Assessment and mitigation of toddlers' personal exposure to black carbon before and during the COVID-19 pandemic: A case study in Singapore.

Black carbon (BC), an important indicator of traffic-related air pollution (TRAP) in urban environments, is receiving increased attention because of its adverse health effects. Personal exposure (PE) of adults to BC has been widely studied, but little is known about the exposure of young children (toddlers) to BC in cities. We carried out a pilot study to investigate the integrated daily PE of toddlers to BC in a city-state with a high population density (Singapore). We studied the impact of urban traffic on the PE of toddlers to BC by comparing and contrasting on-road traffic flow (i.e., volume and composition) in Singapore in 2019 (before the COVID-19 pandemic) and in 2020 (during the COVID-19 pandemic). Our observations indicate that the daily BC exposure levels and inhaled doses increased by about 25% in 2020 (2.9 ± 0.3 µg m-3 and 35.5 µg day-1) compared to that in 2019 (2.3 ± 0.4 µg m-3 and 28.5 µg day-1 for exposure concentration and inhaled dose, respectively). The increased BC levels were associated with the increased traffic volume on both weekdays and weekends in 2020 compared to the same time period in 2019. Specifically, we observed an increase in the number of trucks as well as cars/taxis and motorcycles (private transport) and a decline in the number of buses (public transport) in 2020. The implementation of lockdown measures in 2020 resulted in significant changes in the time, place and duration of PE of toddlers to BC. The recorded daily time-activity patterns indicated that toddlers spent almost all the time in indoor environments during the measurement period in 2020. When we compared different ventilation options (natural ventilation (NV), air conditioning (AC), and portable air cleaner (PAC)) for mitigation of PE to BC in the home environment, we found a significant decrease (>30%) in daily BC exposure levels while using the PAC compared to the NV scenario. Our case study shows that the PE of toddlers to BC is of health concern in indoor environments in 2020 because of the migration of the increased TRAP into naturally ventilated residential homes and more time spent indoors than outdoors. Since toddlers' immune system is weak, technological intervention is necessary to protect their health against inhalation exposure to air pollutants.

Air quality changes in cities during the COVID-19 lockdown: A critical review.

In response to the rapid spread of coronavirus disease-2019 (COVID-19) within and across countries and the need to protect public health, governments worldwide introduced unprecedented measures such as restricted road and air travel and reduced human mobility in 2020. The curtailment of personal travel and economic activity provided a unique opportunity for researchers to assess the interplay between anthropogenic emissions of primary air pollutants, their physical transport, chemical transformation, ultimate fate and potential health impacts. In general, reductions in the atmospheric levels of outdoor air pollutants such as particulate matter (PM), nitrogen dioxide (NO2), carbon monoxide (CO), sulfur dioxide (SO2), and volatile organic compounds (VOCs) were observed in many countries during the lockdowns. However, the levels of ozone (O3), a secondary air pollutant linked to asthma and respiratory ailments, and secondary PM were frequently reported to remain unchanged or even increase. An increase in O3 can enhance the formation of secondary PM2.5, especially secondary organic aerosols, through the atmospheric oxidation of VOCs. Given that the gaseous precursors of O3 (VOCs and NOx) are also involved in the formation of secondary PM2.5, an integrated control strategy should focus on reducing the emission of the common precursors for the co-mitigation of PM2.5 and O3 with an emphasis on their complex photochemical interactions. Compared to outdoor air quality, comprehensive investigations of indoor air quality (IAQ) are relatively sparse. People spend more than 80% of their time indoors with exposure to air pollutants of both outdoor and indoor origins. Consequently, an integrated assessment of exposure to air pollutants in both outdoor and indoor microenvironments is needed for effective urban air quality management and for mitigation of health risk. To provide further insights into air quality, we do a critical review of scientific articles, published from January 2020 to December 2020 across the globe. Finally, we discuss policy implications of our review in the context of global air quality improvement.

Light Absorbing Properties of Primary and Secondary Brown Carbon in a Tropical Urban Environment.

Brown carbon (BrC) has significant climatic impact, but its emission sources and formation processes remain under-represented in climate models. However, there are only limited field studies to quantify the light absorption properties of specific types of primary and secondary organic aerosols (POAs and SOAs) in different environments. This work investigates the light absorption properties of the major OA components in Singapore, a well-developed city in the tropical region, where air quality can be influenced by multiple local urban sources and regional biomass burning events. The source-specific mass absorption cross-section (MAC) and wavelength dependence of different BrC components were quantified based on highly time-resolved aerosol chemical composition and absorption measurements. In particular, the combustion-related emission sources were the primary contributors to BrC light absorption and they were moderately absorbing. The SOA materials, which were freshly formed under atmospheric conditions with industrial influences, were also moderately light absorptive. The aged SOA components that were composed of aged regional emissions, including biomass burning and coal combustion emissions from nearby regions, were weakly light absorbing, highlighting the possibility of photobleaching of BrC during their atmospheric aging and dispersion. Lastly, our estimations illustrate that typical urban POAs and SOAs can contribute up to approximately 36-58% of the BrC absorption, even in some urban locations that are influenced by biomass burning emissions.

Biomass burning-derived airborne particulate matter in Southeast Asia: A critical review.

Smoke haze episodes, resulting from uncontrolled biomass burning (BB) including forest and peat fires, continue to occur in Southeast Asia (SEA), affecting air quality, atmospheric visibility, climate, ecosystems, hydrologic cycle and human health. The pollutant of major concern in smoke haze is airborne particulate matter (PM). A number of fundamental laboratory, field and modeling studies have been conducted in SEA from 2010 to 2020 to investigate potential environmental and health impacts of BB-induced PM. The goal of this review is to bring together the most recent developments in our understanding of various aspects of BB-derived PM based on 127 research articles published from 2010 to 2020, which have not been conveyed in previous reviews. Specifically, this paper discusses the physical, chemical, toxicological and radiative properties of BB-derived PM. It also provides insights into the environmental and health impacts of BB-derived PM, summarizes the approaches taken to do the source apportionment of PM during BB events and discusses the mitigation of exposure to BB-derived PM. Suggestions for future research priorities are outlined. Policies needed to prevent future BB events in the SEA region are highlighted.

Mitigation of indoor human exposure to airborne particles of outdoor origin in an urban environment during haze and non-haze periods.

During the 2019 smoke haze episode in Singapore, elevated levels of fine particulate matter (PM2.5) were observed, deteriorating both ambient and indoor air quality (IAQ). We investigated the mitigation of indoor human exposure to PM2.5 of outdoor origin under diverse exposure scenarios with and without filtration of PM2.5 during both hazy and non-hazy days. The key objective of our study was to make a comparative evaluation of the effectiveness of portable air cleaners (PACs) and air conditioning (AC) systems equipped with particle filters in improving IAQ and to assess related long-term carcinogenic and non-carcinogenic health risks. We conducted real-time measurements of PM2.5, black carbon mass concentrations and particle number concentrations in both indoor and outdoor areas, quantified the relative concentrations of the water-soluble fraction of toxic trace elements in PM2.5 for health risk assessment, and estimated the levels of thermal comfort. In addition, we calculated the total estimated cost of indoor air pollution control. Our findings suggest that indoor air cleaners are more effective at mitigating human exposure to airborne particles and reducing health risk with less consumption of electricity and better cost-effectiveness compared to AC. This information would be beneficial for public health interventions during major air pollution events.