High concentrations of ozone in urban and industrial regions worldwide have long been a major air quality issue. With the rapid increase in fossil fuel consumption in China over the past three ...decades, the emission of chemical precursors to ozone—nitrogen oxides and volatile organic compounds—has increased sharply, surpassing that of North America and Europe and raising concerns about worsening ozone pollution in China. Historically, research and control have prioritized acid rain, particulate matter, and more recently fine particulate matter (PM2.5). In contrast, less is known about ozone pollution, partly due to a lack of monitoring of atmospheric ozone and its precursors until recently. This review summarizes the main findings from published papers on the characteristics and sources and processes of ozone and ozone precursors in the boundary layer of urban and rural areas of China, including concentration levels, seasonal variation, meteorology conducive to photochemistry and pollution transport, key production and loss processes, ozone dependence on nitrogen oxides and volatile organic compounds, and the effects of ozone on crops and human health. Ozone concentrations exceeding the ambient air quality standard by 100–200% have been observed in China's major urban centers such as Jing-Jin-Ji, the Yangtze River delta, and the Pearl River delta, and limited studies suggest harmful effect of ozone on human health and agricultural corps; key chemical precursors and meteorological conditions conductive to ozone pollution have been investigated, and inter-city/region transport of ozone is significant. Several recommendations are given for future research and policy development on ground-level ozone.
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•Studies of atmospheric ozone in urban and rural areas of China are reviewed.•Topics include abundance, chemical and meteorological processes, and effects.•Available data reveals serious and worsening ozone pollution in major areas of China.•Data from national network are needed to get a full picture of ozone pollution and to evaluate its impact.•Strategies for control ozone precursors need to be developed.
Many previous studies have investigated the role of urban greening (especially trees) on thermal comfort condition of open spaces while its influence in street canyons has, however, received ...relatively lower research attention. In this study, numerical experiments involving street canyons of varying aspect ratio (ARB) with embedded trees of varying aspect ratio (ART), leaf area index (LAI), leaf area density (LAD) distribution and trunk height under different wind conditions were conducted using a micro-meteorological model, ENVI-met. Employing physiological equivalent temperature (PET), the in-canyon thermal comfort was characterized. Results show variable magnitude of PET reduction (ΔPET) with trees of different vertical LAD distribution even though they are of similar LAI. While larger tree-covered area (TCA) ensures improved thermal comfort, the magnitude varies with tree-planting pattern. Between the tree-planting patterns in TCA = 0.6, ΔPET with double-rows is higher than centre tree-planting. Between eastern and western side tree-planting (TCA = 0.3), the former is more temporally effective than the latter. Furthermore, strong oblique wind condition (“without trees”) was found to be more efficient in improving thermal comfort than trees under calm perpendicular wind, reinstating the importance of desirable ventilation in achieving a thermally comfortable environment. However, vegetation offers improved PET reduction under calmer perpendicular than stronger oblique wind condition. Finally, recommendations on applications of our key findings were presented.
•Numerical experiments involving street canyons with trees of varying configuration were conducted.•Apart from LAI, vertical LAD distribution influences ΔPET while trunk height is less influential.•Variable temporal ΔPET was observed with similar tree-covered area but different planting pattern.
Well-positioned and configured vegetation barriers (VBs) have been suggested as one of the green infrastructures that could improve near-road (local) air quality. This is because of their influence ...on the underlying mechanisms: dispersion and mass removal (by deposition). Some studies have investigated air quality improvement by near-road vegetation barrier using the dispersion-related method while few studies have done the same using the deposition-related method. However, decision making on vegetation barrier’s configuration and placement for need-based maximum benefit requires a combined assessment with both methods which are not commonly found in a single study. In the present study, we employed a computational fluid dynamics model, ENVI-met, to evaluate the air quality benefit of near-road vegetation barrier using an integrated dispersion–deposition approach. A technique based on distance between source (road) and point of peak concentration before dwindling concentration downwind begins referred to as “distance to maximum concentration (DMC)” has been proposed to determine optimum position from source and thickness of vegetation barrier for improved dispersion and deposition-based benefit, respectively. Generally, a higher volume of vegetation barrier increases the overall mass removal while it weakens dispersion of pollutant within the same domain. Hence, the benefit of roadside vegetation barrier is need-based and can be expressed as either higher mass deposition or higher mass dispersion. Finally, recommendations on applications of our findings were presented.
Biomass burning (BB) is a significant air pollution source, with global, regional and local impacts on air quality, public health and climate. Worldwide an extensive range of studies has been ...conducted on almost all the aspects of BB, including its specific types, on quantification of emissions and on assessing its various impacts. China is one of the countries where the significance of BB has been recognized, and a lot of research efforts devoted to investigate it, however, so far no systematic reviews were conducted to synthesize the information which has been emerging. Therefore the aim of this work was to comprehensively review most of the studies published on this topic in China, including literature concerning field measurements, laboratory studies and the impacts of BB indoors and outdoors in China. In addition, this review provides insights into the role of wildfire and anthropogenic BB on air quality and health globally. Further, we attempted to provide a basis for formulation of policies and regulations by policy makers in China.
Open field biomass burning causes severe air pollution, public health risk and potential climate impact. a) Photo taken in Changzhou rural area on June 10, 2015; b) Photo taken in Hebei rural area on October 23, 2013; c) A traditional indoor burner in rural area in China; d) Tar ball emitted from biomass burning. Display omitted
•This review discusses wildfire and anthropogenic emission from biomass burning in China.•Field observations and laboratory studies on public health and climate impacts of biomass burning•Atmospheric process of biomass burning plumes and their transport•Proposed research priorities and insights about biomass burning in China
Numerical experiments involving street canyons of varying aspect ratio with traffic-induced pollutants (PM
2.5
) and implanted trees of varying aspect ratio, leaf area index, leaf area density ...distribution, trunk height, tree-covered area, and tree planting pattern under different wind conditions were conducted using a computational fluid dynamics (CFD) model, ENVI-met. Various aspects of dispersion and deposition were investigated, which include the influence of various tree configurations and wind condition on dispersion within the street canyon, pollutant mass at the free stream layer and street canyon, and comparison between mass removal by surface (leaf) deposition and mass enhancement due to the presence of trees. Results revealed that concentration level was enhanced especially within pedestrian level in street canyons with trees relative to their tree-free counterparts. Additionally, we found a dependence of the magnitude of concentration increase (within pedestrian level) and decrease (above pedestrian level) due to tree configuration and wind condition. Furthermore, we realized that only ∼0.1–3 % of PM
2.5
was dispersed to the free stream layer while a larger percentage (∼97 %) remained in the canyon, regardless of its aspect ratio, prevailing wind condition, and either tree-free or with tree (of various configuration). Lastly, results indicate that pollutant removal due to deposition on leaf surfaces is potentially sufficient to counterbalance the enhancement of PM
2.5
by such trees under some tree planting scenarios and wind conditions
A total of 14 chemical transport models (CTMs) participated in
the first topic of the Model Inter-Comparison Study for Asia (MICS-Asia)
phase III. These model results are compared with each other and ...an extensive
set of measurements, aiming to evaluate the current CTMs' ability in
simulating aerosol concentrations, to document the similarities and
differences among model performance, and to reveal the characteristics of
aerosol components in large cities over East Asia. In general, these CTMs
can well reproduce the spatial–temporal distributions of aerosols in East
Asia during the year 2010. The multi-model ensemble mean (MMEM) shows
better performance than most single-model predictions, with correlation
coefficients (between MMEM and measurements) ranging from 0.65 (nitrate,
NO3-) to 0.83 (PM2.5). The
concentrations of black carbon (BC), sulfate
(SO42-), and PM10 are
underestimated by MMEM, with normalized mean biases (NMBs) of −17.0 %,
−19.1 %, and −32.6 %, respectively. Positive biases are simulated
for NO3- (NMB = 4.9 %), ammonium
(NH4+) (NMB = 14.0 %), and PM2.5
(NMB = 4.4 %). In comparison with the statistics calculated from
MICS-Asia phase II, frequent updates of chemical mechanisms in CTMs during
recent years make the intermodel variability of simulated aerosol
concentrations smaller, and better performance can be found in reproducing
the temporal variations of observations. However, a large variation (about a
factor of 2) in the ratios of SNA (sulfate, nitrate, and ammonium) to
PM2.5 is calculated among participant models. A more intense secondary
formation of SO42- is simulated by Community Multi-scale Air Quality (CMAQ)
models, because of the higher SOR (sulfur oxidation ratio) than other
models (0.51 versus 0.39). The NOR (nitric oxidation ratio) calculated by all
CTMs has larger values (∼0.20) than the observations,
indicating that overmuch NO3- is
simulated by current models. NH3-limited condition (the mole ratio of
ammonium to sulfate and nitrate is smaller than 1) can be successfully
reproduced by all participant models, which indicates that a small reduction
in ammonia may improve the air quality. A large coefficient of variation
(CV > 1.0) is calculated for simulated coarse particles,
especially over arid and semi-arid regions, which means that current CTMs
have difficulty producing similar dust emissions by using different dust
schemes. According to the simulation results of MMEM in six large Asian
cities, different air-pollution control plans should be taken due to
their different major air pollutants in different seasons. The MICS-Asia
project gives an opportunity to discuss the similarities and differences of
simulation results among CTMs in East Asian applications. In order to acquire
a better understanding of aerosol properties and their impacts, more
experiments should be designed to reduce the diversities among air quality
models.
A synergy of numerical simulation, ground-based measurement and satellite observation was applied to evaluate the impact of biomass burning originating from Southeast Asia (SE Asia) within the ...framework of NASA's 2006 Biomass burning Aerosols in Southeast Asia: Smoke Impact Assessment (BASE-ASIA). Biomass burning emissions in the spring of 2006 peaked in March–April when most intense biomass burning occurred in Myanmar, northern Thailand, Laos, and parts of Vietnam and Cambodia. Model performances were reasonably validated by comparing to both satellite and ground-based observations despite overestimation or underestimation occurring in specific regions due to high uncertainties of biomass burning emission. Chemical tracers of particulate K+, OC concentrations, and OC/EC ratios showed distinct regional characteristics, suggesting biomass burning and local emission dominated the aerosol chemistry. CMAQ modeled aerosol chemical components were underestimated at most circumstances and the converted AOD values from CMAQ were biased low at about a factor of 2, probably due to the underestimation of biomass emissions. Scenario simulation indicated that the impact of biomass burning to the downwind regions spread over a large area via the Asian spring monsoon, which included Southern China, South China Sea, and Taiwan Strait. Comparison of AERONET aerosol optical properties with simulation at multi-sites clearly demonstrated the biomass burning impact via long-range transport. In the source region, the contribution from biomass burning to AOD was estimated to be over 56%. While in the downwind regions, the contribution was still significant within the range of 26%–62%.
► We model biomass burning in Southeast Asia which has been rarely studied. ► Impact of biomass burning in both source and downwind regions are assessed. ► Contributions from biomass burning to aerosol are quantatively determined. ► Model evaluations suggest biomass burning emission inventory should be improved.
Black carbon (BC) emissions play an important role in regional climate
change in the Arctic. It is necessary to pay attention to the impact of
long-range transport from regions outside the Arctic as ...BC emissions from
local sources in the Arctic were relatively small. The task force
Hemispheric Transport of Air Pollution Phase 2 (HTAP2) set up a series of
simulation scenarios to investigate the response of BC in a given region to
different source regions. This study investigated the responses of Arctic BC
concentrations and surface temperature to 20 % anthropogenic emission
reductions from six regions in 2010 within the framework of HTAP2 based on
ensemble modeling results. Emission reductions from East Asia (EAS) had the most
(monthly contributions: 0.2–1.5 ng m−3) significant impact on the
Arctic near-surface BC concentrations, while the monthly contributions from
Europe (EUR), Middle East (MDE), North America (NAM), Russia–Belarus–Ukraine
(RBU), and South Asia (SAS) were 0.2–1.0, 0.001–0.01, 0.1–0.3, 0.1–0.7, and 0.0–0.2 ng m−3,
respectively. The responses of the vertical profiles of the Arctic BC to the
six regions were found to be different due to multiple transport pathways.
Emission reductions from NAM, RBU, EUR, and EAS mainly influenced the BC
concentrations in the low troposphere of the Arctic, while most of the BC in the
upper troposphere of the Arctic derived from SAS. The response of the Arctic
BC to emission reductions in six source regions became less significant with
the increase in the latitude. The benefit of BC emission reductions in terms
of slowing down surface warming in the Arctic was evaluated by using
absolute regional temperature change potential (ARTP). Compared to the
response of global temperature to BC emission reductions, the response of
Arctic temperature was substantially more sensitive, highlighting the need
for curbing global BC emissions.
We used the regional air quality model CMAQ to simulate organic aerosol (OA) concentrations over the Pearl River Delta region (PRD) and compared model results to measurements. Our goals were (1) to ...evaluate the potential contribution of the aqueous reactive uptake of dicarbonyls (glyoxal and methylglyoxal) as a source of secondary organic aerosol (SOA) in an urban environment, and (2) to quantify the sources of SOA in the PRD in fall. We improved the representation of dicarbonyl gas phase chemistry in CMAQ, as well as added SOA formation via the irreversible uptake of dicarbonyls by aqueous aerosols and cloud droplets, characterized by a reactive uptake coefficient γ = 2.9 × 10−3 based on laboratory studies. Our model results were compared to aerosol mass spectrometry (AMS) measurements in Shenzhen during a photochemical smog event in fall 2009. Including the new dicarbonyl SOA source in CMAQ led to an increase in the simulated mean SOA concentration at the sampling site from 4.1 μg m−3 to 9.0 μg m−3 during the smog event, in better agreement with the mean observed oxygenated OA (OOA) concentration (8.0 μg m−3). The simulated SOA reproduced the variability of observed OOA (r = 0.89). Moreover, simulated dicarbonyl SOA was highly correlated with simulated sulfate (r = 0.72), consistent with the observed high correlation between OOA and sulfate (r = 0.84). Including the dicarbonyl SOA source also increased the mean simulated concentrations of total OA from 8.2 μg m−3 to 13.1 μg m−3, closer to the mean observed OA concentration (16.5 μg m−3). The remaining difference between the observed and simulated OA was largely due to impacts from episodic biomass burning emissions, but the model did not capture this variability. We concluded that, for the PRD in fall and outside of major biomass burning events, 75% of the total SOA was biogenic. Isoprene was the most important precursor, accounting for 41% of the total SOA. Aromatics accounted for 13% of the total SOA. Our results show that the aqueous chemistry of dicarbonyls can be an important SOA source, potentially accounting for 53% of the total surface SOA in the PRD in fall.
► CMAQ model performance enhanced with addition of dicarbonyl SOA source. ► 75% of total SOA was biogenic for the PRD in fall and isoprene was the most important precursor. ► The irreversible uptake of dicarbonyls by aqueous particles was an important SOA formation pathway in the PRD in fall.
This study developed updated emission inventories for 2010, 2015, and 2019 base‐year for road transportation in Vietnam with future projections for 2020, 2025, and 2030. In general (2019 base‐year), ...motorcycles contributed a substantial CO, NMVOC, PM10, PM2.5, OC, and CH4 (~53%–89%), while BC, NOx, and SO2 were mostly from diesel‐powered trucks (~42%–76.3%). The countrywide emission contributions from two rapidly growing cities (Hanoi and Ho Chi Minh HCM) were estimated to be 11%–16.2% and 16.6%–20.2%, respectively, and CO2 was found to be the leading pollutant that contributes to the overall Global Warming Potential (41% of CO2e) in Vietnam. In terms of future emission projections, the suggested or planned policy interventions (i.e., banning motorcycles, improved fuel quality, introducing electric vehicles, and public transportation) are expected to lead to 11%–125% (compared to Business As Usual) emission reductions in 2030, pushing Vietnam a step toward environmental sustainability.
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