Utilizing ground-based aerosol observations collected from 19 August 2024 to 19 August 2025, we analyzed the characteristics of aerosol particle size distributions (PSDs) at the summit of Mount Liupan during non-precipitation periods. The average aerosol number concentration spectrum exhibited a trimodal distribution, characterized by the presence of coarse particles exceeding 1000 nm. Distinct diurnal and monthly variations were observed in the number concentrations across the nucleation (5–20 nm), Aitken (20–100 nm), accumulation (100–1000 nm), and coarse (>1000 nm) modes. Generally, diurnal variations were characterized by higher concentrations during the daytime and lower levels at night, although the specific evolutionary patterns varied among different modes. The nucleation mode particles exhibited a unimodal distribution; their number concentration increased rapidly after sunrise and peaked at 14:00 Beijing Time (BJT). The peak for the Aitken and accumulation modes lagged behind that of the nucleation mode. Furthermore, the changes in aerosol PSD under varying precipitation intensities were analyzed, revealing that the reductions in aerosol particle number concentrations generally became more pronounced as precipitation intensity increased. Finally, aerosol PSDs across different times, months, and precipitation stages (before, during, and after precipitation) were fitted using multiple log-normal distributions. Overall, the multiple log-normal fits showed good performance. Except for the during- and post-heavy-rain spectra, which yielded R2 values of 0.8258 and 0.8879, respectively, the R2 values of all other fitted spectra ranged from 0.9121 to 0.9997. This provides localized aerosol spectral parameters as foundational references for numerical simulations in this region.

Accurate descriptions of cloud droplet spectra from aerosol activation to vapor condensation using microphysical parameterization schemes are crucial for numerical simulations of precipitation and climate change in weather forecasting and climate prediction models. Hence, the latest activation and triple-moment condensation schemes were combined to simulate and analyze the evolution characteristics of a cloud droplet spectrum from activation to condensation and compared with a high-resolution Lagrangian bin model and the current double-moment condensation schemes, in which the spectral shape parameter is fixed or diagnosed by an empirical formula. The results demonstrate that the latest schemes effectively capture the evolution characteristics of the cloud droplet spectrum during activation and condensation, which is in line with the performance of the bin model. The simulation of the latest activation and condensation schemes in a parcel model shows that the cloud droplet spectrum gradually widens and exhibits a multimodal distribution during the activation process, accompanied by a decrease in the spectral shape and slope parameters over time. Conversely, during the condensation process, the cloud droplet spectrum gradually narrows, resulting in increases in the spectral shape and slope parameters. However, these double-moment schemes fail to accurately replicate the evolution of the cloud droplet spectrum and its multimodal distribution characteristics. Furthermore, the latest schemes were coupled into a 1.5D cumulus model, and an observation case was simulated. The simulations confirm that the cloud droplet spectrum appears wider at the supersaturated cloud base and cloud top due to activation, while it becomes narrower at the middle altitudes of the cloud due to condensation growth.

Soil passivators can effectively reduce the risk of heavy metals migration in contaminated soil and are widely used for mining soil remediation. The long-term stability of heavy metals by passivators on soil and their transformation mechanisms in unstable and extreme climatic conditions remain largely unexplored. Here, the long-term immobilization effect of As, Sb and Pb in mining soil by Fe-C based passivators was investigated through freeze-thaw (FT) and dry-wet (DW) aging processes. The results revealed that the immobilization effect of As, Sb and Pb in Fe-C based passivators amended soil reduced 3.95 %-13.60 %/17.63 %-25.89 %, 13.67 %25.67 %/3.81-10.87 % and 5.44 %-13.05 %/1.18 %-6.90 % after FT/DW aging, respectively. Meanwhile, the proportion of unstable fractions of As, Sb and Pb also increased. The release of As, Sb and Pb was highly related to soil organic matter (SOM), dissolved organic carbon (DOC), Fe(III, hydr) oxides, pH and soil particle size. Particularly, the decomposition of Fe-SOM associations and Fe(III) reduction contributed to the release of heavy metals during FT and DW aging. In addition, the field trial further confirmed that Fe-C based passivator has longterm stability capacity for As, Sb and Pb under natural conditions. Our findings confirmed the long-term immobilization of As, Sb and Pb by Fe-C based passivators, and provided insights for inhibiting heavy metals release in extreme environments.

PM2.5 poses significant public health risks, with its sources and composition exhibiting pronounced spatial heterogeneity. While extensive research has focused on heavily polluted regions in northern China, the pollution structure of Jiangxi Province remains understudied. This study investigates the chemical composition and source apportionment of PM2.5 during a severe regional haze episode (25 December 2023-20 January 2024) across six cities in Jiangxi Province: Nanchang, Jiujiang, Pingxiang, Ji'an, Xinyu. Observed PM2.5 concentrations ranged from 44.1 to 76.6 mu g/m3, dominated by water-soluble ions, organic matter (OM), and carbonaceous aerosols. Spatial analysis revealed a pollution hotspot centered on Nanchang and Jiujiang, characterized by distinct gradients in SNA (SO4 2-, NO3 -, NH4 +) and OM. Based on local emission patterns and topographic features and the component concentration differences of PM2.5, we speculate that there are three regional patterns: (1) Northern cities, characterized by high loadings of NO3 - (industrial), OM (VOCs-derived), and SO4 2- (promoted by lake air masses with high humidity); (2) Central cities, dominated by local agricultural NH4 + and conversion from industrial gaseous sources precursors enhanced by local photochemistry; (3) Southern Jiangxi, where vehicular NOx-to-NO3 - conversion predominated, exacerbated by topographic stagnation from the Nanling Mountains. Positive Matrix Factorization (PMF) resolved city-specific sources: secondary formation and combustion in Nanchang; industrial and vehicular emissions in Jiujiang; agricultural NH4 + and traffic in Pingxiang; mixed industrial-traffic sources in Ji'an; and vehicle-derived NO3 - with dust in Ganzhou. These findings underscore spatiotemporal heterogeneity in energy structures and regional transport pathways, providing a scientific basis for region-specific PM2.5 control strategies in Jiangxi Province, China.

Despite increasing evidence that cigarette smoke is a significant source of indoor fine particulate matter (PM2.5), quantitative emission factors (EFs) for PM2.5 and its toxic chemical composition in mainstream (MS) and sidestream (SS) smoke are still not well defined. In this study, we employed a custom-designed chamber to separately collect MS (intermittent puff) and SS (continuous sampling) smoke from eleven cigarette models, representing six brands and two product types, under controlled conditions. PM2.5 was collected on quartz-fiber filters and analyzed for carbon fractions (using the thermal–optical IMPROVE-A protocol), nine water-soluble inorganic ions (by ion chromatography), and twelve trace elements (via ICP-MS). SS smoke exhibited significantly higher mass fractions of total analyzed species (84.7% vs. 65.9%), carbon components (50.6% vs. 44.2%), water-soluble ions (17.1% vs. 13.7%), and elements (17.0% vs. 7.0%) compared to MS smoke. MS smoke is characterized by a high proportion of pyrolytic organic carbon fractions (OC1–OC3) and specific elements such as vanadium (V) and arsenic (As), while SS smoke shows elevated levels of elemental carbon (EC1), water-soluble ions (NH4+, NO3−), and certain elements like zinc (Zn) and cadmium (Cd). The toxicity-weighted distribution indicates that MS smoke primarily induces membrane disruption and pulmonary inflammation through semi-volatile organics and elements, whereas SS smoke enhances oxidative stress and cardiopulmonary impairment via EC-mediated reactions and secondary aerosol formation. The mean OC/EC ratio of 132.4 in SS smoke is an order of magnitude higher than values reported for biomass or fossil-fuel combustion, indicative of extensive incomplete combustion unique to cigarettes and suggesting a high potential for oxidative stress generation. Emission factors (µg/g cigarette) revealed marked differences: MS delivered higher absolute EFs for PM2.5 (422.1), OC (8.8), EC (5.0), Na+ (32.6), and V (29.2), while SS emitted greater proportions of NH4+, NO3−, Cl−, and carcinogenic metals (As, Cd, Zn). These findings provide quantitative source profiles suitable for receptor-oriented indoor source-apportionment models and offer toxicological evidence to support the prioritization of comprehensive smoke-free regulations.

Here, a heterogeneous ice nucleation parameterization associated with aerosol acting as ice nucleating particle (INP) was implanted into a two-dimensional numerical cumulus model. To explore the impact of INP on microphysical and electrical processes, a comparison was conducted with the original approach, which employs an empirical formula. Simulation results indicate that INP greatly impacted microphysical evolution in the heterogeneous ice nucleation process, reducing total liquid precipitation amounts and causing a slight precipitation delay, as well as increasing the diameter and mixing ratio of ice crystals and the expansion of the vertical distribution of ice crystals. This led to a notable change in electrification in thunderstorms. The increase of ice crystal diameter was the dominant contributor to the enhancement of electrification using the new parameterization. Additionally, unlike the structure of thunderclouds in a mature stage, which always retains a normal dipole structure adopting the empirical formula, a tripole structure developed a lower positive charge, and the polarity inversion with upper negative and lower positive charges occurred when the new parameterization was adopted. This predominately was the result of abundant ice crystals present below the reversal temperature. The electrification characteristics of thunderstorms may have a close connection with lightning activity. It has been found that charge structure changed significantly in the two cases, with the tripolar charge structure facilitating the production of inverted intra-cloud (IC) flashes and negative Cloud-to-ground (CG) flashes simulated by the new parameterization; additionally, clouds simulated using the empirical formula may be able to develop normal IC lightning and positive CG flashes. Therefore, it will be very meaningful to obtain greater insight into the characteristics of thunderstorms electrification in cumulus model with aerosols.

Progress in numerical models and improved computational capabilities have significantly advanced our comprehension of how aerosol particles impact thunderstorm clouds. Yet, much of this research has focused on employing bulk microphysics models to explain the impacts of aerosol particles acting as cloud condensation nuclei (CCN) on electrical activities in thunderstorm clouds. The bulk thunderstorm models use mean sizes of particles and terminal-fall velocities. This causes calculation deviation in the electrification simulation, which in turn leads to deviations in the simulation of lightning processes. Developing this further, we established a three-dimensional high-resolution cloud–aerosol bin thunderstorm model with electrification and lightning to provide more accurate microphysics and dynamic fields for studying electrical activities. For evaluating the impacts of aerosol particles, specifically CCN, on the properties of continental thunderclouds, aerosols from both clean and polluted continental environments were selected. Cloud simulations indicate that droplets develop a narrower spectrum in polluted continental conditions, and weakened ice crystal growth increases the number of small ice crystals compared to clean conditions. Smaller droplets and ice crystals result in less effective riming and decreased graupel concentration and mass. Consequently, a significant decrease in large ice particles leads to a weakened process of charge separation under conditions of pollution. As a direct result, there is about a 43% reduction in lightning frequency and a delay of approximately 5 min in the lightning process under polluted conditions.

The light absorption enhancement (Eabs) of black carbon (BC) coated with non‐BC materials is crucial in the assessment of radiative forcing, yet its evolution during photochemical aging of plumes from biomass burning, the globe's largest source of BC, remains poorly understood. In this study, plumes from open burning of corn straw were introduced into a smog chamber to explore the evolution of Eabs during photochemical aging. The light absorption of BC was measured with and without coating materials by using a thermodenuder, while the size distributions of aerosols and composition of BC coating materials were also monitored. Eabs was found to increase initially, and then decrease with an overall downward trend. The lensing effect dominated in Eabs at 520 nm, with an estimated contribution percentages of 47.5%–94.5%, which is far greater than light absorption of coated brown carbon (BrC). The effects of thickening and chemical composition changes of the coating materials on Eabs were evaluated through comparing measured Eabs with that calculated by the Mie theory. After OH exposure of 1 × 1010 molecules cm−3 s, the thickening of coating materials led to an Eabs increase by 3.2% ± 1.6%, while the chemical composition changes or photobleaching induced an Eabs decrease by 4.7% ± 0.6%. Simple forcing estimates indicate that coated BC aerosols exhibit warming effects that were reduced after aging. The oxidation of light‐absorbing CxHy compounds, such as polycyclic aromatic hydrocarbons (PAHs), to CxHyO and CxHyO>1 compounds in coating materials may be responsible for the photobleaching of coated BrC.

Increasing air change rate can reduce the performance of air purifiers by acting as a source of outdoor PM into the indoors. Use of multiple air purifiers in a residence reduces PM at a greater rate than use of a single more powerful air purifier.

To investigate the impact of aging on the sources, volatility, and viscosity of organic aerosol (OA) in Chinese outflows, a high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) coupled with a thermodenuder (TD) was deployed in the spring of 2018 in Dongying, which is a regional receptor site of metropolitan emissions in the North China Plain (NCP). The average mass concentration of PM1 is 31.5±22.7 µg m−3, which is mainly composed of nitrate (33 %) and OA (25 %). The source apportionment results show that the OA is mainly contributed by oxygenated OA (OOA) from secondary sources, including background-OOA (33 %) representing a background concentration of OA (2.6 µg m−3) in the NCP area, and transported-OOA (33 %) oxidized from urban emissions. The other two factors include aged hydrocarbon-liked OA (aged-HOA, 28 %) from transported vehicle emissions and biomass burning OA (BBOA, 5 %) from local open burning. The volatility of total OA (average C*=3.2×10-4 µg m−3) in this study is generally lower than that reported in previous field studies, which is mainly due to the high OA oxidation level resulting from aging processes during transport. The volatilities of OA factors follow the order of background-OOA (average C*=2.7×10-5 µg m−3) < transported-OOA (3.7×10-4µgm-3)< aged-HOA (8.1×10-4µgm-3)< BBOA (0.012 µg m−3). Extremely low volatilities in ambient air indicate that oligomers may exist in aged plumes. The viscosity estimation suggests that the majority of ambient OA in this study behaves as semisolid (60 %), liquifies at higher relative humidity (RH) (21 %), and solidifies (19 %) during noon when the RH is low and the oxidation level is high. Finally, the estimated mixing time of molecules in 200 nm OA varies dramatically from minutes at night to years in the afternoon, emphasizing the need to consider its dynamic kinetic limits when modeling OA. In general, the overall results of this study improve our understanding of the impact of aging on OA volatility and viscosity.

The shape parameter of the Gamma size distribution plays a key role in the evolution of the cloud droplet spectrum in the bulk parameterization schemes. However, due to the inaccurate specification of the shape parameter in the commonly used bulk double-moment schemes, the cloud droplet spectra cannot reasonably be described during the condensation process. Therefore, a newly-developed triple-parameter condensation scheme with the shape parameter diagnosed through the number concentration, cloud water content, and reflectivity factor of cloud droplets can be applied to improve the evolution of the cloud droplet spectrum. The simulation with the new parameterization scheme was compared to those with a high-resolution Lagrangian bin scheme, the double-moment schemes in a parcel model, and the observation in a 1.5D Eulerian model that consists of two cylinders. The new scheme with the shape parameter varying with time and space can accurately simulate the evolution of the cloud droplet spectrum. Furthermore, the volume-mean radius and cloud water content simulated with the new scheme match the Lagrangian analytical solutions well, and the errors are steady, within approximately 0.2%.

Double-moment schemes cannot accurately describe the evolution of the cloud droplet spectrum during condensation. Hence, a new triple-moment condensation scheme is developed to describe the evolution of cloud droplet spectra. In this scheme, a three-parameter gamma distribution function of the cloud droplet mass is adopted, and the prognostic equations of the spectral shape parameter and slope parameter are derived by means of the number concentration, cloud water content, and reflectivity factor of cloud droplets. The new parameterization scheme is compared with high-resolution Lagrangian and Eulerian bin schemes, double-moment schemes, and existing triple-moment schemes by performing simulations under different supersaturation values. The new scheme can reduce the cloud spectral error in the cloud water content and reflectivity factor caused by the fixed shape parameter in some bulk schemes. The spectra simulated with the new scheme match the Lagrangian analytical solutions well, with errors within approximately 1% in the cloud water content and reflectivity factor. The effects of curvature and solution on condensation growth are also tested using the new scheme, and a method of using multiple gamma distribution functions to characterize the multimodal spectrum of cloud droplets is proposed in the new condensation scheme. Ultimately, the formation of rain embryos from giant aerosols can be simulated via the new scheme.

Straw burning comprises more than 30% of all types of burned biomass in Asia, while the estimation of the emitted aerosols' direct radiative forcing effect suffers from large uncertainties, especially when atmospheric aging processes are considered. In this study, the light absorption properties of primary and aged straw burning aerosols in open fire were characterized at 7 wavelengths ranging from 370 nm to 950 nm in a chamber. The primary rice, corn and wheat straw burning bulk aerosols together had a mass absorption efficiency (MAE) of 2.43 ± 1.36 m2 g-1 at 520 nm and an absorption Ångström exponent (AAE) of 1.93 ± 0.71, while the primary sorghum straw burning bulk aerosols were characterized by a relatively lower MAE of 0.95 ± 0.54 m2 g-1 and a higher AAE of 4.80 ± 0.68. Both the MAE and AAE of primary aerosols can be well parameterized by the (PM-BC)/BC ratio (in wt.). The MAE of black carbon (BC) increased by 11-190% during photoreactions equivalent to 16-60 h of atmospheric aging, which was positively correlated with the (PM-BC)/(BC) ratio. The MAE of organic aerosols first slightly increased or leveled off, and then decreased. Specifically, at 370 nm, the first growth/plateau stage lasted until OH exposure reached 0.47-1.29 × 1011 molecule cm-3 s, and the following period exhibited decay rates of 1.0-2.8 × 10-12 cm3 molecule-1 s-1 against the OH radical, corresponding to half-lives of 46-134 h in a typical ambient condition. During photoreactions, competition among the lensing effect, growth/bleach of organic chromophores, and particle mass and size growth complicated the evolution of the direct radiative forcing effect. It is concluded that rice and corn straw burning aerosols maintained a warming effect after aging, while the cooling effect of fresh sorghum straw burning aerosols increased with aging.

Cloud droplet nucleation is classically defined as a droplet growing to a size such that its ambient supersaturation exceeds its surface equilibrium water vapor pressure. Unactivated particles are always in equilibrium with the ambient vapor pressure. Further studies showed that such an equilibrium assumption leads to many more cloud droplets being nucleated due to neglecting kinetic growth limitations, including the inertial mechanism, evaporation mechanism, and deactivation mechanism. Moreover, the inertial mechanism results in great discrepancy between the actual size and the critical size of nucleation for large aerosol particles. These issues complicate cloud droplet nucleation parameterization for applications in cloud modeling. To establish a physically based nucleation scheme, we established a highly size-resolved Lagrangian parcel model. Vapor diffusion and heat conduction were calculated according to Maxwell theory, and the surface vapor density and temperature were explicitly simulated. The surface temperature variation of a droplet with its size was considered. The surface supersaturation of a droplet, taking into account the surface temperature variation, is different from its equilibrium supersaturation at its large sizes. The nucleation simulation showed that the inertial and deactivation mechanisms can impact droplet nucleation. Moreover, very large nuclei can trigger rain embryo formation in a short time period. Even though there are kinetic limitations, the classical equilibrium assumption can be applied to determine the primary nucleation number of cloud droplets. Meanwhile, a regression formula for the size of a nucleated droplet and its dry aerosol size was established.

Oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) formation potential of ambient air in Guangzhou, China was investigated using a field-deployed oxidation flow reactor (OFR). The OFR was used to mimic hours to weeks of atmospheric exposure to hydroxyl (OH) radicals within the 2-3 min residence time. A comprehensive investigation on the variation of VOCs and OVOCs as a function of OH exposure is shown. Substantial formation of organic acids and nitrogen-containing OVOC species were observed. Maximum SOA formation in the OFR was observed following 1-4 equiv days' OH exposure. SOA produced from known/measured VOC/IVOC precursors such as single-ring aromatics and long-chain alkanes can account for 52-75% of measured SOA under low NOx and 26-60% under high NOx conditions based on laboratory SOA yield parametrizations. To our knowledge, this is the first time that the contribution (8-20%) of long-chain (C8-C20) alkane oxidation to OFR SOA formation was quantified from direct measurement. By additionally estimating contribution from unmeasured semivolatile and intermediate volatility compounds (S/IVOCs) that are committed with C8-C20 alkanes, 64-100% of the SOA formation observed in the OFR can be explained, signifying the important contribution of S/IVOCs such as large cyclic alkanes to ambient SOA.

Biogenic secondary organic aerosol (BSOA) makes up a significant proportion of organic aerosol, and its formation chemistry, composition, and physical properties can be influenced by anthropogenic emissions, especially in urban areas. Organosulfates (OSs) are an important class of tracers for BSOA and have been well-studied over the past decade, although detailed ambient studies of diurnal variations are still lacking. In this study, fine particulate matter samples were collected eight times a day across summer and winter campaigns at an urban site in Guangzhou, China. Guangzhou is heavily influenced by both biogenic and anthropogenic emissions, allowing for biogenic-anthropogenic interactions to be studied. Individual OSs and nitrooxy OSs (NOSs) species derived from monoterpenes and isoprene were analyzed using ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLC-MS2) and quantified using three authentic and proxy standards. The observations show strong diurnal variations of monoterpene derived OSs and NOSs, which peaked during the night, with concentrations increasing from the early evening, highlighting the role of NO3-oxidation chemistry. Isoprene derived OSs/NOSs showed strong seasonal profiles, with summer and winter average concentrations of 181.8 and 69.5 ng m(-3), respectively, with exponential increases observed at temperatures above 30 degrees C. Low-NO formation pathways were dominant in the summer, while high-NO pathways became more important in the winter. Isoprene OS formation was strongly dependent on the availability of particulate sulfate (SO42-), suggesting an extensive heterogeneous chemistry of oxidized isoprene species. Overall, this study provides further insights into biogenically derived OS and NOS formation within highly anthropogenically influenced environments.

PM2.5 is considered to be the most dangerous form of air pollution and is formed of a complex mixture of both primary and secondary species, from both biogenic and anthropogenic sources. Organic aerosol, comprised of modern carbon has been shown to dominate even in urban settings, but sources and formation mechanism of these biogenic aerosol in the ambient atmosphere remain uncertain. The collection and offline analysis of PM2.5 aerosol samples allows for highly detailed molecular level compositional information to be obtained, but at the cost of time resolution. Previous studies have collected 23-hour offline filters, which although allowing for seasonal changes to be studied, cannot resolve diurnal variations. However, due to recent advances in high-resolution mass spectrometers, the time resolution of offline filters can now be increased. This study utilises high time resolution offline filters collected in Guangzhou, China across two campaigns during summer and winter. Filters were collected every 2 hours during the day (06:00 – 21:00), with a longer collection overnight (21:00-06:00), alongside a suite of complementary gas phase measurements. Guangzhou represents an interesting case study for biogenic secondary organic aerosol (BSOA) especially biogenic-anthropogenic interactions due to its tropical location and high levels of flora, but also located in one of the most densely populated regions of the world within the Guangdong-Hong Kong-Macau Greater Bay area, with a combined population of 71.2 million people. This study presents ultra-high-performance liquid chromatography, high-resolution mass spectrometry measurements of BSOA tracers identified in the ambient PM2.5 samples at the highest time resolution studied so far. A library of 180 potential BSOA tracers from isoprene, monoterpenes and sesquiterpenes was developed containing acid species (CHO), organosulfates (CHOS) and nitrooxy organosulfates (CHOSN). The BSOA tracers were quantified using a mixture of authentic standards, proxy standards and modelled RIE factors for accurate quantification. Matrix suppression factors were also determined for both CHO and CHOS/CHOSN species, splitting the compounds into groups based on their retention time (RT), with species eluting before 2 min showing the largest matrix suppression. Strong diurnal variations were observed for some species while others showed little or no diurnal variation suggesting nonlocal sources, and as such provides insight into how long-range sources can affect BSOA concentrations. Tracers were also correlated to anthropogenic pollutants such as NOX and SO2 as well as sulfate and nitrate measured via ion chromatography, improving our understanding of biogenic-anthropogenic interactions. Comparisons between summer and winter allowed insight into seasonal processes and concentrations, with the potential for different long-range sources. Finally, this study presents comparisons to a growing field of offline BSOA measurements, providing a more comprehensive picture of the contributions BSOA makes to PM2.5 concentrations.

More than 1 million premature deaths in Asia annually are estimated to be associated with indoor air quality. HEPA (high-efficiency particulate air) filter air purifiers (APs) are widely used in urban Chinese residences by the growing middle class, as public awareness of air pollution increases. Currently, understanding of how particle size affects particle removal is inconsistent, and the rate at which different particle types are removed remains largely unknown. Therefore, this investigation aimed to determine the relationship between particle size and the removal efficiency of particles, and how efficiently ambient air is filtered compared to particle types which are typically used in standard tests (tobacco smoke, dust and pollen). Three of the most popular AP models in China were tested in China’s largest indoor controlled chamber laboratory and the removal efficiencies of particles in the 18–514 nm range were identified. Each AP had a distinct profile of removal efficiency against particle size, but the three APs shared similarities in performance, with removal efficiency consistently lowest at 200–250 nm. This size fraction is important in an exposure context as these particles are abundant in ambient air in mega-cities, can penetrate through building shells effectively, remain airborne for long periods of time and can penetrate the deepest areas of the lungs. Ambient air particles were removed at a similar rate to test particles; this confirms that the Association of Home Appliance Manufacturers’ (AHAM) standards are a suitable proxy for “real world” performance.