Expiratory Aerosol pH: The Overlooked Driver of Airborne Virus Inactivation

Respiratory viruses, including influenza virus and SARS-CoV-2, are transmitted by the airborne route. Air filtration and ventilation mechanically reduce the concentration of airborne viruses and are necessary tools for disease mitigation. However, they ignore the potential impact of the chemical environment surrounding aerosolized viruses, which determines the aerosol pH. Atmospheric aerosol gravitates toward acidic pH, and enveloped viruses are prone to inactivation at strong acidity levels. Yet, the acidity of expiratory aerosol particles and its effect on airborne virus persistence have not been examined. Here, we combine pH-dependent inactivation rates of influenza A virus (IAV) and SARS-CoV-2 with microphysical properties of respiratory fluids using a biophysical aerosol model. We find that particles exhaled into indoor air (with relative humidity ≥ 50%) become mildly acidic (pH ∼ 4), rapidly inactivating IAV within minutes, whereas SARS-CoV-2 requires days. If indoor air is enriched with nonhazardous levels of nitric acid, aerosol pH drops by up to 2 units, decreasing 99%-inactivation times for both viruses in small aerosol particles to below 30 s. Conversely, unintentional removal of volatile acids from indoor air may elevate pH and prolong airborne virus persistence. The overlooked role of aerosol acidity has profound implications for virus transmission and mitigation strategies

Inactivation mechanisms of influenza A virus under pH conditions encountered in aerosol particles as revealed by whole-virus HDX-MS

Multiple respiratory viruses, including influenza A virus (IAV), can be transmitted via expiratory aerosol particles, and aerosol pH was recently identified as a major factor influencing airborne virus infectivity. Indoors, small exhaled aerosols undergo rapid acidification to pH ~4. IAV is known to be sensitive to mildly acidic conditions encountered within host endosomes; however, it is unknown whether the same mechanisms could mediate viral inactivation within the more acidic aerosol micro-environment. Here, we identified that transient exposure to pH 4 caused IAV inactivation by a two-stage process, with an initial sharp decline in infectious titers mainly attributed to premature attainment of the post-fusion conformation of viral protein haemagglutinin (HA). Protein changes were observed by hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS) as early as 10 s post-exposure to acidic conditions. Our HDX-MS data are in agreement with other more labor-intensive structural analysis techniques, such as X-ray crystallography, highlighting the ease and usefulness of whole-virus HDX-MS for multiplexed protein analyses, even within enveloped viruses such as IAV. Additionally, virion integrity was partially but irreversibly affected by acidic conditions, with a progressive unfolding of the internal matrix protein 1 (M1) that aligned with a more gradual decline in viral infectivity with time. In contrast, no acid-mediated changes to the genome or lipid envelope were detected. Improved understanding of respiratory virus fate within exhaled aerosols constitutes a global public health priority, and information gained here could aid the development of novel strategies to control the airborne persistence of seasonal and/or pandemic influenza in the future. IMPORTANCE: It is well established that COVID-19, influenza, and many other respiratory diseases can be transmitted by the inhalation of aerosolized viruses. Many studies have shown that the survival time of these airborne viruses is limited, but it remains an open question as to what drives their infectivity loss. Here, we address this question for influenza A virus by investigating structural protein changes incurred by the virus under conditions relevant to respiratory aerosol particles. From prior work, we know that expelled aerosols can become highly acidic due to equilibration with indoor room air, and our results indicate that two viral proteins are affected by these acidic conditions at multiple sites, leading to virus inactivation. Our findings suggest that the development of air treatments to quicken the speed of aerosol acidification would be a major strategy to control infectious bioburdens in the air

Photophoretic spectroscopy in atmospheric chemistry – high-sensitivity measurements of light absorption by a single particle

Light-absorbing organic atmospheric particles, termed brown carbon, undergo chemical and photochemical aging processes during their lifetime in the atmosphere. The role these particles play in the global radiative balance and in the climate system is still uncertain. To better quantify their radiative forcing due to aerosol–radiation interactions, we need to improve process-level understanding of aging processes, which lead to either “browning” or “bleaching” of organic aerosols. Currently available laboratory techniques aim to simulate atmospheric aerosol aging and measure the evolving light absorption, but they suffer from low sensitivity and precision. This study describes the use of electrodynamic balance photophoretic spectroscopy (EDB-PPS) for high-sensitivity and high-precision measurements of light absorption by a single particle. We demonstrate the retrieval of the time-evolving imaginary part of the refractive index for a single levitated particle in the range of 10−4 to 10−5 with uncertainties of less than 25 % and 60 %, respectively. The experimental system is housed within an environmental chamber, in which aging processes can be simulated in realistic atmospheric conditions and lifetimes of days to weeks. This high level of sensitivity enables future studies to explore the major processes responsible for formation and degradation of brown carbon aerosols.ISSN:1867-1381ISSN:1867-854

Early detection of smoldering in silos: Organic material emissions as precursors

The possible use of organic particle emissions as indicators of smoldering fires at low temperatures (early stages, <150 °C) is explored in laboratory experiments. Samples consisting of wood pellets were heated under controlled atmosphere. The sample temperature was slowly ramped, mimicking the spontaneous, slow temperature increase during self-heating in organic materials. As the sample temperature reached 90–100 °C, a two-orders-of-magnitude increase in the number concentration of sub-micrometer particulate matter was measured under both air and nitrogen atmosphere. A detailed analysis of their chemical composition indicated that the particles formed through evaporation of low-volatile organic compounds followed by condensation downstream of the heated volume. The increase in aerosol concentration precedes any significant increase in measured CO and CO2 concentration in both time and temperature. Our results suggest that the sub-micron particle concentration could be considered as an indication of the stages that lead to smoldering fires

Early detection of smoldering in silos: Organic material emissions as precursors

The possible use of organic particle emissions as indicators of smoldering fires at low temperatures (early stages, <150 °C) is explored in laboratory experiments. Samples consisting of wood pellets were heated under controlled atmosphere. The sample temperature was slowly ramped, mimicking the spontaneous, slow temperature increase during self-heating in organic materials. As the sample temperature reached 90–100 °C, a two-orders-of-magnitude increase in the number concentration of sub-micrometer particulate matter was measured under both air and nitrogen atmosphere. A detailed analysis of their chemical composition indicated that the particles formed through evaporation of low-volatile organic compounds followed by condensation downstream of the heated volume. The increase in aerosol concentration precedes any significant increase in measured CO and CO2 concentration in both time and temperature. Our results suggest that the sub-micron particle concentration could be considered as an indication of the stages that lead to smoldering fires.ISSN:0379-7112ISSN:1873-722

Molecular Chemistry of Atmospheric Brown Carbon Inferred from a Nationwide Biomass Burning Event

Lag Ba’Omer, a nationwide bonfire festival in Israel, was chosen as a case study to investigate the influence of a major biomass burning event on the light absorption properties of atmospheric brown carbon (BrC). The chemical composition and optical properties of BrC chromophores were investigated using a high performance liquid chromatography (HPLC) platform coupled to photo diode array (PDA) and high resolution mass spectrometry (HRMS) detectors. Substantial increase of BrC light absorption coefficient was observed during the night-long biomass burning event. Most chromophores observed during the event were attributed to nitroaromatic compounds (NAC), comprising 28 elemental formulas of at least 63 structural isomers. The NAC, in combination, accounted for 50–80% of the total visible light absorption (>400 nm) by solvent extractable BrC. The results highlight that NAC, in particular nitrophenols, are important light absorption contributors of biomass burning organic aerosol (BBOA), suggesting that night time chemistry of •NO₃ and N₂O₅ with particles may play a significant role in atmospheric transformations of BrC. Nitrophenols and related compounds were especially important chromophores of BBOA. The absorption spectra of the BrC chromophores are influenced by the extraction solvent and solution pH, implying that the aerosol acidity is an important factor controlling the light absorption properties of BrC

Evolution of the Complex Refractive Index of Secondary Organic Aerosols during Atmospheric Aging

The wavelength-dependence of the complex refractive indices (RI) in the visible spectral range of secondary organic aerosols (SOA) are rarely studied, and the evolution of the RI with atmospheric aging is largely unknown. In this study, we applied a novel white light-broadband cavity enhanced spectroscopy to measure the changes in the RI (400–650 nm) of β-pinene and <i>p</i>-xylene SOA produced and aged in an oxidation flow reactor, simulating daytime aging under NO_<i>x</i>-free conditions. It was found that these SOA are not absorbing in the visible range, and that the real part of the RI, <i>n</i>, shows a slight spectral dependence in the visible range. With increased OH exposure, <i>n</i> first increased and then decreased, possibly due to an increase in aerosol density and chemical mean polarizability for SOA produced at low OH exposures, and a decrease in chemical mean polarizability for SOA produced at high OH exposures, respectively. A simple radiative forcing calculation suggests that atmospheric aging can introduce more than 40% uncertainty due to the changes in the RI for aged SOA

Expiratory aerosol pH is determined by indoor room trace gases and particle size

ISSN:0027-8424ISSN:1091-649

Expiratory Aerosol pH: The Overlooked Driver of Airborne Virus Inactivation

Respiratory viruses, including influenza virus and SARS-CoV-2, are transmitted by the airborne route. Air filtration and ventilation mechanically reduce the concentration of airborne viruses and are necessary tools for disease mitigation. However, they ignore the potential impact of the chemical environment surrounding aerosolized viruses, which determines the aerosol pH. Atmospheric aerosol gravitates toward acidic pH, and enveloped viruses are prone to inactivation at strong acidity levels. Yet, the acidity of expiratory aerosol particles and its effect on airborne virus persistence have not been examined. Here, we combine pH dependent inactivation rates of influenza A virus (IAV) and SARSCoV-2 with microphysical properties of respiratory fluids using a biophysical aerosol model. We find that particles exhaled into indoor air (with relative humidity >= 50%) become mildly acidic (pH similar to 4), rapidly inactivating IAV within minutes, whereas SARSCoV-2 requires days. If indoor air is enriched with nonhazardous levels of nitric acid, aerosol pH drops by up to 2 units, decreasing 99%-inactivation times for both viruses in small aerosol particles to below 30 s. Conversely, unintentional removal of volatile acids from indoor air may elevate pH and prolong airborne virus persistence. The overlooked role of aerosol acidity has profound implications for virus transmission and mitigation strategies.ISSN:0013-936XISSN:1520-585