A key reaction in the troposphere involves the oxidation of biogenic and anthropogenic alkenes with ozone, which contributes to local photochemical smog. It is generally accepted that this reaction proceeds via a reactive intermediate often called the Criegee intermediate (CI). This reaction is known to produce a plethora of oxidised organic compounds, which contribute to ozone formation and secondary organic aerosol production, two of the main characteristics of a polluted atmosphere. Furthermore, epidemiological studies have shown a close correlation between exposure to ambient organic aerosol and adverse human health effects. The toxicological mechanisms leading to this observation are still poorly characterised, although studies suggest that reactive oxygen species present in organic aerosol are a major contributor. Reactive oxygen species and reactive intermediates represent a large uncertainty in tropospheric chemistry, and pose an analytical challenge due to their high reactivity and typically low concentrations. This emphasises the need for the development of new methods to characterise the chemistry of these species. In this thesis, several novel laboratory based techniques have been developed in order to characterise and quantify reactive intermediates and reactive oxygen species. New methods to facilitate the detection of CIs in both the gas and particle phase are presented. Spin trap molecules are used to scavenge CIs to form stable 1:1 adducts which are subsequently detected and quantified using mass spectrometry. The chemistry of CIs with spin traps is extensively investigated. The unique capability of this technique to simultaneously characterise multiple CIs generated from a variety of atmospherically relevant organic precursors in the gas phase is demonstrated. This technique was further developed to facilitate the detection of CIs in secondary organic aerosol, representing the development of a method capable of characterising low volatility CIs and other reactive intermediates in the condensed phase. Furthermore, two new chemical fluorescence assays have been developed to quantify both organic radicals and reactive oxygen species in organic aerosol. A novel profluorescent spin trap assay was applied to quantify radical concentrations in organic aerosol. A series of experiments were then devised to investigate the lifetime of organic radicals in secondary organic aerosol. A second assay, based on physiologically relevant ascorbic acid chemistry, was also developed to measure the concentrations of toxicologically relevant reactive oxygen species in secondary organic aerosol. The quantitative capability of this assay was extensively characterised. The assay was incorporated into a prototype instrument capable of measuring particle-bound reactive oxygen species on-line, and the assays’ sensitivity to secondary organic aerosol was demonstrated.

Accurate measurements of ammonia, nitric acid and formic acid are important for achieving a complete understanding of their atmospheric role. Models and measurements of formic acid in the atmosphere continue to show disagreements. Also, the contributions of NMHCs and reactive nitrogen (HNO3 and NH3) to organic and inorganic aerosol formation are important to quantify as gaps in the knowledge of atmospheric aerosols are a source of uncertainty in climate science. In this thesis, concentrations of ammonia were measured in the atmosphere and the production of formic acid from the ozonolysis of isoprene was measured in the EXTRA (EXTreme RAnge) chamber. Both gases were studied using chemical ionisation mass spectrometry (CIMS). The kinetics of the reactions of sesquiterpenes and terminal alkenes with ozone were studied in theEXTRA chamber using the relative rates technique and GC-FID. The ozonolysis rate coefficients of a homologous series of terminal alkenes were measured at elevated temperatures and found to be invariant with the carbon number. This led to the conclusion that previous measurements of these rate coefficients were subject to experimental artefacts. The elevated temperature protocol was employed to study the ozonolysis of sesquiterpenes, leading to revisions in ko3 for β-caryophyllene and α-humulene of 3 orders of magnitude. It was thus concluded that ozonolysis only accounts for 9-15% of sesquiterpeneoxidation in the atmosphere. A field intercomparison of CIMS for measuring ammonia was conducted wherein CIMS was found to perform well alongside instruments of comparable time response and limits of detection. This thesis also characterised inlet materials used in atmospheric measurements in the first systematic study on the uptake onto inlet walls in a flow tube system coupled to CIMS. It was found that PFA is the preferable material for atmospheric measurements, both for its kinetic qualities and its ready availability and ease of use. Finally, CIMS was used to measure the yield of formic acid from isoprene ozonolysis as a function of relative humidity. Formic acid yield was found to increase between 0-40% RH to a maximum of 0.18. Using the measured formic acid yields in a global chemistry model leads to an estimate that formic acid production from isoprene ozonolysis is ~9.5 Tg yr-1.

Dans cette thèse, nous présentons des travaux complémentaires conduits à ICARE-CNRS (Orléans), partie A et à l’Université de Fudan (Shanghai), partie B. Partie A : les 2-et 3-carène sont deux composés organiques volatils biogéniques importants présents dans l’atmosphère dont les voies de dégradation sont encore mal connues. Afin de déterminer les constantes de vitesse des réactions de ces espèces avec l’ozone, nous avons utilisé trois systèmes expérimentaux complémentaires : des chambres de simulation d’ICARE de 7300L et 80000L (HELIOS) et un réacteur à flux laminaire. Les rendements de certains produits de réactions, le radical hydroxyle (OH), le formaldéhyde (HCHO) et le monoxyde de carbone (CO) ont aussi été déterminés. D’autre part, pour avoir une meilleure compréhension d’intermédiaires de réaction formés lors de l’ozonolyse, appelés intermédiaires de Criegee, un réacteur à flux laminaire à deux étages a été mis en place pour mesurer leurs constantes de vitesse de réaction avec SO2, NO2 et O3. Partie B : les impacts du “black carbon (BC)” et du “brown carbon (BrC)” constituent une incertitude majeure dans les modèles climatiques actuels. Des rapports récents indiquent que la morphologie et l’évolution des BC et BrC dans l’atmosphère jouent un rôle important sur la capacité d’absorption de ces particules. Afin d’étudier leurs comportements, des mélanges de particules (BC-BrC) ont été exposés en chambre de simulation atmosphérique à l'acide sulfurique, au mélange ammoniac / triéthylamine, et à la vapeur d'eau de manière séquentielle. / In this thesis, we present a complementary work conducted at ICARE-CNRS (Orléans), Part A and at Fudan University (Shanghai), Part B. Part A: 2-and 3-carene are two important biogenic volatile organic compounds present in the atmosphere. The knowledge on their degradation pathways and the corresponding products are still poor. Using complementary reaction systems-ICARE 7300 L and HELIOS 80000 L simulation chambers, vertical laminar flow reactor, their kinetic rate constants for reaction with ozone were determined. Additionally, important product formation yields, hydroxyl radical (OH), formaldehyde (HCHO) and carbon monoxide (CO) have been determined with indication to their corresponding formation routes from the ozonolysis of carene. To have a better understanding on reactions of Criegee intermediates (CIs) generated through ozonolysis in the atmosphere, a horizontal 2-stage laminar flow reactor was set up to measure the rate constants of CIs with SO2, NO2 and O3. Part B: Radiative forcing of black carbon (BC) in the atmosphere, as well as that of brown carbon (BrC), remains to be a major uncertainty in current climate models. Recent reports indicate that the absorption enhancement of BC and BrC particles is determined by evolution of morphology and mixing state during the atmospheric processing. Laboratory-generated BC-BrC mixture particles (BC-BrC) were exposed to sulfuric acid, ammonia/triethylamine, and water vapor sequentially to investigate the alternation in light absorption, morphology and mixing state during simulated atmospheric processing.