Particle filtration efficiency measured using sodium chloride and polystyrene latex sphere test methods.

Standards governing face masks differ in the test methods used to determine sub-micron particle filtration efficiency (PFE), such that the meaning of PFE is not universal. Unifying the meaning of PFE requires data using these different test methods to drive improvements in standards. This simple data set provides the equivalence between two major test methods used to assess PFE: (1) a test method using a neutralized, polydisperse sodium chloride (NaCl) and (2) a test method using an unneutralized, "monodisperse" polystyrene latex sphere (PSL) aerosols. Measurements are made on over 5800 real-world medical masks, leading to the establishment of a relationship between these two kinds of PFE for these products.

Comparison of measurement systems for assessing number- and mass-based particle filtration efficiency.

The particle filtration efficiency (PFE) of a respirator or face mask is one of its key properties. While the physics of particle filtration results in the PFE being size-dependent, measurement standards are specified using a single, integrated PFE, for simplicity. This integrated PFE is commonly defined concerning either the number (NPFE) or mass (MPFE) distribution of particles as a function of size. This relationship is non-trivial; it is influenced by both the shape of the particle distribution and the fact that multiple practical definitions of particle size are used. This manuscript discusses the relationship between NPFE and MPFE in detail, providing a guide to practitioners. Our discussion begins with a description of the theory underlying different variants of PFE. We then present experimental results for a database of size-resolved PFE (SPFE) measurements for several thousand candidate respirators and filter media, including filter media with systematically varied properties and commercial samples that span 20%-99.8% MPFE. The observed relationships between NPFE and MPFE are discussed in terms of the most-penetrating particle size (MPPS) and charge state of the media. For the sodium chloride particles used here, we observed that the MPFE was greater than NPFE for charged materials and vice versa for uncharged materials. This relationship is observed because a shift from NPFE to MPFE weights the distribution toward larger sizes, while charged materials shift the MPPS to smaller sizes. Results are validated by comparing the output of a pair of automated filter testers, which are used in gauging standards compliance, to that of MPFE computed from a system capable of measuring SPFE over the 20 nm-500 nm range.

Source identification and characterization of organic nitrogen in atmospheric aerosols at a suburban site in China.

Despite the fact that atmospheric particulate organic nitrogen (ON) can significantly affect human health, ecosystems and the earth's climate system, qualitative and quantitative chemical characterization of ON remains limited due to its chemical complexity. In this study, the Aerodyne soot particle - high-resolution time-of-flight aerosol mass spectrometer (SP-AMS) was deployed for ambient measurements in Nanjing, China. Positive matrix factorization (PMF) was applied to the ON data to quantify the sources of ON in submicron aerosols. The averaged ON concentration was 1.24 µg m-3, while the averaged total nitrogen (TN) in the aerosol was 20.26 µg m-3. From the PMF ON analysis, a 5-factor solution was selected as the most representative and interpretable solution for the investigated dataset, including oxygenated OA (OOAON), amine-related OAON (AMOAON), hydrocarbon-like OA (HOAON), industry OA (IOAON), and local primary OA (POAON) factors. The quantified ON ions were separated into families, including CxHN, CxHyNO, C3H<6N, CxH2x+2N, CxH2xN and Others, consistent with their contribution to each factor. The CxHyNO family mainly contributed to the OOAON factor and suggested the presence of amides or amino acids. The CxH2x+2N family likely mostly originated from amines only contributing to the AMOAON and HOAON factors. The IOAON and POAON factors were resolved due to significant tracers in the mass spectra. Further, compared with regular organic PMF analysis, PMF ON analysis gave more insights due to improved source separation and interpretability of the OA components, which could be a role model for further atmospheric ON research.

Systematic experimental comparison of particle filtration efficiency test methods for commercial respirators and face masks.

Respirators, medical masks, and barrier face coverings all filter airborne particles using similar physical principles. However, they are tested for certification using a variety of standardized test methods, creating challenges for the comparison of differently certified products. We have performed systematic experiments to quantify and understand the differences between standardized test methods for N95 respirators (NIOSH TEB-APR-STP-0059 under US 42 CFR 84), medical face masks (ASTM F2299/F2100), and COVID-19-related barrier face coverings (ASTM F3502-21). Our experiments demonstrate the role of face velocity, particle properties (mean size, size variability, electric charge, density, and shape), measurement techniques, and environmental preconditioning. The measured filtration efficiency was most sensitive to changes in face velocity and particle charge. Relative to the NIOSH method, users of the ASTM F2299/F2100 method have commonly used non-neutralized (highly charged) aerosols as well as smaller face velocities, each of which may result in approximately 10% higher measured filtration efficiencies. In the NIOSH method, environmental conditioning at elevated humidity increased filtration efficiency in some commercial samples while decreasing it in others, indicating that measurement should be performed both with and without conditioning. More generally, our results provide an experimental basis for the comparison of respirators certified under various international methods, including FFP2, KN95, P2, Korea 1st Class, and DS2.

Ash-Decorated and Ash-Painted Soot from Residual and Distillate-Fuel Combustion in Four Marine Engines and One Aviation Engine.

Soot is typically the dominant component of the nonvolatile particles emitted from internal combustion engines. Although soot is primarily composed of carbon, its chemistry, toxicity, and oxidation rates may be strongly influenced by internally mixed inorganic metal compounds (ash). Here, we describe the detailed microstructure of ash internally mixed with soot from four marine engines and one aviation engine. The engines were operated on different fuels and lubrication oils; the fuels included four residual fuels and five distillate fuels such as diesel, natural gas, and Jet A-1. Using annular-dark-field scanning transmission electron microscopy (ADF-STEM), we observed that ash may occur either as distinct nodules on the soot particle (decorated) or as continuous streaks (painted). Both structures may exist within a single particle. Decorated soot was observed for both distillate and residual fuels and contained elements associated with either the fuel (V, Ni, Fe, S) or with the lubrication oil (Zn, Ca, P). Painted soot was observed only for residual-fuel soot, and only contained elements associated with the fuel. Additional composition measurements by inductively coupled plasma mass spectrometry (ICP-MS) of filter samples indicated that the internal mixing trends of ash with soot were consistent with the overall ash-to-carbon ratio of the sampled combustion aerosols. Painted soot may form when molten ash coagulates with or condenses onto soot within engines.

Brown Carbon in Primary and Aged Coal Combustion Emission.

Smog chamber experiments were conducted to characterize the light absorption of brown carbon (BrC) from primary and photochemically aged coal combustion emissions. Light absorption was measured by the UV-visible spectrophotometric analysis of water and methanol extracts of filter samples. The single-scattering albedo at 450 nm was 0.73 ± 0.10 for primary emissions and 0.75 ± 0.13 for aged emissions. The light absorption coefficient at 365 nm of methanol extracts was higher than that of water extracts by a factor of 10 for primary emissions and a factor of 7 for aged emissions. This suggests that the majority of BrC is water-insoluble even after aging. The mass absorption efficiency of this BrC (MAE365) for primary OA (POA) was dependent on combustion conditions, with an average of 0.84 ± 0.54 m2 g-1, which was significantly higher than that for aged OA (0.24 ± 0.18 m2 g-1). Secondary OA (SOA) dominated aged OA and the decreased MAE365 after aging indicates that SOA is less light absorbing than POA and/or that BrC is bleached (oxidized) with aging. The estimated MAE365 of SOA (0.14 ± 0.08 m2 g-1) was much lower than that of POA. A comparison of MAE365 of residential coal combustion with other anthropogenic sources suggests that residential coal combustion emissions are among the strongest absorbing BrC organics.

Characterization and Reduction of In-Use CH4 Emissions from a Dual Fuel Marine Engine Using Wavelength Modulation Spectroscopy.

In-use exhaust stream CH4 emissions from two dual fuel marine engines were characterized and strategies for CH4 reduction were identified and evaluated. For this, a low-cost, portable, wavelength modulation spectroscopy (WMS) system was developed. The performance of the developed WMS sensor was assessed using gas standards and demonstrated on a heavy-duty, diesel pilot ignited, direct-injection natural gas research engine through comparison to a flame ionization detector. The WMS sensor was subsequently used to measure the exhaust-stream CH4 concentration from two diesel pilot-ignited, port-injected natural gas engines on a coastal vessel while under normal operation. Using cylinder deactivation to reduce the excess air ratio, λ, and vessel operation changes to minimize operation at lower loads, the total CH4 emission were reduced by up to 33%. The measured, load specific CH4 emissions were subsequently used to identify an improved vessel operation strategy, with an estimated 56-60% reduction in CH4 emissions. These results demonstrate the importance of considering the real-world engine operation profile for accurate estimates of the global warming potential, as well as the utility of a WMS sensor for characterizing and mitigating in-use CH4 emissions.

Brown Carbon Aerosol in Urban Xi'an, Northwest China: The Composition and Light Absorption Properties.

Light-absorbing organic carbon (i.e., brown carbon or BrC) in the atmospheric aerosol has significant contribution to light absorption and radiative forcing. However, the link between BrC optical properties and chemical composition remains poorly constrained. In this study, we combine spectrophotometric measurements and chemical analyses of BrC samples collected from July 2008 to June 2009 in urban Xi'an, Northwest China. Elevated BrC was observed in winter (5 times higher than in summer), largely due to increased emissions from wintertime domestic biomass burning. The light absorption coefficient of methanol-soluble BrC at 365 nm (on average approximately twice that of water-soluble BrC) was found to correlate strongly with both parent polycyclic aromatic hydrocarbons (parent-PAHs, 27 species) and their carbonyl oxygenated derivatives (carbonyl-OPAHs, 15 species) in all seasons ( r2 > 0.61). These measured parent-PAHs and carbonyl-OPAHs account for on average ∼1.7% of the overall absorption of methanol-soluble BrC, about 5 times higher than their mass fraction in total organic carbon (OC, ∼0.35%). The fractional solar absorption by BrC relative to element carbon (EC) in the ultraviolet range (300-400 nm) is significant during winter (42 ± 18% for water-soluble BrC and 76 ± 29% for methanol-soluble BrC), which may greatly affect the radiative balance and tropospheric photochemistry and therefore the climate and air quality.

Cloud and fog processing enhanced gas-to-particle partitioning of trimethylamine.

An aerosol time-of-flight mass spectrometer (ATOFMS) was used to detect trimethylamine (TMA) in 0.52-1.9 µm particles at urban and rural sites in Southern Ontario during the summer and winter of 2007. During the summer, TMA-containing particles were observed exclusively during high relative humidity or fog events at both the urban and rural sites. In the wintertime, greater concentrations of TMA-containing particles were linked to cloud processing of aerosol in air masses originating from over agricultural and livestock areas. A laboratory study revealed that, at high relative humidity (∼ 100%), gas phase TMA at concentrations ranging from 2 to 20,000 ppt partitions preferentially to acidic particles present in the ambient air. On the basis of the field and laboratory studies, it appears that gas phase TMA present in ambient air partitions onto pre-existing particles preferentially during periods of acidic cloud/fog processing, leading to the presence of TMA-containing particles in the 0.52-1.9 µm size range.