Vertical column amounts of nitrogen dioxide, C(NO2), are derived from ground‐based direct solar irradiance measurements using two new and independently developed spectrometer systems, Pandora (Goddard Space Flight Center) and MFDOAS (Washington State University). We discuss the advantages of C(NO2) retrievals based on Direct Sun ‐ Differential Optical Absorption Spectroscopy (DS‐DOAS). The C(NO2) data are presented from field campaigns using Pandora at Aristotle University (AUTH), Thessaloniki, Greece; a second field campaign involving both new instruments at Goddard Space Flight Center (GSFC), Greenbelt, Maryland; a Pandora time series from December 2006 to October 2008 at GSFC; and a MFDOAS time series for spring 2008 at Pacific Northwest National Laboratory (PNNL), Richland, Washington. Pandora and MFDOAS were compared at GFSC and found to closely agree, with both instruments having a clear‐sky precision of 0.01 DU (1 DU = 2.67 × 1016 molecules/cm2) and a nominal accuracy of 0.1 DU. The high precision is obtained from careful laboratory characterization of the spectrometers (temperature sensitivity, slit function, pixel to pixel radiometric calibration, and wavelength calibration), and from sufficient measurement averaging to reduce instrument noise. The accuracy achieved depends on laboratory‐measured absorption cross sections and on spectrometer laboratory and field calibration techniques used at each measurement site. The 0.01 DU precision is sufficient to track minute‐by‐minute changes in C(NO2) throughout each day with typical daytime values ranging from 0.2 to 2 DU. The MFDOAS instrument has better noise characteristics for a single measurement, which permits MFDOAS to operate at higher time resolution than Pandora for the same precision. Because Pandora and MFDOAS direct‐sun measurements can be made in the presence of light to moderate clouds, but with reduced precision (∼0.2 DU for moderate cloud cover), a nearly continuous record can be obtained, which is important when matching OMI overpass times for satellite data validation. Comparisons between Pandora and MFDOAS with OMI are discussed for the moderately polluted GSFC site, between Pandora and OMI at the AUTH site, and between MFDOAS and OMI at the PNNL site. Validation of OMI measured C(NO2) is essential for the scientific use of the satellite data for air quality, for atmospheric photolysis and chemistry, and for retrieval of other quantities (e.g., accurate atmospheric correction for satellite estimates of ocean reflectance and bio‐optical properties). Changes in the diurnal variability of C(NO2) with season and day of the week are presented based on the 2‐year time series at GSFC measured by the Pandora instrument.

[1] We review the standard nitrogen dioxide (NO 2 ) data product (Version 1.0.), which is based on measurements made in the spectral region 415-465 nm by the Ozone Monitoring Instrument (OMI) on the NASA Earth Observing System-Aura satellite. A number of ground-and aircraft-based measurements have been used to validate the data product's three principal quantities: stratospheric, tropospheric, and total NO 2 column densities under nearly or completely cloud-free conditions. The validation of OMI NO 2 is complicated by a number of factors, the greatest of which is that the OMI observations effectively average the NO 2 over its field of view (minimum 340 km 2 ), while a ground-based instrument samples at a single point. The tropospheric NO 2 field is often very inhomogeneous, varying significantly over tens to hundreds of meters, and ranges from <10 15 cm À2 over remote, rural areas to >10 16 cm À2 over urban and industrial areas. Because of OMI's areal averaging, when validation measurements are made near NO 2 sources the OMI measurements are expected to underestimate the ground-based, and this is indeed seen. Further, we use several different instruments, both new and mature, which might give inconsistent NO 2 amounts; the correlations between nearby instruments is 0.8-0.9. Finally, many of the validation data sets are quite small and span a very short length of time; this limits the statistical conclusions that can be drawn from them. Despite these factors, good agreement is generally seen between the OMI and ground-based measurements, with OMI stratospheric NO 2 underestimated by about 14% and total and tropospheric columns underestimated by 15-30%. Typical correlations between OMI NO 2 and ground-based measurements are generally >0.6.

Abstract. The importance of nitrogen dioxide in both the troposphere and the stratosphere has been known for some years, and since the early 1970s, spectroscopic determinations have played an important role in understanding NOx chemistry. Spectroscopic measurements of the atmosphere have improved in quality in recent years to the point that an accurate determination of the NO 2 absorption cross section is essential to accurate retrievals of not only NO 2 but also less abundant species in the troposphere and stratosphere. NO2 is such a large absorber (approximately 1% at large air mass) in the stratosphere at twilight or in the troposphere under even mildly polluted conditions, that if it is not properly removed from observed spectra, the spectra of the more subtle species are masked and cannot be measured at all. We present cross sections of NO2 in the spectral region 350-585 nm at four temperatures between 217 and 298 K and total pressures between 100 and 600 torr at a mixing ratio of 84.1 ppmv and at a spectral resolution sufficient for accurate convolution with instruments typically used to measure atmospheric NO 2. Data will be presented to demonstrate the presence of NO 2 pressure dependence in high resolution. A detailed comparison with commonly used literature cross sections is made to show how such instrument parameters as wavelength accuracy, resolution, spectrograph scattered light, and data sampling affect the usefulness of the observed cross section.

Abstract. Because of the extremely short photochemical lifetime of tropospheric OH, comparisons between observations and model calculations should be an effective test of our understanding of the photochemical processes controlling the concentration of OH, the primary oxidant in the atmosphere. However, unambiguous estimates of calculated OH require sufficiently accurate and complete measurements of the key species and physical variables that determine OH concentrations. The Tropospheric OH Photochemistry Experiment (TOHPE) provides an extremely complete set of measurements, sometimes from multiple independent experimental platforms, that allows such a test to be conducted. When the calculations explicitly use observed NO, NO 2, hydrocarbons, and formaldehyde, the photochemical model consistently overpredicts in situ observed OH by -50% for the relatively clean conditions predominantly encountered at Idaho Hill. The model bias is much higher when only CH4-CO chemistry is assumed, or NO is calculated from the steady state assumption. For the most polluted conditions encountered during the campaign, the model results and observations show better agreement. Although the comparison between calculated and observed OH can be considered reasonably good given the +30% uncertainties of the OH instruments and various uncertainties in the model, the consistent bias suggests a fundamental difference between theoretical expectations and the measurements. Several explanations for this discrepancy are possible, including errors in the measurements, unidentified hydrocarbons, losses of HO x to aerosols and the Earth's surface, and unexpected peroxy radical chemistry. Assuming a single unidentified type of hydrocarbon is responsible, the amount of additional hydrocarbon needed to reduce theoretical OH to observed levels is a factor of 2 to 3 greater than the OHreactivity-weighted hydrocarbon content measured at the site. Constraints can be placed on the production and yield of various radicals formed in the oxidation sequence by considering the observed levels of certain key oxidation products such as formaldehyde and acetaldehyde. The model results imply that, under midday clean westerly flow conditions, formaldehyde levels are fairly consistent with the OH and hydrocarbon observations, but observed acetaldehyde levels are a factor of 4 larger than what is expected and also imply a biogenic source. Levels of methacrolein and methylvinylketone are much lower than expected from steady state isoprene chemistry, which implies important removal mechanisms or missing information regarding the kinetics of isoprene oxidation within the model. In a prognostic model application, additional hydrocarbons are added to the model in order to force calculated OH to observed levels. Although the products and oxidation steps related to pinenes and other biogenic hydrocarbons are somewhat uncertain, the addition of a species with an oxidation mechanism similar to that expected from C 10 pinenes would be consistent with the complete set of observa...

The Halogen Occultation Experiment (HALOE) experiment on Upper Atmosphere Research Satellite (UARS) performs solar occultation (sunrise and sunset) measurements to infer the composition and structure of the stratosphere and mesosphere. Two of the HALOE channels, centered at 5.26 gm and 6.25 gm, are designed to infer concentrations of nitric oxide and nitrogen dioxide respectively. The NO measurements extend from the lower stratosphere up to 130 km, while the NO2 results typically range from the lower stratosphere to 50 km and higher near the winter terminator. Comparison with results from various instruments are presented, including satellite-, balloon-, and ground-based measurements. Both NO and NO2 can show large percentage errors in the presence of heavy aerosol concentrations, confined to below 25 km and before 1993. The NO2 measurements show mean differences with correlative measurements of about 10 to 15% over the middle stratosphere. The NO2 precision is about 7.5x10 '13 arm, degrading to 2x10 -12 arm in the lower stratosphere. The NO differences are similar in the middle stratosphere but sometimes show a low bias (as much as 35%) between 30 and 60 km with some correlative measurements. NO precision when expressed in units of density is nearly constant at lx10 '12 atmospheres, or approximately 0.1 ppbv at 10.0 mb or, 1.0 ppbv at 1.0 mb, and so forth when expressed in mixing ratio. Above 65 km, agreement in the mean with Atmospheric Trace Molecule Spectroscopy (ATMOS) NO results is very good, typically + 15%. Model comparisons are also presented, showing good agreement with both expected morphology and diurnal behavior for both NO2 and NO. hydrogen chloride (HCI), hydrogen fluoride (HF), methane (CH4), water vapor (H20), nitric oxide (NO), nitrogen dioxide (NOy), and aerosol extinction. Retrieved profiles cover an altitude range from the upper troposphere, in some cases, to the lower thermosphere for nitric oxide. Fifteen spacecraft sunrises and sunsets are observed daily and usually in opposite hemispheres, although at certain times these measurements occur on the same day and almost overlap in space. Details of the HALOE experiment, including geographic coverage, discussion of the experiment and instrument techniques, instrument ground test results, error mechanisms, in-orbit performance, initial pressure versus latitude cross sections, and orthographic projections are included in the HALOE overview by Russell et. al. [1993]. The purpose of 'this paper is to describe steps taken and 'the status of efforts to validate data from the NO gas correlation channel and the NO2 radiometer channel. All results were inferred using the most current archived HALOE data, version 17, released in November 1994. Version numbers were liberally changed during the continuous validation and evolution of the HALOE processing system. Attainment of research quality results coincided with version 16 in late summer of 1994, which was the first version released to the general science community. However, a second general processing ...