainwater. The ions correlated most significantly with mercury were formate, calcium, and potassium, which suggested that natural sources including vegetation and resuspended soil should be considered as an important factor to affect the wet deposition of mercury in Nanjing. The average Hg P concentration was 1.10± 0.57 ng m−3. A distinct seasonal distribution of HgP concentrations was found to be higher in winter as a result of an increase in the PM 10 concentration. Overall, more than half of the Hg P existed in the particle size range less than 2.1 μm. The highest concentration of Hg P in coarse particles was observed in summer, while Hg P in fine particles dominated in fall and winter. The size distribution of averaged mercury content in particulates was bimodal, with two peaks in the bins of < 0.7 μm and 4.7–5.8 μm. Dry deposition per unit area of Hg P was estimated to be 47.2 μg m −2 using meteorological conditions and a size-resolved particle dry deposition model. This was 16.5 % less than mercury wet deposition. Compared to Hg P in fine particles, HgP in coarse particles contributed more to the total dry deposition due to higher deposition velocities. Negative correlation between precipitation and the Hg P concentration reflected the effect of scavenging of Hg P by precipitation.

Two‐dimensional/three‐dimensional waveform modeling of subducting slab and transition zone beneath Northeast Asia

Subduction plays a fundamental role in dynamics of the mantle convection and in material circulations of the Earths interior. Slabs have been imaged to subduct near horizontally in the transition zone (TZ) beneath the Northeast Asia in seismic tomography. Triplication waveform modeling is an effective tool to study the detailed seismic structure in the TZ. However, TZ triplication modeling has traditionally relied on 1-D models. In this study, we use the spectral element method to explore influences of 2-D/3-D slab structure on TZ triplication waveforms and to model, for the first time, the slab and TZ structures beneath the Northeast Asia. Synthetic tests suggest that, for a subduction zone earthquake, slab structure can have important influences on TZ triplication waveforms and that, even in a narrow azimuth range, the effects from 2-D/3-D slab structure on the wave propagation can lead to erroneous conclusions with 1-D modeling. Our data are high-quality triplicated SH waveforms (at distances of 10°–32°) from a deep event (below 410 km discontinuity) in the Pacific subducting slab. Our 2-D/3-D waveform modeling results suggest that a simple model of the subducting slab (+5% high-velocity anomaly and ~100u2009km thick down to 560u2009km) but normal below 560u2009km can match most of the observed waveforms remarkably well. The bottom of the TZ of the sampling region (north of the Yellow Sea) contains a patch of very slow anomaly. The results indicate that subhorizontal slab above the 660 discontinuity is not everywhere beneath Northeast Asia and the subducting slab is not everywhere continuous to the bottom of the TZ. Compared with the traditional 1-D modeling, our new 2-D/3-D approach provides better fits to the data and allows us to constrain the slab geometry and to separate TZ structure from slab structure

Distribution pattern and mass budget of sedimentary polycyclic aromatic hydrocarbons in shelf areas of the Eastern China Marginal Seas

This study conducted the first extensive and comprehensive investigation of the regional-scale sedimentary polycyclic aromatic hydrocarbons (PAHs) concentration, flux, and budget in the continental shelves of the Eastern China Marginal Seas (ECMSs). Surface sediment samples from multiple sites were collected and assessed, and the latest data from current research was assessed. The spatial distribution pattern of PAHs in the ECMSs was significantly influenced by the regional hydrodynamics, sediment properties (grain-size, total organic carbon (TOC) content, and sedimentation rate), and anthropogenic impacts. Relatively higher PAHs concentrations occurred in areas with fine-grained sediment. Results of source apportionment found that the relative proportions of PAHs showed significant regional variation, mainly influenced by socio-economic differences between north and south China. The PAHs burial flux in the study area ranged from 11.2 to 1308 ng cm−2 y−1 with an average value of 101u2009±u2009104 ng cm−2 y−1. The area-integrated sedimentary PAHs burial flux across the ECMSs was 494 t y−1. A mass budget calculation revealed that riverine input and atmospheric deposition were the most significant sources contributing, 28.4% and 71.6%, respectively. The study demonstrated that net PAHs transportation occurs between the Bohai Sea (BS) and Yellow Sea (YS), with a flux of approximately 10.2 t y−1. PAHs were also transported from YS to the East China Sea (ECS), due to water exchange between the YS and ECS. Additionally, substantial amounts of PAHs in the inner shelf of the ECS were transported out of the shelf area due to cross-shelf plume.