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

Data Descriptor : Collocated observations of cloud condensation nuclei, particle size distributions, and chemical composition

Cloud condensation nuclei (CCN) number concentrations alongside with submicrometer particle number size distributions and particle chemical composition have been measured at atmospheric observatories of the Aerosols, Clouds, and Trace gases Research InfraStructure (ACTRIS) as well as other international sites over multiple years. Here, harmonized data records from 11 observatories are summarized, spanning 98,677 instrument hours for CCN data, 157,880 for particle number size distributions, and 70,817 for chemical composition data. The observatories represent nine different environments, e.g., Arctic, Atlantic, Pacific and Mediterranean maritime, boreal forest, or high alpine atmospheric conditions. This is a unique collection of aerosol particle properties most relevant for studying aerosol-cloud interactions which constitute the largest uncertainty in anthropogenic radiative forcing of the climate. The dataset is appropriate for comprehensive aerosol characterization (e.g., closure studies of CCN), model-measurement intercomparison and satellite retrieval method evaluation, among others. Data have been acquired and processed following international recommendations for quality assurance and have undergone multiple stages of quality assessment.Peer reviewe

Cloud and fog droplet activation of atmospheric black carbon: In-situ observations of the influence of particle size, mixing state and ambient supersaturation

“If I wake up with a nightmare, it is the indirect aerosol effect.” admitted Veerabhadran Ramanathan , one of the most eminent climate scientists worldwide, in year 2000. Progress has been achieved since then, but the quote still summarizes the difficulties met by the scientific community to reach a detailed understanding of the interactions between two highly complex systems: aerosols and clouds. These difficulties arise mainly because aerosol-cloud interactions, compared to e.g. greenhouse gases, entangle complex thermodynamics, microphysics, optics and multi-phase chemistry within local and short-term scales. Today, the radiative forcing caused by anthropogenic aerosols concentrations and interactions with clouds and solar radiation remains the largest obstacle to the understanding and accurate quantification of the global anthropogenic radiative forcing. Attempting to push the degree of complexity a step further in the description of aerosol-cloud interactions could be perceived as unreasonable but it has recently become clear that this is necessary to accurately simulate the climate forcing of aerosols. Black carbon (BC), a subset of the particulate matter known to be both a strong radiation absorber and insoluble in water, has been shown to coagulate with organic and inorganic material and act as a nucleus for these materials to condense on, forming a water-soluble coating surrounding the BC core. In the early 21st century, vigorous efforts were undertaken to identify and quantify the interactions of BC with clouds and radiation, as well as the resulting feedbacks with other components of the climate system. A detailed review published in 2013 ranked BC second only to carbon dioxide (CO2) and slightly ahead of methane (CH4) in promoting global warming, despite its relatively sparse presence in the atmosphere. Moreover, modelling studies that simulate emission inventories revealed that the dominant fraction of atmospheric BC originates from human activities through incomplete combustion of solid and liquid fuels. Controlling BC emissions, by e.g. reducing the incomplete combustion from heat engines, therefore appears as a strategic lever to mitigate anthropogenic climate modifications. However, several aspects linked to the mixing state of BC can explain the current difficulty to bound the climate impacts of BC: the timescale for coating acquisition is still not clear; the modifications of particle optical properties due to coating acquisition is still discussed; and the amount of coating required for cloud droplet activation, a process that strongly facilitates removal from the atmosphere, is still not accurately quantified. The work presented in this thesis firstly addresses the latter aspect before focusing on the characterization of BC properties at a high-altitude site. In this thesis, we strived for a better characterization of BC and a more quantitative description of BC activation in different cloud environments. The relative influences of particle properties and cloud supersaturation on cloud droplet activation of BC were investigated by performing two field campaigns, allowing for the sampling of BC at different degrees of aging as well as different cloud supersaturations. Firstly, a field campaign was conducted at Irchel campus in the city of Zurich during winter 2015/2016. The droplet activation of BC at very low supersaturation (fog) was investigated depending on BC size and mixing state, two highly variable parameters in such an environment because of the variety of sources and atmospheric age (e.g. fresh traffic and residential emissions, aged background aerosol). Secondly, the high-altitude research station Jungfraujoch, positioned on a mountain ridge in the Swiss Alps (3580 m a.s.l.) and frequently located within clouds served as a measurement site during summer 2016 (CLACE2016 campaign). The importance of each factor influencing cloud droplet activation of heavily aged BC (size, mixing state, hygroscopicity, cloud supersaturation) was studied and linked to the fraction of BC-containing particles activated to cloud droplets. Such research questions could be addressed by in-fog/cloud sampling and switching between different inlets. This permitted to selectively sample interstitial (unactivated) particles, cloud droplet residual particles and the total aerosol (sum of interstitial aerosol and droplet residual particles). By comparing instrumental data behind these three inlets, we were able to retrieve important information such as the number fraction of BC-containing, BC-free and total aerosol that activated to fog or cloud droplets (i.e. activated fractions), as well as differences in particle properties. The main instrument utilized during these campaigns was the single-particle soot photometer (SP2), which provides information on the BC core mass-equivalent diameter as well as the optical diameter of BC-containing and BC-free particles. Relating the optical diameter to the BC core diameter provides quantitative information on the BC mixing state at a single-particle level. Activation cut-off diameters from activated fractions combined with cloud condensation nuclei counter (CCNC) data were utilized to retrieve cloud effective peak supersaturation (SSpeak). In addition, information on the bulk aerosol hygroscopicity and mixing state was retrieved. Besides the cloud activation properties of BC, the concentration, size distribution and mixing state of BC were characterized at the Jungfraujoch site both in summer and winter using SP2 data from the aforementioned CLACE2016 campaign and the CLACE2014 campaign, respectively. We also investigated the dependence of these properties on the type of air mass, i.e. free troposphere (FT) versus planetary boundary layer (PBL)-influenced conditions and wind direction. In Zurich, close to emission sources, the aerosol was a mixture of freshly-emitted externally mixed particles and more internally mixed residential or background aerosol. Both degrees of mixing state coexisted with variable respective contributions depending on the time of the day: peak morning emissions during working days caused the major input of freshly emitted particles while residential and background aerosol was mainly sampled at night. At the Jungfraujoch, we observed a strong seasonality in the mixing state of BC, with a large degree of internal mixing in summer when the site is strongly influenced by PBL injections and a very high degree of external mixing in winter when the site is predominantly located in the FT. Although already assumed in some models focusing in BC aging during transport to remote region in order to agree with measurements of BC concentrations, the strong degree of BC external mixing in winter was reported in very few field studies. Whilst the mixing state was found to influence the droplet activation behaviour of BC, we showed that it is not the dominant controlling parameter. Instead, the cloud peak supersaturation (SSpeak) was the main factor driving the fraction of particles that activated to droplet both on number and mass bases. In fog, where very low supersaturations were encountered (SSpeak≈0.05 %), less than 1 % of both BC-free and BC-containing particles could activate to fog droplets; whereas for both subsets, virtually 100 % of the particle population were activated in highly-supersaturated clouds (SSpeak>0.5 %). As SSpeak decreased, the size and mixing state of BC-containing particles became increasingly important criteria modulating the SSpeak–driven droplet activation: large and thickly coated BC activated more efficiently. For example, the number fraction of particles containing a 100 nm BC core acting as cloud condensation nuclei (CCN) in fog reached 50 % with coatings of approximately 150 nm, but BC with no or very thin coating was required to form a cloud droplet at the mountain site. Comparing particles of equal size, those with thicker coatings were more likely to take up water and form a droplet at low and medium supersaturation. These results provide an experimental confirmation of the Kelvin and Raoult effects, which are combined in the κ-Köhler theory describing droplet activation. We tested the ability of this theory, combined with a simplified particle shape representation (spherical core and concentric coating) and the Zdanovski-Stokes-Robinson (ZSR) mixing rule to predict whether a BC-containing particle stays in the interstitial phase or activates to a droplet with knowledge of BC core size, coating thickness, and peak supersaturation. A successful closure between predicted and observed droplet activation of BC-containing particles was achieved both in fog and clouds. This is the first time that the relation between BC particle properties and their droplet activation in real clouds has been quantitatively studied with such detail. These results confirm that atmospheric aging indeed increases the potential of BC-containing particles to activate to cloud droplets as qualitatively expected and quantitatively predicted by theory. These findings inform model simulations how to treat BC activation to cloud droplets, as mixing state information and simplified theory can be applied in a consistent manner. This depends of course on the level of complexity of the aerosol scheme which goes all the way to particle-resolved simulations. The relation between aerosol as well as BC activated fractions (on mass basis) and SSpeak presented in this work, along with characteristic BC particle properties (BC core size and coating thickness distributions), can serve as reference data to assess whether aerosol and BC mass activated fractions obtained in models are within the correct range and for the correct reason. Similarly, the strong seasonality of BC mixing state observed in the FT at the Jungfraujoch could be a first step towards a re-assessment of the climate impact of BC. If this characteristic is verified at large spatial scales, the consideration of the longer lifetime and reduced light absorption due to a higher degree of BC external mixing than previously thought could lead to such a re-assessment. By decreasing the degree of uncertainty of model simulations, these results can contribute to a better assessment of the climate forcing of BC

Jungfraujoch, Switzerland, cloud condensation nuclei concentration, 2012-2014

cloud condensation nuclei concentration, Jungfraujoch, Switzerland, 2012-201

Droplet activation behaviour of atmospheric black carbon particles in fog as a function of their size and mixing state

Among the variety of particle types present in the atmosphere, black carbon (BC), emitted by combustion processes, is uniquely associated with harmful effects to the human body and substantial radiative forcing of the Earth. Pure BC is known to be non-hygroscopic, but its ability to acquire a coating of hygroscopic organic and inorganic material leads to increased diameter and hygroscopicity, facilitating droplet activation. This affects BC radiative forcing through aerosol\u2013cloud interactions (ACIs) and BC life cycle. To gain insights into these processes, we performed a field campaign in winter 2015\u20132016 in a residential area of Zurich which aimed at establishing relations between the size and mixing state of BC particles and their activation to form droplets in fog. This was achieved by operating a CCN counter (CCNC), a scanning mobility particle sizer (SMPS), a single-particle soot photometer (SP2) and an aerosol chemical speciation monitor (ACSM) behind a combination of a total- and an interstitial-aerosol inlet. Our results indicate that in the morning hours of weekdays, the enhanced traffic emissions caused peaks in the number fraction of externally mixed BC particles, which do not act as CCN within the CCNC. The very low effective peak supersaturations (SSpeak) occurring in fog (between approximately 0.03\u2009% and 0.06\u2009% during this campaign) restrict droplet activation to a minor fraction of the aerosol burden (around 0.5\u2009% to 1\u2009% of total particle number concentration between 20 and 593\u2009nm) leading to very selective criteria on diameter and chemical composition. We show that bare BC cores are unable to activate to fog droplets at such low SSpeak, while BC particles surrounded by thick coating have very similar activation behaviour to BC-free particles. Using simplified \u3ba-K\uf6hler theory combined with the ZSR mixing rule assuming spherical core\u2013shell particle geometry constrained with single-particle measurements of respective volumes, we found good agreement between the predicted and the directly observed size- and mixing-state-resolved droplet activation behaviour of BC-containing particles in fog. This successful closure demonstrates the predictability of their droplet activation in fog with a simplified theoretical model only requiring size and mixing state information, which can also be applied in a consistent manner in model simulations.Peer reviewed: YesNRC publication: Ye

Aerosol and dynamical contributions to cloud droplet formation in Arctic low-level clouds

The Arctic is one of the most rapidly warming regions of the globe. Low-level clouds and fog modify the energy transfer from and to space and play a key role in the observed strong Arctic surface warming, a phenomenon commonly termed "Arctic amplification". The response of low-level clouds to changing aerosol characteristics throughout the year is therefore an important driver of Arctic change that currently lacks sufficient constraints. As such, during the NASCENT campaign (Ny-Ålesund AeroSol Cloud ExperimeNT) extending over a full year from October 2019 to October 2020, microphysical properties of aerosols and clouds were studied at the Zeppelin station (475 m a.s.l.), Ny-Ålesund, Svalbard, Norway. Particle number size distributions obtained from differential mobility particle sizers as well as chemical composition derived from filter samples and an aerosol chemical speciation monitor were analyzed together with meteorological data, in particular vertical wind velocity. The results were used as input to a state-of-the-art cloud droplet formation parameterization to investigate the particle sizes that can activate to cloud droplets, the levels of supersaturation that can develop, the droplet susceptibility to aerosol and the role of vertical velocity. We evaluate the parameterization and the droplet numbers calculated through a droplet closure with in-cloud in situ measurements taken during nine flights over 4 d. A remarkable finding is that, for the clouds sampled in situ, closure is successful in mixed-phase cloud conditions regardless of the cloud glaciation fraction. This suggests that ice production through ice-ice collisions or droplet shattering may have explained the high ice fraction, as opposed to rime splintering that would have significantly reduced the cloud droplet number below levels predicted by warm-cloud activation theory. We also show that pristine-like conditions during fall led to clouds that formed over an aerosol-limited regime, with high levels of supersaturation (generally around 1 %, although highly variable) that activate particles smaller than 20 nm in diameter. Clouds formed in the same regime in late spring and summer, but aerosol activation diameters were much larger due to lower cloud supersaturations (ca. 0.5 %) that develop because of higher aerosol concentrations and lower vertical velocities. The contribution of new particle formation to cloud formation was therefore strongly limited, at least until these newly formed particles started growing. However, clouds forming during the Arctic haze period (winter and early spring) can be limited by updraft velocity, although rarely, with supersaturation levels dropping below 0.1 % and generally activating larger particles (20 to 200 nm), including pollution transported over a long range. The relationship between updraft velocity and the limiting cloud droplet number agrees with previous observations of various types of clouds worldwide, which supports the universality of this relationship.ISSN:1680-7375ISSN:1680-736