Kaisa Lakkala

University of Helsinki, Department of Physics, Alumna

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Papers by Kaisa Lakkala

The TROPOMI surface UV product

EGU General Assembly Conference Abstracts, Apr 1, 2018

Record-Breaking Increases in Arctic Solar Ultraviolet Radiation Caused by Exceptionally Large Ozone Depletion in 2020

Measurements of solar ultraviolet radiation (UVR) performed between January and June 2020 at 10 A... more Measurements of solar ultraviolet radiation (UVR) performed between January and June 2020 at 10 Arctic and subarctic locations are compared with historical observations. Differences between 2020 and prior years are also assessed with total ozone column and UVR data from satellites. Erythemal (sunburning) UVR is quantified with the UV Index (UVI) derived from these measurements. UVI data show unprecedently large anomalies, occurring mostly between early March and mid-April 2020. For several days, UVIs observed in 2020 exceeded measurements of previous years by up to 140%. Historical means were surpassed by more than six standard deviations at several locations in the Arctic. In northern Canada, the average UVI for March was about 75% larger than usual. UVIs in April 2020 were elevated on average by about 25% at all sites. However, absolute anomalies remained below 3.0 UVI units because the enhancements occurred during times when the solar elevation was still low. Plain Language Summary The ultraviolet radiation (UVR) from the Sun depends on the amount of ozone in the atmosphere. During March and April 2020, ozone concentrations in the Arctic atmosphere were exceptionally low and this led to large increases in UV radiation at the surface. These increases were quantified by analyzing data from ground-based instruments at 10 Arctic and subarctic locations, as well as satellite data. UV levels were quantified with the UV Index (UVI), which measures the amount of "sunburning" UVR. The greatest UVI increases were observed between early March and mid-April 2020. For several days, UVIs observed in 2020 exceeded measurements of previous years by up to 140%. In northern Canada, the average UVI for March was about 75% larger than usual. UVIs in April 2020 were elevated on average by about 25%. However, these large relative anomalies occurred early in the year when the Sun is still low in the sky. UVI increases remained therefore small in absolute terms and did not exceed typical summertime UVI values under clear skies.

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Assessment of TOMS UV bias due to absorbing aerosols

Proceedings of SPIE, Oct 14, 2004

We compared NASA Total Ozone Mapping Spectrometer (TOMS) overpass irradiances against ground-base... more We compared NASA Total Ozone Mapping Spectrometer (TOMS) overpass irradiances against ground-based Brewer measurements at Ispra, Italy, and Thessaloniki, Greece, with the main objective of evaluating the effect of absorbing aerosols on TOMS UV bias using direct measurements of aerosol optical properties. Dependence of the TOMS/Brewer bias on aerosol absorption optical depth is significant. Our study demonstrates that the bias between TOMS and measured Brewer 324 nm irradiances increases with increasing aerosol absorption optical depth t abs. The correlation coefficients between the ratio TOMS/Brewer and t abs at Ispra and Thessaloniki are $0.7 or more and $0.8 or more, respectively, depending on the range of solar zenith angle values selected for the analysis. We found that the correlation with single-scattering albedo w or aerosol optical depth is significantly smaller than with t abs. If measurements or climatology of t abs are available in the UVB range, the TOMS UV product can be postcorrected for absorbing aerosols using similar site-dependent regressions as established in our study. If no correction is applied, the mean positive biases between TOMS and Brewer 324 nm irradiances in Thessaloniki and Ispra are 19.2% and 29.7%, respectively, while the standard deviations are 8.9% and 16.5%. Depending on the level of correction, the mean positive bias can be essentially removed, and the standard deviations can be reduced down to $6% and $10%, respectively.

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Performance of the FMI cosine error correction method for the Brewer spectral UV measurements

Atmospheric Measurement Techniques, Sep 11, 2018

Non-ideal angular response of a spectroradiometer is a well-known error source of spectral UV mea... more Non-ideal angular response of a spectroradiometer is a well-known error source of spectral UV measurements and for that reason instrument specific cosine error correction is applied. In this paper, the performance of the cosine error correction method of Brewer spectral UV measurements in use at the Finnish Meteorological Institute (FMI) is studied. Ideally, the correction depends on the actual sky radiation distribution, which can change even during one spectral scan due to rapid changes in cloudiness. The FMI method has been developed to take into account the changes in the ratio of direct to diffuse sky radiation and it derives a correction coefficient for each measured wavelength. Measurements of five Brewers were corrected for the cosine error and the results were compared to the reference travelling spectroradiometer (QASUME). Measurements were performed during the RBCC-E (Regional Brewer Calibration Center-Europe) X Campaign held at El Arenosillo, Huelva (37 • N, 7 • W), Spain, in 2015. In addition, results of site audits of FMI's Brewers in Sodankylä (67 • N, 27 • E) and Jokioinen (61 • N, 24 • E) during 2002-2014 were studied. The results show that the spectral cosine error correction varied between 4 and 14 %. After that the correction was applied to Brewer UV spectra the relative differences between the QASUME and the Brewer diminished even by 10 %. The study confirms that the method, originally developed for measurements at high latitudes, can be used at mid-latitudes as well. The method is applicable to other Brewers as far as the required input parameters, i.e. total ozone, aerosol information, albedo, instrument specific angular response and slit function are available.

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Temperature dependence of the Brewer global UV measurements

Atmospheric Measurement Techniques, Nov 22, 2017

Spectral measurements of global UV irradiance recorded by Brewer spectrophotometers can be signif... more Spectral measurements of global UV irradiance recorded by Brewer spectrophotometers can be significantly affected by instrument-specific optical and mechanical features, thus proper corrections are needed in order to reduce the associated uncertainties to within acceptable levels. The present study aims to contribute to the reduction of uncertainties originating from changes in the Brewer's internal temperature, which affect the performance of the optical and electronic parts, and subsequently the response of the instrument. Until now, measurements of the irradiance from various types of lamps at different temperatures are used to characterize the instruments' temperature dependence. The use of 50 Watt lamps was found to induce errors in the characterization due to changes in the transmissivity of the Teflon diffuser as it warms up by the heat of the lamp. In contrast the use of 200 or 1000 Watt lamps is considered more appropriate because they are positioned at longer distances from the diffuser so that warming is negligible. Temperature gradients inside the instrument can cause mechanical stresses which can affect the instrument's optical characteristics. Therefore during the temperature-dependence characterization procedure warming or cooling must be slow enough to allow all parts of the instrument to be nearly equilibrated. In this study, results of the temperature characterization of eight different Brewer spectrophotometers operating in Greece, Finland, Germany and Spain are presented. It was found that the instruments' response is changing differently in different temperature regions due to different responses of the diffusers' transmittance. The temperature correction factors derived for the Brewer spectrophotometers operating at Thessaloniki, Greece and Sodankylä, Finland were evaluated and were found to remove the temperature dependence of the instruments' sensitivity. 1 Introduction The Brewer spectrophotometer was developed in the 1970s and became commercially available in the beginning of the 1980s (Brewer, 1973; Kerr et al., 1985b). They were initially designed to measure the total columns of ozone (O 3) and sulfur dioxide

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Supplementary material to "Temperature dependence of the Brewer global UV measurements&quot

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Reconstruction of Solar Spectral Surface UV Irradiances Using Radiative Transfer Simulations

Photochemistry and Photobiology, Sep 1, 2009

UV radiation exerts several effects concerning life on Earth, and spectral information on the pre... more UV radiation exerts several effects concerning life on Earth, and spectral information on the prevailing UV radiation conditions is needed in order to study each of these effects. In this paper, we present a method for reconstruction of solar spectral UV irradiances at the Earth's surface. The method, which is a further development of an earlier published method for reconstruction of erythemally weighted UV, relies on radiative transfer simulations, and takes as input (1) the effective cloud optical depth as inferred from pyranometer measurements of global radiation (300-3000 nm); (2) the total ozone column; (3) the surface albedo as estimated from measurements of snow depth; (4) the total water vapor column; and (5) the altitude of the location. Reconstructed daily cumulative spectral irradiances at Jokioinen and Sodankyla¨in Finland are, in general, in good agreement with measurements. The mean percentage difference, for instance, is mostly within ±8%, and the root mean square of the percentage difference is around 10% or below for wavelengths over 310 nm and daily minimum solar zenith angles (SZA) less than 70°. In this study, we used pseudospherical radiative transfer simulations, which were shown to improve the performance of our method under large SZA (low Sun).

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A new method for estimating UV fluxes at ground level in cloud-free conditions

A new method has been developed to estimate the global and direct solar irradiance in the UV-A an... more A new method has been developed to estimate the global and direct solar irradiance in the UV-A and UV-B at ground level in cloud-free conditions. It is based on a resampling technique applied to the results of the kdistribution method and the correlated-k approximation of Kato et al. (1999) over the UV band. Its inputs are the aerosol properties and total column ozone that are produced by the Copernicus Atmosphere Monitoring Service (CAMS). The estimates from this new method have been compared to instantaneous measurements of global UV irradiances made in cloud-free conditions at five stations at high latitudes in various climates. For the UV-A irradiance, the bias ranges between −0.8 W m −2 (−3 % of the mean of all data) and −0.2 W m −2 (−1 %). The root mean square error (RMSE) ranges from 1.1 W m −2 (6 %) to 1.9 W m −2 (9 %). The coefficient of determination R 2 is greater than 0.98. The bias for UV-B is between −0.04 W m −2 (−4 %) and 0.08 W m −2 (+13 %) and the RMSE is 0.1 W m −2 (between 12 and 18 %). R 2 ranges between 0.97 and 0.99. This work demonstrates the quality of the proposed method combined with the CAMS products. Improvements, especially in the modeling of the reflectivity of the Earth's surface in the UV region, are necessary prior to its inclusion into an operational tool.

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Comparing OMI UV index to ground-based measurements at two Finnish sites with focus on cloud-free and overcast conditions

Satellite based surface UV product of the Ozone Monitoring Instrument OMI was validated using gro... more Satellite based surface UV product of the Ozone Monitoring Instrument OMI was validated using ground based UV measurements from the two Finnish sites Jokioinen and Sodankylä. The goal was to further investigate the observed positive UV bias of the OMI UV product focusing on how it may be connected to cloudiness during the overpass of the Aura satellite. A total of seven years of summer time data was used to compare OMI UV index to a reference UVI observed on the ground with Solar Light 501 broadband radiometers. Cloudiness during satellite overpass was determined with auxiliary ground based observations on sunshine duration, cloud cover and global radiation as well as the satellite based MODIS cloud cover estimates. The analysis aimed to minimize the error sources from temporal discrepancies and from the differences in the field of view of OMI and its ground based reference data. As a result, OMI UV product was seen to overestimate surface UV index by 21 % in average and overcast UV index up to 56 %. The study confirms that OMI UV index is overestimated compared to ground based reference, and shows, that the bias is related to cloudiness and is higher during well defined overcast conditions.

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UV-Indien Network ground-based measurements: comparisons with satellite and model estimates of UV radiation over the Western Indian Ocean

Within the framework of the UV-Indien network, nine ground stations have been equipped with ultra... more Within the framework of the UV-Indien network, nine ground stations have been equipped with ultraviolet broadband radiometers, five of them have also been equipped with an all-sky camera, and the main station in Saint-Denis de la Réunion is also equipped with a spectroradiometer. These stations are spatially distributed to cover a wide range of latitudes, longitudes, altitudes, and environmental conditions in five countries of the western Indian Ocean region (Comoros, France, Madagascar, Mauritius, and Seychelles), a part of the world where almost no measurements have been made so far. The distribution of the stations is based on the scientific interest of studying ultraviolet radiation not only in relation to atmospheric processes but also in order to provide data relevant to fields such as biology, health (prevention of skin cancer), and agriculture. The main scientific objectives of this network are to study the annual and inter-annual variability in the ultraviolet (UV) radiation in this area, to validate the output of numerical models and satellite estimates of ground-based UV measurements, and to monitor UV radiation in the context of climate change and projected ozone depletion in this region. A calibration procedure including three types of calibrations responding to the various constraints of sustaining the network has been put in place, and a data processing chain has been set up to control the quality and the format of the files sent to the various data centres. A method of clear-sky filtering of the data is also applied. Here, we present an intercomparison with other datasets, as well as several daily or monthly representations of the UV index (UVI) and cloud fraction data, to discuss the quality of the data and their range of values for the older stations (Antananarivo, Anse Quitor, Mahé, and Saint-Denis). Ground-based measurements of the UVI are used to validate satellite estimates-Ozone Monitoring Instrument (OMI), the TROPOspheric Monitoring Instrument (TROPOMI), and the Global Ozone Monitoring Experiment (GOME)-and model forecasts of UVI-Tropospheric Emission Monitoring Internet Service (TEMIS) and Copernicus Atmospheric Monitoring Service (CAMS). The median relative differences between satellite or model estimates and ground-based measurements of clear-sky UVI range between −34.5 % and 15.8 %. Under clear skies, the smallest UVI median difference between the satellite or model estimates and the measurements made by ground-based instruments is found to be 0.02 (TROPOMI), 0.04 (OMI), −0.1 (CAMS), and −0.4 (CAMS) at Saint-Denis, Antananarivo,

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Supplementary material to "Validation of TROPOMI Surface UV Radiation Product&quot

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Radiation measurements at the Pallas-Sodankylä Global Atmosphere Watch station — diurnal and seasonal cycles of ultraviolet, global and photosynthetically-active radiation

We present time series of solar ultraviolet radiation (UVR), global radiation, and photosynthetic... more We present time series of solar ultraviolet radiation (UVR), global radiation, and photosynthetically-active radiation (PAR) measurements from the Pallas-Sodankylä Global Atmosphere Watch (GAW) station at Sodankylä for the period 1990-2013. Measurements were performed both in an open area and in the surrounding boreal forest, which had a total leaf area index of 3.6. We present a method to homogenize multifilter radiometer UV time series. Using this method, the relative mean differences between the multifilter radiometer and the reference spectroradiometer were 5% for UVB and erythemally-weighted dose rates, for all sky conditions and solar zenith angles (SZA) smaller than 60° during the period 2008-2012. Our results show that daily doses measured in the forest were four times smaller than those measured in the nearby open area. Maximum UVB (280-320 nm) and UVA (320-400 nm) daily doses of 75.0 kJ m-2 and 1.74 MJ m-2 , respectively, were measured during the period of multifilter radiometer measurements, 2002-2012. The maximum daily sum of global radia

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Results From The Nordic Intercomparison of Ultraviolet Spectroradiometers In Sweden 2000

EGSGA, 2002

Fourteen spectroradiometers measuring ultraviolet radiation participated in a Nordic intercompari... more Fourteen spectroradiometers measuring ultraviolet radiation participated in a Nordic intercomparison in Halmstad, Sweden, 10-15 June, 2000. Measurements were performed at both clear and cloudy conditions. Protocols and tools for analysing measurements were much the same as in previous large campaigns (Bais et al. 2001, Kjeldstad et al. 1997), for instance two days were blind day measurements, establishment of a common

The PROMOTE UV Record: Toward a Global Satellite-Based Climatology of Surface Ultraviolet Irradiance

IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Sep 1, 2009

This paper describes the PROMOTE UV Record, which aims to provide a global long-term record of th... more This paper describes the PROMOTE UV Record, which aims to provide a global long-term record of the surface UV radiation. The algorithm developed takes as input cloud information from the International Satellite Cloud Climatology Project (ISCCP) and a recently developed multisensor assimilated record of the total ozone column. Aerosols and surface albedo are based on climatologies. Here, first validation results

UV radiation and photoprotective pigments in Scots pine saplings (Pinus sylvestris L.)

Arctic Monitoring and Assesment Programme (AMAP), 2004

Supplementary material to "Comparison of OMI UV observations with ground-based measurements at high northern latitudes

Datasets of UV irradiance, visible and photosynthetically active radiation in Marambio, Antarctica from March 2017 to May 2019

GUV radiometer data from Marambio station for the period March 2017 - May 2019. UV-B and UV-A dai... more GUV radiometer data from Marambio station for the period March 2017 - May 2019. UV-B and UV-A daily maximum irradiances and dailydoses. Daily noon UV index and total column ozone. Daily maximum irradiances at 555 nm (VIS) and daily maximum of photosynthetically active radiation. If you use the data, please contact Kaisa Lakkala, Finnish Meteorological Institute.

State of the Climate in 2012 (SCI)

For the first time in serveral years, the El Nino-Southern Oscillation did not dominate regional ... more For the first time in serveral years, the El Nino-Southern Oscillation did not dominate regional climate conditions around the globe. A weak La Ni a dissipated to ENSOneutral conditions by spring, and while El Nino appeared to be emerging during summer, this phase never fully developed as sea surface temperatures in the eastern conditions. Nevertheless, other large-scale climate patterns and extreme weather events impacted various regions during the year. A negative phase of the Arctic Oscillation from mid-January to early February contributed to frigid conditions in parts of northern Africa, eastern Europe, and western Asia. A lack of rain during the 2012 wet season led to the worst drought in at least the past three decades for northeastern Brazil. Central North America also experienced one of its most severe droughts on record. The Caribbean observed a very wet dry season and it was the Sahel's wettest rainy season in 50 years. Overall, the 2012 average temperature across glo...

UV radiation and defence mechanisms of Scots pine (Pinus sylvestris L.) at the subarctic: photoprotective pigments

The effects fire, post-fire salvage logging, and revegetation on nutrient budgets were estimated ... more The effects fire, post-fire salvage logging, and revegetation on nutrient budgets were estimated for a site in the eastern Sierra Nevada Mountains that burned in a wildfire in 1981. Approximately two decades after the fire, the shrub (former fire) ecosystem contained less C and more N than the adjacent forest ecosystem (Johnson et al., 2004). Fig. 1. Estimated losses of C and N by volatilization and salvage logging in the Little Valley fire and gains in soil, O horizon and vegetation over the subsequent two decades.

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Irradiance scale of long term UV measurements at Sodankylä and Jokioinen, Finland

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