Technical note: Entrainment-limited kinetics of bimolecular reactions in clouds

The method of entrainment-limited kinetics enables atmospheric chemistry models that do not resolve clouds to simulate heterogeneous (surface and multiphase) cloud chemistry more accurately and efficiently than previous numerical methods. The method, which was previously described for reactions with first-order kinetics in clouds, incorporates cloud entrainment into the kinetic rate coefficient. This technical note shows how bimolecular reactions with second-order kinetics in clouds can also be treated with entrainment-limited kinetics, enabling efficient simulations of a wider range of cloud chemistry reactions. Accuracy is demonstrated using oxidation of SO2 to S(VI) – a key step in formation of acid rain – as an example. Over a large range of reaction rates, cloud fractions, and initial reactant concentrations, the numerical errors in the entrainment-limited bimolecular reaction rates are typically << 1 % and always < 4 %, which is far smaller than the errors found in several commonly used methods of simulating cloud chemistry with fractional cloud cover.

The roles of polycyclic aromatic hydrocarbons in dark cloud chemistry: new constraints on sulphur-bearing species

ABSTRACT The recent detection of c-C6H5CN in the dark cloud TMC-1 provides a new evidence of polycyclic aromatic hydrocarbons (PAHs) in dark clouds. However, knowledge of PAHs in dark cloud chemistry is still very limited. In this study, we investigate the effects of PAHs on the chemistry in dark clouds by coupling published PAH-related chemistry with a gas–grain reaction network. We found that abundances of some ice species, such as OCS, OCN, C2H5OH and HCOOCH3, are enhanced by more than two orders of magnitude due to the inclusion of PAHs in the gas–grain chemistry through accretion and subsequent reactions in ice. Especially, combining PAHs with gas–grain chemistry in dark clouds provides an alternative way to reach a good overall agreement of sulphur-bearing species in both the gas and solid phases using the cosmic value of sulphur. With the inclusion of PAHs, CS + O $\longrightarrow$ OCS makes a contribution of ∼5–10 per cent to solid-phase OCS together with CO + S $\longrightarrow$ OCS (∼80−90 per cent), depending on the initial abundances, chemical age and PAH parameters.

CAPRAM reduction towards an operational multiphase halogen and dimethyl sulfide chemistry treatment in the chemistry transport model COSMO-MUSCAT(5.04e)

A condensed multiphase halogen and dimethyl sulfide (DMS) chemistry mechanism for application in chemistry transport models is developed by reducing the CAPRAM DMS module 1.0 (CAPRAM-DM1.0) and the CAPRAM halogen module 3.0 (CAPRAM-HM3.0). The reduction is achieved by determining the main oxidation pathways from analysing the mass fluxes of complex multiphase chemistry simulations with the air parcel model SPACCIM (SPectral Aerosol Cloud Chemistry Interaction Model). These simulations are designed to cover both pristine and polluted marine boundary layer conditions. Overall, the reduced CAPRAM-DM1.0 contains 32 gas-phase reactions, 5 phase transfers, and 12 aqueous-phase reactions, of which two processes are described as equilibrium reactions. The reduced CAPRAM-HM3.0 contains 199 gas-phase reactions, 23 phase transfers, and 87 aqueous-phase reactions. For the aqueous-phase chemistry, 39 processes are described as chemical equilibrium reactions. A comparison of simulations using the complete CAPRAM-DM1.0 and CAPRAM-HM3.0 mechanisms against the reduced ones indicates that the relative deviations are below 5 % for important inorganic and organic air pollutants and key reactive species under pristine ocean and polluted conditions. The reduced mechanism has been implemented into the chemical transport model COSMO-MUSCAT and tested by performing 2D simulations under prescribed meteorological conditions that investigate the effect of stable (stratiform cloud) and more unstable meteorological conditions (convective clouds) on marine multiphase chemistry. The simulated maximum concentration of HCl is of the order of 109 molecules cm−3 and that of BrO is around 1×107 molecules cm−3, reproducing the range of ambient measurements. Afterwards, the oxidation pathways of DMS in a cloudy marine atmosphere have been investigated in detail. The simulations demonstrate that clouds have both a direct and an indirect photochemical effect on the multiphase processing of DMS and its oxidation products. The direct photochemical effect is related to in-cloud chemistry that leads to high dimethyl sulfoxide (DMSO) oxidation rates and a subsequently enhanced formation of methane sulfonic acid compared to aerosol chemistry. The indirect photochemical effect is characterized by cloud shading, which occurs particularly in the case of stratiform clouds. The lower photolysis rate affects the activation of Br atoms and consequently lowers the formation of BrO radicals. The corresponding DMS oxidation flux is lowered by up to 30 % under thick optical clouds. Moreover, high updraught velocities lead to a strong vertical mixing of DMS into the free troposphere predominately under cloudy conditions. The photolysis of hypohalous acids (HOX, X = Cl, Br, or I) is reduced as well, resulting in higher HOX-driven sulfite-to-sulfate oxidation in aerosol particles below stratiform clouds. Altogether, the present model simulations have demonstrated the ability of the reduced mechanism to be applied in studying marine aerosol–cloud processing effects in regional models such as COSMO-MUSCAT. The reduced mechanism can be used also by other regional models for more adequate interpretations of complex marine field measurement data.

H₂O₂ modulates the energetic metabolism of the cloud microbiome

Chemical reactions in clouds lead to oxidation processes driven by radicals (mainly HO⚫, NO3⚫, or HO2⚫) or strong oxidants such as H2O2, O3, nitrate, and nitrite. Among those species, hydrogen peroxide plays a central role in the cloud chemistry by driving its oxidant capacity. In cloud droplets, H2O2 is transformed by microorganisms which are metabolically active. Biological activity can therefore impact the cloud oxidant capacity. The present article aims at highlighting the interactions between H2O2 and microorganisms within the cloud system. First, experiments were performed with selected strains studied as a reference isolated from clouds in microcosms designed to mimic the cloud chemical composition, including the presence of light and iron. Biotic and abiotic degradation rates of H2O2 were measured and results showed that biodegradation was the most efficient process together with the photo-Fenton process. H2O2 strongly impacted the microbial energetic state as shown by adenosine triphosphate (ATP) measurements in the presence and absence of H2O2. This ATP depletion was not due to the loss of cell viability. Secondly, correlation studies were performed based on real cloud measurements from 37 cloud samples collected at the PUY station (1465 m a.s.l., France). The results support a strong correlation between ATP and H2O2 concentrations and confirm that H2O2 modulates the energetic metabolism of the cloud microbiome. The modulation of microbial metabolism by H2O2 concentration could thus impact cloud chemistry, in particular the biotransformation rates of carbon compounds, and consequently can perturb the way the cloud system is modifying the global atmospheric chemistry.