A. Kraus

Field Measurements of Atmospheric Photolysis Frequencies for O3, NO2, HCHO, CH3CHO, H2O2, and HONO by UV Spectroradiometry

A calibrated spectroradiometer was used for the measurement of spectra of the absolute actinic flux Fλ during the POPCORN field campaign in Pennewitt (53.8° N, 11.7° E, sea level) in August 1994. The obtained set of actinic flux spectra was used to determine the photolysis frequencies J(O1D), J(NO2), J(HCHO), J(H2O2), J(HONO), and J(CH3CHO), using molecular photodissociation data from literature. The accuracy of the actinic flux measurement was about ±5%. The accuracy of the photolysis frequency determination is limited by the uncertainties of the molecular absorption cross section and quantum yield data. A good agreement within the experimental uncertainties was found in comparison with measurements of J(O1D) and J(NO2) by filterradiometer which were calibrated absolutely against chemical actinometer. A comparison of this works photolysis frequency measurements at 40° solar zenith angle with respective measured and modeled data from the literature also shows good agreement for most of the processes considered in this work. However, in the case of J(NO2) data reported in the literature as a function of solar zenith angle differences up to a factor of 1.6 with respect to this works J(NO2) data are observed. Since this is far beyond the estimated experimental uncertainties, other atmospheric variables, such as aerosols, seem to affect J(NO2) to an extent that is underestimated by now and make indirect comparisons of J(NO2) measurements difficult.

Photolysis frequency of NO2: Measurement and modeling during the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI)

[1] The photolysis frequency of NO2, j(NO2), was determined by various instrumental techniques and calculated using a number of radiative transfer models for 4 days in June 1998 at the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) in Boulder, Colorado. Experimental techniques included filter radiometry, spectroradiometry, and chemical actinometry. Eight research groups participated using 14 different instruments to determine j(NO2). The blind intercomparison experimental results were submitted to the independent experimental referee and have been compared. Also submitted to the modeling referee were the results of NO2 photolysis frequency calculations for the same time period made by 13 groups who used 15 different radiative transfer models. These model results have been compared with each other and also with the experimental results. The model calculation of clear-sky j(NO2) values can yield accurate results, but the accuracy depends heavily on the accuracy of the molecular parameters used in these calculations. The instrumental measurements of j(NO2) agree to within the uncertainty of the individual instruments and indicate the stated uncertainties in the instruments or the uncertainties of the molecular parameters may be overestimated. This agreement improves somewhat with the use of more recent NO2 cross-section data reported in the literature. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: photolysis, NO2 (nitrogen dioxide), radiative transfer, intercomparison Citation: Shetter, R. E., et al., Photolysis frequency of NO2: Measurement and modeling during the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI), J. Geophys. Res., 108(D16), 8544, doi:10.1029/2002JD002932, 2003.

The global tropospheric distribution of NO x estimated by a three-dimensional chemical tracer model

The global distribution of NOx in the troposphere is calculated using a simple three-dimensional chemical tracer model. This model includes a simplified chemistry scheme for the tracers NOx ≡ NO + NO2 and HNO3, which are redistributed by advection, dry and wet convection, and large-scale diffusion. The sources of NOx considered are fossil fuel combustion, emissions from soil microbial activity, biomass burning, lightning discharges, emissions by aircraft, and downward transport from the stratosphere. Dry and wet deposition act as final sinks. At northern middle and high latitudes the calculated tropospheric NOx content is dominated by the surface sources, fossil fuel combustion in particular. In the tropical free troposphere, lightning discharges provide about 80% of the total NOx throughout the year. The zonally averaged fractional contribution of aircraft emissions strongly depends on the season. The largest contribution of this source, over 60%, occurs during January in the upper troposphere between 45°N and 60°N. The NO mixing ratios determined by the model show good overall agreement with vertical profiles measured during the Stratospheric Ozone Experiment (STRATOZ) III aircraft campaign.

Actinic Radiation and Photolysis Processes in the Lower Troposphere: Effect of Clouds and Aerosols

Within the German Tropospheric Research Program (TFS) a series of projects were performed focussing on aspects of radiation transfer and the effects of UV-radiation on air chemistry. The individual projects covered laboratory investigations, instrument development for photolysis processes as well as field studies of actinic radiation and comparison to model calculations. One and three-dimensional models were tested against field campaign data. The results confirm the improvement of measurement technology achieved through deployment of new techniques like spectroradiometry that offer a wider range of investigations than was previously attainable using chemical actinometry or fixed wavelength filter radiometry. Reasonable agreement was also found between measurements and models for a few selected and well defined cloudy conditions. On the other hand, using simple stratiform geometry models yielded significant deviations between measurement and model in both directions particularly in the case of high zenith angles and with high aerosol load. Further tools both for experimental investigations and for model calculations were developed within the framework of the Troposphere Research Program (TFS) and deficiencies were identified demanding further investigations when broken clouds and more complex cloud layers prevail.

Meteorological aspects, ozone, and solar radiation measurements during POPCORN 1994

Meteorological data, ozone mixing ratios, and photolysis frequencies for the period August 2 to August 24, 1994, are presented and discussed in support of the field campaign POPCORN (Photochemistry of Plant Emitted Compounds and OH Radicals in Northeastern Germany). Measurements of temperature, ozone, and wind speed at different heights are used to evaluate micro-meteorological parameters. The observations provide information about local influences on the air mass composition. The analysis of radio sonde data of nearby stations provides the height of the planetary boundary layer.

The passive transport of NOx emissions from aircraft studied with a hierarchy of models

Abstract The passive transport of aircraft emissions of nitrogen oxides (NOx = NO + NO2) has been studied with a hierarchy of models ranging from two-dimensional and three-dimensional chemistry transport models up to three-dimensional models of the general circulation. The sink of NOx was parameterized by an exponential decay process with a globally constant half-lifetime of 10 days. By performing a simple experiment the importance of the various transport processes has been studied. The three-dimensional models show that the monthly mean volume mixing ratio of NOx varies by a factor of three in the longitudinal direction and the temporal variability is of the order of 30%. In view of the nonlinearity of the chemical processes leading to ozone formation in the presence of NOx this implies that the assessment of the effects of subsonic aircraft emissions of NOx should be done with three-dimensional models. Vertical redistribution by convection strongly affects the maximum NOx mixing ratio at cruise altitudes, but due to the limited lifetime of NOx of the order of ten days the most important contribution to the mixing ratio at a certain level usually stems from emissions around that level. The strong static stability in the stratosphere hampers significant dispersion of the subsonic aircraft emissions above the height where the emissions take place for the lifetimes considered here. Some model deficiencies and biases have been identified and discussed. Examples are the oscillatory signature of NOx distributions obtained with a spectral advection scheme, the strong diffusion of one of the GCMs into the polar regions, and the too intense interhemispheric exchange of one of the two-dimensional CTMs. For the vertical redistribution of the emissions it may be necessary to include not only updrafts but also downdrafts in the convective parametrization of the transport model.