Arabian incense (Bakhoor) burning is a widely practiced fragrancing and ceremonial activity, yet how the Bakhoor composition controls particle emissions and oxidative potential remains poorly constrained, especially under repeated use in low-ventilation settings. Here we characterized emissions from Bakhoor burning in a controlled chamber using a charcoal-assisted heating configuration representative of common practice and quantified aerosol oxidative potential using complementary acellular dithiothreitol (DTT) activity and a macrophage-based intracellular oxidative-stress response, with smoldering sidestream cigarette smoke as a protocol-matched indoor combustion reference. Normalized by the initial Bakhoor mass per burn, Bakhoor burning produced particle mass and number emission rates of 670–1690 µg min−1 g−1 and (6–7) × 1011 particles min−1 g−1, respectively. Ultrafine particles contributed 70 %–75 % of the total particle number, and their emission rates substantially exceeded those from sidestream cigarette smoke. Across Bakhoor materials, emission magnitude followed a nonlinear power-law relationship with the loading of the hexane-soluble fraction, indicating that this fraction is an important control on particle production. In the acellular assay, the water-soluble particle mass-normalized DTT consumption rate (OPDTT, mWS) was approximately 32 pmol min−1 µg−1, modestly lower than that of cigarette smoke particles, whereas Bakhoor burning particles elicited stronger intracellular oxidative-stress responses. Ozone aging increased oxidative potential for both sources, and the acellular and cellular responses remained evident after aging equivalent to days of indoor exposure. Overall, Bakhoor burning represents a previously underrecognized source of ultrafine aerosol with substantial oxidative potential.

Indoor sulfur dioxide (SO2) is a common air pollutant that may affect surface-associated bacterial communities. While high concentrations (≥100 ppm) are known to act as disinfectants, the effects of typical indoor concentrations (≤100 ppb) remain unclear. This study investigated SO2 impacts on bacterial viability, biofilm formation, and community composition across nutrient gradients and relative humidity (RH, 20%-97%) using controlled chamber exposures with Escherichia coli to probe mechanistic responses and real-world kitchen surface communities to assess ecological relevance. Bactericidal effects were strongest on loosely adherent E. coli under nutrient-poor, low-RH (20%) conditions and low cell density (106 CFU/cm2), likely due to increased acidification and sulfate adsorption. At this density and nutrient level, ≥30 ppb of SO2 significantly reduced viability across all RH levels, while ≤100 ppb did not affect nutrient-rich surfaces at 97% RH or at higher densities (107 CFU/cm2). Biofilm inhibition required 100 ppb, indicating greater resistance than loosely adherent cells. In kitchen surface communities, bacterial abundance declined at 30 ppb on cooking surfaces and at ≥10 ppb on noncooking surfaces at ≤60% RH, with no effect at 97% RH. At 30 ppb, SO2 reduced bacterial diversity and altered microbial composition, independent of surface type or RH. Ambient SO2 evidently has an underrecognized impact on indoor-surface microbial communities.

Peroxide species, including hydrogen peroxide (H2O2) and organic peroxides, are key oxidants in shaping the atmospheric oxidative capacity. However, their formation pathways remain elusive under high-NOx conditions, where gas-phase mechanisms are suppressed. Herein, we report an "in-particle" peroxide formation pathway driven by photosensitization reactions in biomass burning organic aerosol. This mechanism remains highly efficient even in polluted, high-NOx environments, leading to orders-of-magnitude increase in particulate H2O2 concentrations in the presence of sunlight than that expected from gas-phase partitioning. These findings suggest that intensifying wildfires in our warming world, beyond their primary emissions, may substantially alter the atmospheric oxidation chemistry and exacerbate air quality degradation.

Biomass burning (BB) is increasingly relevant to air quality as global warming intensifies wildfire activity. Levoglucosan (LEVO) is a widely used tracer for BB, but its oxidative decay is still less understood. While gas-phase hydroxyl radicals (•OH) have been considered the primary oxidants for LEVO decay in biomass burning organic aerosol (BBOA) particles, high particulate viscosity at low relative humidity (RH) limits •OH uptake. In this study, we demonstrate a less recognized pathway that leads to LEVO degradation in BBOA particles under light, which is primarily driven by triplet excited states (3BBOA*) via hydrogen transfer in photosensitization reactions. Unlike •OH oxidation that relies on mass transfer of oxidants from the gas phase, 3BBOA* oxidation remains effective in viscous particles at low RH and accounts for ∼50% of LEVO decay, shortening its chemical lifetime in previous estimates by a half to ∼1.8 days. These findings highlight the importance of processes that generate oxidants within particles, such as photosensitization, and enhance our understanding of the atmospheric fate of LEVO to facilitate BB tracking.

Volatile chemical products (VCPs) are increasingly recognized as significant sources of volatile organic compounds (VOCs) in urban atmospheres, potentially serving as key precursors for secondary organic aerosol (SOA) formation. This study investigates the formation and physicochemical transformations of VCP-derived SOA, produced through ozonolysis of VOCs evaporated from a representative room deodorant air freshener, focusing on the effects of aerosol evaporation on its molecular composition, light absorption properties, and reactive oxygen species (ROS) generation. Following aerosol evaporation, solutes become concentrated, accelerating reactions within the aerosol matrix that lead to a 42% reduction in peroxide content and noticeable browning of the SOA. This process occurs most effectively at moderate relative humidity (∼40%), reaching a maximum solute concentration before aerosol solidification. Molecular characterization reveals that evaporating VCP-derived SOA produces highly conjugated nitrogen-containing products from interactions between existing or transformed carbonyl compounds and reduced nitrogen species, likely acting as chromophores responsible for the observed brownish coloration. Additionally, the reactivity of VCP-derived SOA was elucidated through heterogeneous oxidation of sulfur dioxide (SO2), which revealed enhanced photosensitized sulfate production upon drying. Direct measurements of ROS, including singlet oxygen (1O2), superoxide (O2•-), and hydroxyl radicals (•OH), showed higher abundances in dried versus undried SOA samples under light exposure. Our findings underscore that drying significantly alters the physicochemical properties of VCP-derived SOA, impacting their roles in atmospheric chemistry and radiative balance.

222 nm UV indoor disinfection using KrCl* excimer lamps has been gaining popularity due to claims of minimal ocular and skin damage from direct irradiation. However, the secondary aerosol formation under irradiation of KrCl* excimer lamps, which could be an inhalation hazard, is less explored. SO2, a well-known precursor of outdoor sulfate aerosol, is also ubiquitous in indoor environments in urban cities in northern China where coal is used for domestic heating and cooking. In this work, we studied secondary aerosol formation by 222 nm irradiation on SO2, using a Go: PAM flow reactor, a scanning mobility particle sizer (SMPS), and a time-of-flight aerosol chemical composition monitor (ToF-ACSM). The formation of sulfate nanoparticles was found much more effective at 222 nm than at 254 nm and under fluorescent lamp (FL) irradiation at the same UV doses and RH, likely due to different SO2 oxidation mechanisms. We have also found that NH3 and cooking volatile organic compounds (CVOC), as other indoor-relevant gases, promoted the formation of secondary aerosols by 222 nm radiation on SO2. Overall, 222 nm disinfection can generate secondary pollutants in indoor environments. Caution should be taken during its indoor application, especially in areas with high SO2 concentrations such as coal-fueled households.

Biomass-burning organic aerosol(s) (BBOA) are rich in brown carbon, which significantly absorbs solar irradiation and potentially accelerates global warming. Despite its importance, the multiphase photochemistry of BBOA after light absorption remains poorly understood due to challenges in determining the oxidant concentrations and the reaction kinetics within aerosol particles. In this study, we explored the photochemical reactivity of BBOA particles in multiphase S(IV) oxidation to sulfate. We found that sulfate formation in BBOA particles under light is predominantly driven by photosensitization involving the triplet excited states (3BBOA*) instead of iron, nitrate, and S(IV) photochemistry. Rates in BBOA particles are three orders of magnitude higher than those observed in the bulk solution, primarily due to the fast interfacial reactions. Our results highlight that the chemistry of 3BBOA* in particles can greatly contribute to the formation of sulfate, as an example of the secondary pollutants. Photosensitization of BBOA will likely become increasingly crucial due to the intensified global wildfires.

. Atmospheric oxidation of sulfur dioxide (SO 2 ) to sulfate has been widely investigated by means of gas phase and

Particulate free amino acids (FAAs) are essential components of organonitrogen that have critical climate impacts, and they are usually considered stable end-products from protein degradation. In this work, we investigated the decay of glycine (GC) as a model FAA under the photolysis of different particulate nitrate salts using an in situ Micro-Raman system. Upon cycling the relative humidity (RH) between 3 % and 80 % RH, ammonium nitrate (AN) and GC mixed particles did not exhibit any phase change, whereas sodium nitrate (SN) and GC mixed particles crystallized at 60 % and deliquesced at 82 % RH. Under light illumination at 80 % RH, AN + GC particles showed almost no spectral changes, while rapid decays of glycine and nitrate were observed in SN + GC particles. The interactions between nitrate and glycine in AN + GC particles suppressed crystallization but also hindered nitrate photolysis and glycine decay. On the other hand, glycine may form a complex with Na + in deliquescent SN + GC particles and allow unbonded nitrate to undergo photolysis and trigger glycine decay, though nitrate photolysis was greatly hindered upon particle crystallization. Our work provides insights into how FAAs may interact with different nitrate salts under irradiation and lead to distinct decay rates, which facilitates their atmospheric lifetime estimation.

Atmospheric oxidation of sulfur dioxide (SO2) to sulfate has been widely investigated by means of gas-phase and in-cloud chemistry studies. Recent field measurements have shown significant sulfate formation in cloud-free environments with high aerosol loadings. As an important fraction of biomass burning aerosol components, particulate phenolic and non-phenolic aromatic carbonyls may initiate photosensitized multiphase oxidation of SO2 in aerosols, of which our knowledge however is still in its nascent stage. In this study, on the basis of single-particle aerosol mass spectrometry (SPAMS) measurements, we find evident sulfate formation in the biomass-burning-derived photosensitizer particles under UV and SO2 exposure, attributable to photosensitized oxidation of S(IV), while almost no sulfate was observed under dark conditions. The efficiency of sulfate production by photosensitizer particles under UV irradiation, represented by the number percentage of sulfate-containing particles (99 %-43 %) and the relative peak area (RPA) of sulfate (0.67-0.12) in single-particle spectra, in descending order, were 3,4-dimethoxybenzaldehyde (DMB), vanillin (VL) and syringaldehyde (SyrAld). Internal mixtures of VL and potassium nitrate (KNO3) gave a slightly lower number percentage and RPA of sulfate than VL particles alone. In externally mixed VL and KNO3 particles, sulfate was predominantly formed on the former, confirming that sulfate formation via photosensitization prevails over that via nitrate photolysis. Our results suggest that photosensitized oxidation of S(IV) could make an important contribution to aerosol sulfate formation, especially in areas influenced by biomass burning.

Far-UVC irradiation (222 nm) is an emerging approach for disinfection due to its effectiveness and potentially harmless nature to humans by direct irradiation compared with other UV wavelengths.

Incense burning is a common religious activity that emits abundant gaseous and particulate pollutants into the 18 atmosphere. During their atmospheric lifetime, these gases and particles are subjected to oxidation, leading to the formation 19 of secondary pollutants. We examined the oxidation of incense burning plumes under O3 exposure and dark condition using 20 an oxidation flow reactor connected to a single particle aerosol mass spectrometer (SPAMS). Nitrate formation was observed 21 in incense burning particles, mainly attributable to the ozonolysis of nitrogen-containing organic compounds. With UV on, 22 nitrate formation was significantly enhanced, likely due to HNO3/HNO2/NOx uptake triggered by OH chemistry, which is 23 more effective than ozone oxidation. The extent of nitrate formation is insensitive to O3 and OH exposure, which can be 24 explained by the diffusion limitation on interfacial uptake. The O3-UV-aged particles are more oxygenated and functionalized 25 than O3-Dark-aged particles. Oxalate and malonate, two typical secondary organic aerosols (SOA), were found in O3-UV-aged 26 particles. Our work reveals that nitrate, accompanied by SOA, can rapidly form in incense-burning particles upon 27 photochemical oxidation in the atmosphere, which could deepen our understanding of air pollution caused by religious 28 activities. 29

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.

Incense burning is a common religious activity that emits abundant gaseous and particulate pollutants into the atmosphere. During their atmospheric lifetime, these gases and particles are subjected to oxidation, leading to the formation of secondary pollutants. We examined the oxidation of incense burning plumes under O3 exposure and dark condition using an oxidation flow reactor connected to a single particle aerosol mass spectrometer (SPAMS). Nitrate formation was observed in incense burning particles, mainly attributable to the ozonolysis of nitrogen-containing organic compounds. With UV on, nitrate formation was significantly enhanced, likely due to HNO3/HNO2/NOx uptake triggered by OH chemistry, which is more effective than ozone oxidation. The extent of nitrate formation is insensitive to O3 and OH exposure, possibly due to the diffusion limitation on interfacial uptake. The O3-UV-aged particles are more oxygenated and functionalized than O3-Dark-aged particles. Oxalate and malonate, two typical secondary organic aerosol (SOA) components, were found in O3-UV-aged particles. Our work reveals that nitrate, accompanied by SOA, can rapidly form in incense-burning particles upon photochemical oxidation in the atmosphere, which could deepen our understanding of air pollution caused by religious activities.

Incense burning is a common religious activity that emits abundant gaseous and particulate pollutants into the 18 atmosphere. During their atmospheric lifetime, these gases and particles are subjected to (photo-)oxidation, leading to the 19 formation of secondary pollutants. We examined the oxidation of incense burning plumes under O3 exposure and dark 20 condition using an oxidation flow reactor connected to a single particle aerosol mass spectrometer (SPAMS). Nitrate formation 21 was observed in incense burning particles, mainly attributable to the ozonolysis of nitrogen-containing organic compounds. 22 With UV on, nitrate formation was significantly enhanced, likely due to HNO3/HNO2/NOx uptake triggered by OH chemistry, 23 which is more effective than ozone oxidation. The extent of nitrate formation is insensitive to O3 and OH exposure, which can 24 be explained by the diffusion limitation on interfacial uptake. The OH-aged particles are more oxygenated and functionalized 25 than O3-aged particles. Oxalate and malonate, two typical secondary organic aerosols (SOA), were found in OH-aged particles. 26 Our work reveals that nitrate, accompanied by SOA, can rapidly form in incense-burning particles upon photochemical 27 oxidation in the atmosphere, which could deepen our understanding of air pollution caused by religious activities. 28

Particulate nitrate photolysis can lead to the formation of secondary inorganic and organic aerosols that affect climate, air quality, and human health.