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.

Nitryl chloride (ClNO2) plays potentially important roles in atmospheric chemistry, but its abundance and effect are not fully understood due to the small number of ambient observations of ClNO2 to date. In late autumn 2013, ClNO2 was measured with a chemical ionization mass spectrometer (CIMS) at a mountain top (957 m above sea level) in Hong Kong. During 12 nights with continuous CIMS data, elevated mixing ratios of ClNO2 (>400 parts per trillion by volume) or its precursor N2O5 (>1000 pptv) were observed on six nights, with the highest ever reported ClNO2 (4.7 ppbv, 1 min average) and N2O5 (7.7 ppbv, 1 min average) in one case. Backward particle dispersion calculations driven by winds simulated with a mesoscale meteorological model show that the ClNO2/N2O5‐laden air at the high‐elevation site was due to transport of urban/industrial pollution north of the site. The highest ClNO2/N2O5 case was observed in a later period of the night and was characterized with extensively processed air and with the presence of nonoceanic chloride. A chemical box model with detailed chlorine chemistry was used to assess the possible impact of the ClNO2 in the well‐processed regional plume on next day ozone, as the air mass continued to downwind locations. The results show that the ClNO2 could enhance ozone by 5–16% at the ozone peak or 11–41% daytime ozone production in the following day. This study highlights varying importance of the ClNO2 chemistry in polluted environments and the need to consider this process in photochemical models for prediction of ground‐level ozone and haze.

We analyze a photochemical smog episode to understand the oxidative capacity and radical chemistry of the polluted atmosphere in Hong Kong and the Pearl River Delta (PRD) region. A photochemical box model based on the Master Chemical Mechanism (MCM v3.2) is constrained by an intensive set of field observations to elucidate the budgets of RO x (RO x = OH+HO 2 +RO 2 ) and NO 3 radicals. Highly abundant radical precursors (i.e. O 3 , HONO and carbonyls), nitrogen oxides (NO x ) and volatile organic compounds (VOCs) facilitate strong production and efficient recycling of RO x radicals. The OH reactivity is dominated by oxygenated VOCs (OVOCs), followed by aromatics, alkenes and alkanes. Photolysis of OVOCs (except for formaldehyde) is the dominant primary source of RO x with average daytime contributions of 34-47 %. HONO photolysis is the largest contributor to OH and the second-most significant source (19-22 %) of RO x . Other considerable RO x sources include O 3 photolysis (11-20 %), formaldehyde photolysis (10-16 %), and ozonolysis reactions of unsaturated VOCs (3.9-6.2 %). In one case when solar irradiation was attenuated, possibly by the high aerosol loadings, NO 3 became an important oxidant and the NO 3 -initiated VOC oxidation presented another significant RO x source (6.2 %) even during daytime. This study suggests the possible impacts of daytime NO 3 chemistry in the polluted atmospheres under con-ditions with the co-existence of abundant O 3 , NO 2 , VOCs and aerosols, and also provides new insights into the radical chemistry that essentially drives the formation of photochemical smog in the high-NO x environment of Hong Kong and the PRD region. recycling (e.g. OH→RO 2 →RO→HO 2 →OH) and produce O 3 and oxygenated VOCs (OVOCs) .

No abstract

No abstract