Spatiotemporal Gradients of PAH Concentrations in Greek Cities and Associated Exposure Impacts

To study the spatiotemporal variability of particle-bound polycyclic aromatic hydrocarbons (PAHs) and assess their carcinogenic potential in six contrasting urban environments in Greece, a total of 305 filter samples were collected and analyzed. Sampling sites included a variety of urban background, traffic (Athens, Ioannina and Heraklion), rural (Xanthi) and near-port locations (Piraeus and Volos). When considering the sum of 16 U.S. EPA priority PAHs, as well as that of the six EU-proposed members, average concentrations observed across locations during summer varied moderately (0.4-2.2 ng m-3) and independently of the population of each site, with the highest values observed in the areas of Piraeus and Volos that are affected by port and industrial activities. Winter levels were significantly higher and more spatially variable compared to summer, with the seasonal enhancement ranging from 7 times in Piraeus to 98 times in Ioannina, indicating the large impact of PAH emissions from residential wood burning. Regarding benzo(a)pyrene (BaP), an IARC Group 1 carcinogen and the only EU-regulated PAH, the winter/summer ratios were 24-33 in Athens, Volos, Heraklion and Xanthi; 60 in Piraeus; and 480 in Ioannina, which is afflicted by severe wood-burning pollution events. An excellent correlation was observed between organic carbon (OC) and benzo(a)pyrene (BaP) during the cold period at all urban sites (r2 > 0.8) with stable BaP/OC slopes (0.09-0.14 × 10-3), highlighting the potential use of OC as a proxy for the estimation of BaP in winter conditions. The identified spatiotemporal contrasts, which were explored for the first time for PAHs at such a scale in the Eastern Mediterranean, provide important insights into sources and controlling atmospheric conditions and reveal large deviations in exposure risks among cities that raise the issue of environmental injustice on a national level.

Chemical Fingerprinting of Biomass Burning Organic Aerosols from Sugar Cane Combustion: Complementary Findings from Field and Laboratory Studies

Agricultural fires are a major source of biomass-burning organic aerosols (BBOAs) with impacts on health, the environment, and climate. In this study, globally relevant BBOA emissions from the combustion of sugar cane in both field and laboratory experiments were analyzed using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry. The derived chemical fingerprints of fresh emissions were evaluated using targeted and nontargeted evaluation approaches. The open-field sugar cane burning experiments revealed the high chemical complexity of combustion emissions, including compounds derived from the pyrolysis of (hemi)cellulose, lignin, and further biomass, such as pyridine and oxime derivatives, methoxyphenols, and methoxybenzenes, as well as triterpenoids. In comparison, laboratory experiments could only partially model the complexity of real combustion events. Our results showed high variability between the conducted field and laboratory experiments, which we, among others, discuss in terms of differences in combustion conditions, fuel composition, and atmospheric processing. We conclude that both field and laboratory studies have their merits and should be applied complementarily. While field studies under real-world conditions are essential to assess the general impact on air quality, climate, and environment, laboratory studies are better suited to investigate specific emissions of different biomass types under controlled conditions.

Uncovering the dominant contribution of intermediate volatility compounds in secondary organic aerosol formation from biomass-burning emissions

Organic vapors from biomass burning are a major source of secondary organic aerosols (SOAs). Previous smog chamber studies found that the SOA contributors in biomass-burning emissions are mainly volatile organic compounds (VOCs). While intermediate volatility organic compounds (IVOCs) are efficient SOA precursors and contribute a considerable fraction of biomass-burning emissions, their contribution to SOA formation has not been directly observed. Here, by deploying a newly-developed oxidation flow reactor to study SOA formation from wood burning, we find that IVOCs can contribute ∼70% of the formed SOA, i.e. >2 times more than VOCs. This previously missing SOA fraction is interpreted to be due to the high wall losses of semi-volatile oxidation products of IVOCs in smog chambers. The finding in this study reveals that SOA production from biomass burning is much higher than previously thought, and highlights the urgent need for more research on the IVOCs from biomass burning and potentially other emission sources.

Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols

Phenols emitted from biomass burning contribute significantly to secondary organic aerosol (SOA) formation through the partitioning of semivolatile products formed from gas-phase chemistry and multiphase chemistry in aerosol liquid water and clouds. The aqueous-phase SOA (aqSOA) formed via hydroxyl radical (•OH), singlet molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*), which oxidize dissolved phenols in the aqueous phase, might play a significant role in the evolution of organic aerosol (OA). However, a quantitative and predictive understanding of aqSOA has been challenging. Here, we develop a stand-alone box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets and aerosol liquid water. We investigate four phenolic compounds, i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which represent some of the key potential sources of aqSOA from biomass burning in clouds. For the same initial precursor organic gas that dissolves in aerosol/cloud liquid water and subsequently reacts with aqueous phase oxidants, we predict that the aqSOA formation potential (defined as aqSOA formed per unit dissolved organic gas concentration) of these phenols is higher than that of isoprene-epoxydiol (IEPOX), a well-known aqSOA precursor. Cloud droplets can dissolve a broader range of soluble phenols compared to aqueous aerosols, since the liquid water contents of aerosols are orders of magnitude smaller than cloud droplets. Our simulations suggest that highly soluble and reactive multifunctional phenols like GA would predominantly undergo cloud chemistry within cloud layers, while gas-phase chemistry is likely to be more important for less soluble phenols. But in the absence of clouds, the condensation of low-volatility products from gas-phase oxidation followed by their reversible partitioning to organic aerosols dominates SOA formation, while the SOA formed through aqueous aerosol chemistry increases with relative humidity (RH), approaching 40% of the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our model developments of biomass-burning phenols and their aqueous chemistry can be readily implemented in regional and global atmospheric chemistry models to investigate the aqueous aerosol and cloud chemistry of biomass-burning organic gases in the atmosphere.

Health burden of sugarcane burning on agricultural workers and nearby communities

Sugarcane is the most widely cultivated crop in the world, with equatorial developing nations performing most of this agriculture. Burning sugarcane is a common practice to facilitate harvest, producing extremely high volumes of respirable particulate matter in the process. These emissions are known to have deleterious effects on agricultural workers and nearby communities, but the extent of this exposure and potential toxicity remain poorly characterized. As the epidemicof chronic kidney disease of an unknown etiology (CKDu) and its associated mortality continue to increase along with respiratory distress, there is an urgent need to investigate the causes, determine viable interventions to mitigate disease andimprove outcomes for groups experiencing disproportionate impact. The goal of this review is to establish the state of available literature, summarize what is known in terms of human health risk, and provide recommendations for what areas should be prioritized in research.

Residential Wood Combustion in Germany: A Twin-Site Study of Local Village Contributions to Particulate Pollutants and Their Potential Health Effects

Residential wood combustion contributing to airborne particulate matter (PM10) was studied for 1 year at two sites in the village of Melpitz. Significant excess pollution was observed at the Melpitz center compared to that at the TROPOS research station Melpitz reference site, situated only 700 m away. Local concentration increments at the village site for the combustion PM constituents organic carbon, elemental carbon, levoglucosan, and benzo[a]pyrene were determined under appropriate wind directions, and their winter mean values were 0.7 μg m-3, 0.3 μg m-3, 0.1 μg m-3, and 0.4 ng m-3, representing relative increases over the regional background concentration of 24, 70, 61, and 107%, respectively. Yearly, weekly, and diurnal profiles of village increments suggest residential heating as the dominant source of this excess pollution, mainly originating from wood combustion. Receptor modeling using positive matrix factorization quantified 4.5 μg m-3 wood combustion PM at the village site, representing an increment of 1.9 μg m-3 and an increase of ∼75% over the 2.6 μg m-3 regional background wood combustion PM. This increment varied with season, temperature, and boundary layer height and reached daily mean values of 4-6 μg m-3 during unfavorable meteorological conditions. Potential health effects were estimated and resulted in an all-cause mortality from short-term exposure to wood combustion PM of 2.1 cases per 100,000 inhabitants and year for areas with similar wood smoke levels as observed in Melpitz. The excess cancer risk from the concentrations of polycyclic aromatic hydrocarbons was 6.4 per 100,000. For both health metrics, the very local contributions from the village itself were about 40-50%, indicating a strong potential for mitigation through local-scale policies. A compilation of literature data demonstrates wood combustion to represent a major source of PM pollution in Germany, with average winter-time contributions of 10-20%. The present study quantifies the negative impacts of heating with wood in rural residential areas, where the continuous monitoring of air quality is typically lacking. Further regulation of this PM source is warranted in order to protect human health.

Fossil and Nonfossil Sources of Winter Organic Aerosols in the Regional Background Atmosphere of China

Carbonaceous aerosols (CA) from anthropogenic emissions have been significantly reduced in urban China in recent years. However, the relative contributions of fossil and nonfossil sources to CA in rural and background regions of China remain unclear. In this study, the sources of different carbonaceous fractions in fine aerosols (PM2.5) from five background sites of the China Meteorological Administration Atmosphere Watch Network during the winter of 2019 and 2020 were quantified using radiocarbon (14C) and organic markers. The results showed that nonfossil sources contributed 44-69% to total carbon at these five background sites. Fossil fuel combustion was the predominant source of elemental carbon at all sites (73 ± 12%). Nonfossil sources dominated organic carbon (OC) in these background regions (61 ± 13%), with biomass burning or biogenic-derived secondary organic carbon (SOC) as the most important contributors. However, the relative fossil fuel source to OC in China (39 ± 13%) still exceeds those at other regional/background sites in Asia, Europe, and the USA. SOC dominated the fossil fuel-derived OC, highlighting the impact of regional transport from anthropogenic sources on background aerosol levels. It is therefore imperative to develop and implement aerosol reduction policies and technologies tailored to both the anthropogenic and biogenic emissions to mitigate the environmental and health risks of aerosol pollution across China.

Attributing Atmospheric Phosphorus in the Himalayas: Biomass Burning vs Mineral Dust

Atmospheric phosphorus is a vital nutrient for ecosystems whose sources and fate are still debated in the fragile Himalayan region, hindering our comprehension of its local ecological impact. This study provides novel insights into atmospheric phosphorus based on the study of total suspended particulate matter at the Qomolangma station. Contrary to the prevailing assumptions, we show that biomass burning (BB), not mineral dust, dominates total dissolved phosphorus (TDP, bioavailable) deposition in this arid region, especially during spring. While total phosphorus is mainly derived from dust (77% annually), TDP is largely affected by the transport of regional biomass-burning plumes from South Asia. During BB pollution episodes, TDP causing springtime TDP fluxes alone accounts for 43% of the annual budget. This suggests that BB outweighs dust in supplying bioavailable phosphorus, a critical nutrient, required to sustain Himalayas' ecological functions. Overall, this first-hand field evidence refines the regional and global phosphorus budget by demonstrating that BB emission, while still unrecognized, is a significant source of P, even in the remote mountains of the Himalayas. It also reveals the heterogeneity of atmospheric phosphorus deposition in that region, which will help predict changes in the impacted ecosystems as the deposition patterns vary.

Polycyclic Aromatic Hydrocarbons (PAHs) in Wildfire Smoke Accumulate on Indoor Materials and Create Postsmoke Event Exposure Pathways

Wildfire smoke contains PAHs that, after infiltrating indoors, accumulate on indoor materials through particle deposition and partitioning from air. We report the magnitude and persistence of select surface associated PAHs on three common indoor materials: glass, cotton, and mechanical air filter media. Materials were loaded with PAHs through both spiking with standards and exposure to a wildfire smoke proxy. Loaded materials were aged indoors over ∼4 months to determine PAH persistence. For materials spiked with standards, total PAH decay rates were 0.010 ± 0.002, 0.025 ± 0.005, and 0.051 ± 0.009 day-1, for mechanical air filter media, glass, and cotton, respectively. PAH decay on smoke-exposed samples is consistent with that predicated by decay constants from spiked materials. Decay curves of smoke loaded samples show that PAH surface concentrations are elevated above background for ∼40 days after the smoke clears. Cleaning processes efficiently remove PAHs, with reductions of 71% and 62% after cleaning smoke-exposed glass with ethanol and a commercial cleaner, respectively. Laundering smoke-exposed cotton in a washing machine and heated drying removed 48% of PAHs. An exposure assessment indicates that both inhalation and dermal PAH exposure pathways may be relevant following wildfire smoke events.

Broadband Cavity-Enhanced Absorption Spectroscopy (BBCEAS) Coupled with an Interferometer for On-Band and Off-Band Detection of Glyoxal

Glyoxal (CHOCHO) is a trace gas in the atmosphere, often used as an indicator of biogenic emissions. It is frequently compared to formaldehyde concentrations, which serve as indicators of anthropogenic emissions, to gain insights into the characteristics of the environmental source. This study employed broadband cavity-enhanced absorption spectroscopy to detect gaseous CHOCHO, methylglyoxal, and NO2. Two different detection methods are compared. Spectrograph and CCD Detection: This approach involves coupling the system to a spectrograph with a charge-coupled device (CCD) detector. It achieved a 1 min 1-σ detection limit of 2.5 × 108 molecules/cm3, or 10 parts per trillion (ppt). Methylglyoxal and NO2 achieved 1 min 1-σ detection limits of 34 ppt and 22 ppt, respectively. Interferometer and PMT Detection: In this method, an interferometer is used in conjunction with a photomultiplier tube (PMT) detector. It resulted in a 2 min 1-σ detection limit of 1.5 × 1010 molecules/cm3, or 600 ppt. The NO2 2 min 1-σ detection limit was determined to be 900 ppt. Concentrations of methylglyoxal were difficult to determine using this method, as they appeared to be below the detection limit of the instrument. This study discusses the advantages and limitations of each of these detection methods.

Dissecting the contributions of organic nitrogen aerosols to global atmospheric nitrogen deposition and implications for ecosystems

Atmospheric deposition of particulate organic nitrogen (ONp) is a significant process in the global nitrogen cycle and may be pivotally important for N-limited ecosystems. However, past models largely overlooked the spatial and chemical inhomogeneity of atmospheric ONp and were thus deficient in assessing global ONp impacts. We constructed a comprehensive global model of atmospheric gaseous and particulate organic nitrogen (ON), including the latest knowledge on emissions and secondary formations. Using this model, we simulated global atmospheric ONp abundances consistent with observations. Our estimated global atmospheric ON deposition was 26 Tg N yr-1, predominantly in the form of ONp (23 Tg N yr-1) and mostly from wildfires (37%), oceans (22%) and aqueous productions (17%). Globally, ONp contributed as much as 40% to 80% of the total N deposition downwind of biomass-burning regions. Atmospheric ONp deposition thus constituted the dominant external N supply to the N-limited boreal forests, tundras and the Arctic Ocean, and its importance may be amplified in a future warming climate.

Phase Behavior and Viscosity in Biomass Burning Organic Aerosol and Climatic Impacts

Smoke particles generated by burning biomass consist mainly of organic aerosol termed biomass burning organic aerosol (BBOA). BBOA influences the climate by scattering and absorbing solar radiation or acting as nuclei for cloud formation. The viscosity and the phase behavior (i.e., the number and type of phases present in a particle) are properties of BBOA that are expected to impact several climate-relevant processes but remain highly uncertain. We studied the phase behavior of BBOA using fluorescence microscopy and showed that BBOA particles comprise two organic phases (a hydrophobic and a hydrophilic phase) across a wide range of atmospheric relative humidity (RH). We determined the viscosity of the two phases at room temperature using a photobleaching method and showed that the two phases possess different RH-dependent viscosities. The viscosity of the hydrophobic phase is largely independent of the RH from 0 to 95%. We use the Vogel-Fulcher-Tamman equation to extrapolate our results to colder and warmer temperatures, and based on the extrapolation, the hydrophobic phase is predicted to be glassy (viscosity >1012 Pa s) for temperatures less than 230 K and RHs below 95%, with possible implications for heterogeneous reaction kinetics and cloud formation in the atmosphere. Using a kinetic multilayer model (KM-GAP), we investigated the effect of two phases on the atmospheric lifetime of brown carbon within BBOA, which is a climate-warming agent. We showed that the presence of two phases can increase the lifetime of brown carbon in the planetary boundary layer and polar regions compared to previous modeling studies. Hence, the presence of two phases can lead to an increase in the predicted warming effect of BBOA on the climate.

Assessing Lifetime Cancer Risk Associated with Population Exposure to PM-Bound PAHs and Carcinogenic Metals in Three Mid-Latitude Metropolitan Cities

Lifetime cancer risk characterization of ambient PM-bound carcinogenic metals and polycyclic aromatic hydrocarbons (PAHs) were examined in the cities of Los Angeles (USA), Thessaloniki (Greece) and Milan (Italy), which share similar Mediterranean climates but are different in their urban emission sources and governing air quality regulations. The samples in Milan and Thessaloniki were mostly dominated by biomass burning activities whereas the particles collected in Los Angeles were primary impacted by traffic emissions. We analyzed the ambient PM2.5 mass concentration of Cadmium (Cd), Hexavalent Chromium (Cr(VI)), Nickel (Ni), Lead (Pb), as well as 13 PAH compounds in the PM samples, collected during both cold and warm periods at each location. Pb exhibited the highest annual average concentration in all three cities, followed by Ni, As, Cr(VI), Cd and PAHs, respectively. The cancer risk assessment based on outdoor pollutants was performed based on three different scenarios, with each scenario corresponding to a different level of infiltration of outdoor pollutants into the indoor environment. Thessaloniki exhibited a high risk associated with lifetime inhalation of As, Cr(VI), and PAHs, with values in the range of (0.97-1.57) × 10-6, (1.80-2.91) × 10-6, and (0.77-1.25) × 10-6, respectively. The highest cancer risk values were calculated in Milan, exceeding the US EPA standard by a considerable margin, where the lifetime risk values of exposure to As, Cr(VI), and PAHs were in the range of (1.29-2.08) × 10-6, (6.08-9.82) × 10-6, and (1.10-1.77) × 10-6, respectively. In contrast, the estimated risks associated with PAHs and metals, except Cr(VI), in Los Angeles were extremely lower than the guideline value, even when the infiltration factor was assumed to be at peak. The lifetime cancer risk values associated with As, Cd, Ni, Pb, and PAHs in Los Angeles were in the range of (0.04-0.33) × 10-6. This observation highlights the impact of local air quality measures in improving the air quality and lowering the cancer risks in Los Angeles compared to the other two cities.

Sulfate Formation by Photosensitization in Mixed Incense Burning-Sodium Chloride Particles: Effects of RH, Light Intensity, and Aerosol Aging

Elevated particulate sulfate concentrations have been frequently observed in coastal areas when air masses are influenced by continental emissions, especially combustion sources like biomass burning. We studied the SO₂ uptake by laboratory-generated droplets containing incense smoke extracts and sodium chloride (IS-NaCl) under irradiation and found enhanced sulfate production over pure NaCl droplets, attributable to photosensitization induced by constituents in IS. Low relative humidity and high light intensity facilitated sulfate formation and increased the SO₂ uptake coefficient by IS-NaCl particles. Aging of the IS particles further enhanced sulfate production, attributable to the enhanced secondary oxidant production promoted by increased proportions of nitrogen-containing CHN and oxygen- and nitrogen-containing CHON species under light and air. Experiments using model compounds of syringaldehyde, pyrazine, and 4-nitroguaiacol verified the enhancements of CHN and CHON species in sulfate formation. This work provides experimental evidence of enhanced sulfate production in laboratory-generated IS-NaCl droplets via enhanced secondary oxidant production triggered by photosensitization in multiphase oxidation processes under light and air. Our results can shed light on the possible interactions between sea salt and biomass burning aerosols in enhancing sulfate production.

Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau

Himalayas and Tibetan Plateau (HTP) is important for global biodiversity and regional sustainable development. While numerous studies have revealed that the ecosystem in this unique and pristine region is changing, their exact causes are still poorly understood. Here, we present a year-round (23 March 2017 to 19 March 2018) ground- and satellite-based atmospheric observation at the Qomolangma monitoring station (QOMS, 4276 m a.s.l.). Based on a comprehensive chemical and stable isotope (¹⁵N) analysis of nitrogen compounds and satellite observations, we provide unequivocal evidence that wildfire emissions in South Asia can come across the Himalayas and threaten the HTP's ecosystem. Such wildfire episodes, mostly occurring in spring (March-April), not only substantially enhanced the aerosol nitrogen concentration but also altered its composition (i.e., rendering it more bioavailable). We estimated a nitrogen deposition flux at QOMS of ∼10 kg N ha^-1 yr^-1, which is approximately twice the lower value of the critical load range reported for the Alpine ecosystem. Such adverse impact is particularly concerning, given the anticipated increase of wildfire activities in the future under climate change.

Phototransformation of Vanillin in Artificial Snow by Direct Photolysis and Mediated by Nitrite

The photodegradation of vanillin, as a proxy of methoxyphenols emitted by biomass burning, was investigated in artificial snow at 243 K and in liquid water at room temperature. Nitrite (NO₂^-) was used as a photosensitizer of reactive oxygen and nitrogen species under UVA light, because of its key photochemical role in snowpacks and atmospheric ice/waters. In snow and in the absence of NO₂^-, slow direct photolysis of vanillin was observed due to back-reactions taking place in the quasi-liquid layer at the ice-grain surface. The addition of NO₂^- made the photodegradation of vanillin faster, because of the important contribution of photoproduced reactive nitrogen species in vanillin phototransformation. These species triggered both nitration and oligomerization of vanillin in irradiated snow, as the identified vanillin by-products showed. Conversely, in liquid water, direct photolysis was the main photodegradation pathway of vanillin, even in the presence of NO₂^-, which had negligible effects on vanillin photodegradation. The results outline the different role of iced and liquid water in the photochemical fate of vanillin in different environmental compartments.

Chemical Signatures of Seasonally Unique Anthropogenic Influences on Organic Aerosol Composition in the Central Amazon

Urbanization and fires perturb the quantities and composition of fine organic aerosol in the central Amazon, with ramifications for radiative forcing and public health. These disturbances include not only direct emissions of particulates and secondary organic aerosol (SOA) precursors but also changes in the pathways through which biogenic precursors form SOA. The composition of ambient organic aerosol is complex and incompletely characterized, encompassing millions of potential structures relatively few of which have been synthesized and characterized. Through analysis of submicron aerosol samples from the Green Ocean Amazon (GoAmazon2014/5) field campaign by two-dimensional gas chromatography coupled with machine learning, ∼1300 unique compounds were traced and characterized over two seasons. Fires and urban emissions produced chemically and interseasonally distinct impacts on product signatures, with only ∼50% of compounds observed in both seasons. Seasonally unique populations point to the importance of aqueous processing in Amazonian aerosol aging, but further mechanistic insights are impeded by limited product identity knowledge. Less than 10% of compounds were identifiable at an isomer-specific level. Overall, the findings (i) provide compositional characterization of anthropogenic influence on submicron organic aerosol in the Amazon, (ii) identify key season-to-season differences in chemical signatures, and (iii) highlight high-priority knowledge gaps in current speciated knowledge.

Brown Coal and Logwood Combustion in a Modern Heating Appliance: The Impact of Combustion Quality and Fuel on Organic Aerosol Composition

Residential heating with solid fuels is one of the major drivers for poor air quality in Central and Eastern Europe, and coal is still one of the major fuels in countries, such as Poland, the Czech Republic, and Hungary. In this work, emissions from a single-room heater fueled with brown coal briquettes (BCBs) and spruce logs (SLs) were analyzed for signatures of inorganic as well as semivolatile aromatic and low-volatile organic constituents. High variations in organic carbon (OC) emissions of BCB emissions, ranging from 5 to 22 mg MJ^-1, were associated to variations in carbon monoxide (CO) emissions, ranging from 900 to 1900 mg MJ^-1. Residential BCB combustion turned out to be an equally important source of levoglucosan, an established biomass burning marker, as spruce logwood combustion, but showed distinct higher ratios to manosan and galactosan. Signatures of polycyclic aromatic hydrocarbons emitted by BCB combustion exhibited defunctionalization and desubstitution with increasing combustion quality. Lastly, the concept of island and archipelago structural motifs adapted from petroleomics is used to describe the fraction low-volatile organic compounds in particulate emissions, where a transition from archipelago to island motifs in relation with decreasing CO emissions was observed in BCB emissions, while emissions from SL combustion exhibited the island motif.

Mapping the Contribution of Biomass Burning to Persistent Organic Pollutants in the Air of the Indo-China Peninsula Based on a Passive Air Monitoring Network

Biomass burning (BB) is an important source of atmospheric persistent organic pollutants (POPs) across the world. However, there are few field-based regional studies regarding the POPs released from BB. Due to the current limitations of emission factors and satellites, the contribution of BB to airborne POPs is still not well understood. In this study, with the simultaneous monitoring of BB biomarkers and POPs based on polyurethane foam-based passive air sampling technique, we mapped the contribution of BB to polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) in the Indo-China Peninsula. Spearman correlations between levoglucosan and 16 PCBs (r_s = 0.264-0.767, p < 0.05) and 2 OCPs (r_s = 0.250-0.328, p < 0.05) confirmed that BB may facilitate POP emissions. Source apportionment indicated that BB contributed 9.3% to the total PCB and OCP mass. The high contribution of positive matrix factorization-resolved BB to PCBs and OCPs was almost consistent with their concentration distributions in the open BB season but not completely consistent with those in the pre-monsoon and/or monsoon seasons. Their contribution distributions may reflect the use history and geographic distribution in secondary sources of POPs. The field-based contribution dataset of BB to POPs is significant in improving regional BB emission inventories and model prediction.

The Effects of Trash, Residential Biofuel, and Open Biomass Burning Emissions on Local and Transported PM2.5 and Its Attributed Mortality in Africa

Long-term exposure to ambient fine particulate matter (PM_2.5) is the second leading risk factor of premature death in Sub-Saharan Africa. We use GEOS-Chem to quantify the effects of (a) trash burning, (b) residential solid-fuel burning, and (c) open biomass burning (BB) (i.e., landscape fires) on ambient PM_2.5 and PM_2.5-attributable mortality in Africa. Using a series of sensitivity simulations, we excluded each of the three combustion sources in each of five African regions. We estimate that in 2017 emissions from these three combustion sources within Africa increased global ambient PM_2.5 by 2%, leading to 203,000 (95% confidence interval: 133,000-259,000) premature mortalities yr^-1 globally and 167,000 premature mortalities yr^-1 in Africa. BB contributes more ambient PM_2.5-related premature mortalities per year (63%) than residential solid-fuel burning (29%) and trash burning (8%). Open BB in Central Africa leads to the largest number of PM_2.5-attributed mortalities inside the region, while trash burning in North Africa and residential solid-fuel burning in West Africa contribute the most regional mortalities for each source. Overall, Africa has a unique ambient air pollution profile because natural sources, such as windblown dust and BB, contribute strongly to ambient PM_2.5 levels and PM_2.5-related mortality. Air pollution policies may need to focus on taking preventative measures to avoid exposure to ambient PM_2.5 from these less-controllable sources.

Influence of the Long-Range Transport of Siberian Biomass Burnings on Air Quality in Northeast China in June 2017

Biomass burning (BB) emits a large volume of trace gases and aerosols into the atmosphere, which can significantly affect the earth's radiative balance and climate and has negative impacts on air quality and even human health. In late June 2017, an intense BB case, dominated by forest and savanna fires, occurred in Siberia, and it affected the air quality of Northeast China through long-range transport. Here, multisatellite remote-sensing products and ground-based PM_2.5 measurements are used to evaluate the influence of the Siberian smoky plume on Northeast China. The results show that the BB was intense at the early stage when the daily fire count and average fire radiative power exceeded 300 and 200 MW, respectively. The maximum daily fire count reached 1350 in Siberia, and the peak value of instantaneous fire radiative power was as high as 3091.5 MW. High concentrations of CO and aerosols were emitted into the atmosphere by the BB in Siberia. The maximum daily mean values of the CO column concentration and aerosol optical depth (AOD) increased by 3 × 10¹⁷ molec·cm² and 0.5 compared with that during the initial BB stage. In addition, the BB released a large number of absorptive aerosols into the atmosphere, and the UV aerosol index (UVAI) increased by five times at the peak of the event in Siberia. Under the appropriate synoptic conditions and, combined with pyroconvection, the smoky plume was lifted into the upper air and transported to Northeast China, affecting the air quality of Northeast China. The daily mean values of CO concentration, AOD, and UVAI in Northeast China increased by 6 × 10¹⁷ molec·cm², 0.5, and 1.4, respectively, after being affected. Moreover, the concentration of the surface PM_2.5 in Northeast China approximately doubled after being affected by the plume. The results of this study indicate that the air quality of Northeast China can be significantly affected by Siberian BBs under favorable conditions.

Influence of Wildfire on Urban Ozone: An Observationally Constrained Box Modeling Study at a Site in the Colorado Front Range

Increasing trends in biomass burning emissions significantly impact air quality in North America. Enhanced mixing ratios of ozone (O3) in urban areas during smoke-impacted periods occur through transport of O3 produced within the smoke or through mixing of pyrogenic volatile organic compounds (PVOCs) with urban nitrogen oxides (NOx = NO + NO2) to enhance local O3 production. Here, we analyze a set of detailed chemical measurements, including carbon monoxide (CO), NOx, and speciated volatile organic compounds (VOCs), to evaluate the effects of smoke transported from relatively local and long-range fires on O3 measured at a site in Boulder, Colorado, during summer 2020. Relative to the smoke-free period, CO, background O3, OH reactivity, and total VOCs increased during both the local and long-range smoke periods, but NOx mixing ratios remained approximately constant. These observations are consistent with transport of PVOCs (comprised primarily of oxygenates) but not NOx with the smoke and with the influence of O3 produced within the smoke upwind of the urban area. Box-model calculations show that local O3 production during all three periods was in the NOx-sensitive regime. Consequently, this locally produced O3 was similar in all three periods and was relatively insensitive to the increase in PVOCs. However, calculated NOx sensitivities show that PVOCs substantially increase O3 production in the transition and NOx-saturated (VOC-sensitive) regimes. These results suggest that (1) O3 produced during smoke transport is the main driver for O3 increases in NOx-sensitive urban areas and (2) smoke may cause an additional increase in local O3 production in NOx-saturated (VOC-sensitive) urban areas. Additional detailed VOC and NOx measurements in smoke impacted urban areas are necessary to broadly quantify the effects of wildfire smoke on urban O3 and develop effective mitigation strategies.

Reaction of HOCl with Wood Smoke Aerosol: Impacts on Indoor Air Quality and Outdoor Reactive Chlorine

High loadings of biomass burning (BB) aerosol particles from wildfire or residential heating sources can be present in both outdoor and indoor environments, where they deposit onto surfaces such as walls and furniture. These pollutants can interact with oxidants in both the aerosol and deposited forms. Hypochlorous acid (HOCl), a strong oxidant emitted during cleaning with chlorine-cleaning agents such as bleach, can attain mixing ratios of hundreds of ppbv indoors; moreover, lower mixing ratios are naturally present outdoors. Here, we report the heterogeneous reactivity of HOCl with wood smoke aerosol particles. After exposure to gas-phase HOCl, the particle chlorine content increased reaching chlorine-to-organic mass ratios of 0.07 with the chlorine covalently bound as organochlorine species, many of which are aromatic. Investigating individual potential BB components, we observed that unsaturated species such as coniferaldehyde and furfural react efficiently with HOCl. These observations indicate that organochlorine pollutants will form indoors when bleach cleaning a wildfire impacted space. The chlorine component of particles internally mixed with BB material and chloride initially increased, upon HOCl exposure, indicating that active chlorine recycling in the outdoor environment will be suppressed in the presence of BB emissions.

Oxidation of Phenolic Aldehydes by Ozone and Hydroxyl Radicals at the Air-Solid Interface

Biomass burning emissions contain abundant phenolic aldehydes (e.g., syringaldehyde, vanillin, and 4-hydroxybenaldehyde) that are oxidized during atmospheric transport, altering the physicochemical properties of particulates. Herein, the oxidative processing of thin films made of syringaldehyde, vanillin, and 4-hydroxybenaldehyde is studied at the air-solid interface under a variable O3(g) molar ratio (410 ppbv-800 ppmv) and relative humidity (0-90%). Experiments monitored the absorption changes of C=C, C=O, and -COOH vibration changes during the oxidation of thin films by transmission Fourier transform infrared spectroscopy (FTIR). Selected spectroscopic features of aromatic ring cleavage by O3(g) revealed the production of carboxylic acids. Instead, monitoring O-H stretching provided a comparison of a hydroxylation channel from in situ produced hydroxyl radical. The overall oxidation reactivity trend syringaldehyde > vanillin > 4-hydroxybenzladehyde can be explained based on the additional electron density from methoxide substituents to the ring. The reactive uptake coefficient of O3(g) increases for higher relative humidity, e.g., for syringaldehyde by 18 and 215 times at 74% and 90% relative humidity (RH), respectively, as compared to dry conditions. A Langmuir-Hinshelwood mechanism fits well the kinetics of oxidation under a variable O3(g) molar ratio at 74% RH, providing useful information that should be included in atmospheric chemistry models.

Aerosol Mass Spectral Profiles from NAMaSTE Field-Sampled South Asian Combustion Sources

Unit mass resolution mass spectral profiles of nonrefractory submicron aerosol were retrieved from undersampled atmospheric emission sources common to South Asia using a "mini" aerosol mass spectrometer. Emission sources including wood- and dung-fueled cookstoves, agricultural residue burning, garbage burning, engine exhaust, and coal-fired brick kilns were sampled during the 2015 Nepal Ambient Monitoring and Source Testing Experiment (NAMaSTE) campaign. High-resolution peak fitting estimates of the mass spectra were used to characterize ions found within each source profile and help identify mass spectral signatures unique to aerosol emissions from the investigated source types. The first aerosol mass spectral profiles of dung burning, charcoal burning, garbage burning, and brick kilns are provided in this work. The online aerosol mass spectra show that organics were generally the dominant component of the nonrefractory aerosol. However, inorganic aerosol components including ammonium and chloride were significant in dung- and charcoal-fired cookstove emissions and sulfate compounds were major components of the coal-fired brick kiln emissions. Organic mass spectra from both the charcoal burning and zigzag brick kiln were dominated by nitrogen-containing ions thought to be from the electron ionization of amines and amides contained in the emissions. The mixed garbage burning emissions profiles were dominated by plastic combustion with very low fractions of organic markers associated with biomass burning. The plastic burning emissions were associated with enhanced organic signal at mass-to-charge (m/z) 104 and m/z 166, which could be useful fragment ion indicators for garbage burning in ambient aerosol profiles. Finally, a framework for the identification of emission sources using the unit mass resolution organic mass fractions at m/z 55 (f ₅₅), m/z 57 (f ₅₇), and m/z 60 (f ₆₀) is proposed in this work. Plotting the ratio of f ₅₅ to f ₅₇ versus f ₆₀ is found to be effective for the identification of emissions by the fuel type and even useful in separating emissions of similar source types. Although the sample size was limited, these results give further context to the aerosol and gas-phase emission factors presented in other NAMaSTE works and provide a critical reference for future aerosol composition measurements in South Asia.

Assessing Vertical Allocation of Wildfire Smoke Emissions Using Observational Constraints From Airborne Lidar in the Western U.S

Wildfire emissions are a key contributor of carbonaceous aerosols and trace gases to the atmosphere. Induced by buoyant lifting, smoke plumes can be injected into the free troposphere and lower stratosphere, which by consequence significantly affects the magnitude and distance of their influences on air quality and radiation budget. However, the vertical allocation of emissions when smoke escapes the planetary boundary layer (PBL) and the mechanism modulating it remain unclear. We present an inverse modeling framework to estimate the wildfire emissions, with their temporal and vertical evolution being constrained by assimilating aerosol extinction profiles observed from the airborne Differential Absorption Lidar-High Spectral Resolution Lidar during the Fire Influence on Regional to Global Environments and Air Quality field campaign. Three fire events in the western U.S., which exhibit free-tropospheric injections are examined. The constrained smoke emissions indicate considerably larger fractions of smoke injected above the PBL (f _>PBL, 80%-94%) versus the column total, compared to those estimated by the WRF-Chem model using the default plume rise option (12%-52%). The updated emission profiles yield improvements for the simulated vertical structures of the downwind transported smoke, but limited refinement of regional smoke aerosol optical depth distributions due to the spatiotemporal coverage of flight observations. These results highlight the significance of improving vertical allocation of fire emissions on advancing the modeling and forecasting of the environmental impacts of smoke.

Wildfire Smoke Influence on Cloud Water Chemical Composition at Whiteface Mountain, New York

Wildfires significantly impact air quality and climate, including through the production of aerosols that can nucleate cloud droplets and participate in aqueous-phase reactions. Cloud water was collected during the summer months (June-September) of 2010-2017 at Whiteface Mountain, New York and examined for biomass burning influence. Cloud water samples were classified by their smoke influence based on backward air mass trajectories and satellite-detected smoke. A total of 1,338 cloud water samples collected over 485 days were classified by their probability of smoke influence, with 49% of these days categorized as having moderate to high probability of smoke influence. Carbon monoxide and ozone levels were enhanced during smoke influenced days at the summit of Whiteface Mountain. Smoke-influenced cloud water samples were characterized by enhanced concentrations of potassium, sulfate, ammonium, and total organic carbon, compared to samples lacking identified influence. Five cloud water samples were examined further for the presence of dissolved organic compounds, insoluble particles, and light-absorbing components. The five selected cloud water samples contained the biomass burning tracer levoglucosan at 0.02-0.09 μM. Samples influenced by air masses that remained aloft, above the boundary layer during transport, had lower insoluble particle concentrations, larger insoluble particle diameters, and larger oxalate:sulfate ratios, suggesting cloud processing had occurred. These findings highlight the influence that local and long-range transported smoke have on cloud water composition.

The Impact of Meteorological Conditions and Agricultural Waste Burning on PM Levels: A Case Study of Avellino (Southern Italy)

In this work, the effect of the meteorological conditions and the agricultural waste burning on PM air pollution levels has been investigated in the city of Avellino, located in the Sabato Valley (southern Italy). Avellino has been described among the most polluted towns in Italy in terms of particulate matter (PM) during the last 10 years. The main aim of this study was to analyze the air quality data collected in Avellino and its surroundings during September 2021. In this period, the air quality in the Sabato Valley has been adversely affected by agricultural practices, which represent a significant source of PM. The impact of agricultural waste burning on PM levels in Avellino has been determined through an integrated monitoring network, consisting of two fixed urban reference stations and by several low-cost sensors distributed in the Sabato Valley. In the considered period, the two reference stations recorded several exceedances of the daily average PM₁₀ legislative limit value (50 µg m^-3) in addition to high concentrations of PM_2.5. Moreover, we provide a detailed description of the event that took place on 25 September 2021, when the combined effect of massive agricultural practices and very stable atmospheric conditions produced a severe pollution episode. Results show PM exceedances in Avellino concurrent with high PM values in the areas bordering the city due to agricultural waste burning and adverse meteorological conditions, which inhibit PM dispersion in the atmosphere.

Rate of atmospheric brown carbon whitening governed by environmental conditions

Biomass burning organic aerosol (BBOA) in the atmosphere contains many compounds that absorb solar radiation, called brown carbon (BrC). While BBOA is in the atmosphere, BrC can undergo reactions with oxidants such as ozone which decrease absorbance, or whiten. The effect of temperature and relative humidity (RH) on whitening has not been well constrained, leading to uncertainties when predicting the direct radiative effect of BrC on climate. Using an aerosol flow-tube reactor, we show that the whitening of BBOA by oxidation with ozone is strongly dependent on RH and temperature. Using a poke-flow technique, we show that the viscosity of BBOA also depends strongly on these conditions. The measured whitening rate of BrC is described well with the viscosity data, assuming that the whitening is due to oxidation occurring in the bulk of the BBOA, within a thin shell beneath the surface. Using our combined datasets, we developed a kinetic model of this whitening process, and we show that the lifetime of BrC is 1 d or less below ∼1 km in altitude in the atmosphere but is often much longer than 1 d above this altitude. Including this altitude dependence of the whitening rate in a chemical transport model causes a large change in the predicted warming effect of BBOA on climate. Overall, the results illustrate that RH and temperature need to be considered to understand the role of BBOA in the atmosphere.

Airborne Emission Rate Measurements Validate Remote Sensing Observations and Emission Inventories of Western U.S. Wildfires

Carbonaceous emissions from wildfires are a dynamic mixture of gases and particles that have important impacts on air quality and climate. Emissions that feed atmospheric models are estimated using burned area and fire radiative power (FRP) methods that rely on satellite products. These approaches show wide variability and have large uncertainties, and their accuracy is challenging to evaluate due to limited aircraft and ground measurements. Here, we present a novel method to estimate fire plume-integrated total carbon and speciated emission rates using a unique combination of lidar remote sensing aerosol extinction profiles and in situ measured carbon constituents. We show strong agreement between these aircraft-derived emission rates of total carbon and a detailed burned area-based inventory that distributes carbon emissions in time using Geostationary Operational Environmental Satellite FRP observations (Fuel2Fire inventory, slope = 1.33 ± 0.04, r² = 0.93, and RMSE = 0.27). Other more commonly used inventories strongly correlate with aircraft-derived emissions but have wide-ranging over- and under-predictions. A strong correlation is found between carbon monoxide emissions estimated in situ with those derived from the TROPOspheric Monitoring Instrument (TROPOMI) for five wildfires with coincident sampling windows (slope = 0.99 ± 0.18; bias = 28.5%). Smoke emission coefficients (g MJ^-1) enable direct estimations of primary gas and aerosol emissions from satellite FRP observations, and we derive these values for many compounds emitted by temperate forest fuels, including several previously unreported species.

Chemical Structure Regulates the Formation of Secondary Organic Aerosol and Brown Carbon in Nitrate Radical Oxidation of Pyrroles and Methylpyrroles

Nitrogen-containing heterocyclic volatile organic compounds (VOCs) are important components of wildfire emissions that are readily reactive toward nitrate radicals (NO₃) during nighttime, but the oxidation mechanism and the potential formation of secondary organic aerosol (SOA) and brown carbon (BrC) are unclear. Here, NO₃ oxidation of three nitrogen-containing heterocyclic VOCs, pyrrole, 1-methylyrrole (1-MP), and 2-methylpyrrole (2-MP), was investigated in chamber experiments to determine the effect of precursor structures on SOA and BrC formation. The SOA chemical compositions and the optical properties were analyzed using a suite of online and offline instrumentation. Dinitro- and trinitro-products were found to be the dominant SOA constituents from pyrrole and 2-MP, but not observed from 1-MP. Furthermore, the SOA from 2-MP and pyrrole showed strong light absorption, while that from 1-MP were mostly scattering. From these results, we propose that NO₃-initiated hydrogen abstraction from the 1-position in pyrrole and 2-MP followed by radical shift and NO₂ addition leads to light-absorbing nitroaromatic products. In the absence of a 1-position hydrogen, NO₃ addition likely dominates the 1-MP chemistry. We also estimate that the total SOA mass and light absorption from pyrrole and 2-MP are comparable to those from phenolic VOCs and toluene in biomass burning, underscoring the importance of considering nighttime oxidation of pyrrole and methylpyrroles in air quality and climate models.

Saccharides as Particulate Matter Tracers of Biomass Burning: A Review

The adverse effects of atmospheric particulate matter (PM) on health and ecosystems, as well as on meteorology and climate change, are well known to the scientific community. It is therefore undeniable that a good understanding of the sources of PM is crucial for effective control of emissions and to protect public health. One of the major contributions to atmospheric PM is biomass burning, a practice used both in agriculture and home heating, which can be traced and identified by analyzing sugars emitted from the combustion of cellulose and hemicellulose that make up biomass. In this review comparing almost 200 selected articles, we highlight the most recent studies that broaden such category of tracers, covering research publications on residential wood combustions, open-fire or combustion chamber burnings and ambient PM in different regions of Asia, America and Europe. The purpose of the present work is to collect data in the literature that indicate a direct correspondence between biomass burning and saccharides emitted into the atmosphere with regard to distinguishing common sugars attributed to biomass burning from those that have co-causes of issue. In this paper, we provide a list of 24 compounds, including those most commonly recognized as biomass burning tracers (i.e., levoglucosan, mannosan and galactosan), from which it emerges that monosaccharide anhydrides, sugar alcohols and primary sugars have been widely reported as organic tracers for biomass combustion, although it has also been shown that emissions of these compounds depend not only on combustion characteristics and equipment but also on fuel type, combustion quality and weather conditions. Although it appears that it is currently not possible to define a single compound as a universal indicator of biomass combustion, this review provides a valuable tool for the collection of information in the literature and identifies analytes that can lead to the determination of patterns for the distribution between PM generated by biomass combustion.

Asthma and rhinitis in wet and dry season among students in upper Northern Thailand: the role of building dampness and household air pollution

We investigated associations between domestic exposure and respiratory health in students inNorthern Thailand in wet and dry season (1159 participants), calculating odds ratios (OR) with 95% confidence intervals (CI) by multilevel logistic regression. Totally 6.0% had wheeze, 23.0% dyspnoea, 4.0% current asthma, 54.6% rhinitis and 31.5% rhinoconjunctivitis. Girls had less wheeze (OR 0.66; 95% CI 0.43-1.00) and current asthma (OR 0.32; 95% CI 0.19-0.54). Water leakage was associated with wheeze (OR 2.35; 95% CI 1.09-5.06), dyspnea (OR 2.00; 95% CI 1.24-3.23) and rhinoconjunctivitis (OR 1.62; 95% CI 1.11-2.38). Mould was associated with rhinitis (OR 1.89; 95% CI 1.06-3.38). Window pane condensation was associated with wheeze (OR 2.60; 95% CI 1.13-5.98) and rhinoconjunctivitis (OR 1.70; 95% CI 1.08-2.67). Biomass burning was associated with wheeze, dyspnoea, rhinitis and rhinoconjunctivitis. In conclusion, household dampness and pollution from domestic biomass burning can increase asthma symptoms and rhinitis symptoms in students in northern Thailand.

Acute differences in blood lipids and inflammatory biomarkers following controlled exposures to cookstove air pollution in the STOVES study

Household air pollution is a leading risk factor for morbidity and premature mortality. Numerous cookstoves have been developed to reduce household air pollution, but it is unclear whether such cookstoves meaningfully improve health. In a controlled exposure study with a crossover design, we assessed the effect of pollution emitted from multiple cookstoves on acute differences in blood lipids and inflammatory biomarkers. Participants (n = 48) were assigned to treatment sequences of exposure to air pollution emitted from five cookstoves and a filtered-air control. Blood lipids and inflammatory biomarkers were measured before and 0, 3, and 24 hours after treatments. Many of the measured outcomes had inconsistent results. However, compared to control, intercellular adhesion molecule-1 was higher 3 hours after all treatments, and C-reactive protein and serum amyloid-A were higher 24 hours after the highest treatment. Our results suggest that short-term exposure to cookstove air pollution can increase inflammatory biomarkers within 24 hours.

Aging of Volatile Organic Compounds in October 2017 Northern California Wildfire Plumes

In the western United States, the number and severity of large wildfires have been growing for decades. Biomass burning (BB) is a major source of volatile organic compounds (VOCs) to the atmosphere both globally and regionally. Following emission, BB VOCs are oxidized while being transported downwind, producing ozone, secondary organic aerosols, and secondary hazardous VOCs. In this research, we measured VOCs using proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) in an urban area 55-65 km downwind of the October 2017 Northern California wildfires. Nonaromatic oxygenated compounds were the dominant component of BB VOCs measured. In the smoke plumes, the VOCs account for 70-75% of the total observed organic carbon, with the remainder being particulate matter (with a diameter of <2.5 μm, PM_2.5). We show that the correlation of VOCs with furan (primary BB VOC) and maleic anhydride (secondary BB VOC) can indicate the origin of the VOCs. This was further confirmed by the diurnal variations of the VOCs and their concentration-weighted trajectories. Oxidation during transport consumed highly reactive compounds including benzenoids, furanoids, and terpenoids and produced more oxygenated VOCs. Furthermore, wildfire VOCs altered the ozone formation regime and raised the O₃ levels in the San Francisco Bay Area.

Large contribution of biomass burning emissions to ozone throughout the global remote troposphere

Ozone is the third most important anthropogenic greenhouse gas after carbon dioxide and methane but has a larger uncertainty in its radiative forcing, in part because of uncertainty in the source characteristics of ozone precursors, nitrogen oxides, and volatile organic carbon that directly affect ozone formation chemistry. Tropospheric ozone also negatively affects human and ecosystem health. Biomass burning (BB) and urban emissions are significant but uncertain sources of ozone precursors. Here, we report global-scale, in situ airborne measurements of ozone and precursor source tracers from the NASA Atmospheric Tomography mission. Measurements from the remote troposphere showed that tropospheric ozone is regularly enhanced above background in polluted air masses in all regions of the globe. Ozone enhancements in air with high BB and urban emission tracers (2.1 to 23.8 ppbv [parts per billion by volume]) were generally similar to those in BB-influenced air (2.2 to 21.0 ppbv) but larger than those in urban-influenced air (-7.7 to 6.9 ppbv). Ozone attributed to BB was 2 to 10 times higher than that from urban sources in the Southern Hemisphere and the tropical Atlantic and roughly equal to that from urban sources in the Northern Hemisphere and the tropical Pacific. Three independent global chemical transport models systematically underpredict the observed influence of BB on tropospheric ozone. Potential reasons include uncertainties in modeled BB injection heights and emission inventories, export efficiency of BB emissions to the free troposphere, and chemical mechanisms of ozone production in smoke. Accurately accounting for intermittent but large and widespread BB emissions is required to understand the global tropospheric ozone burden.

Polycyclic Aromatic Carbon: A Key Fraction Determining the Light Absorption Properties of Methanol-Soluble Brown Carbon of Open Biomass Burning Aerosols

The composition and radiative forcing of light-absorbing brown carbon (BrC) aerosol remain poorly understood. Polycyclic aromatics (PAs) are BrC chromophores with fused benzene rings. Understanding the occurrence and significance of PAs in BrC is challenging due to a lack of standards for many PAs. In this study, we quantified polycyclic aromatic carbon (PAC), defined as the carbon of fused benzene rings, based on molecular markers (benzene polycarboxylic acids, BPCAs). Open biomass burning aerosols (OBBAs) of 22 rainforest plants were successively extracted with water and methanol for the analysis of water- and methanol-soluble PAC (WPAC and MPAC, respectively). PAC is an important fraction of water- and methanol-soluble organic carbon (WSOC and MSOC, respectively). WPAC/WSOC ranged from 0.03 to 0.18, and MPAC/MSOC was even higher (range: 0.16-0.80). The priority polycyclic aromatic hydrocarbons contributed less than 1% of MPAC. The mass absorption efficiency (MAE) of MSOC showed a strong linear correlation with MPAC/MSOC (r = 0.60-0.95, p < 0.01). The absorption Ångström exponent (AAE) of methanol-soluble BrC showed a strong linear correlation with the degree of aromatic condensation of MPAC, which was described by the average number of carboxylic groups of BPCA (r = -0.79, p < 0.01). This result suggested that PAC was a key fraction determining the light absorption properties (i.e., light absorptivity and wavelength dependence) of methanol-soluble BrC in OBBAs.

Novel Analysis to Quantify Plume Crosswind Heterogeneity Applied to Biomass Burning Smoke

We present a novel method, the Gaussian observational model for edge to center heterogeneity (GOMECH), to quantify the horizontal chemical structure of plumes. GOMECH fits observations of short-lived emissions or products against a long-lived tracer (e.g., CO) to provide relative metrics for the plume width (w_i/w_CO) and center (b_i/w_CO). To validate GOMECH, we investigate OH and NO₃ oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO (w_HONO/w_CO = 0.73-0.84 ± 0.01) and maleic anhydride (an OH oxidation product) is enhanced on plume edges (w_{maleicanhydride}/w_CO = 1.06-1.12 ± 0.01). By contrast, NO₃ production [P(NO₃)] occurs mainly at the plume center (w_P(NO3)/w_CO = 0.91-1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO₃, are narrower than CO (w_phenol/w_CO = 0.96 ± 0.03, w_catechol/w_CO = 0.91 ± 0.01, and w_{methylcatechol}/w_CO = 0.84 ± 0.01), suggesting that plume edge phenolic losses are the greatest. Yet, nitrophenolic aerosol, their oxidation product, is the greatest at the plume center (w_{nitrophenolicaerosol}/w_CO = 0.95 ± 0.02). In a large plume case study, GOMECH suggests that nitrocatechol aerosol is most associated with P(NO₃). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO₃ in our case study.

Source Apportionment of Fine Organic Particulate Matter (PM2.5) in Central Addis Ababa, Ethiopia

The development of infrastructure, a rapidly increasing population, and urbanization has resulted in increasing air pollution levels in the African city of Addis Ababa. Prior investigations into air pollution have not yet sufficiently addressed the sources of atmospheric particulate matter. This study aims to identify the major sources of fine particulate matter (PM2.5) and its seasonal contribution in Addis Ababa, Ethiopia. Twenty-four-hour average PM2.5 mass samples were collected every 6th day, from November 2015 through November 2016. Chemical species were measured in samples and source apportionment was conducted using a chemical mass balance (CMB) receptor model that uses particle-phase organic tracer concentrations to estimate source contributions to PM2.5 organic carbon (OC) and the overall PM2.5 mass. Vehicular sources (28%), biomass burning (18.3%), plus soil dust (17.4%) comprise about two-thirds of the PM2.5 mass, followed by sulfate (6.5%). The sources of air pollution vary seasonally, particularly during the main wet season (June-September) and short rain season (February-April): From motor vehicles, (31.0 ± 2.6%) vs. (24.7 ± 1.2%); biomass burning, (21.5 ± 5%) vs. (14 ± 2%); and soil dust, (11 ± 6.4%) vs. (22.7 ± 8.4%), respectively, are amongst the three principal sources of ambient PM2.5 mass in the city. We suggest policy measures focusing on transportation, cleaner fuel or energy, waste management, and increasing awareness on the impact of air pollution on the public's health.

Associations between indoor environment in residential buildings in wet and dry seasons and health of students in upper northern Thailand

We performed a repeated questionnaire study on home environment and health (six medical symptoms) in 1159 junior high school students (age 12.8 ± 0.7 years) in upper northern Thailand in wet and dry seasons. Data on outdoor temperature, relative humidity (RH), and air pollution were collected from nearest monitoring station. Odds ratios (OR) were calculated by multi-level logistic regression. Most common symptoms were rhinitis (62.5%), headache (49.8%), throat (42.8%), and ocular symptoms (42.5%). Ocular symptoms were more common at lower RH and rhinitis more common in dry season. Water leakage (28.2%), indoor mold (7.1%), mold odor (4.1%), and windowpane condensation (13.6%) were associated with all six symptoms (ORs: 1.3-3.5). Other risk factors included cat keeping, environmental tobacco smoke (ETS), other odor than mold odor, gas cooking, and cooking with biomass fire. Biomass burning inside and outside the home for other reasons than cooking was associated with all six symptoms (ORs: 1.5-2.6). Associations between home environment exposure and rhinitis were stronger in wet season. In conclusion, dampness-related exposure, windowpane condensation, cat keeping, ETS, gas cooking, and biomass burning can impair adolescent health in upper northern Thailand. In subtropical areas, environmental health effects should be investigated in wet and dry seasons.

Formation Mechanisms and Source Apportionments of Airborne Nitrate Aerosols at a Himalayan-Tibetan Plateau Site: Insights from Nitrogen and Oxygen Isotopic Compositions

Formation pathways and sources of atmosphere nitrate (NO₃^-) have attracted much attention as NO₃^- had detrimental effects on Earth's ecosystem and climate change. Here, we measured nitrogen (δ¹⁵N-NO₃^-) and oxygen (δ¹⁸O-NO₃^- and Δ¹⁷O-NO₃^-) isotope compositions in nitrate aerosols at the Qomolangma station (QOMS) over the Himalayan-Tibetan Plateau (HTP) to quantify the formation mechanisms and emission sources of nitrate at the background site. At QOMS, the enhanced NO₃^- concentrations were observed in the springtime. The average δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and Δ¹⁷O-NO₃^- values were 0.4 ± 4.9, 64.7 ± 11.5 and 27.6 ± 6.9‰, respectively. Seasonal variations of isotope ratios at QOMS can be explained by the different emissions and formation pathways to nitrate. The average fractions of NO₂ + OH and N₂O₅ + H₂O to nitrate production were estimated to be 43 and 52%, respectively, when the NO₃ + hydrocarbon (HC)/dimethyl sulfide (DMS) (NO₃ + HC/DMS) pathway was assumed to be 5%. Using stable isotope analysis in the R (SIAR) model, the relative contributions of biomass burning (BB), biogenic soil emission, traffic, and coal combustion to nitrate were estimated to be 28, 25, 24, and 23%, respectively, on yearly basis. By FLEXible PARTicle (FLEXPART) dispersion model, we highlighted that NO_x from BB emission over South Asia that had undergone N₂O₅ + H₂O processes enhanced the nitrate concentrations in the springtime over the HTP region.

Wintertime Air Quality in Megacity Dhaka, Bangladesh Strongly Affected by Influx of Black Carbon Aerosols from Regional Biomass Burning

Clean air is a key parameter for a sustainable society, and currently, megacity Dhaka has among the worst air qualities in the world. This results from poorly constrained contributions of a variety of sources from both local emissions and regional influx from the highly polluted Indo-Gangetic Plain, impacting the respiratory health of the 21 million inhabitants in the Greater Dhaka region. An important component of the fine particulate matter (PM_2.5) is black carbon (BC) aerosols. In this study, we investigated the combustion sources of BC using a dual carbon isotope (δ¹³C and Δ¹⁴C) in Dhaka during the high-loading winter period of 2013/14 (regular and lockdown/hartal period) in order to guide mitigation policies. On average, BC (13 ± 6 μg m^-3) contributed about 9% of the PM_2.5 (145 ± 79 μg m^-3) loadings. The relative contribution from biomass combustion under regular conditions was 44 ± 1% (with the rest from fossil combustion), while during periods of politically motivated large-scale lockdown of business and traffic, the biomass burning contribution increased to 63 ± 1%. To reduce the severe health impact of BC and other aerosol pollution in Dhaka, mitigation should therefore target regional-scale biomass/agricultural burning in addition to local traffic.

Wildfire smoke impacts on indoor air quality assessed using crowdsourced data in California

Wildfires have become an important source of particulate matter (PM_2.5 < 2.5-µm diameter), leading to unhealthy air quality index occurrences in the western United States. Since people mainly shelter indoors during wildfire smoke events, the infiltration of wildfire PM_2.5 into indoor environments is a key determinant of human exposure and is potentially controllable with appropriate awareness, infrastructure investment, and public education. Using time-resolved observations outside and inside more than 1,400 buildings from the crowdsourced PurpleAir sensor network in California, we found that the geometric mean infiltration ratios (indoor PM_2.5 of outdoor origin/outdoor PM_2.5) were reduced from 0.4 during non-fire days to 0.2 during wildfire days. Even with reduced infiltration, the mean indoor concentration of PM_2.5 nearly tripled during wildfire events, with a lower infiltration in newer buildings and those utilizing air conditioning or filtration.

Quantifying the Health Benefits of Face Masks and Respirators to Mitigate Exposure to Severe Air Pollution

Familiarity with the use of face coverings to reduce the risk of respiratory disease has increased during the coronavirus pandemic; however, recommendations for their use outside of the pandemic remains limited. Here, we develop a modeling framework to quantify the potential health benefits of wearing a face covering or respirator to mitigate exposure to particulate air pollution. This framework accounts for the wide range of available face coverings and respirators, fit factors and efficacy, air pollution characteristics, and exposure-response data. Our modeling shows that N95 respirators offer robust protection against different sources of particulate matter, reducing exposure by more than a factor of 14 when worn with a leak rate of 5%. Synthetic-fiber masks offer less protection with a strong dependence on aerosol size distribution (protection factors ranging from 4.4 to 2.2), while natural-fiber and surgical masks offer reductions in the exposure of 1.9 and 1.7, respectively. To assess the ability of face coverings to provide population-level health benefits to wildfire smoke, we perform a case study for the 2012 Washington state fire season. Our models suggest that although natural-fiber masks offer minor reductions in respiratory hospitalizations attributable to smoke (2%-11%) due to limited filtration efficiency, N95 respirators and to a lesser extent surgical and synthetic-fiber masks may lead to notable reductions in smoke-attributable hospitalizations (22%-39%, 9%-24%, and 7%-18%, respectively). The filtration efficiency, bypass rate, and compliance rate (fraction of time and population wearing the device) are the key factors governing exposure reduction potential and health benefits during severe wildfire smoke events.

Molecular Dynamics and Light Absorption Properties of Atmospheric Dissolved Organic Matter

The light-absorbing organic aerosol referred to as brown carbon (BrC) affects the global radiative balance. The linkages between its molecular composition and light absorption properties and how environmental factors influence BrC composition are not well understood. In this study, atmospheric dissolved organic matter (ADOM) in 55 aerosol samples from Guangzhou was characterized using Fourier transform ion cyclotron resonance mass spectrometry and light absorption measurements. The abundant components in ADOM were aliphatics and peptide-likes (in structure), or nitrogen- and sulfur-containing compounds (in elemental composition). The light absorption properties of ADOM were positively correlated with the levels of unsaturated and aromatic structures. Particularly, 17 nitrogen-containing species, which are identified by a random forest, characterized the variation of BrC absorption well. Aggregated boosted tree model and nonmetric multidimensional scaling analysis show that the BrC composition was largely driven by meteorological conditions and anthropogenic activities, among which biomass burning (BB) and OH radical were the two important factors. BrC compounds often accumulate with elevated BB emissions and related secondary processes, whereas the photolysis/photooxidation of BrC usually occurs under high solar radiance/^•OH concentration. This study first illuminated how environmental factors influence BrC at the molecular level and provided clues for the molecular-level research of BrC in the future.

Qualitative Study on the Observations of Emissions, Transport, and the Influence of Climatic Factors from Sugarcane Burning: A South African Perspective

There are two methods of harvesting sugarcane—manual or mechanical. Manual harvesting requires the burning of the standing sugarcane crop. Burning of the crop results in the emission of aerosols and harmful trace gases into the atmosphere. This work makes use of a long-term dataset (1980–2019) to study (1) the atmospheric spatial and vertical distribution of pollutants; (2) the spatial distribution and temporal change of biomass emissions; and (3) the impact/influence of climatic factors on temporal change in atmospheric pollutant loading and biomass emissions over the Mpumalanga and KwaZulu Natal provinces in South Africa, where sugarcane farming is rife. Black carbon (BC) and sulfur dioxide (SO₂) are two dominant pollutants in the JJA and SON seasons due to sugarcane burning. Overall, there was an increasing trend in the emissions of BC, SO₂, and carbon dioxide (CO₂) from 1980 to 2019. Climatic conditions, such as warm temperature, high wind speed, dry conditions in the JJA, and SON season, favor the intensity and spread of the fire, which is controlled. The emitted pollutants are transported to neighboring countries and can travel over the Atlantic Ocean, as far as ~6600 km from the source site.

Indoor fine particulate matter and demographic, household, and wood stove characteristics among rural US homes heated with wood fuel

Household heating using wood stoves is common practice in many rural areas of the United States (US) and can lead to elevated concentrations of indoor fine particulate matter (PM_2.5 ). We collected 6-day measures of indoor PM_2.5 during the winter and evaluated household and stove-use characteristics in homes at three rural and diverse study sites. The median indoor PM_2.5 concentration across all homes was 19 µg/m³ , with higher concentrations in Alaska (median = 30, minimum = 4, maximum = 200, n = 10) and Navajo Nation homes (median = 29, minimum = 3, maximum = 105, n = 23) compared with Montana homes (median = 16, minimum = 2, maximum = 139, n = 59). Households that had not cleaned the chimney within the past year had 65% higher geometric mean PM_2.5 compared to those with chimney cleaned within 6 months (95% confidence interval [CI]: -1, 170). Based on a novel wood stove grading method, homes with low-quality and medium-quality stoves had substantially higher PM_2.5 compared to homes with higher-quality stoves (186% higher [95% CI: 32, 519] and 161% higher; [95% CI:27, 434], respectively). Our findings highlight the need for, and complex nature of, regionally appropriate interventions to reduce indoor air pollution in rural wood-burning regions. Higher-quality stoves and behavioral practices such as regular chimney cleaning may help improve indoor air quality in such homes.

Elucidating an Atmospheric Brown Carbon Species-Toward Supplanting Chemical Intuition with Exhaustive Enumeration and Machine Learning

Brown carbon (BrC) is involved in atmospheric light absorption and climate forcing and can cause adverse health effects. Understanding the formation mechanisms and molecular structure of BrC is of key importance in developing strategies to control its environment and health impact. Structure determination of BrC is challenging, due to the lack of experiments providing molecular fingerprints and the sheer number of molecular candidates with identical mass. Suggestions based on chemical intuition are prone to errors due to the inherent bias. We present an unbiased algorithm, using graph-based molecule generation and machine learning, which can identify all molecular structures of compounds involved in biomass burning and the composition of BrC. We apply this algorithm to C₁₂H₁₂O₇, a light-absorbing "test case" molecule identified in chamber experiments on the aqueous photo-oxidation of syringol, a prevalent marker in wood smoke. Of the 260 million molecular graphs, the algorithm leaves only 36,518 (0.01%) as viable candidates matching the spectrum. Although no unique molecular structure is obtained from only a chemical formula and a UV/vis absorption spectrum, we discuss further reduction strategies and their efficacy. With additional data, the method can potentially more rapidly identify isomers extracted from lab and field aerosol particles without introducing human bias.

[Pollution Characteristics and Risk Assessment of Nitro Polycyclic Aromatic Hydrocarbons in PM2.5 of Nanjing, China]

Nitro polycyclic aromatic hydrocarbons (NPAHs) are important pollutants with carcinogenic effects present in PM_2.5. To analyze the pollution characteristics and sources of NPAHs, 14 samples of PM_2.5 were collected in Nanjing from November 2017 to March 2018. The results showed that, 2,8-dinitrodibenzothiophene (743 pg·m^-3), 2, 7-dinitrofluorene (331 pg·m^-3), 9-nitroanthracene (326 pg·m^-3), 3-nitrofluoranthene (217 pg·m^-3), and 1,8-dinitropyrene (193 pg·m^-3) were dominant, and the detection concentrations notably varied between seasons; the highest concentrations occurred in winter (3082 pg·m^-3) followed by autumn (1553 pg·m^-3) and spring (1218 pg·m^-3). The ratio of nitrofluoranthene and 1-nitropyrene concentrations, and 9-nitroanthracene and 1-nitropyrene concentrations, indicated that the main sources of NPAHs in the PM_2.5 of Nanjing were photooxidation of polycyclic aromatic hydrocarbons and biomass burning. NPAHs were more typically associated with smaller particles, which further indicated that secondary formation is an important source. The current carcinogenic risk of NPAHs in PM_2.5 in Nanjing is controllable, and dinitro-polycyclic aromatic hydrocarbons represent the highest level of risk. The data presented in this study provide important baseline information that can inform the management of risks associated with NPAHs in PM_2.5 in Nanjing.

ENSO and Southeast Asian biomass burning modulate subtropical trans-Pacific ozone transport

Trans-Pacific transport of enhanced ozone plumes has been mainly attributed to fossil fuel combustion in Asia in spring, but less attention has been paid to vegetation fires in Asia. Here we show that the El Niño-Southern Oscillation (ENSO)-modulated fires in Southeast Asia, rather than Asian fossil fuel plumes, dominate the interannual variability of springtime trans-Pacific transport of ozone across the entire North Pacific Ocean. During El Niño springs, the intensified fires from both the Indochinese Peninsula and Indonesia, together with large-scale circulation anomalies, result in enhanced ozone plumes that stretch over 15 000 km in both the lower-middle and upper troposphere. This enhancement is also observed in the in situ measurements of ozone concentration, with an almost 10% increase at Mauna Loa Observatory, Hawaii, a unique site to monitor the long-distance transport over the North Pacific. This study reports an unexpectedly strong influence of vegetation fires, linked with climate variability, on global tropospheric chemistry and proves once more how complex the interactions in the climate system are.