Measurement of pernitric acid, hydrogen chloride, and sulfur dioxide during the Intercontinental Chemical Transportation Experiment campaign
This study presents airborne measurements (0 km < z < 12 km) of HO2NO2, HCl and SO<sub>2</sub> 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 HOx 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 HOx 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.