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 Analysis of Temperature and Humidity Conditions on Electrochemical Sensor Response in Ambient Air Quality Monitoring.

The increasing applications of low-cost air sensors promises more convenient and cost-effective systems for air monitoring in many places and under many conditions. However, the data quality from such systems has not been fully characterized and may not meet user expectations in research and regulatory uses, or for use in citizen science. In our study, electrochemical sensors (Alphasense B4 series) for carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO2), and oxidants (Ox) were evaluated under controlled laboratory conditions to identify the influencing factors and quantify their relation with sensor outputs. Based on the laboratory tests, we developed different correction methods to compensate for the impact of ambient conditions. Further, the sensors were assembled into a monitoring system and tested in ambient conditions in Hong Kong side-by-side with regulatory reference monitors, and data from these tests were used to evaluate the performance of the models, to refine them, and validate their applicability in variable ambient conditions in the field. The more comprehensive correction models demonstrated enhanced performance when compared with uncorrected data. One over-arching observation of this study is that the low-cost sensors may promise excellent sensitivity and performance, but it is essential for users to understand and account for several key factors that may strongly affect the nature of sensor data. In this paper, we also evaluated factors of multi-month stability, temperature, and humidity, and considered the interaction of oxidant gases NO2 and ozone on a newly introduced oxidant sensor.

The roles of scientific research and stakeholder engagement for evidence-based policy formulation on shipping emissions control in Hong Kong.

Shipping emissions control is critical to air quality management and improved public health for coastal port cities and regions with heavy marine traffic. However, Asian port cities have been slow in introducing regulations on marine fuels for two main reasons - firstly, due to a lack of information and therefore appreciation on the air quality and public health benefits that could be derived; and secondly, due to sensitivity as to whether there may be negative impacts on port competitiveness and trade opposition. Hong Kong, one of the top-ten international container ports in the world, has been proactive in reducing shipping emissions in the past decade. The Ocean Going Vessels Fuel at Berth regulation, enforced since July 2015 in Hong Kong, is the first marine fuel control regulation for ocean going vessels in Asia. This regulation has been adopted nationally by China for its coastal ports, followed by the establishment of domestic emission control areas in its coastal waters that will come into force in 2019. This paper describes the decade-long journey where scientific research led to evidence-based policy changes. New insights and understanding arising from the research enabled cross-sectoral engagement and dialogue among the key stakeholders in government, industry and civil society, which resulted in the political consensus needed for a change in policy and legislation. Similar evidence-based policy formulation, together with public-private sectors dialogue could be useful to other jurisdictions in pursuing a "win-win" path to improve environmental protection and public health through regulating shipping emissions. The same combination of science-to-engagement-to-policy approach could also become part of a knowledge-and-consensus-building process for other environmental policy areas as well.

Ambient Ozone Control in a Photochemically Active Region: Short-Term Despiking or Long-Term Attainment?

China has made significant progress decreasing the ambient concentrations of most air pollutants, but ozone (O3) is an exception. O3 mixing ratios during pollution episodes are far higher than the national standard in the Pearl River Delta (PRD), thus greater evidence-based control efforts are needed for O3 attainment. By using a validated O3 modeling system and the latest regional emission inventory, this study illustrates that control strategies for short-term O3 despiking and long-term attainment in the PRD may be contradictory. VOC-focused controls are more efficient for O3 despiking in urban and industrial areas, but significant NOx emission reductions and a subsequent transition to a NOx-limited regime are required for O3 attainment. By tracking O3 changes along the entire path toward long-term attainment, this study recommends to put a greater focus on NOx emission controls region-wide. Parallel VOC reductions around the Nansha port are necessary in summertime and should be extended to the urban and industrial areas in fall with a flexibility to be strengthened on days forecasted to have elevated O3. Contingent VOC-focused controls on top of regular NOx-focused controls would lay the groundwork for striking a balance between short-term despiking and long-term attainment of O3 concentrations in the PRD.

Development and Application of a Next Generation Air Sensor Network for the Hong Kong Marathon 2015 Air Quality Monitoring.

This study presents the development and evaluation of a next generation air monitoring system with both laboratory and field tests. A multi-parameter algorithm was used to correct for the impact of environmental conditions on the electrochemical sensors for carbon monoxide (CO) and nitrogen dioxide (NO2) pollutants. The field evaluation in an urban roadside environment in comparison to designated monitors showed good agreement with measurement error within 5% of the pollutant concentrations. Multiple sets of the developed system were then deployed in the Hong Kong Marathon 2015 forming a sensor-based network along the marathon route. Real-time air pollution concentration data were wirelessly transmitted and the Air Quality Health Index (AQHI) for the Green Marathon was calculated, which were broadcast to the public on an hourly basis. The route-specific sensor network showed somewhat different pollutant patterns than routine air monitoring, indicating the immediate impact of traffic control during the marathon on the roadside air quality. The study is one of the first applications of a next generation sensor network in international sport events, and it demonstrated the usefulness of the emerging sensor-based air monitoring technology in rapid network deployment to supplement existing air monitoring.

Short-term associations of cause-specific emergency hospitalizations and particulate matter chemical components in Hong Kong.

Despite an increasing number of recent studies, the overall epidemiologic evidence associating specific particulate matter chemical components with health outcomes has been mixed. The links between components and hospitalizations have rarely been examined in Asia. We estimated associations between exposures to 18 chemical components of particulate matter with aerodynamic diameter less than 10 µm (PM10) and daily emergency cardiorespiratory hospitalizations in Hong Kong, China, between 2001 and 2007. Carbonaceous particulate matter, sulfate, nitrate, and ammonium accounted for two-thirds of the PM10 mass. After adjustment for time-varying confounders, a 3.4-µg/m(3) increment in 2-day moving average of same-day and previous-day nitrate concentrations was associated with the largest increase of 1.32% (95% confidence interval: 0.73, 1.92) in cardiovascular hospitalizations; elevation in manganese level (0.02 µg/m(3)) was linked to a 0.91% (95% confidence interval: 0.19, 1.64) increase in respiratory hospitalizations. Upon further adjustment for gaseous copollutants, nitrate, sodium ion, chloride ion, magnesium, and nickel remained significantly associated with cardiovascular hospitalizations, whereas sodium ion, aluminum, and magnesium, components abundantly found in coarser PM10, were associated with respiratory hospitalizations. Most positive links were seen during the cold season. These findings lend support to the growing body of literature concerning the health associations of particulate matter composition and provide important insight into the differential health risks of components found in fine and coarse modes of PM10.

Ambient carbon monoxide and the risk of hospitalization due to chronic obstructive pulmonary disease.

Data from recent experimental and clinical studies have indicated that lower concentrations of inhaled carbon monoxide might have beneficial antiinflammatory effects. Inhaled carbon monoxide has the potential to be a therapeutic agent for chronic obstructive pulmonary diseases (COPD). However, population-based epidemiologic studies of environmentally relevant carbon monoxide exposure have generated mixed findings. We conducted a time-series study in Hong Kong to estimate the association of short-term exposure to ambient carbon monoxide with emergency hospitalizations for COPD. We collected daily emergency hospital admission data and air pollution data from January 2001 to December 2007. We used log-linear Poisson models to estimate the associations between daily hospital admissions for COPD and the average daily concentrations of carbon monoxide while controlling for the traffic-related co-pollutants nitrogen dioxide and particulate matter with an aerodynamic diameter less than 2.5 µm. Results showed that ambient carbon monoxide was negatively associated with the risk of hospitalizations for COPD. After adjustment for levels nitrogen dioxide or particulate matter with an aerodynamic diameter less than 2.5 µm, the negative associations of carbon monoxide with COPD hospitalizations became stronger. The risk estimates were similar for female and male subjects. In conclusion, short-term exposure to ambient carbon monoxide was associated with a decreased risk of hospitalization for COPD, which suggests that carbon monoxide exposure provides some acute protection of against exacerbation of COPD.

Increasing external effects negate local efforts to control ozone air pollution: a case study of Hong Kong and implications for other Chinese cities.

It is challenging to reduce ground-level ozone (O3) pollution at a given locale, due in part to the contributions of both local and distant sources. We present direct evidence that the increasing regional effects have negated local control efforts for O3 pollution in Hong Kong over the past decade, by analyzing the daily maximum 8 h average O3 and Ox (=O3+NO2) concentrations observed during the high O3 season (September-November) at Air Quality Monitoring Stations. The locally produced Ox showed a statistically significant decreasing trend over 2002-2013 in Hong Kong. Analysis by an observation-based model confirms this decline in in situ Ox production, which is attributable to a reduction in aromatic hydrocarbons. However, the regional background Ox transported into Hong Kong has increased more significantly during the same period, reflecting contributions from southern/eastern China. The combined result is a rise in O3 and a nondecrease in Ox. This study highlights the urgent need for close cross-boundary cooperation to mitigate the O3 problem in Hong Kong. China's air pollution control policy applies primarily to its large cities, with little attention to developing areas elsewhere. The experience of Hong Kong suggests that this control policy does not effectively address secondary pollution, and that a coordinated multiregional program is required.

Atmospheric peroxides in a polluted subtropical environment: seasonal variation, sources and sinks, and importance of heterogeneous processes.

Hydrogen peroxide (H2O2) and organic peroxides play an important role in atmospheric chemistry, but knowledge of their abundances, sources, and sinks from heterogeneous processes remains incomplete. Here we report the measurement results obtained in four seasons during 2011-2012 at a suburban site and a background site in Hong Kong. Organic peroxides were found to be more abundant than H2O2, which is in contrast to most previous observations. Model calculations with a multiphase chemical mechanism suggest important contributions from heterogeneous processes (primarily transition metal ion [TMI]-HOx reactions) to the H2O2 budget, accounting for about one-third and more than half of total production rate and loss rate, respectively. In comparison, they contribute much less to organic peroxides. The fast removal of H2O2 by these heterogeneous reactions explains the observed high organic peroxide fractions. Sensitivity analysis reveals that the role of heterogeneous processes depends on the abundance of soluble metals in aerosol, serving as a net H2O2 source at low metal concentrations, but as a net sink with high metal loading. The findings of this study suggest the need to consider the chemical processes in the aerosol aqueous phase when examining the chemical budget of gas-phase H2O2.

Concurrent measurements of nitrate at urban and suburban sites identify local nitrate formation as a driver for urban episodic PM2.5 pollution.

Nitrate (NO3-) is often among the leading components of urban particulate matter (PM) during PM pollution episodes. However, the factors controlling its prevalence remain inadequately understood. In this work, we analyzed concurrent hourly monitoring data of NO3- in PM2.5 at a pair of urban and suburban locations (28 km apart) in Hong Kong for a period of two months. The concentration gradient in PM2.5 NO3- was 3.0 ± 2.9 (urban) vs. 1.3 ± 0.9 µg m-3 (suburban) while that for its precursors nitrogen oxides (NOx) was 38.1 vs 4.1 ppb. NO3- accounted for 45 % of the difference in PM2.5 between the sites. Both sites were characterized to have more available NH3 than HNO3. Urban nitrate episodes, defined as periods of urban-suburban NO3- difference exceeding 2 µg m-3, constituted 21 % of the total measurement hours, with an hourly NO3- average gradient of 4.2 and a peak value of 23.6 µg m-3. Our comparative analysis, together with 3-D air quality model simulations, indicates that the high NOx levels largely explain the excessive NO3- concentrations in our urban site, with the gas phase HNO3 formation reaction contributing significantly during the daytime and the N2O5 hydrolysis pathway playing a prominent role during nighttime. This study presents a first quantitative analysis that unambiguously shows local formation of NO3- in urban environments as a driver for urban episodic PM2.5 pollution, suggesting effective benefits of lowering urban NOx.

Risk tradeoffs between nitrogen dioxide and ozone pollution during the COVID-19 lockdowns in the Greater Bay area of China.

Photochemical regime for ozone (O3) formation is complicated in the sense that reducing emission of nitrogen oxides (NOx) may increase O3 concentration. The lockdown due to COVID-19 pandemic affords a unique opportunity to use real observations to explore the O3 formation regime and the effectiveness of NOx emission control strategies. In this study, observations from ground networks during the lockdowns were used to assess spatial disparity of the Ratio of Ozone Formation (ROF) for nitrogen dioxide (NO2) reduction in the Greater Bay Area (GBA) of China. The health risk model from Air Quality Health Index (AQHI) system in Hong Kong was adopted to evaluate the risk tradeoffs between NO2 and O3. Results show that the levels of O3 increase and NO2 reduction were comparable due to high ROF values in urban areas of central GBA. The ozone reactivity to NO2 reduction gradually declined outwards from central GBA. Despite the O3 increases, the NOx emission controls reduced the Integrated Health Risk (IHR) of NO2 and O3 in most regions of the GBA. When risk coefficients from the AQHI in Canada or the global review were adopted in the risk analyses, the results are extremely encouraging because the controls of NOx emission reduced the IHR of NO2 and O3 almost everywhere in the GBA. Our results underscore the importance of using a risk-based method to assess the effectiveness of emission control measures and the overall health benefit from NOx emission controls in the GBA.

Combined effects of increased O3 and reduced NO2 concentrations on short-term air pollution health risks in Hong Kong.

The reduction of NOx emissions in a VOC-limited region can lead to an increase of the local O3 concentration. An evaluation of the net health effects of such pollutant changes is therefore important to ascertain whether the emission control measures effectively improve the overall protection of public health. In this study, we use a short-term health risk (added health risk or AR) model developed for the multi-pollutant air quality health index (AQHI) in Hong Kong to examine the overall health impacts of these pollutant changes. We first investigate AR changes associated with NO2 and O3 changes, followed by those associated with changes in all four AQHI pollutants (NO2, O3, SO2, and particulate matter (PM)). Our results show that for the combined health effects of NO2 and O3 changes, there is a significant reduction in AR in urban areas with dense traffic, but no statistically significant changes in other less urbanized areas. The increase in estimated AR for higher O3 concentrations is offset by a decrease in the estimated AR for lower NO2 concentrations. In areas with dense traffic, the reduction in AR as a result of decreased NO2 is substantially larger than the increase in AR associated with increased O3. When additionally accounting for the change in ambient SO2 and PM, we found a statistically significant reduction in total AR everywhere in Hong Kong. Our results show that the emission control measures resulting in NO2, SO2, and PM reductions over the past decade have effectively reduced the AR over Hong Kong, even though these control measures may have partially contributed to an increase in O3 concentrations. Hence, efforts to reduce NOx, SO2, and PM should be continued.

Air quality and synergistic health effects of ozone and nitrogen oxides in response to China's integrated air quality control policies during 2015-2019.

O3 pollution had been worsening in mainland China in the past decade, posing significant human health challenges. The NOx control would trigger increasing O3 concentrations in response to a series of released China emission reduction policies. This study used sensitivity analysis methodology to explore the effectiveness of integrated sectoral emission control policies that have been expanded throughout China. Air quality and synergistic health effects of O3 and NO2 were investigated to obtain an in-depth understanding of the O3 control, especially under a VOC-limited regime. The findings demonstrated that although the NOx-titration effect triggered an increase in O3, the combined health effects of two pollutants tended to improve in most regions of China under a VOC-limited regime. The region-based annual average NO2 concentrations exhibited a larger reduction in Hong Kong (HK) than in the Pearl River Delta Economic Zone (PRD EZ). The short-term measures led to substantial health benefits for Shenzhen and HK. The sectoral emission controls demonstrated a considerable health improvement for the major PRD EZ cities. Joint national control efforts confined the domain-wide health risks below the safety line in China. National cooperative efforts in China could avoid more than 1.5-2% of the emergency hospital admissions for cardiovascular and respiratory diseases attributed to NO2 and O3 exposure. The observed O3 increases due to the NOx-titration effect for calculating the integral health effects of emission control on concentration reduction called for simultaneously strengthened controls on both NOx and VOC in areas subject to a VOC-limited regime.

Role of export industries on ozone pollution and its precursors in China.

This study seeks to estimate how global supply chain relocates emissions of tropospheric ozone precursors and its impacts in shaping ozone formation. Here we show that goods produced in China for foreign markets lead to an increase of domestic non-methane volatile organic compounds (NMVOCs) emissions by 3.5 million tons in 2013; about 13% of the national total or, equivalent to half of emissions from European Union. Production for export increases concentration of NMVOCs (including some carcinogenic species) and peak ozone levels by 20-30% and 6-15% respectively, in the coastal areas. It contributes to an estimated 16,889 (3,839-30,663, 95% CI) premature deaths annually combining the effects of NMVOCs and ozone, but could be reduced by nearly 40% by closing the technology gap between China and EU. Export demand also alters the emission ratios between NMVOCs and nitrogen oxides and hence the ozone chemistry in the east and south coast.

Estimating contributions of vehicular emissions to PM2.5 in a roadside environment: A multiple approach study.

Vehicular emissions (VE) are among the major sources of airborne fine particulate matter (PM2.5) in urban atmospheres, which adversely impact the environment and public health. Receptor models are widely used for estimating PM2.5 source contributions from VE (PMvehicle), but often give inconsistent results due to different modelling principles and assumptions. During December 2015-May 2017, we collected nine-months of hourly organic carbon (OC) and elemental carbon (EC) data, as well as 24-h PM2.5 speciation data including major species and organic tracers on select days from an ad hoc roadside site in Hong Kong. The weekday vs. holiday and diurnal variations of EC tracked closely with those of traffic flow volume, indicating EC as a reliable tracer for PMvehicle in this area. We applied multiple approaches to estimate the PMvehicle, including the EC-tracer method with the hourly OC-EC data, and chemical mass balance (CMB) and positive matrix factorization (PMF) analyses with the filter-based speciation data. Considering source profile variability, CMB gave the lowest PMvehicle estimate among the three approaches, possibly due to the degradation of organic markers (i.e., hopanes). The PMvehicle derived from the EC-tracer method and PMF were comparable, accounting for ~12% (3.4-4.0 µg/m3) of PM2.5 averaged across 20 samples in both approaches, but a larger sample size is needed for a more robust PMF solution. The monthly PMvehicle derived from the EC-tracer method was in the range of 3.2-6.6 µg/m3. The continuous measurement reveals a decreasing trend in PMvehicle throughout the entire sampling period, indicating the effectiveness of a recent vehicle control measures implemented by the Government in phasing out pre-Euro IV diesel commercial vehicles. This work implies that hourly OC-EC monitoring at strategically located spots is an effective way of monitoring vehicle control measures. It provides reasonable estimate of PMvehicle through comparing with other more sophisticated receptor models.

Reconciling discrepancies in the source characterization of VOCs between emission inventories and receptor modeling.

Emission inventory (EI) and receptor model (RM) are two of the three source apportionment (SA) methods recommended by Ministry of Environment of China and used widely to provide independent views on emission source identifications. How to interpret the mixed results they provide, however, were less studied. In this study, a cross-validation study was conducted in one of China's fast-developing and highly populated city cluster- the Pearl River Delta (PRD) region. By utilizing a highly resolved speciated regional EI and a region-wide gridded volatile organic compounds (VOCs) speciation measurement campaign, we elucidated underlying factors for discrepancies between EI and RM and proposed ways for their interpretations with the aim to achieve a scientifically plausible source identification. Results showed that numbers of species, temporal and spatial resolutions used for comparison, photochemical loss of reactive species, potential missing sources in EI and tracers used in RM were important factors contributed to the discrepancies. Ensuring the consensus of species used in EIs and RMs, utilizing a larger spatial coverage and longer time span, addressing the impacts of photochemical losses, and supplementing emissions from missing sources could help reconcile the discrepancies in VOC source characterizations acquired using both approaches. By leveraging the advantages and circumventing the disadvantages in both methods, the EI and RM could play synergistic roles to obtain robust SAs to improve air quality management practices.

Oxidizing capacity of the rural atmosphere in Hong Kong, Southern China.

Atmospheric oxidizing capacity (AOC), dominated by the hydroxyl radical (OH), is an important index of the self-cleaning capacity of atmosphere and plays a vital role in the tropospheric chemistry. To better understand the key processes governing the chemistry of rural atmosphere of southern China, we analyzed the oxidation capacity and radical chemistry at a regional background site in Hong Kong from 23 August to 22 December 2012, which covered the summer, autumn and winter seasons. A chemical box model built on the latest Master Chemical Mechanism (v3.3) was used to elucidate the OH reactivity and sources of ROX radicals (ROX=OH+HO2+RO2). The AOC showed a clear seasonal pattern with stronger intensity in late summer compared to autumn and winter. Reactions with NO2 (30%) and oxygenated volatile organic compounds (OVOCs) (31%) together dominated the OH loss in summer, while reactions with CO (38% in autumn and 39% in winter) and OVOCs (34% in autumn and 25% in winter) made larger contributions in autumn and winter. Photolysis of O3 (36%-47%) presented the major ROX source during all three seasons. The second largest ROx source was HONO photolysis (25%) in summer compared to HCHO photolysis in autumn (20%) and winter (21%). Besides, photolysis of other OVOCs was another important primary source of ROx radicals with average contributions of 14%, 13% and 20% for the summer, autumn and winter cases, respectively. Overall, the present study evaluates the oxidizing capacity of the rural atmosphere of South China and elucidates the varying characteristics of photochemical processes in different air masses.

Seasonal variations of C1-C4 alkyl nitrates at a coastal site in Hong Kong: Influence of photochemical formation and oceanic emissions.

Five C1-C4 alkyl nitrates (RONO2) were measured at a coastal site in Hong Kong in four selected months of 2011 and 2012. The total mixing ratios of C1-C4 RONO2 (Σ5RONO2) ranged from 15.4 to 143.7 pptv with an average of 65.9 ± 33.0 pptv. C3-C4 RONO2 (2-butyl nitrate and 2-propyl nitrate) were the most abundant RONO2 during the entire sampling period. The mixing ratios of C3-C4 RONO2 were higher in winter than those in summer, while the ones of methyl nitrate (MeONO2) were higher in summer than those in winter. Source analysis suggests that C2-C4 RONO2 were mainly derived from photochemical formation along with biomass burning (58.3-71.6%), while ocean was a major contributor to MeONO2 (53.8%) during the whole sampling period. The photochemical evolution of C2-C4 RONO2 was investigated, and found to be dominantly produced by the parent hydrocarbon oxidation. The notable enrichment of MeONO2 over C3-C4 RONO2 was observed in a summer episode when the air masses originating from the South China Sea (SCS) and MeONO2 was dominantly derived from oceanic emissions. In order to improve the accuracy of ozone (O3) prediction in coastal environment, the relative contribution of RONO2 from oceanic emissions versus photochemical formation and their coupling effects on O3 production should be taken into account in future studies.

Applications of low-cost sensing technologies for air quality monitoring and exposure assessment: How far have they gone?

Over the past decade, a range of sensor technologies became available on the market, enabling a revolutionary shift in air pollution monitoring and assessment. With their cost of up to three orders of magnitude lower than standard/reference instruments, many avenues for applications have opened up. In particular, broader participation in air quality discussion and utilisation of information on air pollution by communities has become possible. However, many questions have been also asked about the actual benefits of these technologies. To address this issue, we conducted a comprehensive literature search including both the scientific and grey literature. We focused upon two questions: (1) Are these technologies fit for the various purposes envisaged? and (2) How far have these technologies and their applications progressed to provide answers and solutions? Regarding the former, we concluded that there is no clear answer to the question, due to a lack of: sensor/monitor manufacturers' quantitative specifications of performance, consensus regarding recommended end-use and associated minimal performance targets of these technologies, and the ability of the prospective users to formulate the requirements for their applications, or conditions of the intended use. Numerous studies have assessed and reported sensor/monitor performance under a range of specific conditions, and in many cases the performance was concluded to be satisfactory. The specific use cases for sensors/monitors included outdoor in a stationary mode, outdoor in a mobile mode, indoor environments and personal monitoring. Under certain conditions of application, project goals, and monitoring environments, some sensors/monitors were fit for a specific purpose. Based on analysis of 17 large projects, which reached applied outcome stage, and typically conducted by consortia of organizations, we observed that a sizable fraction of them (~ 30%) were commercial and/or crowd-funded. This fact by itself signals a paradigm change in air quality monitoring, which previously had been primarily implemented by government organizations. An additional paradigm-shift indicator is the growing use of machine learning or other advanced data processing approaches to improve sensor/monitor agreement with reference monitors. There is still some way to go in enhancing application of the technologies for source apportionment, which is of particular necessity and urgency in developing countries. Also, there has been somewhat less progress in wide-scale monitoring of personal exposures. However, it can be argued that with a significant future expansion of monitoring networks, including indoor environments, there may be less need for wearable or portable sensors/monitors to assess personal exposure. Traditional personal monitoring would still be valuable where spatial variability of pollutants of interest is at a finer resolution than the monitoring network can resolve.

Comparison of PM2.5 carbon measurement methods in Hong Kong, China.

Samples from Hong Kong, China, were analyzed for organic carbon (OC), elemental carbon (EC), and total carbon (TC) by three thermal protocols (low-temperature IMPROVE and high-temperature STN and NIOSH) and two optical monitoring methods: reflectance and transmittance. Good agreement (+/-10%) for TC among the three protocols was observed for sample loadings of 1-55 microg m(-3). The two protocols using a reflectance pyrolysis correction showed best agreement for EC, with <20% differences found for approximately 80% of the samples. Hong Kong has a large diesel fleet, and for some heavily loaded samples the light transmittance was too low for quantitative detection, resulting in large uncertainties in the OC/EC split based on transmittance. Hong Kong experienced OC levels similar to those at US sites, but has much higher EC concentrations. OC/EC ratios range from 2 to 5 at two US sites and from 0.2 to 1.2 at three Hong Kong sites.