Abstract. Working with measurement data in atmospheric science often necessitates the collocation of observations from instruments or platforms at different locations, with different geographical and/or temporal data coverage. Varying complexity and abundance of the different data sets demand a consolidation of the observations. This paper presents a tool for (i) finding temporally and spatially resolved intersections between two- or three-dimensional geographical tracks (trajectories) and (ii) extracting of observations and other derived parameters in the vicinity of intersections to achieve the optimal combination of various data sets and measurement techniques. The TrackMatcher tool has been designed specifically for matching height-resolved remote-sensing observations along the ground track of a satellite with position data of aircraft (flight tracks) and clouds (cloud tracks) and intended extension for ships (shiptracks) and air parcels (forward and backward trajectories). The open-source algorithm is written in the Julia programming language. The core of the matching algorithm consist of interpolating tracks of different objects with a piecewise cubic Hermite interpolating polynomial with subsequent identification of an intercept point by minimising the norm between the different track point coordinate pairs. The functionality wrapped around the two steps allows for application of the TrackMatcher tool to a wide range of scenarios. Here, we present three examples of matching satellite tracks with the position of individual aircraft and clouds that demonstrate the usefulness of TrackMatcher for application in atmospheric science.

Working with measurement data in atmospheric science often necessitates the co-location of observations from instruments or platforms at different locations with different geographical and/or temporal data coverage. The varying complexity and abundance of the different data sets demand a consolidation of the observations. This paper presents a tool for (i) finding temporally and spatially resolved intersections between two- or three-dimensional geographical tracks (trajectories) and (ii) extracting observations and other derived parameters in the vicinity of intersections to achieve the optimal combination of various data sets and measurement techniques. The TrackMatcher tool has been designed specifically for matching height-resolved remote sensing observations along the ground track of a satellite with position data of aircraft (flight tracks) and clouds (cloud tracks) and is intended to be an extension for ships (ship tracks) and air parcels (forward and backward trajectories). The open-source algorithm is written in the Julia programming language. The core of the matching algorithm consist of interpolating tracks of different objects with a piecewise cubic Hermite interpolating polynomial with the subsequent identification of an intercept point by minimising the norm between the different track point coordinate pairs. The functionality wrapped around the two steps allows for the application of the TrackMatcher tool to a wide range of scenarios. Here, we present three examples of matching satellite tracks with the position of individual aircraft and clouds that demonstrate the usefulness of TrackMatcher for application in atmospheric science.

Aviation outflow is the only anthropogenic source of pollution that is directly emitted into the upper troposphere. This emission has the potential to modify the cloudiness directly by forming linear contrails and indirectly by injecting aerosols, which can act as cloud condensation nuclei (CCN) and ice nucleating particles (INP). Contrail cirrus can persist either in cloud-free supersaturated air, increasing high-cloud cover or inside natural cirrus cloud, and therefore modifying the microphysical properties of already existing cirrus clouds. Even though the situation that an aircraft flies through a natural cirrus is one of the highly probable situations in the upper troposphere, its subsequent impact is unclear with the present state of knowledge. Quantifying such impact is necessary if we are to properly account for the influence of aviation on climate. One main limitation preventing us from better identifying these impacts is the lack of height resolved measurements inside the cirrus clouds. In this study, we used new retrievals from combined satellite cloud radar and lidar (CloudSat/CALIPSO; DARDAR-Nice algorithm), which provide height resolved information of ice crystal number concentration, at intercepts between the CALIPSO ground track and the position of civil aircraft operating between the west coast of the continental United States (Seattle, San Francisco and Los Angeles) and Hawaii during 2010 and 2011 from an earlier study. Comparing cloudy air behind the aircraft inside the flight track to the adjacent regions and to ahead of the aircraft revealed a notable difference in ice number concentration at 300 m to 540 m beneath the flight height. These differences are derived from the reduction of ice number concentrations as we proceed toward the cloud base in regions unaffected by aviation and the increase of ice crystals as we distance a few hundreds of meters beneath the flight level in the regions affected by aviation.

Aerosol particles affect the climate directly through interaction with radiation. They also can cause a radiative forcing due to aerosol-cloud interactions (ACI), by acting as cloud condensation nuclei (CCN) in the formation of warm clouds or as ice nucleating particles (INP) during the phase change in mixed-phase clouds. Spaceborne remote sensing is a promising approach for quantifying ACI at a global scale and a useful technique for assessing and improving the performance of climate models. A more than 14-year data set of height-resolved measurements of aerosol optical properties from the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite (Winker at al., 2009) is utilized for estimating the vertical distributions of cloud-relevant particle concentrations. More specifically in this satellite-based study, conversion factors are applied to extinction coefficient observations to obtain vertical profiles of dry particle number and surface area concentration (Mamouri and Ansmann, 2015; 2016; Marinou et al., 2019). The last two are then used as input in measurement-based INP parameterizations in order to retrieve the INP active fractions for different aerosol types. Second part of this methodology is to combine images of geostationary sensors which provide continuously the history of cloud development with polar-orbiting observations on aerosol and cloud parameters that will allow the quantification of ACI. The spaceborne-based findings are crucial for identifying the effects of changes in aerosol concentration on the glaciation of warm and cold clouds.

Aviation outflow is the only anthropogenic source of pollution that is directly emitted into the upper troposphere. This emission has the potential to modify the cloudiness directly by forming linear contrails and indirectly by injecting aerosols, which can act as cloud condensation nuclei (CCN) and ice nucleating particles (INP). Contrail cirrus can persist either in cloud-free supersaturated air, increasing high-cloud cover or inside natural cirrus cloud, and therefore modifying the microphysical properties of already existing cirrus clouds. Even though the situation that an aircraft flies through a natural cirrus is one of the highly probable situations in the upper troposphere, its subsequent impact is unclear with the present state of knowledge. Quantifying such impact is necessary if we are to properly account for the influence of aviation on climate. One main limitation preventing us to better identify these impacts is the lack of height resolved measurements inside the cirrus clouds. In this study, we used new retrievals from combined satellite cloud radar and lidar (Cloud- Sat/CALIPSO; DARDAR-Nice algorithm), which provide height resolved information of ice crystal number concentration, at intercepts between the CALIPSO ground track and the position of civil aircraft operating between the west coast of the continental United States (Seattle, San Francisco and Los Angeles) and Hawaii during 2010 and 2011 from an earlier study. Comparing cloudy air behind the aircraft inside the flight track to the adjacent regions and to ahead of the aircraft revealed a notable difference in ice number concentration at 300 m to 540 m beneath the flight height. These differences are derived from the reduction of ice number concentrations as we proceed toward the cloud base in regions unaffected by aviation and the increase of ice crystals as we distance a few hundreds of meters beneath the flight level in the regions affected by aviation.

Effects of aviation on the Earth’s radiation budget and climate related to CO2 emissions and from the formation of linear contrails and contrail cirrus have been the focus of detailed studies. Aviation effects on existing cirrus clouds are much less investigated. Contrail formation in existing cirrus clouds has the potential to increase the cloud optical thickness (COT) of optically thin cirrus, which might result in a net cooling effect. Spaceborne remote sensing generally provides the means for studying the impact of aviation on climate. However, only active instruments such as lidar or radar can be used to study the effect of contrails that form within existing cirrus clouds. For such an investigation, the location of an aircraft at a given time needs to be matched with information on cloud coverage, cloud type, cloud layer height, and COT as can be retrieved from spaceborne CALIPSO lidar data. We have developed an algorithm to find intersections of aircraft flight tracks with satellite tracks. Besides the spatial coordinates, the time difference between the passing of the aircraft and the satellite at the intersection is monitored and relevant aircraft data and satellite recordings are retrieved at the intersection. The algorithm is highly adjustable so that it can be adapted for other applications such as investigation of ship tracks or cloud tracking. The new algorithm has been used to identify aircraft flying through cirrus clouds in remote regions of the Earth to study the effects of individual aircraft on existing cirrus.

Atmospheric aerosol particles are of great importance for cloud formation in the atmosphere because they are needed to act as cloud condensation nuclei (CCN) in liquid-water clouds and as ice nucleating particles (INP) in ice-containing clouds. Changes in aerosol concentration affect the albedo, development, phase, lifetime and rain rate of clouds. These aerosol-cloud interactions (ACI) and the resulting climate effects still cause the largest uncertainty in assessing climate change as they are understood only with medium confidence.The PACIFIC project, which is embedded in the French-German Make Our Planet Great Again (MOPGA) initiative, aims to improve our understanding of ACI by enhancing the representation of those aerosols that are relevant for cloud processes and by quantifying temporal changes in cloud properties throughout the cloud life cycle. PACIFIC uses a three-fold approach for studying ACI based on spaceborne observations by (i) using spaceborne lidar data to obtain unprecedented insight in CCN and INP concentrations at cloud level opposed to using column-integrated parameters, (ii) characterizing the development of clouds by tracking them in time-resolved geostationary observations opposed to resorting to the snap-shot view of polar-orbiting sensors, and (iii) combining the detailed observations from polar-orbiting sensors with the time-resolved observations of geostationary sensors – for clouds observed by both – to study the effects of CCN and INP on the albedo, liquid and ice water content, droplet and crystal size, development, phase and rain rate of clouds within different regimes carefully accounting for the meteorological background.This contribution will present the scope of the MOPGA-GRI project PACIFIC and illustrate the first findings.

This paper presents a new CAPRAM–GECKO-A protocol for mechanism auto-generation of aqueous-phase organic processes. For the development, kinetic data in the literature were reviewed and a database with 464 aqueous-phase reactions of the hydroxyl radical with organic compounds and 130 nitrate radical reactions with organic compounds has been compiled and evaluated. Five different methods to predict aqueous-phase rate constants have been evaluated with the help of the kinetics database: gas–aqueous phase correlations, homologous series of various compound classes, radical reactivity comparisons, Evans–Polanyi-type correlations, and structure–activity relationships (SARs). The quality of these prediction methods was tested as well as their suitability for automated mechanism construction. Based on this evaluation, SARs form the basis of the new CAPRAM–GECKO-A protocol. Evans–Polanyi-type correlations have been advanced to consider all available H atoms in a molecule besides the H atoms with only the weakest bond dissociation enthalpies (BDEs). The improved Evans–Polanyi-type correlations are used to predict rate constants for aqueous-phase NO3 and organic compounds reactions. Extensive tests have been performed on essential parameters and on highly uncertain parameters with limited experimental data. These sensitivity studies led to further improvements in the new CAPRAM–GECKO-A protocol but also showed current limitations. Biggest uncertainties were observed in uptake processes and the estimation of Henry's law coefficients as well as radical chemistry, in particular the degradation of alkoxy radicals. Previous estimation methods showed several deficits, which impacted particle growth. For further evaluation, a 1,3,5-trimethylbenzene oxidation experiment has been performed in the aerosol chamber “Leipziger Aerosolkammer” (LEAK) at high relative humidity conditions and compared to a multiphase mechanism using the Master Chemical Mechanism (MCMv3.2) in the gas phase and using a methylglyoxal oxidation scheme of about 600 reactions generated with the new CAPRAM–GECKO-A protocol in the aqueous phase. While it was difficult to evaluate single particle constituents due to concentrations close to the detection limits of the instruments applied, the model studies showed the importance of aqueous-phase chemistry in respect to secondary organic aerosol (SOA) formation and particle growth. The new protocol forms the basis for further CAPRAM mechanism development towards a new version 4.0. Moreover, it can be used as a supplementary tool for aerosol chambers to design and analyse experiments of chemical complexity and help to understand them on a molecular level.

Chlorine atoms (Cl) are highly reactive toward hydrocarbons in the Earth's troposphere, including the greenhouse gas methane (CH4). However, the regional and global CH4 sink from Cl is poorly quantified as tropospheric Cl concentrations ([Cl]) are uncertain by ~2 orders of magnitude. Here we describe the addition of a detailed tropospheric chlorine scheme to the TOMCAT chemical transport model. The model includes several sources of tropospheric inorganic chlorine (Cly), including (i) the oxidation of chlorocarbons of natural (CH3Cl, CHBr2Cl, CH2BrCl, and CHBrCl2) and anthropogenic (CH2Cl2, CHCl3, C2Cl4, C2HCl3, and CH2ClCH2Cl) origin and (ii) sea‐salt aerosol dechlorination. Simulations were performed to quantify tropospheric [Cl], with a focus on the marine boundary layer, and quantify the global significance of Cl atom CH4 oxidation. In agreement with observations, simulated surface levels of hydrogen chloride (HCl), the most abundant Cly reservoir, reach several parts per billion (ppb) over polluted coastal/continental regions, with sub‐ppb levels typical in more remote regions. Modeled annual mean surface [Cl] exhibits large spatial variability with the largest levels, typically in the range of 1–5 × 104 atoms cm−3, in the polluted northern hemisphere. Chlorocarbon oxidation provides a tropospheric Cly source of up to ~4320 Gg Cl/yr, sustaining a background surface [Cl] of <0.1 to 0.5 × 103 atoms cm−3 over large areas. Globally, we estimate a tropospheric methane sink of ~12–13 Tg CH4/yr due the CH4 + Cl reaction (~2.5% of total CH4 oxidation). Larger regional effects are predicted, with Cl accounting for ~10 to >20% of total boundary layer CH4 oxidation in some locations.

To quantify the effects of an advanced iron photochemistry scheme, the chemical aqueous-phase radical mechanism (CAPRAM 3.0i) has been updated with several new Fe(III)-carboxylate complex photolysis reactions. Newly introduced ligands are malonate, succinate, tartrate, tartronate, pyruvate, and glyoxalate. Model simulations show that more than 50% of the total Fe(III) is coordinated by oxalate and up to 20% of total Fe(III) is bound in the newly implemented 1:1 complexes with tartronate, malonate, and pyruvate. Up to 20% of the total Fe(III) is found in hydroxo and sulfato complexes. The fraction of [Fe(oxalate)2](-) and [Fe(pyruvate)](2+) is significantly higher during nighttime than during daytime, which points toward a strong influence of photochemistry on these species. Fe(III) complex photolysis is an important additional sink for tartronate, pyruvate, and oxalate, with a complex photolysis contribution to overall degradation of 46, 40, and 99%, respectively, compared to all possible sink reactions with atmospheric aqueous-phase radicals, such as (•)OH, NO3(•), and SO4(•) (-). Simulated aerosol particles have a much lower liquid water content than cloud droplets, thus leading to high concentrations of species and, consequently, an enhancement of the photolysis sink reactions in the aerosol particles. The simulations showed that Fe(III) photochemistry should not be neglected when considering the fate of carboxylic acids, which constitute a major part of aqueous secondary organic aerosol (aqSOA) in tropospheric cloud droplets and aqueous particles. Failure to consider this loss pathway has the potential to result in a significant overestimate of aqSOA production.

Tropospheric clouds and deliquescent aerosol particles are a complex multiphase and multi-component environment with simultaneously occurring gas and aqueous phase chemical transformations. Such aqueous phase chemical processes in cloud droplets are expected to proceed very efficient on short timescales and hence they are able to alter the chemical aerosol composition and the deduced physical properties on a global scale. Besides the physico-chemical aerosol processing, chemical aerosol-cloud interactions have significant effects on the whole multiphase oxidation budget. In order to improve the still limited understanding of the aerosol-cloud interactions, Lagrangian-type field experiments, where an orographic cloud is used as a natural flow-through reactor, are used for studying such processes in more detail. In Sept./Oct. 2010, the Lagrangian-type cloud passage experiment HCCT-2010 (Hill Cap Cloud Thuringia 2010) was conducted at Mt. Schmücke in Thuringia, Germany to study aerosol cloud interactions (see Figure 1). As known from former cloud passage experiments, particularly associated model investigations (see e.g. Tilgner et al., 2005) including comparisons of model results with observations have considerably contributed to the interpretation of the measured field data.

Modelling studies were performed with the multiphase mechanism RACM-MIM2ext/CAPRAM 3.0i to investigate the tropospheric multiphase chemistry in deliquesced particles and non-precipitating clouds using the SPACCIM model framework. Simulations using a non-permanent cloud scenario were carried out for two different environmental conditions focusing on the multiphase chemistry of oxidants and other linked chemical subsystems. Model results were analysed by time-resolved reaction flux analyses allowing advanced interpretations. The model shows significant effects of multiphase chemical interactions on the tropospheric budget of gas-phase oxidants and organic compounds. In-cloud gas-phase OH radical concentration reductions of about 90 % and 75 % were modelled for urban and remote conditions, respectively. The reduced in-cloud gas-phase oxidation budget increases the tropospheric residence time of organic trace gases by up to about 30 %. Aqueous-phase oxidations of methylglyoxal and 1,4-butenedial were identified as important OH radical sinks under polluted conditions. The model revealed that the organic C 3 and C 4 chemistry contributes with about 38 %/48 % and 8 %/9 % considerably to the urban and remote cloud / aqueous particle OH sinks. Furthermore, the simulations clearly implicate the potential role of deliquescent particles to operate as a reactive chemical medium due to an efficient TMI/HO x,y chemical processing including e.g. an effective in-situ formation of OH radicals. Considerable chemical differences between deliquescent particles and cloud droplets, e.g. a circa 2 times more efficient daytime iron processing in the urban deliquescent particles, were identified. The in-cloud oxidation of methylglyoxal and its oxidation products is identified as efficient sink for NO 3 radicals in the aqueous phase.

A new detailed multiphase halogen mechanism, the CAPRAM Halogen Module 2.0 (HM2), has been developed and coupled to the multiphase chemistry mechanism RACM-MIM2ext/CAPRAM 3.0n. The overall mechanism comprises 1,705 reactions including 595 reactions of the HM2. Halogen chemistry box model studies have been, for the first time, performed with a non-permanent cloud scenario for pristine open ocean regions in mid-latitudes. Moreover, detailed time-resolved reaction flux analysis has been used to investigate the multiphase halogen reaction cycles in more detail. Clouds significantly change the multiphase halogen chemical system and new reaction cycles are proposed for in-cloud conditions. While most gas phase concentrations are decreased for chlorine and iodine species, they are increased for bromine. Flux analyses determined the relative contributions of the methylene dihalides CH 2 IX (X = Cl, Br, I) as the main I atom source with a contribution of about 80 % to the total iodocarbon sources. Furthermore, HOI was confirmed to be important for chlorine activation. It is shown that 25 % of the ozone loss can be attributed to halogens. VOC oxidation by halogens is important as halogens account for about 20 % of the methane oxidation and up to 80 % of the oxidation of other VOCs. In other cases, enhanced VOC and VOC oxidation product concentration levels were found. For example, 15 % of the methyl peroxyl radicals are formed after the reaction of chlorine atoms with methane or methyl hydroperoxide. In the aqueous phase, changes in the oxidation of organics do only occur for highly oxidised organics without a C-H bond. For example, over 80 % of oxalic acid are oxidised by electron transfer with Cl 2 − in deliquescent particles during non-cloud periods.