Global association between satellite-derived nitrogen dioxide (NO2) and lockdown policies under the COVID-19 pandemic.

The COVID-19 pandemic has severely affected various aspects of life, at different levels and in different countries on almost every continent. In response, many countries have closed their borders and imposed lockdown policies, possibly bringing benefits to people's health with significantly less emission from air pollutants. Currently, most studies or reports are based on local observations at the city or country level. There remains a lack of systematic understanding of the impacts of different lockdown policies on the air quality from a global perspective. This study investigates the impacts of COVID-19 pandemic towards global air quality through examining global nitrogen dioxide (NO2) dynamics from satellite observations between 1 January and 30 April 2020. We used the Apriori algorithm, an unsupervised machine learning method, to investigate the association among confirmed cases of COVID-19, NO2 column density, and the lockdown policies in 187 countries. The findings based on weekly data revealed that countries with new cases adopted various lockdown policies to stop or prevent the virus from spreading whereas those without tended to adopt a wait-and-see attitude without enforcing lockdown policies. Interestingly, decreasing NO2 concentration due to lockdown was associated with international travel controls but not with public transport closure. Increasing NO2 concentration was associated with the "business as usual" strategy as evident from North America and Europe during the early days of COVID-19 outbreak (late January to early February 2020), as well as in recent days (in late April) after many countries have started to resume economic activities. This study enriches our understanding of the heterogeneous patterns of global associations among the COVID-19 spreading, lockdown policies and their environmental impacts on NO2 dynamics.

Solar-powered air quality monitor applied under subtropical conditions in Hong Kong: Performance evaluation and application for pollution source tracking.

Air monitoring is desirable in many places to understand dynamic pollution trends and sources and improve knowledge of population exposure. While highly miniaturized low cost sensor technology is quickly evolving, there is also a need for the advancement of mid-tier systems that are closer to reference-grade technologies in their longevity and performance, but also feature compactness that requires less significant infrastructure. This project evaluated the performance of a prototype solar-powered air monitoring system known as a Village Green Project (VGP) system with wireless data transmission that was deployed on a school rooftop in Hong Kong and operated for over one year. The system provided highly time-resolved and long-term data utilizing mid-tier cost ozone, PM2.5 and meteorological instruments. It operated with very minimal maintenance but shading by a nearby building reduced solar radiation, thus battery run time, over the 16-months measurement period, approximately 330,000 1-min observations were recorded (data completeness of ~62%). The monitoring data were evaluated by comparison with a nearby Hong Kong Environment Protection Department (EPD) station and exhibited good performance for 1-h resolution (R 2 = 0.74 for PM2.5 and R 2 = 0.76 for ozone). Furthermore as a demonstration, a nonparametric regression (NPR) model was applied for identifying the location of pollution source, combining air pollution and meteorological measurements. In addition, based on the high time-resolution wind data, local-scale back-trajectories were calculated as an input for receptor-oriented Nonparametric Trajectory Analysis (NTA) model. The combination of the VGP air monitoring system and NTA model identified apparent local sources in urban area. The demonstration was largely successful and operational improvements are clearly suggested to insure better siting and configurations to insure adequate power and air flow.

Impact of tropical cyclone track change on regional air quality.

There has been an increase in tropical cyclones (TCs) in the western North Pacific (WNP) that traverse with a northward recurving track towards East Asia and a decrease in TC tracks entering the South China Sea (SCS) in the past few decades. To investigate the potential impact of the prevailing TC track change on Hong Kong air quality, an analysis has been carried out based on historical data (1991 to 2010) of TC tracks and air quality. Compared to TCs in other regions, TCs in the vicinity of Taiwan (Region 2, R2) have the greatest impact on Hong Kong air quality due to regional transport of air pollutants from the highly industrialized Pearl River Delta (PRD). In the last twenty years, the number of days with TCs in R2 (May to October) has increased by 45% from 111days in the period 1991-2000 to 161days in 2001-2010, during which there was an increase in yearly TC-related pollution episodes of approximately 3 episodes per year in Hong Kong. The enhancement of mean O3 concentration due to TCs in R2 is reported as 82% (~50.8µg/m3 at a rural station) and 58% (~16.8µg/m3 at an urban station) higher than the summer averages. A similar enhancement is also observed for PM10 (called RSP) and SO2 with an average of 70% (i.e., 22.2µg/m3) and 100% (i.e., 15.2µg/m3) increases, respectively. Overall, the 20years of historical data show that the O3 concentrations on the TC-affected days are increasing at the estimated rates of 0.5µg/m3 and 2.6µg/m3 per year, respectively, in the urban and remote areas, which are significantly higher than the increase of 0.3µg/m3 and 0.4µg/m3 per year in the average summer concentrations.

Ozone pollution in China: A review of concentrations, meteorological influences, chemical precursors, and effects.

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.

A review of biomass burning: Emissions and impacts on air quality, health and climate in China.

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.

Study of traffic-related pollutant removal from street canyon with trees: dispersion and deposition perspective.

Numerical experiments involving street canyons of varying aspect ratio with traffic-induced pollutants (PM2.5) 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 PM2.5 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 PM2.5 by such trees under some tree planting scenarios and wind conditions.

Evaluating the role of green infrastructures on near-road pollutant dispersion and removal: Modelling and measurement.

To enhance the quality of human life in a rapidly urbanized world plagued with high transportation, the masterful contribution of improved urban and local air quality cannot be overemphasized. In order to reduce human exposure to near-road air pollution, several approaches including the installation of roadside structural barriers especially in open street areas, such as city entrances are being applied. In the present study, the air quality around real world and idealized green infrastructures was investigated by means of numerical simulation and a short field measurement campaign. Fair agreement was found between ENVI-met modelled and measured particulate matter's concentration data around a realistic vegetation barrier indicating a fair representation of reality in the model. Several numerical experiments were conducted to investigate the influence of barrier type (vegetation/hedge and green wall) and dimensions on near-road air quality. The results show different horizontal/vertical patterns and magnitudes of upwind and downwind relative concentration (with and without a barrier) depending on wind condition, barrier type and dimension. Furthermore, an integrated dispersion-deposition approach was employed to assess the impact on air quality of near-road vegetation barrier. At last, recommendations to city and urban planners on the implementation of roadside structural barriers were made.

Simulation study of dispersion and removal of particulate matter from traffic by road-side vegetation barrier.

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.

Multi-purpose rainwater harvesting for water resource recovery and the cooling effect.

The potential use of rainwater harvesting in conjunction with miscellaneous water supplies and a rooftop garden with rainwater harvesting facility for temperature reduction have been evaluated in this study for Hong Kong. Various water applications such as toilet flushing and areal climate controls have been systematically considered depending on the availability of seawater toilet flushing using the Geographic Information System (GIS). For water supplies, the district Area Precipitation per Demand Ratio (APDR) has been calculated to quantify the rainwater utilization potential of each administrative district in Hong Kong. Districts with freshwater toilet flushing prove to have higher potential for rainwater harvest and utilization compared to the areas with seawater toilet flushing. Furthermore, the effectiveness of using rainwater harvesting for miscellaneous water supplies in Hong Kong and Tokyo has been analyzed and compared; this revives serious consideration of diurnal and seasonal patterns of rainfall in applying such technology. In terms of the cooling effect, the implementation of a rooftop rainwater harvesting garden has been evaluated using the ENVI-met model. Our results show that a temperature drop of 1.3 °C has been observed due to the rainwater layer in the rain garden. This study provides valuable insight into the applicability of the rainwater harvesting for sustainable water management practice in a highly urbanized city.

Sensitivity and linearity analysis of ozone in East Asia: the effects of domestic emission and intercontinental transport.

UNLABELLED: In this study, ozone (O3) sensitivity and linearity over East Asia (EA) and seven urban areas are examined with an integrated air quality modeling system under two categories of scenarios: (1) The effects of domestic emission are estimated under local emission reduction scenarios, as anthropogenic NO(x) and volatile organic compounds (VOC) emissions are reduced by 20%, 50%, and 100%, respectively and independently; and (2) the influence of intercontinental transport is evaluated under Task Force on Hemispheric Transport of Air Pollution (TF HTAP) emission reduction scenarios, as anthropogenic NO(x) emission is reduced by 20% in Europe (EU), North America (NA), and South Asia (SA), respectively. Simulations are conducted for January and July 2001 to examine seasonal variation. Through the domestic O3 sensitivity investigation, we find O3 sensitivity varies dynamically depending on both time and location: North EA is VOC limited in January and NO(x) limited in July, except for the urban areas Beijing, Shanghai, Tokyo, and Seoul, which are VOC limited in both months; south EA is NO(x) limited in both January and July, except for the urban areas Taipei, which is VOC-limited in both months, and Pearl River Delta, which is VOC limited in January. Surface O3 change is found to be affected more by NO(x) than by VOC over EA in both January and July. We also find different O3 linearity characteristics among urban areas in EA: O3 at Beijing, Tokyo, and Seoul shows a strong negative linear response to NO(x) emission in January; O3 at Shanghai, Pearl River Delta, and Taipei shows a strong positive response to VOC emission in both January and July. Through the long-range transport investigation, monthly O3 changes over EA resulting from different source regions indicate the largest source contribution comes from NA (0.23 ppb), followed by SA (0.11 ppb) and EU (0.10 ppb). All of the three regions show higher impacts in January than in July. IMPLICATIONS: This study examine O3 sensitivities and linear response of NO(x) and VOC emission over EA and seven urban areas based on regional air quality modeling system MM5/CMAQ. We also quantify the intercontinental transport effect from EU, SA, and NA over EA. The result provide a theoretical basis for emission control strategy design in EA, and also reveal the O3 special nonlinearity features for further related studies that are applicable to other continents. The HTAP multimodel experiments need to examine the potential impacts on ground-level O3 of changes in meteorology and transport patterns expected as a result of the regional scale.