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.