W. Junkermann

Actinometric and radiometric measurement and modeling of the photolysis rate coefficient of ozone to O (^1D) during Mauna Loa Observatory Photochemistry Experiment 2

The in situ photolysis rate coefficient of O3 to O(1D) has been measured at Mauna Loa Observatory using a new actinometric instrument based on the reaction of O(1D) with N2O and with a hemispherical radiometer. One minute averaged photolysis rate coefficients were determined with an overall uncertainty of approximately ±11% at the 1 σ level for the actinometer and ±15% at the 1 σ level for the radiometer. Over 120 days of data were collected with varying cloud cover, aerosol loadings, and overhead ozone representing the first set of long term measurements. Clear sky solar noon values vary between approximately 3.0 × 10−5 and 4.5 × 10−5 sec−1. Modeling of the photolysis rate coefficients was done using a discrete ordinate radiative transfer scheme and results were compared with the actinometric measurements. The quantum yields for O(1D) production are reevaluated from existing data and reported here. The comparisons were done using the quantum yields for the photolysis of ozone recommended by DeMore et al. [1994], the newer evaluation of Michelsen et al. [1994], and also with reevaluated values in this paper. An analysis of the measured photolysis rate coefficient of O3 to O(1D) and model simulations of the photolysis rate coefficient data from clear days during the study provides added insight into the choice of quantum yield data for use in photochemical models of the troposphere.

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.

Cloud impacts on UV spectral actinic flux observed during the International Photolysis Frequency Measurement and Model Intercomparison (IPMMI)

[1] Observations collected during the International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) have enabled a semiquantitative assessment of cloud impacts on UV spectral actinic flux. Critical to this analysis is the availability of spectroradiometer data accompanied by sky camera images. Data reveal several interesting aspects of cloud influences. Enhancements in UV actinic flux of up to 40% over clear-sky values are observed for cloudy conditions when the solar disk is unoccluded. When the solar disk is occluded, reductions in actinic flux appear to vary inversely with cloud fraction in some instances. A wavelength dependence is also observed with shorter wavelengths generally exhibiting lower variability for both enhancements and reductions (e.g., 20-30% less variability at 320 versus 420 nm). Enhancements in actinic flux show a stronger wavelength dependence with increasing cloud fraction. By contrast, wavelength dependence appears to weaken with increasing cloud fraction for actinic flux reduction. At low Sun, however, a wavelength dependence appears to occur even for overcast conditions. Despite these differences, integration over longer periods shows cloud impacts on actinic flux to be similar for all UV wavelengths, suggesting that differences in wavelength response are localized phenomena with impacts that are minimized when integrated over the surrounding area. Further studies at other latitudes and in other seasons are needed to verify this behavior.

Photolysis frequency of O to O( D): Measurements and modeling during the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI)

R. E. Shetter, W. Junkermann, W. H. Swartz, G. J. Frost, J. H. Crawford, B. L. Lefer, J. D. Barrick, S. R. Hall, A. Hofzumahaus, A. Bais, J. G. Calvert, C. A. Cantrell, S. Madronich, M. Müller, A. Kraus, P. S. Monks, G. D. Edwards, R. McKenzie, P. Johnston, R. Schmitt, E. Griffioen, M. Krol, A. Kylling, R. R. Dickerson, S. A. Lloyd, T. Martin, B. Gardiner, B. Mayer, G. Pfister, E. P. Röth, P. Koepke, A. Ruggaber, H. Schwander, and M. van Weele

Altitude‐resolved shortwave and longwave radiative effects of desert dust in the Mediterranean during the GAMARF campaign: Indications of a net daily cooling in the dust layer

Desert dust interacts with shortwave (SW) and longwave (LW) radiation, influencing the Earth radiation budget and the atmospheric vertical structure. Uncertainties on the dust role are large in the LW spectral range, where few measurements are available and the dust optical properties are not well constrained. The first airborne measurements of LW irradiance vertical profiles over the Mediterranean were carried out during the Ground-based and Airborne Measurements of Aerosol Radiative Forcing (GAMARF) campaign, which took place in spring 2008 at the island of Lampedusa. The experiment was aimed at estimating the vertical profiles of the SW and LW aerosol direct radiative forcing (ADRF) and heating rates (AHR), taking advantage of vertically resolved measurements of irradiances, meteorological parameters, and aerosol microphysical and optical properties. Two cases, characterized respectively by the presence of a homogeneous dust layer (3 May, with aerosol optical depth, AOD, at 500 nm of 0.59) and by a low aerosol burden (5 May, with AOD of 0.14), are discussed. A radiative transfer model was initialized with the measured vertical profiles and with different aerosol properties, derived from measurements or from the literature. The simulation of the irradiance vertical profiles, in particular, provides the opportunity to constrain model-derived estimates of the AHR. The measured SW and LW irradiances were reproduced when the model was initialized with the measured aerosol size distributions and refractive indices. For the dust case, the instantaneous (solar zenith angle, SZA, of 55.1°) LW-to-SW ADRF ratio was 23% at the surface and 11% at the top of the atmosphere (TOA), with a more significant LW contribution on a daily basis (52% at the surface and 26% at TOA), indicating a relevant reduction of the SW radiative effects. The AHR profiles followed the aerosol extinction profile, with comparable peaks in the SW (0.72 ± 0.11 K d−1) and in the LW (−0.52 ± 0.12 K d−1) for the considered SZA. On a daily basis, the absolute value of the heating rate was larger in the LW than in the SW, producing a net cooling effect at specific levels. These are quite unexpected results, emphasizing the important role of LW radiation.

Aircraft-borne measurements of peroxy radicals by Chemical Conversion/Ion Molecule Reaction Mass Spectrometry: Calibration, diagnostics, and results

We describe a novel instrument for aircraft-borne measurements of peroxy radicals developed and built at the Max-Planck-Institut fur Kernphysik in Heidelberg. The instrument measures peroxy radicals using the chemical conversion of peroxy radicals to sulfuric acid and the detection of sulfuric acid by ion molecule reaction mass spectrometry. The peroxy radical measurement system was calibrated at the ground using the UV photolysis of H2O and was further characterized by laboratory and in-flight diagnostic measurements. Particular attention was paid to the effects of wall losses in the flow reactor and sampling system and the measurement efficiency for organic peroxy radicals. The conversion efficiency of methyl peroxy radicals to sulfuric acid was investigated by detailed laboratory studies and was found to be >80% for the conditions of our experiment. The instrument was employed for aircraft-borne measurements of peroxy radicals in the free troposphere over south Germany in combination with measurements of O3, NO, H2O, and J(O1D). Peroxy radical mixing ratios between 10 and 40 pptv were measured at altitudes between 3.8 and 7.8 km. A comparison of the measured peroxy radical concentrations with the results of a simple steady state estimate is presented.

The SCALEX Campaign: Scale-Crossing Land Surface and Boundary Layer Processes in the TERENO-preAlpine Observatory

AbstractScaleX is a collaborative measurement campaign, collocated with a long-term environmental observatory of the German Terrestrial Environmental Observatories (TERENO) network in the mountainous terrain of the Bavarian Prealps, Germany. The aims of both TERENO and ScaleX include the measurement and modeling of land surface–atmosphere interactions of energy, water, and greenhouse gases. ScaleX is motivated by the recognition that long-term intensive observational research over years or decades must be based on well-proven, mostly automated measurement systems, concentrated in a small number of locations. In contrast, short-term intensive campaigns offer the opportunity to assess spatial distributions and gradients by concentrated instrument deployments, and by mobile sensors (ground and/or airborne) to obtain transects and three-dimensional patterns of atmospheric, surface, or soil variables and processes. Moreover, intensive campaigns are ideal proving grounds for innovative instruments, methods, and...

Measuring the 3-D wind vector with a weight-shift microlight aircraft

This study investigates whether the 3-D wind vector can be measured reliably from a highly transportable and low-cost weight-shift microlight aircraft. Therefore we draw up a transferable procedure to accommodate flow distortion originating from the aircraft body and -wing. This procedure consists of the analysis of aircraft dynamics and seven successive calibration steps. For our aircraft the horizontal wind components receive their greatest single amendment (14 %, relative to the initial uncertainty) from the correction of flow distortion magnitude in the dynamic pressure computation. Conversely the vertical wind component is most of all improved (31 %) by subsequent steps considering the 3-D flow distortion distribution in the flow angle computations. Therein the influences of the aircraft’s trim (53 %), as well as changes in the aircraft lift (16 %) are considered by using the measured lift coefficient as explanatory variable. Three independent lines of analysis are used to evaluate the quality of the wind measurement: (a) A wind tunnel study in combination with the propagation of sensor uncertainties defines the systems input uncertainty to ≈0.6 m s−1 at the extremes of a 95 % confidence interval. (b) During severe vertical flight manoeuvres the deviation range of the vertical wind component does not exceed 0.3 m s −1. (c) The comparison with ground based wind measurements yields an overall operational uncertainty (root mean square error) of ≈0.4 m s−1 for the horizontal and≈0.3 m s−1 for the vertical wind components. No conclusive dependence of the uncertainty on the wind magnitude ( <8 m s−1) or true airspeed (ranging from 23–30 m s −1) is found. Hence our analysis provides the necessary basis to study the wind measurement precision and spectral quality, which is prerequisite for reliable Eddy-Covariance flux measurements. Correspondence to: W. Junkermann (wolfgang.junkermann@kit.edu)

Ultrafine particles over Eastern Australia: an airborne survey

Ultrafine particles (UFP) in the atmosphere may have significant impacts on the regional water and radiation budgets through secondary effects on cloud microphysics. Yet, as these particles are invisible for current remote sensing techniques, knowledge about their three-dimensional distribution, source strengths and budgets is limited. Building on a 40-yr-old Australia-wide airborne survey which provides a reference case study of aerosol sources and budgets, this study presents results from a new airborne survey over Eastern Australia, northern New South Wales and Queensland. Observations identified apparent changes in the number and distribution of major anthropogenic aerosol sources since the early 1970s, which might relate to the simultaneously observed changes in rainfall patterns over eastern Queensland. Coal-fired power stations in the inland areas between Brisbane and Rockhampton were clearly identified as the major sources for ultrafine particulate matter. Sugar mills, smelters and shipping along the coast close to the Ports of Townsville and Rockhampton were comparable minor sources. Airborne Lagrangian plume studies were applied to investigate source strength and ageing properties within power station plumes. Significant changes observed, compared to the measurements in the 1970s, included a significant increase in the number concentration of UFP related to coal-fired power station emissions in the sparsely populated Queensland hinterland coincident with the area with the most pronounced reduction in rainfall.

Using airborne technology to quantify and apportion emissions of CH4 and NH3 from feedlots

A novel airborne approach using the latest technology in concentration measurements of methane (CH4) and ammonia (NH3), with quantum cascade laser gas analysers (QCLAs) and high-resolution wind, turbulence and other atmospheric parameters integrated into a low- and slow-flying modern airborne platform, was tested at a 17 000 head feedlot near Charlton, Victoria, Australia, in early 2015. Aircraft flights on 7 days aimed to define the lateral and vertical dimensions of the gas plume above and downwind of the feedlot and the gas concentrations within the plume, allowing emission rates of the target gases to be calculated. The airborne methodology, in the first instance, allowed the emissions to be qualitatively apportioned to individual rows of cattle pens, effluent ponds and manure piles. During each flight, independent measurements of emissions were conducted by ground-based inverse-dispersion and eddy covariance techniques, simultaneously. The aircraft measurements showed good agreement with earlier studies using more traditional approaches and the concurrent ground-based measurements. It is envisaged to use the aircraft technology for determining emissions from large-scale open grazing farms with low cattle densities. Our results suggested that this technique is able to quantify emissions from various sources within a feedlot (pens, manure piles and ponds), as well as the whole feedlot. Furthermore, the airborne technique enables tracing emissions for considerable distances downwind. In the current case, it was possible to detect elevated CH4 to at least 25 km and NH3 at least 7 km downwind of the feedlot.