This study presents airborne measurements (0 km < z < 12 km) of HO₂NO₂, HCl and SO₂ using chemical ionization mass spectrometry (CIMS) during the Intercontinental Chemical Transport Experiment (INTEX) field campaign, an intensive study to characterize the chemical composition of the troposphere in the eastern United States, Mexico City, and the North Pacific which is the outflow region of Asia.

The first direct in situ measurements of HO₂NO₂ were made in the free troposphere over the eastern U.S. during summer 2004. The highest mean mixing ratio of 76 pptv (median = 77 pptv, σ= 39 pptv) was observed in the altitude range of 8-9 km. Highly constrained steady state calculations of HO₂NO₂ using measured HO_x levels are poorly correlated with observed HO₂NO₂ in the upper troposphere (8 km < z < 12 km; the median ratio of [HO ₂NO₂]_SS-MEA/[HO₂NO₂] _MEA = 2.9). However, steady state HO₂NO₂ using model-derived HO_x shows reasonable agreement with measurements in the free troposphere ([HO₂NO₂]_SS-MEA/[HO ₂NO₂]_MEA = 1.3). These results indicate that observed HO₂ and HO₂NO₂ are in poor agreement in the upper troposphere but that HO₂NO₂ levels are consistent with current photochemical theory.

The vertical distribution of HCl was measured over the north Pacific during May 2006 from the marine boundary layer (MBL) up to lower stratosphere. Recent stratospheric influence in the upper troposphere (8 km < z < 12 km) was efficiently identified from enhanced HCl (up to ∼100 pptv) relative to very low background levels (< 2pptv). In the remote MBL, the acidification of seasalt aerosols by HNO₃ appeared to be the major source of HCl, with level consistently over 20 pptv (up to 400 pptv). Although HCl was generally under detection limit (< 2pptv) in the mid troposphere (4 km < z < 8 km), a case study suggests that HCl may be produced in this altitude range by the dechlorination of Cl-containing dust aerosols.

The distribution of SO₂ was measured in the outflow region of the eastern U.S. and Asia; two major anthropogenic SO₂ source regions. This study presents vertical and horizontal distributions of SO ₂ and relevant gas phase and aerosol parameters to characterize SO ₂ transport in the troposphere. SO₂ in the boundary layer was efficiently transported to the upper troposphere by deep convection and frontal uplift processes. High SO₂ in convective plume in the upper troposphere were strongly correlated with ultrafine aerosols (diameter less that< 0.01 μm).Conversely, SO₂ from frontal uplift shows a strong correlation with non-volatile aerosols (0.01 μm to 7 μm). Comparisons of SO₂ products from global 3-D chemical transportation models (GEOS-CHEM and MOZART) with observations suggest that sulfur sources are relatively well described but that the oxidation mechanism needs refinement.