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 (NO 2 ), carbon monoxide (CO), sulfur dioxide (SO 2 ), and volatile organic compounds (VOCs) were observed in many countries during the lockdowns. However, the levels of ozone (O 3 ), 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 O 3 can enhance the formation of secondary PM 2.5 , especially secondary organic aerosols, through the atmospheric oxidation of VOCs. Given that the gaseous precursors of O 3 (VOCs and NO x ) are also involved in the formation of secondary PM 2.5, an integrated control strategy should focus on reducing the emission of the common precursors for the co-mitigation of PM 2.5 and O 3 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 provide 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.

Abstract. São Paulo in Brazil has relatively relaxed regulations for ambient air pollution standards and often experiences high air pollution levels due to emissions of particulate pollutants from local sources and long-range transport of air masses impacted by biomass burning. In order to evaluate the sources of particulate air pollution and related health risks, a year-round sampling was done at the University of São Paulo campus (20 m a.g.l.), a green area near an important expressway. The sampling was performed for PM 2.5 (≤ 2.5 µm) and PM 10 (≤ 10 µm) in 2014 through intensive (everyday sampling in wintertime) and extensive campaigns (once a week for the whole year) with 24 h of sampling. This year was characterized by having lower average precipitation compared to meteorological data, and high-pollution episodes were observed all year round, with a significant increase in pollution level in the intensive campaign, which was performed during wintertime. Different chemical constituents, such as carbonaceous species, polycyclic aromatic hydrocarbons (PAHs) and derivatives, water-soluble ions, and biomass burning tracers were identified in order to evaluate health risks and to apportion sources. The species such as PAHs, inorganic and organic ions, and monosaccharides were determined using chromatographic techniques and carbonaceous species using thermal-optical analysis. Trace elements were determined using inductively coupled plasma mass spectrometry. The risks associated with particulate matter exposure based on PAH concentrations were also assessed, along with indexes such as the benzo[a]pyrene equivalent (BaPE) and lung cancer risk (LCR). High BaPE and LCR were observed in most of the samples, rising to critical values in the wintertime. Also, biomass burning tracers and PAHs were higher in this season, while secondarily formed ions presented low variation throughout the year. Meanwhile, vehicular tracer species were also higher in the intensive campaign, suggesting the influence of lower dispersion conditions in that period. Source apportionment was performed using positive matrix factorization (PMF), which indicated five different factors: road dust, industrial emissions, vehicuPublished by Copernicus Publications on behalf of the European Geosciences Union. 11944 G. M. Pereira et al.: Particulate pollutants in the Brazilian city of São Paulo lar exhaust, biomass burning and secondary processes. The results highlighted the contribution of vehicular emissions and the significant input from biomass combustion in wintertime, suggesting that most of the particulate matter is due to local sources, in addition to the influence of pre-harvest sugarcane burning.

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Globally consistent measurements of airborne metal concentrations in fine particulate matter (PM2.5) are important for understanding potential health impacts, prioritizing air pollution mitigation strategies, and enabling global chemical transport model development. PM2.5 filter samples (N ~ 800 from 19 locations) collected from a globally distributed surface particulate matter sampling network (SPARTAN) between January 2013 and April 2019 were analyzed for particulate mass and trace metals content. Metal concentrations exhibited pronounced spatial variation, primarily driven by anthropogenic activities. PM2.5 levels of lead, arsenic, chromium, and zinc were significantly enriched at some locations by factors of 100–3000 compared to crustal concentrations. Levels of metals in PM2.5 and PM10 exceeded health guidelines at multiple sites. For example, Dhaka and Kanpur sites exceeded the US National Ambient Air 3-month Quality Standard for lead (150 ng m−3). Kanpur, Hanoi, Beijing and Dhaka sites had annual mean arsenic concentrations that approached or exceeded the World Health Organization’s risk level for arsenic (6.6 ng m−3). The high concentrations of several potentially harmful metals in densely populated cites worldwide motivates expanded measurements and analyses.

Landscape fires in Indonesia during the 2015 resulted in large-scale emissions of airborne particulate matter (PM) that degraded ambient air quality of several countries in Southeast Asia (SEA) including Singapore. During this transboundary haze episode, the general public was advised to remain indoors as much as possible in order to mitigate their exposure to high concentrations of PM in the outdoor environment. To understand the quantitative relationship between outdoor and indoor air quality, we measured PM 2.5 as well as the size-fractionated PM (coarse, accumulation and quasi-ultrafine (q-UF) particles) simultaneously inside and outside a naturally ventilated apartment and studied the potential health risk associated with exposure to PM of different sizes under the three levels of smoke haze (light, moderate and severe). PM mass concentrations increased with a decrease in particle size, and the q-UF particles (diameter ≤ 250 nm) were observed to be as high as 80 to 85 µg m -3 both indoors and outdoors. Estimation of PM deposition patterns along the human respiratory tract revealed that q-UF particles were mainly deposited in the deeper alveolar region, thereby posing severe health threats. Potential human health risk assessment results based on bioavailable concentrations of toxic elements in PM raised further concerns about health impacts of q-UF particles deposited in the alveolar region. Moreover, uncertainty analysis of exposure parameters used in potential carcinogenic health risk assessment model indicated much higher exceedance of potential health risk than the threshold limit for 95 th percentile values of the health risk (11.5 times higher for PM 2.5 ) during severe-haze episodes. The potential health risk estimated in this study indicates the need to conduct further studies focused upon mitigation of human exposure to achieve health benefits during haze episodes.

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