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This thesis reports the use of an aerosol chamber to investigate Secondary Organic Aerosol (SOA) transformation and formation in biogenic and anthropogenic VOC mixtures. Experiments conducted to comprehensively describe and characterise the Manchester aerosol chamber (MAC) are reported. Photo-oxidation chamber experiments were conducted using online and offline mass spectrometric instrumentation, including Gas Chromatography-Mass Spectrometer (GC-MS), Aerosol Mass Spectrometer (AMS), Chemical Ionisation Mass spectrometer (CIMS) and liquid chromatography ultra-high resolution mass spectrometry (UPLC-orbitrap-MS). These were used to explore SOA transformation and formation in anthropogenic (o-cresol) and biogenic (a-pinene and isoprene) mixed precursors in the presence of NOx and inorganic seed particles (ammonium sulphate) through investigation of the SOA particle mass yield and characterisation of SOA composition in mixed precursor systems. The MAC characterisation study highlights the importance of conducting regular characterisation experiments to track the performance of the simulation chamber and emphasises the need to develop a routine set of standardised experiments for chambers, elucidating the particular characteristics of each facility, thus enabling the interpretation of the results. The study of SOA formation from mixed precursors established a novel chamber experimental design associated with initial "iso-reactivity" of the systems towards the dominant oxidant (OH), achieved by adjustment of the initial concentration of precursors in the mixed system. The SOA particle mass yield exhibited suppression when compared with that of a-pinene when mixed with isoprene, but a minor yield enhancement was tentatively found while adding isoprene to o-cresol. The a-pinene/o-cresol system exhibited a higher measured SOA particle mass yield than that calculated based on additivity. The measured and predicted yields were comparable in ternary systems. The ambiguity in reference for yield evaluation present challenges to the evaluation of changes in SOA formation when more than one precursor contributed to the SOA mass, as do differences in the prevailing chemical regime. An automated non-targeted accurate mass analysis of LC-Orbitrap-MS data was used to characterise the particulate products from mixed precursor systems. a-pinene oxidation products were shown to dominate the binary mixed a-pinene/isoprene system in terms of fractional signal contribution and the number of particle components detected. o-cresol oxidation products (methyl-nitrocatechol and methy-nitrophenol) dominated the SOA particle composition in both o-cresol/isoprene and o-cresol/a-pinene binary systems in negative ionisation mode. This thesis further stressed the significance of unique-to-mixture products formation, which contributes to molecular composition and signal abundance in o-cresol-containing binary systems in positive ionisation mode. Compounds uniquely identified in each mixture were identified, with the potential to be used as tracers in SOA source attribution in future ambient studies. Finally this thesis reports the alteration of average carbon oxidation state of SOA particle components in all systems, by employing HR-TOF-AMS, FIGAERO-CIMS and LC-Orbitrap MS to characterise the SOA composition. The oxidation state of nitrogen (OSN) for compounds that composed of Carbon (C), Hydrogen (H), Oxygen (O) and Nitrogen (N) element significantly influenced average OSc in single a-pinene and o-cresol system in FIGAERO-CIMS and LC-Orbitrap MS measurement. A substantial discrepancy in the estimated OSc between the FIGAERO-CIMS and HR-ToF-AMS and LC-Orbitrap MS techniques was found, likely associated with the characteristic of each technique and their limitation. a-pinene driven SOA and o-cresol oxidation products were shown to dominate the average OSc of SOA in binary a-pinene/isoprene system and binary o-cresol /isoprene system. In contrast, both a-pinene and o-cresol oxidation products contributed to the alteration of the average OSc during SOA formation in the binary a-pinene/o-cresol system. In the ternary precursor system, the OSc of SOA is not dominantly controlled by any single precursors but is associated with molecular interaction of the product.

Highly oxygenated molecules (HOM) are crucial in the processes of secondary organic aerosol (SOA). In this thesis, two studies on HOM are carried out. The first one applies a simplified HOM scheme to PyCHAM, a newly developed box mode, to simulate alpha-pinene ozonolysis experiments in the dark environment in two different chambers, and concludes the essential role of the second-generation autoxidation. The second study develops and tests a more mechanistic HOM scheme in PyCHAM to investigate the binary system containing alpha-pinene and isoprene in terms of SOA production. These two studies can contribute to a better understanding of HOM as the precursors of SOA production. A command-line tool that can co-operate with PyCHAM is developed and used in both studies.

Secondary organic aerosols (SOA) contribute significantly to the total organic aerosol mass in the atmosphere. Understanding the SOA formation is crucial in order to estimate its impact on the air quality, climate and human health. SOA is typically produced from the oxidation of anthropogenic volatile organic compounds (AVOCs) or biogenic volatile organic compounds (BVOCs) by the oxidants such as OH radical, O3, or NO3 radical. SOA formation from single BVOCs or AVOCs has been widely investigated, while there are limited studies of the SOA formation from VOCs mixtures. There are many uncertainties in the physicochemical properties of SOA formed in the mixed VOC precursors. This work designed a series of experiments to explore SOA formation from the mixed AVOC (o-cresol) and BVOCs (a-pinene and isoprene) under the presence of NOx in the Manchester Aerosol Chamber. The online and offline analytical techniques were applied to investigate the chemical and physical properties of SOA. The gas- and particle-phase oxidation products were monitored by the near real-time online iodide chemical ionization mass spectrometry (CIMS) coupled with Filter Inlet for Gases and AEROsols (FIGAERO). The collected filter at the end of each experiment was characterised by the offline liquid chromatography orbitrap mass spectrometry (LC-Orbitrap MS) to investigate particle-phase chemical components. This study found that the chemical components of SOA formed from various VOCs systems showed big differences. In the a-pinene and a-pinene/isoprene mixture, the products with carbon, hydrogen and oxygen (CHO group) dominated the signals in the particle phase, broadly consistent with the LC-Orbitrap MS negative mode analysis which was able to better identify the sulphur-containing fraction. By contrast, o-cresol containing systems were dominated by the CHON signal fraction (>60%) from offline negative mode analysis. More compounds with high carbon numbers (nC>=16) were detected by the LC-Orbitrap MS positive ionisation mode, which indicated a fraction missed by the negative mode and CIMS measurements. Additionally, unique-to-the-mixture products were observed in the mixture, suggesting molecular interactions in the mixture systems. This work brought an insight into the SOA chemical composition from the mixed volatile precursors by the combination of online FIGAERO-CIMS and offline LC-Orbitrap MS analytical techniques and highlighted the importance of SOA studies in the mixed volatile precursors.

Aerosol particles are ubiquitous in the atmosphere, impacting air quality, global climate and threatening public health. These aerosol-induced environmental impacts are tightly bounded by their water uptake. However, it is not yet well understood how complex chemical composition influences water uptake and the related impacts. In addition, direct observations (especially the long-term) of aerosol water uptake worldwide are still lacking due to expensive expenses. The aim of the presented thesis is to demonstrate the chemical controls of aerosol water uptake, its impacts on aerosol phase state and air pollution, and to retrieve hygroscopicity parameter from open-access data using a combination of laboratory chamber experiments, field observational data and theoretical calculations. Chamber experiments on the secondary organic aerosol (SOA) from mixed biogenic and anthropogenic precursors, showed that the water saturation ratio and organic mass fraction are the key factors of water uptake whereas the SOA composition plays a second-order role. For the reconciliation of sub- and super-saturated water uptake, the hygroscopicity parameter ksub/ksuper ratio increased with organic mass fraction. The factors which influence the aerosol water uptake, control the aerosol phase transition between the liquid and non-liquid. In addition, aerosol water uptake was found to play a key role in Beijing winter haze development via a positive feedback between aerosol liquid water and particulate nitrate formation, where the aerosol liquid water can contribute up to 24 % to visibility reduction. To resolve the shortage of hygroscopicity observations worldwide, a novel method was developed, based on k-Kohler and Mie theory, to retrieve aerosol hygroscopicity from open access PM2.5 mass concentration, ambient RH and visibility data. The method was applied in Delhi, India and reported the high aerosol hygroscopicity for the first time. To sum up, the presented thesis improved our understanding on water uptake of multi-component aerosol particles and the relevant impacts. More interestingly, the developed method is applicable in wider regions worldwide and make the aerosol hygroscopicity knowledge achievable. Furthermore, this method shed lights on the quantification of co-condensation of condensable vapours on aerosol hygroscoscopicity in future after its successful case in Delhi.

Atmospheric aerosols are complex combinations of different chemical species, and they have an important impact on the climate system by scattering and absorbing solar radiation and interacting with clouds. Atmospheric aerosols also negatively affect air quality and thus human health and ecosystems. With the rapid development of the world economy and urbanization, emissions of anthropogenic aerosols has increased significantly. Thus, it is important to characterise the physicochemical properties of aerosols. However, detailed characterisation of aerosol mixing state and its impact is challenging due to both the instrument limitations and the complicated emission and aging mechanisms. This thesis addresses this issue through three in-situ measurements: two ground-based measurement campaigns and one aircraft measurement campaign. The two ground measurements were conducted in Beijing and its suburban region to investigate the aerosol mixing state in a polluted environment, and the first ground measurement contributes to the Air Pollution and Human Health (APHH) Beijing project. Two novel size classifiers, a Centrifugal Particle Mass Analyzer (CPMA) and an Aerodynamic Aerosol Classifier (AAC), were provided the size-resolved measurements in these two studies respectively. A number of different measurement techniques were placed downstream of the aerosol classifier to measure the aerosol chemical composition, number concentrations and cloud condensation nuclei (CCN) abilities. For the first time, the application of CPMA together with an inversion method provided a new morphology independent measurement method to characterise the BC mixing state in the ambient atmosphere. Black carbon (BC) particles were found to be between an internal and external mixture in urban Beijing. The BC mixing state in urban Beijing was influenced by the air mass in winter, while no such strong correlation was found during the summer. For broader implications, the AAC combination provides the aerodynamic size-resolved measurement of both refractory and non-refractory composition in a suburb of Beijing. While organic matter accounted for a large fraction of particulate mass, a higher contribution of particulate nitrate at larger sizes was found in polluted conditions. The number fraction of BC was found to increase with aerodynamic diameter, and these thickly coated BC may act as source of CCN. This size regime in a polluted sub-urban environment may exhibit a higher deposition rate. Aircraft observations were performed over the Atlantic Ocean to measure the shipping emissions as part of the Atmospheric Composition and Radiative forcing change due to the International Ship Emissions regulations (ACRUISE) project. Sulfate was found to be an important contributor to the overall ship emitted aerosol compositions, and the reduction of fuel sulfur content (FSC) due to the new global emission regulation is estimated to reduce the CCN abilities of ship emitted aerosols significantly. However, the emission of less hygroscopic compounds, BC and organic matter, is less influenced by the change of FSC. In summary, the results from the new measurement techniques and methodologies in this work offer an in-depth insight into the physicochemical properties of anthropogenic aerosols. These key findings build the connection between the aerosol mixing state and cloud condensation nuclei activities. This increasing understanding of aerosol properties can contribute to the improvement of model simulations and the development of policies to mitigate the effects of anthropogenic pollution in future studies.

Aerosol particles originate from both primary and secondary sources with implications on climate, air quality and human health. Secondary organic aerosols (SOA) comprise a large fraction of the total aerosol mass and are formed from the oxidation of anthropogenic and biogenic volatile organic compounds (AVOC and BVOC, respectively). The VOC emission rates and sources are diverse, leading to a highly complex SOA composition that can substantially vary temporarily and spatially. Until recently, the SOA mass was thought to be independently formed from the oxidation of each VOC precursor, however recent evidence demonstrated that the molecular interactions of the oxidised products may alter the SOA formation potential. This could partly explain our inability to predict the SOA loadings and impacts that are still highly uncertain. This thesis explores the photochemical SOA formation, composition and volatility derived from the mixing of various key AVOC (o-cresol) and BVOC (a-pinene and isoprene) by conducting experiments in an atmospheric simulation chamber. In order to assess the capabilities and limitation of the facilities used in this thesis, a detailed characterisation of the Manchester Aerosol Chamber (MAC) was conducted. This revealed the importance of regularly characterising such facilities for losses of particles and gases and highlighted the need for the development of a unified framework to characterise the atmospheric simulation chamber facilities. The concept of initial iso-reactivity was conceived that enabled the preparation of VOC mixtures that had comparable reactivity towards the assumed dominat oxidant (OH) in all the systems examined. The SOA formation potential was found to be enhanced, supressed or unaffected by the mixing of the precursors, suggesting that the effect of mixing is system-dependent and not straightforward. Similarly, certain mixed systems showed to have higher SOA particle volatility than that observed in single precursor experiments (e.g., o-cresol/isoprene), others showed lower (e.g., a-pinene/o-cresol and a-pinene/o-cresol/isoprene), while others appeared to be unaffected (e.g., a-pinene/isoprene). All the mixed systems however showed clear differences in the SOA chemical composition. Two main processes were observed in all the mixed VOC systems; the suppression in the formation of products that formed in single precursor systems and the formation of unique products in each mixture. The trade-off between these two processes are likely, at least partly, defining the SOA composition in mixed VOC systems that in turn may affect the SOA formation and volatility.

Air pollution is a major global health concern, contributing to an estimated 7 million premature deaths per year. 91% of the world's population live in areas with unsafe air which is a particular issue in urban areas. This is due to high levels of anthropogenic emissions from sectors such as: industry, transport, heating and biomass burning. However, natural emissions from wildfires, volcanoes, sea spray and desert dust can also contribute to poor air quality. Pollutants in the atmosphere undergo physical and chemical changes which can greatly affect their physical and chemical properties. Furthermore, they can interact with radiation to impact the climate and cause changes in meteorology, which can enhance atmospheric pollution. Rapid urbanisation and industrialisation in countries like China and India has led to large populations living in urban environments with poor air quality. Beijing, a megacity in North Eastern China, is well known for its air quality problems. This is due to high anthropogenic emissions combined with unfavourable meteorology and topography. Despite policy interventions improving average annual air quality in Beijing, it still experiences extreme pollution episodes or haze. During haze episodes, aerosol particles accumulate in a shallow planetary boundary layer (PBL), to reduce visibility < 10 km. The interactions of aerosol particles with radiation in Beijing is believed to suppress turbulent motion, inhibit pollutant dispersion and allow for high aerosol concentrations to accumulate in a shallow PBL. This further increases the extent of aerosol-radiation interactions. The feedback between aerosols, radiation and PBL meteorology is believed to contribute significantly to the intensity and longevity of haze episodes in Beijing. However, quantifying this effect has proven difficult through observational and regional modelling studies alone. These studies struggle to fully characterise the urban PBL and directly elucidate some of the important processes and variables affecting the aerosol-PBL feedback mechanism. This work presents the development and use of a fully coupled LES-aerosol radiation model, which allows for isolation of processes and variables that impact the aerosol-PBL feedback. This has allowed for further understanding of the contribution of this process to Beijing haze episodes.

Atmospheric aerosols are important in the atmosphere and affect global climate and human health. Aerosols are a large area of uncertainty in the climate system, in part, because there are considerable analytical challenges to understanding the properties of micron and nano sized particles and the processes governing aerosol transformation. This thesis uses a range of spectroscopy, spectrometry and imaging methods and combinations thereof to probe material properties of two types of aerosol, organic aerosol and black carbon. The properties explored include vapour pressure, mass diffusion, and absorbance. Organic aerosol partitioning is dependent on the vapour pressures of its' components. The vapour pressures of organic isomers, as measured with a Knudsen Effusion Mass Spectrometer (KEMS), are reported here. The results indicate a difference of up to seven orders of magnitude between measured and group contribution method predicted values. Using KEMS measured vapour pressures instead of predicted vapour pressures results in a larger fraction of a species in the condensed phase. The predictive methods largely do not account for functional group positioning. With organic aerosol consisting of complex mixtures of potentially hundreds of thousands of organics, understanding multicomponent diffusion is important as it affects aerosol partitioning. Diffusion ordered spectroscopy nuclear magnetic resonance was used for the first time to measure multicomponent diffusion coefficients for organic aerosol. Component solubility, mutual diffusion and self-diffusion in NMR experiments, binary and multicomponent diffusion, and finally models of obstruction were all explored using this technique. Black carbon's effect on climate is largely due to its ability to absorb radiation. Absorption, in carbon materials, is related to sp2 hybridization, or graphitic bonding. Raman spectroscopy and near edge X-ray absorption structure (NEXAFS) spectroscopy were applied to a range of reference standards to understand systematic uncertainties in determining carbon bonding. A chosen method for determining graphitization was then used in combination with transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and scanning tunnelling x-ray microscopy (STXM) to analyse anthropogenic aerosol and African biomass burning samples. Using this combination of methods, the morphology, elemental composition, sp2 hybridization, and carbon to oxygen ratios were found for each particle for a detailed characterization of aerosol mixing state.

This thesis investigates the use of aerosol activation as the determining factor between an aerosol particle and a cloud droplet numerical models. The formation of cloud droplets occurs through the condensation of water vapour on ambient aerosol particles. The point at which an aerosol particle becomes a cloud droplet is called activation, and is calculated according to Kohler (or similar) theory. In this work the current representation of cloud droplet activation in state-of-the-art climate and weather models is evaluated using a detailed cloud parcel model. It is found that there is a systematic tendency to over estimate the number of activated droplets when the median aerosol particle diameter is relatively large. It is also demonstrated that the most popular method to representing cloud droplet activation in climate models is not however the most accurate approach available. Although the calculation of cloud droplet activation is important in accurately determining cloud droplet number concentration in weather and climate simulations, this work demonstrates a newly identified aerosol-cloud interaction in which the growth of aerosol particles before activation is the determining factor. It is shown that the competition for water vapour between ambient aerosol particles can lead to the suppression of ice formation by limiting the ability of ice nucleating particles to activate into cloud droplets and therefore freeze. A common assumption in models which include state-of-the-art representation of heterogeneous ice nucleation is that immersion freezing may only occur if an ice nucleating particle has activated into a cloud droplet. Observations from cloud chamber experiments and results from a detailed cloud parcel model indicate that ice nucleation in the immersion mode may take place on ice nucleating particles before they become activated. This means that the number concentration of ice crystals may be significantly under estimated in current weather and climate model simulations where ice nucleation can only take place in activated cloud droplets. Un-activated aerosol particles are not only often excluded from ice nucleation processes but also from precipitation formation. The calculation of rain drop formation, for example by autoconversion parameterisations following Manton and Cotton (1997) or Khairoutdinov and Kogan (2000), in some models depends on the number of concentration of cloud droplets determined by cloud droplet activation. In this work un-activated aerosol particles, which are able to swell to considerable sizes before they activate, are shown to contribute significantly to collision-coalescence which leads to precipitation formation in warm clouds. Overall the work in this thesis highlights the need to consider not just activated, but also un-activated aerosol particles as cloud droplets in calculations of cloud microphysical processes in weather and climate models.

Accurate models of aerosol transformation including partitioning between the gas- and particle-phase are needed for estimating their effects on climate and air quality. In this thesis, the process of partitioning between phases and its determining factors are introduced and investigated. Three studies assess: the accuracy of estimation methods for aerosol component volatility and its effect on particulate concentration and composition; the consistency of different solutions to models of particle-phase diffusion; and, a method to analytically solve particle-phase diffusion. These studies contribute to the ongoing effort of improving aerosol models, such that their wide-ranging effects can be accurately estimated.

Atmospheric trace components play a critical role in the earth–atmosphere system through their interaction and perturbation to global atmospheric chemistry. They perturb the climate through scattering and absorbing of solar radiation (direct effects), thereby impacting on the heat energy balance of the atmosphere, and alter cloud microphysical properties affecting cloud formation, cloud lifetime and precipitation formation (indirect effects). These trace components can also have adverse effects on human health, visibility and air quality (AQ) composition, including various feedback processes on the state of the atmosphere. As well as their direct and indirect effects, aerosols are important for cloud formation. They serve as cloud condensation and ice nuclei (CCN and IN) during cloud droplet and ice crystal formations. Although many connections between clouds and aerosol effects have been established in cloud physics and climate modelling, aerosol–cloud interaction (ACI) is still one of the areas of large uncertainties in modern climate and weather projections. Different models have been developed placing much emphasis on ACIs, to have robust and more consistent description processes within the meteorological and chemical variables to account for ACIs and feedback processes. Because pollutant distributions are controlled by a specific meteorology that promotes residence times and vertical mixing in the atmosphere, reliable chemical composition measurements are required to understand the changes occurring in the earth–atmosphere system. Also, because atmospheric pollution is a combination of both natural and man-made (anthropogenic) sources, to direct controlled and/or mitigation procedures efficiently, contributions of different sources need to be considered. Occasionally these are explored from a particular region or global environment, depending on a specific area of interest. A fully coupled online meteorology–chemistry model framework (WRF-Chem) is used to investigate atmospheric ACIs over southern Africa –a region characterized by a strong and intense seasonal biomass burning (BB) cycle. The large transport of aerosol plumes originating from the seasonal burning from agriculture, land-use management and various activities give rise to a unique situation warranting special scrutiny. Simulations are conducted for the 2008 dry season BB episode, implementing a chemical dataset from various emission sources (anthropogenic, BB, biogenic, dust and sea salt) with the meteorological conditions. A base line (CNTRL) simulation was conducted with all emission sources from 26 August to 10 September 2008. To probe the contribution of BB on the regional pollution and influence on ACIs, a sensitivity (TEST) simulation was conducted without BB emissions and compared to the base line. The impact of natural and anthropogenic aerosol particles is studied and quantified for the two simulations, focusing on aerosol concentration and cloud responses under different model resolutions. A statistical analysis of pollutant concentration of major regulated species and cloud variables is conducted and the percentage difference used to assess the contribution due to BB emissions. Results confirm the high variability of spatial and temporal patterns of chemical species, with the greatest discrepancies occurring in the tropical forests whereas the subtropics show more urban/industrial related emissions. Whilst CO and O3 show statistically significant increases over a number of cities/towns, the trend and spatial variability is much less uniform with NO2 and PM in most urban and populous cities. Statistical analysis of major chemical pollutants was mainly influenced by BB emissions. O3, NOx, CO and PM increase by 24%, 76%, 51%, 46% and 41% over the main source regions, whereas in the less affected regions concentrations increased by 5%, 5%, 5%, 3% and 2% when BB emissions are included. This study sheds new light on the response of cloud processes to changing aerosol concentrations and different model resolutions. In the parameterised case (dx = 20 km), clouds become more cellular, correlated with high supersaturations, whereas in the resolved case (dx = 4 km), they become more faint with relatively lower supersaturations. Aerosol effects on cloud properties were further studied and statistical analysis conducted on CCN, cloud droplet number concentration (CDNC), supersaturation and aerosol optical depth (AOD) at two different grid spacings. Most clouds occur to the west of the domain coincident with increase in aerosol concentration and AOD, while single scattering albedo (SSA) decreases. A considerable cloud ‘burn-off’ occurs in tropical west Africa, where aerosols can also be lofted up to 500-hPa level when BB emissions are included in the simulation. Due to BB, absorbing aerosol increased by 76% and 23% over tropical west and subtropical southeast, while tropical east shows no change. The study shows that tropical central Africa is characterized by an increased build-up in biomass burning aerosols (BBAs), forming a regional haze with high AOD; this becomes stronger near active burning areas with a significant proportion occurring to the west. AOD enhancement increases up to 38%, 31% and 11% in the west, east and south respectively. Although CDNC increased in areas with high aerosol concentration, supersaturation decreases (in the small domains) since increase in aerosol number concentration decreases maximum supersaturation Smax. Changes in absorbed radiation increased by +56 Wm-2, +23 Wm-2 and +14 Wm-2 in the west, east and southeast. To further evaluate the model sensitivity and its skill, an analysis was conducted by comparing the model performance with measurement data. Simulated AOD, surface concentrations of CO and O3, ozonesondes and liquid water path (LWP) were compared with measured data from MODIS satellite, SAFARI2000 field study and Cape Point WMO. The model shows a good skill in capturing and reproducing the trends as that measured. However, a severe lack of measurement data over southern Africa makes it more difficult to effectively evaluate WRF-Chem over southern Africa. There is a need for increased availability of measurements to adequately compare with models. This study is one of the first WRF-Chem studies conducted over southern Africa to simulate the weather and pollution interaction. The novelty of the present study is the combined analysis of ACI sensitivity to aerosol loading and cloud response in a regime-based approach. The study concludes with a brief discusssion of future directions for work on AQ and modelling interactions between pollution and weather over southern Africa.

The Weather Research and Forecasting with Chemistry (WRF-Chem) Model is an “online” regional scale prediction system designed to simulate many detailed meteorological, gas-phase chemical and aerosol processes, with full coupling between the different components. The impacts of aerosol particles are complex and spatially heterogeneous, their impacts varying greatly at the regional scale. Modelling the properties and impacts in a systematic manner requires the coupling between different chemical phases, meteorological and physical parameterisations a model such as WRF-Chem offers. This manuscript documents several developments, and their evaluation, that have been made to the WRF-chem model to improve its representation of detailed gas-phase chemical and aerosol processes. The first study gives an overview of developments made for modeling the North-West European region, including the addition of a new semi-explicit chemical mechanism, N2O5 heterogeneous chemistry and modifications to the sea-spray emissions routine to include fine-mode organic material. The broad impacts of these developments were assessed in the study, while a follow up paper (included in supplementary material) investigated more deeply the impacts of N2O5 heterogeneous chemistry. The second study discusses modifications to WRF-Chem and emission products to improve modelled representation of biomass burning aerosol particles over Brazil. Model results were compared with aircraft measurements and found to represent aerosol particle size distributions and cloud condensation nuclei concentrations reasonably well, but too much biomass burning aerosol were transported up to high altitudes (4-8 km) by the model. In the third study, nested simulations (at higher resolutions than those used in the second study) over Brazil were used to evaluate the impact of aerosol particles on the local radiative balance, by comparing model results from simulations with and with- out aerosol-radiative feedbacks. The instantaneous clear sky aerosol-radiation forcings were found to have a net cooling of -5.0 W m−2 at the top of the atmosphere. Issues with resolving aerosol–cloud interactions, because of the convective parameterisation and differences in model setup across scales, made evaluating semi- and indirect effects impossible.