Flow reactors, denuders, and sampling tubes are essential tools for many applications in analytical and physical chemistry and engineering. We derive a new method for determining radial diffusion effects and the penetration or transmission of gas molecules and aerosol particles through cylindrical tubes under laminar flow conditions using explicit analytical equations. In contrast to the traditional Brown method [Brown, R. L. J. Res. Natl. Bur. Stand. (U. S.) 1978, 83, 1-8] and CKD method (Cooney, D. O.; Kim, S. S.; Davis, E. J. Chem. Eng. Sci. 1974, 29, 1731-1738), the new approximation developed in this study (known as the KPS method) does not require interpolation or numerical techniques. The KPS method agrees well with the CKD method under all experimental conditions and also with the Brown method at low Sherwood numbers. At high Sherwood numbers corresponding to high uptake on the wall, flow entry effects become relevant and are considered in the KPS and CKD methods but not in the Brown method. The practical applicability of the KPS method is demonstrated by analysis of measurement data from experimental studies of rapid OH, intermediate NO3, and slow O3 uptake on various organic substrates. The KPS method also allows determination of the penetration of aerosol particles through a tube, using a single equation to cover both the limiting cases of high and low deposition described by Gormley and Kennedy (Proc. R. Ir. Acad., Sect. A. 1949, 52A, 163-169). We demonstrate that the treatment of gas and particle diffusion converges in the KPS method, thus facilitating prediction of diffusional loss and penetration of gases and particles, analysis of chemical kinetics data, and design of fluid reactors, denuders, and sampling lines.

Multiphase reactions of OH radicals are among the most important pathways of chemical aging of organic aerosols in the atmosphere. Reactive uptake of OH by organic compounds has been observed in a number of studies, but the kinetics of mass transport and chemical reaction are still not fully understood. Here we apply the kinetic multilayer model of gas-particle interactions (KM-GAP) to experimental data from OH exposure studies of levoglucosan and abietic acid, which serve as surrogates and molecular markers of biomass burning aerosol (BBA). The model accounts for gas-phase diffusion within a cylindrical coated-wall flow tube, reversible adsorption of OH, surface-bulk exchange, bulk diffusion, and chemical reactions at the surface and in the bulk of the condensed phase. The nonlinear dependence of OH uptake coefficients on reactant concentrations and time can be reproduced by KM-GAP. We find that the bulk diffusion coefficient of the organic molecules is approximately 10(-16) cm(2) s(-1), reflecting an amorphous semisolid state of the organic substrates. The OH uptake is governed by reaction at or near the surface and can be kinetically limited by surface-bulk exchange or bulk diffusion of the organic reactants. Estimates of the chemical half-life of levoglucosan in 200 nm particles in a biomass burning plume increase from 1 day at high relative humidity to 1 week under dry conditions. In BBA particles transported to the free troposphere, the chemical half-life of levoglucosan can exceed 1 month due to slow bulk diffusion in a glassy matrix at low temperature.

Air pollution is a potential driver for the increasing prevalence of allergic disease, and post-translational modification by air pollutants can enhance the allergenic potential of proteins. Here, the kinetics and mechanism of protein oligomerization upon ozone (O3) exposure were studied in coated-wall flow tube experiments at environmentally relevant O3 concentrations, relative humidities and protein phase states (amorphous solid, semisolid, and liquid). We observed the formation of protein dimers, trimers, and higher oligomers, and attribute the cross-linking to the formation of covalent intermolecular dityrosine species. The oligomerization proceeds fast on the surface of protein films. In the bulk material, reaction rates are limited by diffusion depending on phase state and humidity. From the experimental data, we derive a chemical mechanism and rate equations for a kinetic multilayer model of surface and bulk reaction enabling the prediction of oligomer formation. Increasing levels of tropospheric O3 in the Anthropocene may promote the formation of protein oligomers with enhanced allergenicity and may thus contribute to the increasing prevalence of allergies.

The chemical kinetics of organic nitrate production during new particle formation and growth of secondary organic aerosols (SOA) were investigated using the short-lived radioactive tracer (13)N in flow-reactor studies of α-pinene oxidation with ozone. Direct and quantitative measurements of the nitrogen content indicate that organic nitrates accounted for ∼40% of SOA mass during initial particle formation, decreasing to ∼15% upon particle growth to the accumulation-mode size range (>100 nm). Experiments with OH scavengers and kinetic model results suggest that organic peroxy radicals formed by α-pinene reacting with secondary OH from ozonolysis are key intermediates in the organic nitrate formation process. The direct reaction of α-pinene with NO3 was found to be less important for particle-phase organic nitrate formation. The nitrogen content of SOA particles decreased slightly upon increase of relative humidity up to 80%. The experiments show a tight correlation between organic nitrate content and SOA particle-number concentrations, implying that the condensing organic nitrates are among the extremely low volatility organic compounds (ELVOC) that may play an important role in the nucleation and growth of atmospheric nanoparticles.

The condensational growth of submicrometer aerosol particles to climate relevant sizes is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. We have simulated the mass accommodation and evaporation processes of water using molecular dynamics, and the results are compared to the condensation equations derived from the kinetic gas theory to shed light on the compatibility of the two. Molecular dynamics simulations were performed for a planar TIP4P-Ew water surface at four temperatures in the range 268-300 K as well as two droplets, with radii of 1.92 and 4.14 nm at T = 273.15 K. The evaporation flux from molecular dynamics was found to be in good qualitative agreement with that predicted by the simple kinetic condensation equations. Water droplet growth was also modeled with the kinetic multilayer model KM-GAP of Shiraiwa et al. [Atmos. Chem. Phys. 2012, 12, 2777]. It was found that, due to the fast transport across the interface, the growth of a pure water droplet is controlled by gas phase diffusion. These facts indicate that the simple kinetic treatment is sufficient in describing pure water condensation and evaporation. The droplet size was found to have minimal effect on the value of the mass accommodation coefficient. The mass accommodation coefficient was found to be unity (within 0.004) for all studied surfaces, which is in agreement with previous simulation work. Additionally, the simulated evaporation fluxes imply that the evaporation coefficient is also unity. Comparing the evaporation rates of the mass accommodation and evaporation simulations indicated that the high collision flux, corresponding to high supersaturation, present in typical molecular dynamics mass accommodation simulations can under certain conditions lead to an increase in the evaporation rate. Consequently, in such situations the mass accommodation coefficient can be overestimated, but in the present cases the corrected values were still close to unity with the lowest value at ≈0.99.

Terbuthylazine (TBA) is a widely used herbicide, and its heterogeneous reaction with OH radicals is important for assessing its potential to undergo atmospheric long-range transport and to affect the environment and public health. The apparent reaction rate coefficients obtained in different experimental investigations, however, vary by orders of magnitude depending on the applied experimental techniques and conditions. In this study, we used a kinetic multilayer model of aerosol chemistry with reversible surface adsorption and bulk diffusion (KM-SUB) in combination with a Monte Carlo genetic algorithm to simulate the measured decay rates of TBA. Two experimental data sets available from different studies can be described with a consistent set of kinetic parameters resolving the interplay of chemical reaction, mass transport, and shielding effects. Our study suggests that mass transport and shielding effects can substantially extend the atmospheric lifetime of reactive pesticides from a few days to weeks, with strong implications for long-range transport and potential health effects of these substances.

Air pollution is a major risk factor for human health. Chemical reactions in the epithelial lining fluid (ELF) of the human respiratory tract result in the formation of reactive oxygen species (ROS), which can lead to oxidative stress and adverse health effects. We use kinetic modeling to quantify the effects of fine particulate matter (PM2.5), ozone (O₃), and nitrogen dioxide (NO₂) on ROS formation, interconversion, and reactivity, and discuss different chemical metrics for oxidative stress, such as cumulative production of ROS and hydrogen peroxide (H₂O₂) to hydroxyl radical (OH) conversion. All three air pollutants produce ROS that accumulate in the ELF as H₂O₂, which serves as reservoir for radical species. At low PM2.5 concentrations (<10 μg m^-3), we find that less than 4% of all produced H₂O₂ is converted into highly reactive OH, while the rest is intercepted by antioxidants and enzymes that serve as ROS buffering agents. At elevated PM2.5 concentrations (>10 μg m^-3), however, Fenton chemistry overwhelms the ROS buffering effect and leads to a tipping point in H₂O₂ fate, causing a strong nonlinear increase in OH production. This shift in ROS chemistry and the enhanced OH production provide a tentative mechanistic explanation for how the inhalation of PM2.5 induces oxidative stress and adverse health effects.

Reactive oxygen species (ROS) play a central role in adverse health effects of air pollutants. Respiratory deposition of fine air particulate matter can lead to the formation of ROS in epithelial lining fluid, potentially causing oxidative stress and inflammation. Secondary organic aerosols (SOA) account for a large fraction of fine particulate matter, but their role in adverse health effects is unclear. Here, we quantify and compare the ROS yields and oxidative potential of isoprene, β-pinene, and naphthalene SOA in water and surrogate lung fluid (SLF). In pure water, isoprene and β-pinene SOA were found to produce mainly OH and organic radicals, whereas naphthalene SOA produced mainly H₂O₂ and O₂^•-. The total molar yields of ROS of isoprene and β-pinene SOA were 11.8% and 8.2% in water and decreased to 8.5% and 5.2% in SLF, which can be attributed to ROS removal by lung antioxidants. A positive correlation between the total peroxide concentration and ROS yield suggests that organic (hydro)peroxides may play an important role in ROS formation from biogenic SOA. The total molar ROS yields of naphthalene SOA was 1.7% in water and increased to 11.3% in SLF. This strong increase is likely due to redox reaction cycles involving environmentally persistent free radicals (EPFR) or semiquinones, antioxidants, and oxygen, which may promote the formation of H₂O₂ and the adverse health effects of anthropogenic SOA from aromatic precursors.

Nitration of the major birch pollen allergen Bet v 1 alters the immune responses toward this protein, but the underlying chemical mechanisms are not yet understood. Here we address the efficiency and site-selectivity of the nitration reaction of recombinant protein samples of Bet v 1.0101 with different nitrating agents relevant for laboratory investigations (tetranitromethane, TNM), for physiological processes (peroxynitrite, ONOO(-)), and for the health effects of environmental pollutants (nitrogen dioxide and ozone, O₃/NO₂). We determined the total tyrosine nitration degrees (ND) and the NDs of individual tyrosine residues (NDY). High-performance liquid chromatography coupled to diode array detection and HPLC coupled to high-resolution mass spectrometry analysis of intact proteins, HPLC coupled to tandem mass spectrometry analysis of tryptic peptides, and amino acid analysis of hydrolyzed samples were performed. The preferred reaction sites were tyrosine residues at the following positions in the polypeptide chain: Y83 and Y81 for TNM, Y150 for ONOO(-), and Y83 and Y158 for O₃/NO₂. The tyrosine residues Y83 and Y81 are located in a hydrophobic cavity, while Y150 and Y158 are located in solvent-accessible and flexible structures of the C-terminal region. The heterogeneous reaction with O₃/NO₂ was found to be strongly dependent on the phase state of the protein. Nitration rates were about one order of magnitude higher for aqueous protein solutions (∼20% per day) than for protein filter samples (∼2% per day). Overall, our findings show that the kinetics and site-selectivity of nitration strongly depend on the nitrating agent and reaction conditions, which may also affect the biological function and adverse health effects of the nitrated protein.