Abstract. We assess the standard operational nitrogen dioxide (NO 2 ) data product (OMNO2, version 2.1) retrieved from the Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite using a combination of aircraft and surface in situ measurements as well as ground-based column measurements at several locations and a bottom-up NO x emission inventory over the continental US. Despite considerable sampling differences, NO 2 vertical column densities from OMI are modestly correlated (r = 0.3-0.8) with in situ measurements of tropospheric NO 2 from aircraft, ground-based observations of NO 2 columns from MAX-DOAS and Pandora instruments, in situ surface NO 2 measurements from photolytic converter instruments, and a bottom-up NO x emission inventory. Overall, OMI retrievals tend to be lower in urban regions and higher in remote areas, but generally agree with other measurements to within ± 20 %. No consistent seasonal bias is evident. Contrasting results between different data sets reveal complexities behind NO 2 validation. Since validation data sets are scarce and are limited in space and time, validation of the global product is still limited in scope by spatial and temporal coverage and retrieval conditions. Monthly mean vertical NO 2 profile shapes from the Global Modeling Initiative (GMI) chemistry-transport model (CTM) used in the OMI retrievals are highly consistent with in situ aircraft measurements, but these measured profiles exhibit considerable day-to-day variation, affecting the retrieved daily NO 2 columns by up to 40 %. This assessment of OMI tropospheric NO 2 columns, together with the comparison of OMI-retrieved and model-simulated NO 2 columns, could offer diagnostic evaluation of the model.

The electrochemical and physical properties of a class of 1-n-alkyl-3-methylimidazolium (C n mim) bis͑perfluoroalkylsulfonyl͒imide room-temperature ionic liquids ͑RTILs͒ is reported. By varying the chain length on the cation (n ϭ 6, 8, and 10͒ and the perfluorinated carbon chains of the imide anions, -CF 3 ͑BMSI͒ to -CF 2 CF 3 ͑BETI͒, the effect on water content, viscosity, conductivity, and potential window has been examined. The water content of the RTILs, both ambient and water-equilibrated, varies linearly with the imidazolium alkyl chain length. The viscosities of the water-equilibrated BMSI-and BETI-based RTILs were accurately calculated using a linear combination of the viscosity of water and the ambient RTILs. The conductivities for this series ranges from 2.9 mS/cm (C 6 mimBMSI) to 0.92 mS/cm (C 10 mimBETI) and were accurately estimated based on the viscosity, density, and molecular weight. The potential windows of the ambient RTILs are 4.3 and 4.9 V for the BMSI and BETI salts, respectively. The window is reduced to 3.9 and 4.3 V upon equilibration with water. The reversible electrochemistry of ferrocene (⌬E p ϭ 70 mV) and decamethylferrocene (⌬E p ϭ 63 mV) has been measured in water-equilibrated imide-based RTILs with as much as 23 mol% H 2 O.

We describe and show results from a series of field campaigns that used balloonborne instruments launched from India and Saudi Arabia during the summers 2014–17 to study the nature, formation, and impacts of the Asian Tropopause Aerosol Layer (ATAL). The campaign goals were to i) characterize the optical, physical, and chemical properties of the ATAL; ii) assess its impacts on water vapor and ozone; and iii) understand the role of convection in its formation. To address these objectives, we launched 68 balloons from four locations, one in Saudi Arabia and three in India, with payload weights ranging from 1.5 to 50 kg. We measured meteorological parameters; ozone; water vapor; and aerosol backscatter, concentration, volatility, and composition in the upper troposphere and lower stratosphere (UTLS) region. We found peaks in aerosol concentrations of up to 25 cm–3 for radii > 94 nm, associated with a scattering ratio at 940 nm of ∼1.9 near the cold-point tropopause. During medium-duration balloon flights near the tropopause, we collected aerosols and found, after offline ion chromatography analysis, the dominant presence of nitrate ions with a concentration of about 100 ng m–3. Deep convection was found to influence aerosol loadings 1 km above the cold-point tropopause. The Balloon Measurements of the Asian Tropopause Aerosol Layer (BATAL) project will continue for the next 3–4 years, and the results gathered will be used to formulate a future National Aeronautics and Space Administration–Indian Space Research Organisation (NASA–ISRO) airborne campaign with NASA high-altitude aircraft.