Michael Kotkamp

Solar ultraviolet spectroradiometry in New Zealand: instrumentation and sample results from 1990.

In 1988 the New Zealand Department of Scientific and Industrial Research initiated a program to characterize the spectrum of solar ultraviolet radiation reaching the ground in New Zealand and to identify the extent and causes of its variability. Routine measurements began at Lauder (45 degrees S 170 degrees E) in December 1989. The instrumentation, measurement strategy, and calibration procedures are discussed and uncertainties in the measurements are analyzed. With the present system useful measurements at 1-nm resolution are limited to irradiances greater than 10(-3) microW cm(-2) nm(-1), which corresponds to a lower limit in wavelength in the region 290-295 nm (depending on the Sun angle and ozone amount). This is a useful lower limit for many applications of relevance to the biosphere. Results from the first year of operation are presented and discussed.

First southern hemisphere intercomparison of measured solar UV spectra

Three UV spectroradiometers from the National Institute of Water and Atmospheric Research (NIWA) New Zealand, the Fraunhofer Institute (IFU) Germany, and the Australian Radiation Laboratory (ARL) Australia were intercompared at Lauder NZ on 23 February 1993. Over the spectral range 290–400 nm, the agreement between the IFU and NIWA instruments was better than 5%. At noon on this day, the irradiances measured by all three instruments agreed within ±10%, except at wavelengths shorter than 300 nm, where the ARL instrument gave higher readings. At larger solar zenith angles (SZA) the differences at short wavelengths were more pronounced, and at wavelengths above 300 nm the ARL measurements were systematically lower. The reasons for these differences are discussed. Having established the differences between the sets of instrumentation, spectra of maximum clear sky UV irradiances observed by these groups in New Zealand, Australia, and Europe are compared. The erythemally weighted irradiance observed in Melbourne Australia was the highest (0.35 W m−2). Respective maxima for Lauder NZ and for Neuherberg Germany were 85% and 66% of that in Australia. Differences are larger for DNA-weighted UV.

Upwelling UV spectral irradiances and surface albedo measurements at Lauder, New Zealand

Simultaneous measurements of upwelling and downwelling ultraviolet (UV) spectral irradiance were used to deduce the wavelength dependence and the solar zenith angle dependence of the albedo of the ground surface (long grass) at Lauder, New Zealand (45.05°S, 169.68°E). In the UVB region the deduced albedos are approximately 1%. At longer wavelengths the albedo increases to approximately 2% at 400 nm, and 4.5% at 450 nm. These albedos are significantly smaller than those generally quoted for the visible region. The deduced albedos tend to increase at larger solar zenith angles, demonstrating that the surface is not strictly Lambertian.

Relationship between UVB and erythemally weighted radiation

We discuss the move from reporting damaging UV radiation in terms of UVB to the now widely accepted erythemally weighted UV radiation (UV(Ery)) and the UV Index (UVI). The relationship between these quantities is given: to a good approximation, it is found that UVB(280-315 nm)= 7.55 [times] UV(Ery). In terms of the UV Index, the estimated UVB(280-315 nm) in units of W m(-2) is 18.9 times the UVI. These approximations generally hold to within approximately 10% for all solar zenith angles (sza) less than 70 degrees. For most practical purposes, this is a sufficient range, since for larger sza, the intensity of UVB is less than 10% of that for overhead sun conditions. The simple relationship above is verified using spectral measurements. However, tables are provided to enable calculation of the conversion with greater accuracy under such conditions. Similar model calculations are provided to estimate UVB(280-320 nm). Correction tables to convert erythemally weighted UV to other biological weightings are also presented.

Verification techniques for N2O emission at the paddock scale in New Zealand: FarmGas2006

High-precision micrometeorological measurement with tunable diode laser (TDL)-based trace gas analysers provides a continuous spatially integrating and non-intrusive measurement technique that is capable of detecting and quantifying episodic N2O emission at the paddock scale. Results are presented from the FarmGas2006 measurement campaign conducted on a commercial dairy farm in North Canterbury, New Zealand, over 3 weeks in October 2006. This was the first field deployment of a TDL instrument for paddock-based N2O flux measurement in New Zealand. A goal of this campaign was assessment of a range of atmospheric N2O sensing technologies and micrometeorological approaches. In this paper the capabilities of TDL technology are compared with gas chromatography (GC) in flux-gradient measurements. Baseline emission was <100 ng N/m2.s and increased to <250 ng N/m2.s following grazing by the dairy herd. There was very good correlation between GC- and TDL-determined fluxes and also good agreement between the instruments in the mean emission in 10 days before (45–50 ng N/m2.s) and after (75–80 ng N/m2.s) paddock grazing. The flux was characterised by events of high emission lasting several hours such that half of the total N2O was emitted in ~10% of the time over the duration of the campaign. We discuss the implications of this and advantages of high-precision techniques as tools for ‘top-down’ verification and for the assessment of N2O emission mitigation options.

Spectral UV Measurements of Global Irradiance, Solar Radiance, and Actinic Flux in New Zealand: Intercomparison between Instruments and Model Calculations

Abstract Presented here are the results of a short but intense measurement campaign at Lauder, New Zealand, in which spectral irradiance from instruments operated by the National Institute of Water and Atmospheric Research (NIWA) and Austria/Innsbruck (ATI) were traced to different irradiance standards and compared. The observed spectral differences for global irradiance were relatively small (<5%) and were consistent with those expected from observed differences in the radiation standards used by each group. Actinic fluxes measured by both groups were also intercompared and found to agree at the 10% level. The ATI instrument had the additional capability of measuring solar direct beam irradiance and sky radiances. These provided the first series of sky radiance measurements at this pristine Network for the Detection of Atmospheric Composition Change (NDACC) site. The polarization of sky radiance results were compared with estimates from a radiative transfer model without any aerosols and was found to be ...

Peak UV: Spectral contributions from cloud enhancements

We use multi-year datasets of UV spectral irradiances, measured to the exacting standards required by the Network for the Detection of Atmospheric Composition Change (NDACC), to investigate the enhancement effects of clouds and their wavelength dependence at sites that span a wide range of altitudes. These enhancements are derived by comparing weighted UV irradiance measurements with corresponding model calculations at each site for clear skies. We find that the frequency, magnitude, and wavelength-dependence of cloud enhancements are insufficient to explain the repeatedly high values of UVB and UVI observed by Cabrol et al., 2014, and that ozone amounts lower than have ever been seen there would be required.

International Intercomparison of Solar UVR Spectral Measurement Systems in Melbourne in 2013

Monitoring ambient solar UVR levels provides information on how much there is in both real time and historically. Quality assurance of ambient measurements of solar UVR is critical to ensuring accuracy and stability and this can be achieved by regular intercomparisons of spectral measurement systems with those of other organizations. In October and November of 2013 a solar UVR spectroradiometer from Public Health England (PHE) was brought to Melbourne for a campaign of intercomparisons with a new Bentham spectrometer of Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) and one at the Australian Bureau of Meteorology (BOM), supported by New Zealands National Institute for Water and Atmosphere (NIWA). Given all three spectroradiometers have calibrations that are traceable to various national standards, the intercomparison provides a chance to determine measurement uncertainties and traceability that support UV measurement networks in Australia, New Zealand and the UK. UV Index measurements from all three systems were compared and ratios determined for clear sky conditions when the scans from each instrument were within 2 min of each other. While wavelengths below 305 nm showed substantial differences between the PHE unit and the two other systems, overall the intercomparison results were encouraging, with mean differences in measured UV Index between the BOM/NIWA and those of PHE and ARPANSA of <0.1% and 7.5%, respectively.

25 years of solar spectral UV measurements at 45° S

We review 25 years of solar spectral UV at Lauder, measured to the exacting standards of the Network for the Detection of Atmospheric Composition Change (NDACC) with the best systems available at the time. Recent reanalysis of the alignment, calibration, and data processing from all nine of the UV spectroradiometers used at Lauder and other NDACC sites has better characterised the effects of changes in instrument technology. There has been no detectable trend in solar UV radiation other than that resulting from ozone variation.