Properties and Processing of Aviation Exhaust Aerosol at Cruise Altitude Observed from the IAGOS-CARIBIC Flying Laboratory.

The characteristics of aviation-induced aerosol, its processing, and effects on cirrus clouds and climate are still associated with large uncertainties. Properties of aviation-induced aerosol, however, are crucially needed for the assessment of aviation's climate impacts today and in the future. We identified more than 1100 aircraft plume encounters during passenger aircraft flights of the IAGOS-CARIBIC Flying Laboratory from July 2018 to March 2020. The aerosol properties inside aircraft plumes were similar, independent of the altitude (i.e., upper troposphere, tropopause region, and lowermost stratosphere). The exhaust aerosol was found to be mostly externally mixed compared to the internally mixed background aerosol, even at a plume age of 1 to 3 h. No enhancement of accumulation mode particles (diameter >250 nm) could be detected inside the aircraft plumes. Particle number emission indices (EIs) deduced from the observations in aged plumes are in the same range as values reported from engine certifications. This finding, together with the observed external mixing state inside the plumes, indicates that the aviation exhaust aerosol almost remains in its emission state during plume expansion. It also reveals that the particle number EIs used in global models are within the range of the EIs measured in aged plumes.

Correction: Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry.

Correction for 'Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry' by Michelle Färber et al., Phys. Chem. Chem. Phys., 2024, https://doi.org/10.1039/d3cp04163h.

Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry.

The formation of peroxynitrates (RO2NO2) from the reaction of peroxy radicals (RO2) and nitrogen dioxide (NO2) and their subsequent redissociation are typically not included in chemical mechanisms. This is often done to save computational time as the assumption is that the equilibrium is strongly towards the RO2 + NO2 reaction for most conditions. Exceptions are the reactions of the methyl peroxy radical due to its abundance in the atmosphere and of acyl-RO2 radicals due to the long lifetime of peroxyacyl nitrates RO2NO2 (PANs). In this study, the nighttime oxidation of cis-2-butene and trans-2-hexene in the presence of NO2 is investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum Jülich, Germany, at atmospherically-relevant conditions at different temperatures (≈276 K, ≈293 K, ≈305 K). Measured concentrations of peroxy and hydroperoxy radicals as well as other trace gases (ozone, NO2, volatile organic compounds) are compared to state-of-the-art zero-dimensional box model calculations. Good model-measurement agreement can only be achieved when reversible RO2 + NO2 reactions are included for all RO2 species using literature values available from the latest SAR by [Jenkin et al., Atmos. Chem. Phys., 2019, 19, 7691]. The good agreement observed gives confidence that the SAR, derived originally for aliphatic RO2, can be applied to a large range of substituted RO2 radicals, simplifying generalised implementation in chemical models. RO2NO2 concentrations from non-acyl RO2 radicals of up to 2 × 10 cm-3 are predicted at 276 K, impacting effectively the kinetics of RO2 radicals. Under these conditions, peroxy radicals are slowly regenerated downwind of the pollution source and may be lost in the atmosphere through deposition of RO2NO2. Based on this study, 60% of RO2 radicals would be stored as RO2NO2 at a temperature of 10 °C and in the presence of a few ppbv of NO2. The fraction increases further at colder temperatures and/or higher NO2 mixing ratios. This does not only affect the expected concentrations of RO2 radicals but, as the peroxynitrates can react with OH radicals or photolyse, they could comprise a net sink for RO2 radicals as well as increase the production of NOx (= NO + NO2) in different locations depending on their lifetime. Omitting this chemistry from the kinetic model can lead to misinterpreted product formation and may prevent reconciling observations and model predictions.

Multiple Impacts of Aerosols on O3 Production Are Largely Compensated: A Case Study Shenzhen, China.

Tropospheric ozone (O3) is a harmful gas compound to humans and vegetation, and it also serves as a climate change forcer. O3 is formed in the reactions of nitrogen oxides and volatile organic compounds (VOCs) with light. In this study, an O3 pollution episode encountered in Shenzhen, South China in 2018 was investigated to illustrate the influence of aerosols on local O3 production. We used a box model with comprehensive heterogeneous mechanisms and empirical prediction of photolysis rates to reproduce the O3 episode. Results demonstrate that the aerosol light extinction and NO2 heterogeneous reactions showed comparable influence but opposite signs on the O3 production. Hence, the influence of aerosols from different processes is largely counteracted. Sensitivity tests suggest that O3 production increases with further reduction in aerosols in this study, while the continued NOx reduction finally shifts O3 production to an NOx-limited regime with respect to traditional O3-NOx-VOC sensitivity. Our results shed light on the role of NOx reduction on O3 production and highlight further mitigation in NOx not only limiting the production of O3 but also helping to ease particulate nitrate, as a path for cocontrol of O3 and fine particle pollution.

Highly Oxygenated Organic Nitrates Formed from NO3 Radical-Initiated Oxidation of ß-Pinene.

The reactions of biogenic volatile organic compounds (BVOC) with the nitrate radicals (NO3) are major night-time sources of organic nitrates and secondary organic aerosols (SOA) in regions influenced by BVOC and anthropogenic emissions. In this study, the formation of gas-phase highly oxygenated organic molecules-organic nitrates (HOM-ON) from NO3-initiated oxidation of a representative monoterpene, ß-pinene, was investigated in the SAPHIR chamber (Simulation of Atmosphere PHotochemistry In a large Reaction chamber). Six monomer (C = 7-10, N = 1-2, O = 6-16) and five accretion product (C = 17-20, N = 2-4, O = 9-22) families were identified and further classified into first- or second-generation products based on their temporal behavior. The time lag observed in the peak concentrations between peroxy radicals containing odd and even number of oxygen atoms, as well as between radicals and their corresponding termination products, provided constraints on the HOM-ON formation mechanism. The HOM-ON formation can be explained by unimolecular or bimolecular reactions of peroxy radicals. A dominant portion of carbonylnitrates in HOM-ON was detected, highlighting the significance of unimolecular termination reactions by intramolecular H-shift for the formation of HOM-ON. A mean molar yield of HOM-ON was estimated to be 4.8% (-2.6%/+5.6%), suggesting significant HOM-ON contributions to the SOA formation.

Chemical Production of Oxygenated Volatile Organic Compounds Strongly Enhances Boundary-Layer Oxidation Chemistry and Ozone Production.

Photolysis of oxygenated volatile organic compounds (OVOCs) produces a primary source of free radicals, including OH and inorganic and organic peroxy radicals (HO2 and RO2), consequently increasing photochemical ozone production. The amplification of radical cycling through OVOC photolysis provides an important positive feedback mechanism to accelerate ozone production. The large production of OVOCs near the surface helps promote photochemistry in the whole boundary layer. This amplifier effect is most significant in regions with high nitrogen oxides (NOx) and VOC concentrations such as Wangdu, China. Using a 1-D model with comprehensive observations at Wangdu and the Master Chemical Mechanism (MCM), we find that OVOC photolysis is the largest free-radical source in the boundary layer (46%). The condensed chemistry mechanism we used severely underestimates the OVOC amplifier effect in the boundary layer, resulting in a lower ozone production rate sensitivity to NOx emissions. Due to this underestimation, the model-simulated threshold NOx emission value, below which ozone production decreases with NOx emission decrease, is biased low by 24%. The underestimated OVOC amplifier effect in a condensed mechanism implies a low bias in the current 3-D model-estimated efficacy of NOx emission reduction on controlling ozone in polluted urban and suburban regions of China.

No Evidence for a Significant Impact of Heterogeneous Chemistry on Radical Concentrations in the North China Plain in 2014.

The oxidation of nitric oxide to nitrogen dioxide by hydroperoxy (HO2) and organic peroxy radicals (RO2) is responsible for the chemical net ozone production in the troposphere and for the regeneration of hydroxyl radicals, the most important oxidant in the atmosphere. In Summer 2014, a field campaign was conducted in the North China Plain, where increasingly severe ozone pollution has been experienced in the last years. Chemical conditions in the campaign were representative for this area. Radical and trace gas concentrations were measured, allowing for calculating the turnover rates of gas-phase radical reactions. Therefore, the importance of heterogeneous HO2 uptake on aerosol could be experimentally determined. HO2 uptake could have suppressed ozone formation at that time because of the competition with gas-phase reactions that produce ozone. The successful reduction of the aerosol load in the North China Plain in the last years could have led to a significant decrease of HO2 loss on particles, so that ozone-forming reactions could have gained importance in the last years. However, the analysis of the measured radical budget in this campaign shows that HO2 aerosol uptake did not impact radical chemistry for chemical conditions in 2014. Therefore, reduced HO2 uptake on aerosol since then is likely not the reason for the increasing number of ozone pollution events in the North China Plain, contradicting conclusions made from model calculations reported in the literature.

Wintertime N2O5 uptake coefficients over the North China Plain.

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) plays an important role in regulating NOx. The N2O5 uptake coefficient, γ(N2O5), was determined using an iterative box model that was constrained to observational data obtained in suburban Beijing from February to March 2016. The box model determined 2289 individual γ(N2O5) values that varied from <0.001 to 0.02 with an average value of 0.0046 ± 0.0039 (and a median value of 0.0032). We found the derived winter γ(N2O5) values in Beijing were relatively low as compared to values reported in previous field studies conducted during winter in Hong Kong (average value of 0.014) and the eastern U.S. coast (median value of 0.0143). In our study, field evidence of the suppression of γ(N2O5) values due to pNO3- content, organics and the enhancement by aerosol liquid water content (ALWC) is in line with previous laboratory study results. Low ALWC, high pNO3- content, and particle morphology (inorganic core with an organic shell) accounted for the low γ(N2O5) values in the North China Plain (NCP) during wintertime. The field-derived γ(N2O5) values are well reproduced by a revised parameterization method, which includes the aerosol size distribution, ALWC, nitrate and organic coating, suggesting the feasibility of comprehensive parameterization in the NCP during wintertime.

Fast Photochemistry in Wintertime Haze: Consequences for Pollution Mitigation Strategies.

In contrast to summer smog, the contribution of photochemistry to the formation of winter haze in northern mid-to-high latitude is generally assumed to be minor due to reduced solar UV and water vapor concentrations. Our comprehensive observations of atmospheric radicals and relevant parameters during several haze events in winter 2016 Beijing, however, reveal surprisingly high hydroxyl radical oxidation rates up to 15 ppbv/h, which is comparable to the high values reported in summer photochemical smog and is two to three times larger than those determined in previous observations during winter in Birmingham (Heard et al. Geophys. Res. Lett. 2004, 31, (18)), Tokyo (Kanaya et al. J. Geophys. Res.: Atmos. 2007, 112, (D21)), and New York (Ren et al. Atmos. Environ. 2006, 40, 252-263). The active photochemistry facilitates the production of secondary pollutants. It is mainly initiated by the photolysis of nitrous acid and ozonolysis of olefins and maintained by an extremely efficiently radical cycling process driven by nitric oxide. This boosted radical recycling generates fast photochemical ozone production rates that are again comparable to those during summer photochemical smog. The formation of ozone, however, is currently masked by its efficient chemical removal by nitrogen oxides contributing to the high level of wintertime particles. The future emission regulations, such as the reduction of nitrogen oxide emissions, therefore are facing the challenge of reducing haze and avoiding an increase in ozone pollution at the same time. Efficient control strategies to mitigate winter haze in Beijing may require measures similar as implemented to avoid photochemical smog in summer.

Winter photochemistry in Beijing: Observation and model simulation of OH and HO2 radicals at an urban site.

A field campaign was conducted from November to December 2017 at the campus of Peking University (PKU) to investigate the formation mechanism of the winter air pollution in Beijing with the measurement of hydroxyl and hydroperoxyl radical (OH and HO2) with the support from comprehensive observation of trace gases compounds. The extent of air pollution depends on meteorological conditions. The daily maximum OH radical concentrations are on average 2.0 × 106 cm-3 and 1.5 × 106 cm-3 during the clean and polluted episodes, respectively. The daily maximum HO2 radical concentrations are on average 0.4 × 108 cm-3 and 0.3 × 108 cm-3 during the clean and polluted episodes, respectively (diurnal averaged for one hour bin). A box model based on RACM2-LIM1 mechanism can reproduce the OH concentrations but underestimate the HO2 concentrations by 50% during the clean episode. The OH and HO2 concentrations are underestimated by 50% and 12 folds during the polluted episode, respectively. Strong dependence on nitric oxide (NO) concentration is found for both observed and modeled HO2 concentrations, with the modeled HO2 decreasing more rapidly than observed HO2, leading to severe HO2 underestimation at higher NO concentrations. The OH reactivity is calculated from measured and modeled species and inorganic compounds (carbon monoxide (CO), NO, and nitrogen dioxide (NO2)) make up 69%-76% of the calculated OH reactivity. The photochemical oxidation rate denoted by the OH loss rate increases by 3 times from the clean to polluted episodes, indicating the strong oxidation capacity in polluted conditions. The comparison between measurements at PKU site and a suburban site from one previous study shows that chemical conditions are similar in both urban and suburban areas. Hence, the strong oxidation capacity and its potential contribution to the pollution bursts are relatively homogeneous over the whole Beijing city and its surrounding areas.

Explicit diagnosis of the local ozone production rate and the ozone-NOx-VOC sensitivities.

In the troposphere, ozone is a harmful gas compound to both human health and vegetation. Ozone is produced from the reaction of NOx (NO + NO2) and VOCs (volatile organic compounds) with light. Due to the highly nonlinear relationships between ozone and its precursors, proper ozone mitigation relies on the knowledge of chemical mechanisms. In this study, an observation-based method is used to simulate ozone formation and elucidate its controlling factors for a rural site on the North China Plain. The instantaneous ozone production rate is calculated utilizing a box model using the dataset obtained from the Wangdu campaign. First, the model was operated in a time-dependent mode to calculate the ozone production rate at each time stamp. The calculated ozone formation rate showed a diurnal average maximum value of 17 ppbv/h (1-h diurnal averaged). The contribution of individual peroxy radicals to ozone production was analyzed. In addition, the functional dependence of calculated P(O3) reveals that ozone production was in a NOx-limited regime during the campaign. Furthermore, the missing peroxy radical source will further extend NOx-limited conditions to earlier in the day, making NOx limitation dominate more of a day than the current chemical model predicts. Finally, a multiple scenarios mode, also known as EKMA (empirical kinetic modeling approach), was used to simulate the response of P(O3) to the imaginary change in precursor concentrations. We found that ozone production was in the NOx-limited region. However, the use of NO2 measured by the molybdenum converter and/or the absence of a peroxy radical source in the current chemical model could over-emphasize the VOC-limited effect on ozone production.

Upper tropospheric water vapour and its interaction with cirrus clouds as seen from IAGOS long-term routine in situ observations.

IAGOS (In-service Aircraft for a Global Observing System) performs long-term routine in situ observations of atmospheric chemical composition (O3, CO, NOx, NOy, CO2, CH4), water vapour, aerosols, clouds, and temperature on a global scale by operating compact instruments on board of passenger aircraft. The unique characteristics of the IAGOS data set originate from the global scale sampling on air traffic routes with similar instrumentation such that the observations are truly comparable and well suited for atmospheric research on a statistical basis. Here, we present the analysis of 15 months of simultaneous observations of relative humidity with respect to ice (RHice) and ice crystal number concentration in cirrus (Nice) from July 2014 to October 2015. The joint data set of 360 hours of RHice-Nice observations in the global upper troposphere and tropopause region is analysed with respect to the in-cloud distribution of RHice and related cirrus properties. The majority of the observed cirrus is thin with Nice < 0.1 cm-3. The respective fractions of all cloud observations range from 90% over the mid-latitude North Atlantic Ocean and the Eurasian Continent to 67% over the subtropical and tropical Pacific Ocean. The in-cloud RHice distributions do not depend on the geographical region of sampling. Types of cirrus origin (in situ origin, liquid origin) are inferred for different Nice regimes and geographical regions. Most importantly, we found that in-cloud RHice shows a strong correlation to Nice with slightly supersaturated dynamic equilibrium RHice associated with higher Nice values in stronger updrafts.

The Essential Role for Laboratory Studies in Atmospheric Chemistry.

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.