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Residential solid fuel combustion significantly impacts air quality and human health. Pelletized biomass fuels are promoted as a cleaner alternative, particularly for those who cannot afford the high costs of gas/electricity, but their emission characteristics and potential effects remain poorly understood. The present laboratory-based study evaluated pollution emissions from pelletized biomass burning, including CH 4 (methane), NMHC (nonmethane hydrocarbon compounds), CO, SO 2 , NO x , PM 2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm), OC (organic carbon), EC (element carbon), PAHs (polycyclic aromatic hydrocarbons), EPFRs (environmentally persistent free radicals), and OP (oxidative potential) of PM 2.5 , and compared with those from raw biomass burning. For most targets, except for SO 2 and NO x , the mass-based emission factors for pelletized biomass were 62-96% lower than those for raw biomass. SO 2 and NO x levels were negatively correlated with other air pollutants ( p < 0.05). Based on real-world daily consumption data, this study estimated that households using pelletized biomass could achieve significant reductions (51-95%) in emissions of CH 4 , NMHC, CO, PM 2.5 , OC, EC, PAHs, and EPFRs compared to those using raw biomass, while the differences in emissions of NO x and SO 2 were statistically insignificant. The reduction rate of benzo(a)pyrene-equivalent emissions was only 16%, much lower than the reduction in the total PAH mass (78%). This is primarily attributed to the more PAHs with high toxic potentials, such as dibenz(a,h)anthracene, in the pelletized biomass emissions. Consequently, impacts on human health associated with PAHs might be overestimated if only the mass of total PAHs was counted. The OP of particles from the pellet burning was also significantly lower than that from raw biomass by 96%. The results suggested that pelletized biomass could be a transitional substitution option that can significantly improve air quality and mitigate human exposure.

Household air pollution associated with solid fuel use is a long-standing public concern. The global population mainly using solid fuels for cooking remains large. Besides cooking, large amounts of coal and biomass fuels are burned for space heating during cold seasons in many regions. In this study, a wintertime multiple-region field campaign was carried out in north China to evaluate indoor PM 2.5 variations. With hourly resolved data from ∼1600 households, key influencing factors of indoor PM 2.5 were identified from a machine learning approach, and a random forest regression (RFR) model was further developed to quantitatively assess the impacts of household energy transition on indoor PM 2.5 . The indoor PM 2.5 concentration averaged at 120 μg/m 3 but ranged from 16 to ∼400 μg/m 3 . Indoor PM 2.5 was ∼60% lower in families using clean heating approaches compared to those burning traditional coal or biomass fuels. The RFR model had a good performance ( R 2 = 0.85), and the interpretation was consistent with the field observation. A transition to clean coals or biomass pellets can reduce indoor PM 2.5 by 20%, and further switching to clean modern energies would reduce it an additional 30%, suggesting many significant benefits in promoting clean transitions in household heating activities.

Indoor PM 2.5 , particulate matter no more than 2.5 μm in aerodynamic equivalent diameter, has very high spatiotemporal variabilities; and exploring the key factors influencing the variabilities is critical for purifying air and protecting human health. Here, we conducted a longer-term field monitoring campaign using low-cost sensors and evaluated inter- and intra-household PM 2.5 variations in rural areas where energy or stove stacking is common. Household PM 2.5 varied largely across different homes but also within households. Using generalized linear models and dominance analysis, we estimated that outdoor PM 2.5 explained 19% of the intrahousehold variation in indoor daily PM 2.5 , whereas factors like the outdoor temperature and indoor-outdoor temperature difference that was associated with energy use directly or indirectly, explained 26% of the temporal variation. Inter-household variation was lower than intrahousehold variation. The inter-household variation was strongly associated with distinct internal sources, with energy-use-associated factors explaining 35% of the variation. The statistical source apportionment model estimated that solid fuel burning for heating contributed an average of 31%-55% of PM 2.5 annually, whereas the contribution of sources originating from the outdoors was ≤10%. By replacing raw biomass or coal with biomass pellets in gasifier burners for heating, indoor PM 2.5 could be significantly reduced and indoor temperature substantially increased, providing thermal comforts in addition to improved air quality.

Nitroaromatic compounds (NACs) not only are strongly absorbing chromophores but also adversely affect human health. NACs can be emitted from incomplete combustions and can derive secondarily through photochemical reactions. Here, emission experiments were conducted for 31 fuel-stove combinations to elucidate variations in, and influencing factors of, NAC emission factors (EF ∑NACs ) and to explore potential tracers for different combustion sources. EF ∑NACs varied by 2 orders of magnitude among different combinations. Differences in fuel type contributed more than the stove difference to the observed variation. EF ∑NACs for biomass pellets was approximately 66% lower than that for raw biomass, although the bulk organic and brown carbon EFs were 95% lower. 2-Nitro-1-naphthol was the most abundant individual compound, followed by 4-nitrocatechol, while acid compounds (salicylic acid and benzoic acid) were low in abundance (<1%). Substantially different profiles were observed between coal and biomass burning emissions. Biomass burning had more single-ring-based phenolic compounds with more 4-nitrocatechol, while in coal combustion, more two-ring products were produced. This study demonstrated much lower ratios of 2-nitro-1-naphthol/4-nitrocatechol for biomass in both traditional (2.0 ± 3.5) and improved stoves (3.0 ± 2.1) than for coals (15 ± 6). Coal and biomass burning differed in not only EF ∑NACs but also compound profile, consequently leading to distinct health and climate impacts; moreover, the ratio of 2-nitro-1-naphthol/4-nitrocatechol may be used in source apportionment of NACs.

Ultrafine particles (UFPs) significantly affect human health and climate. UFPs can be produced largely from the incomplete burning of solid fuels in stoves; however, indoor UFPs are less studied compared to outdoor UFPs, especially in coal-combustion homes. In this study, indoor and outdoor UFP concentrations were measured simultaneously by using a portable instrument, and internal and outdoor source contributions to indoor UFPs were estimated using a statistical approach based on highly temporally resolved data. The total concentrations of indoor UFPs in a rural household with the presence of coal burning were as high as 1.64 × 10 5 (1.32 × 10 5 -2.09 × 10 5 as interquartile range) #/cm 3 , which was nearly one order of magnitude higher than that of outdoor UFPs. Indoor UFPs were unimodal, with the greatest abundance of particles in the size range of 31.6-100 nm. The indoor-to-outdoor ratio of UFPs in a rural household was about 6.4 (2.7-16.0), while it was 0.89 (0.88-0.91) in a home without strong internal sources. A dynamic process illustrated that the particle number concentration increased by ~5 times during the coal ignition period. Indoor coal combustion made up to over 80% of indoor UFPs, while in an urban home without coal combustion sources indoors, the outdoor sources may contribute to nearly 90% of indoor UFPs. A high number concentration and a greater number of finer particles in homes with the presence of coal combustion indicated serious health hazards associated with UFP exposure and the necessity for future controls on indoor UFPs.

A large population does not have access to modern household energy and relies on solid fuels such as coal and biomass fuels. Burning of these solid fuels in low-efficiency home stoves produces high amounts of multiple air pollutants, causing severe air pollution and adverse health outcomes. In evaluating impacts on human health and climate, it is critical to understand the formation and emission processes of air pollutants from these combustion sources. Air pollutant emission factors (EFs) from indoor solid fuel combustion usually highly vary among different testing protocols, fuel-stove systems, sampling and analysis instruments, and environmental conditions. In this critical review, we focus on the latest developments in pollutant emission factor studies, with emphases on the difference between lab and field studies, fugitive emission quantification, and factors that contribute to variabilities in EFs. Field studies are expected to provide more realistic EFs for emission inventories since lab studies typically do not simulate real-world burning conditions well. However, the latter has considerable advantages in evaluating formation mechanisms and variational influencing factors in observed pollutant EFs. One main challenge in field emission measurement is the suitable emission sampling system. Reasons for the field and lab differences have yet to be fully elucidated, and operator behavior can have a significant impact on such differences. Fuel properties and stove designs affect emissions, and the variations are complexly affected by several factors. Stove classification is a challenge in the comparison of EF results from different studies. Lab- and field-based methods for quantifying fugitive emissions, as an important contributor to indoor air pollution, have been developed, and priority work is to develop a database covering different fuel-stove combinations. Studies on the dynamics of the combustion process and evolution of air pollutant formation and emissions are scarce, and these factors should be an important aspect of future work., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Indoor air quality is critically important to the human as people spend most time indoors. Indoor PM 2.5 is related to the outdoor levels, but more directly influenced by internal sources. Severe household air pollution from solid fuel use has been recognized as one major risk for human health especailly in rural area, however, the issue is significantly overlooked in most national air quality controls and intervention policies. Here, by using low-cost sensors, indoor PM 2.5 in rural homes burning coals was monitored for ~4 months and analyzed for its temporal dynamics, distributions, relationship with outdoor PM 2.5 , and quantitative contributions of internal sources. A bimodal distribution of indoor PM 2.5 was identified and the bimodal characteristic was more significant at the finer time resolution. The bimodal distribution maxima were corresponding to the emissions from strong internal sources and the influence of outdoor PM 2.5 , respectively. Indoor PM 2.5 was found to be correlated with the outdoor PM 2.5 , even though indoor coal combustion for heating was thought to be predominant source of indoor PM 2.5 . The indoor-outdoor relationship differed significantly between the heating and non-heating seasons. Impacts of typical indoor sources like cooking, heating associated with coal use, and smoking were quantitatively analyzed based on the highly time-resolved PM 2.5 . Estimated contribution of outdoor PM 2.5 to the indoor PM 2.5 was ~48% during the non-heating period, but decreased to about 32% during the heating period. The contribution of indoor heating burning coals comprised up to 47% of the indoor PM 2.5 during the heating period, while the other indoor sources contributed to ~20%. The study, based on a relatively long-term timely resolved PM 2.5 data from a large number of rural households, provided informative results on temporal dynamics of indoor PM 2.5 and quantitative contributions of internal sources, promoting scientific understanding on sources and impacts of household air pollution., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Black carbon (BC) emissions, derived primarily from incomplete fuel combustion, significantly affect the global and regional climate. Mass absorption efficiency (MAE) is one important parameter in evaluating the climate impacts of BC. Here, values and variabilities in the MAE of BC (MAE BC ) from real-world residential emissions were investigated from a field campaign covering 163 burning events for different fuel-stove combinations. MAE BC (average: 12 ± 5 m 2 /g) was normally distributed and varied greatly by 2 orders of magnitude. Statistically significant differences in MAE BC were found for various fuels, while no significant differences were observed among different stoves. The fuel difference explained 72 ± 7% of the MAE BC variation. MAE BC did not correlate with the modified combustion efficiency but positively correlated with the ratio of organic carbon (OC) to elemental carbon (EC) and negatively correlated with char-EC. The OC/EC ratio was not always lower in coal emissions in comparison to biomass burning emissions. Coal- and biomass-burning emissions had different profiles of carbon fractions. Char-EC, OC, OC/EC, and char-EC/soot-EC can explain 68.7% of the MAE BC variation, providing the potential for predicting MAE BC from the carbon fractions, since they are more commonly measured and available.

The residential sector is a major source of air pollutant emission inventory uncertainties. A nationwide field emission measurement campaign was conducted in rural China to evaluate the variabilities of realistic emission factors (EFs) from indoor solid fuel combustion. For a total of 1313 burning events, the overall average EFs (±standard deviation) of PM 2.5 were 8.93 ± 6.95 and 7.33 ± 9.01 g/kg for biomass and coals, respectively, and 89.3 ± 51.2 and 114 ± 87 g/kg for CO. Higher EFs were found from burning of uncompressed straws, while lower EFs were found from processed biomass pellets, coal briquettes, and relatively clean anthracite coals. Modified combustion efficiency was found to be the most significant factor associated with variations in CO EFs, whereas for PM 2.5 , fuel and stove differences determined its variations. Weak correlations between PM 2.5 and CO indicated high uncertainties in using CO as a surrogate for PM 2.5 . EFs accurately fit log-normal distributions, and obvious spatial heterogeneity was observed attributed to different fuel-stove combinations across the country. Emission estimation variabilities, which are determined by the interquartile ranges divided by the median values, were notably reduced when spatially resolved EFs were adopted in the inventory.

Rapid industrialization and urbanization have resulted in widespread pollution of airborne particulate matter (PM) containing various heavy metals with adverse human-health effects. Health risk assessment of PM calls for accurate evaluation of the bioaccessibility, instead of the total content, of heavy metals in PM. Here, we demonstrated that the leachable fraction of particle-bound As, Pb, Cr, Mn, Cd, Cu, Ni and Zn in lung fluid within the typical retention duration of particles in human lungs varied drastically among particles originated from different air pollution sources, including coal combustion, biomass combustion, fugitive dust, road dust, construction dust, cement and soil. Moreover, bioaccessibility of heavy metals, particularly in biomass combustion, cement and soil particles, was strongly dependent on pollution sources, and the particulate Cu, Ni, Pb and Cd appeared to be the primary indicators of the source dependence of heavy metal bioaccessibility. Using total rather than bioaccessible concentrations of particle-bound heavy metals not only led to overestimation of the health risk of source particles, but more importantly, inaccurate identification of the high-risk pollution sources and the priority metal pollutants in the source particles. When considering bioaccessibility of particle-bound heavy metals examined in this study, coal combustion products exhibited the highest carcinogenic and noncarcinogenic risks among all source particles, whereas cement particles would be the source with highest risk based on total metal content. As and Mn appeared to be the main drivers for the noncarcinogenic risks of source particles, while As, Ni and Cr were the major contributors to the carcinogenic risks of source particles, significantly different from those based on total contents. This research underlines the importance of incorporating bioaccessibility into health risk indexes of frequently occurring particle-bound heavy metals from specific air pollution sources, which will facilitate risk-based assessment of source contribution and hence effective source regulation of airborne PM., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Airborne carbonaceous fine particles, such as soot and biochar, represent a significant fraction of air particulate matter and have received widespread concern due to their health effects. Atmospheric carbonaceous particles can contain high concentration of polycyclic aromatic hydrocarbons (PAHs), and may pose significant health risks when carried into respiratory system from inhalation of particulates. In this study, the bioaccessibility of two PAH compounds, phenanthrene and pyrene, bound to biochar fine particles was assessed by examining their release in two simulated lung fluids: Gamble's solution and artificial lysosomal fluid (ALF). We observed that only 0.47 to 0.75% of biochar-bound PAHs were released in the simulated lung fluids, most likely due to the physical entrapment of PAH molecules in the micropore regimes of biochar, resulting in strong desorption hysteresis, even though apparent desorption equilibrium was reached within 30 min, well within the average clearance time of particulate matter in lung system. The inorganic and organic salts in the simulated lung fluids were found to inhibit the release of PAHs by exerting the pore blockage effect and salting-out effect. Moreover, the low molecular weight organic acids (LMWOAs) in the lung fluids further inhibited PAH release by increasing the micropore volume and surface area of biochar fine particles. When taking into account the inhibited release, the estimated carcinogenic risks of biochar-bound PAHs are typically low, even under extreme conditions wherein both biochar concentrations and PAH loadings on biochar are very high. An important implication is that contaminant bioavailability needs to be taken into account when assessing the risks of the contaminants bound to airborne carbonaceous materials., (Copyright © 2018 Elsevier B.V. All rights reserved.)