Ultraviolet Radiation Flux Research Articles

Non-thermal water loss of the early Mars: 3D multi-ion hybrid simulations

In this study we analyze the non-thermal loss rates of O +, O 2 + and CO 2 + ions over the last 4.5 billion years (Gyr) in the Martian history by using a 3D hybrid model. For this reason we derived the past solar wind conditions in detail. We take into account the intensified particle flux of the early Sun as well as an Martian atmosphere, which was exposed to a sun's extreme ultraviolet (EUV) radiation flux 4.5 Gyr ago that was 100 times stronger than today. Furthermore, we model the evolution of the interplanetary magnetic field by a Weber & Davis solar wind model. The ‘external’ influences of the Sun's radiation flux and solar wind flux lead to the formation of an ionospheric obstacle by photoionization, charge exchange and electron impact. For the early Martian conditions we could show that charge exchange was the dominant ionization mechanism. Several hybrid simulations for different stages in the evolution of the Martian atmosphere, at 1, 2, 5, 10, 30 and 100 EUV, were performed to analyze the non-thermal escape processes by ion pick-up, momentum transfer from the solar wind to the ionosphere and detached ionospheric plasma clouds. Our results show a non-linear evolution of the loss rates. Using mean solar wind parameters the simulations result in an oxygen loss equivalent to the depth of a global Martian ocean of about 2.6 m over the last 4.5 Gyr. The induced magnetic field strength could be increased up to about 2000 nT. A simulation run with high solar wind density results in an oxygen loss of a Martian ocean up to 205 m depth during 150 million years after the sun reached the zero age mean sequence (ZAMS).

Analysis of the experimental possibility of forming a conducting channel in the atmosphere using ultraviolet radiation of pulsed discharge

Preliminary experimental investigations are performed of photoionization of air by directed flux of ultraviolet radiation (UVR) generated by a compact pulsed discharge. The plasma temperature of discharge is estimated by the basic characteristics of this discharge, and the electron concentration and current in the conducting channel are measured. It is demonstrated that the optical system of focusing of radiation must be significantly improved for attaining the required levels of electron concentration and current in the channel.

Space Solar Patrol data and changes in weather and climate, including global warming**This article was originally submitted for inclusion with the papers from the 9th International Symposium on Measurement Science and Intelligent Instruments (ISMTII-2009), published in the May 2010 issue.

In this paper, the results obtained during the execution of several ISTC projects are presented. The general aim of these projects has been the study of global changes in the environment, connected with solar activity. A brief description of the optical apparatus of the Space Solar Patrol (SSP) developed and built in the framework of the ISTC projects 385, 385.2, 1523 and 2500 is given. The SSP is intended for permanent monitoring of spectra and absolute fluxes of soft x-ray and extreme ultraviolet (x-ray/EUV) radiation from the full disk of the Sun which ionizes the upper atmosphere of the Earth. Permanent solar monitoring in the main part of the ionizing radiation spectra 0.8–115 (119) nm does not exist. The apparatus of the SSP was developed in the years 1996–2005 with multiyear experience of developing such apparatus in S I Vavilov State Optical Institute. The basis of this apparatus is the use of unique detectors of ionizing radiation—open secondary electron multipliers, which are ‘solar blind’ to near UV, visible and IR radiation from the Sun, and new methodology of these solar spectroradiometric absolute measurements. The prospects are discussed of using the SSP data for the investigation and forecast of the influence of solar variability on the weather and climate including global warming and also on the biosphere including human beings (proposal 3878).

Rf plasma system as an atomic oxygen exposure facility

The materials of spacecraft external surfaces in low Earth orbit (LEO) are exposed to the various constituents of the space environment, including atomic oxygen (AO) and solar ultra violet (UV) radiation. Material degradation and erosion by LEO are simulated in ground laboratories using a variety of experimental facilities, each with their respective limitations. rf oxygen plasma is a simulation facility widely used for materials screening for LEO application. However, the complex plasma environment, which contains, in addition to the neutral oxygen atoms, excited species, electrons, and ions as well as vacuum ultraviolet (VUV) radiation, might lead to erroneous determination of materials reactivity with respect to LEO. This paper describes the development of a simple, low cost rf plasma system to produce a well-defined AO and VUV environment. The new system constrained the afterglow flow through two right-angle turns. The afterglow was characterized at three specific locations by (i) optical emission spectroscopy for assessment of electronically excited states, (ii) current measurements, and (iii) UV radiation measurements. KaptonR samples were exposed at the three specific locations in the system and characterized by mass loss for etch rate evaluation, and atomic force microscopy for surface modification. It was found that there is a significant reduction in ionic species, excited species, and UV radiation as the afterglow advances through the right-angle turns. The reduction in charged particle flux is due to recombination within the afterglow as well as neutralization by colliding with the grounded metal chamber walls; similar decrease in UV radiation flux occurs through radiation absorption by the chamber walls. Finally, it is shown that the ground state AO is the dominant reactive specie of the plasma afterglow after passing through the two right-angle turns.

Modeling impacts of geomagnetic field variations on middle atmospheric ozone responses to solar proton events on long timescales

Strength and structure of the Earth's magnetic field control the deflection of energetic charged particles of solar and cosmic origin. Therefore variations of the geomagnetic field occurring on geological timescales affect the penetration of charged particles into the atmosphere. During solar proton events (SPEs) the flux of high‐energy protons from the Sun is markedly increased. In order to investigate the impact of SPEs on the middle atmospheric ozone on longer timescales, two‐dimensional atmospheric chemistry and transport simulations have been performed using simulated time series of SPEs covering 200 years. Monte Carlo calculations were used to obtain ionization rates, which were then applied to the atmosphere under the consideration of different shielding properties of the geomagnetic field. The present‐day magnetic field configuration and four other scenarios were analyzed. For the first time, field configurations representing possible realistic situations during reversals have been investigated with respect to SPE‐caused ozone losses. With decreasing magnetic field strength the impacts on the ozone are found to significantly increase especially in the Southern Hemisphere, and subsequently, the flux of harmful ultraviolet radiation increases at the Earth's surface. The ozone destructions are most pronounced in the polar regions, and for some field configurations they exceed the values of ozone hole situations after large SPEs. In contrast to ozone holes the depletions due to SPEs are not restricted to winter and spring times but persist into polar summer.

Neural network for the estimation of UV erythemal irradiance using solar broadband irradiance

AbstractIn recent years there has been a substantial increase in attempts to model radiative flux of ultraviolet radiation, UV. In this paper we present the development of an artificial neural network (ANN) model that can be used to estimate solar UV erythemal irradiance, UVER, based on optical air mass, ozone columnar content and broadband solar irradiance. The study was developed at seven stations in the Iberian Peninsula using data recorded at A Coruña, Málaga, Murcia and Santander, during 2000–2003; at Madrid in the period 2000–2002; at Valencia in 2000, 2001 and 2003; and at Zaragoza, from 2001 to 2003. The UVER observations are recorded as half‐hour average values. The measurements were performed in the framework of the Spanish UV‐B radiometric network operated and maintained by the Spanish Meteorological Institute. In order to train and validate the Multi‐layer Perceptron neural networks, independent subsets of data were extracted from the complete database at each station. The networks developed at each place when applied to an independent data set recorded at the same location provide estimates with mean bias deviation less than 1% and root mean square deviation below 17% for all the sites. The generalization network developed using data registered at A Coruña, Madrid, Murcia and Zaragoza provides estimates at all the locations with RMSD below 19%. According to these results, the use of solar broadband irradiances for the estimation of ultraviolet erythemal irradiance provides a tool that can solve the difficulties associated to the retrieval of appropriate information on the cloud field by human observers. In this sense, the proposed method seems appropriate to use the widespread networks of solar broadband irradiance to obtain ultraviolet erythemal irradiance data sets in places where this radiative flux is not measured or to extend back in time the existing data sets. Copyright © 2007 Royal Meteorological Society

Nanophase iron oxides as a key ultraviolet sunscreen for ancient photosynthetic microbes

We propose that nanophase iron-oxide-bearing materials provided important niches for ancient photosynthetic microbes on the Earth that ultimately led to the oxygenation of the Earth's atmosphere and the formation of iron-oxide deposits. Atmospheric oxygen and ozone attenuate ultraviolet radiation on the Earth today providing substantial protection for photosynthetic organisms. With ultraviolet radiation fluxes likely to have been even higher on the early Earth than today, accessing solar radiation was particularly risky for early organisms. Yet, we know that photosynthesis arose early and played a critical role in subsequent evolution. Of primary importance was protection below 290 nm, where peak nucleic acid (~260 nm) and protein (~280 nm) absorptions occur. Nanophase ferric oxide/oxyhydroxide minerals absorb, and thus block, the lethal ultraviolet radiation, while transmitting light through much of the visible and near-infrared regions of interest to photosynthesis (400 to 1100 nm). Furthermore, they were available in early environments, and are synthesized by many organisms. Based on experiments using nanophase ferric oxide/oxyhydroxide minerals as a sunscreen for photosynthetic microbes, we suggest that iron, an abundant element widely used in biological mechanisms, may have provided the protection that early organisms needed in order to be able to use photosynthetically active radiation while being protected from ultraviolet-induced damage. The results of this study are broadly applicable to astrobiology because of the abundance of iron in other potentially habitable bodies and the evolutionary pressure to utilize solar radiation when available as an energy source. This model could apply to a potential life form on Mars or other bodies where liquid water and ultraviolet radiation could have been present at significant levels. Based on ferric oxide/oxyhydroxide spectral properties, likely geologic processes, and the results of experiments with the photosynthetic organisms, Euglena sp. and Chlamydomonas reinhardtii, we propose a scenario where photosynthesis, and ultimately the oxygenation of the atmosphere, depended on the protection of early microbes by nanophase ferric oxides/oxyhydroxides.

Model of Risk of Cortical Cataract in the US Population with Exposure to Increased Ultraviolet Radiation due to Stratospheric Ozone Depletion

The authors modeled the possible consequences for US cataract incidence of increases in ultraviolet B radiation due to ozone depletion. Data on the dose-response relation between ocular exposure to ultraviolet B radiation and cortical cataract were derived from a population-based study (the Salisbury Eye Evaluation Project, Salisbury, Maryland) in which extensive data on cataract and ultraviolet radiation were collected in persons aged 65-84 years. Exposure estimates for the US population were derived using estimated ultraviolet radiation fluxes as a function of wavelength. US Census data were used to obtain the age, ethnicity, and sex distribution of the population. Predicted probabilities of cataract were derived from the age-, sex-, and ethnicity-specific ocular ultraviolet exposure data and were modeled under conditions of 5-20% ozone depletion. The analysis indicated that by 2050, the prevalence of cortical cataract will increase above expected levels by 1.3-6.9%. The authors estimate that with 5-20% ozone depletion, there will be 167,000-830,000 additional cases of cortical cataract by 2050. Because of the high prevalence of cataract in older persons, at a 2003 cost of 3,370 dollars per cataract operation, this increase could represent an excess cost of 563 million dollars to 2.8 billion dollars.

Standard Receivers for Ultraviolet Radiation

Primary standard receivers in the spectral band 30–400 nm based on the use of an ionization chamber and Golay pneumatic detector are developed. Sources of pulse and continuous radiation are used to generate the flux of ultraviolet radiation.

The influence of the solar particle and radiation environment on Titan’s atmosphere evolution

Abstract Measurements of the 15N/14N isotope ratio in Titan’s atmosphere by Earth-based millimetric wavelength spectroscopic observations of HC15N/HC14N indicate that the bulk nitrogen may be enriched in the heavy isotope 15N by about 3.9–4.5 times relative to the terrestrial value. For the explanation of this isotope anomaly, one finds that the mass of Titan’s early atmosphere was at least 30 times larger than the present value. In our study we use recent astrophysical observations on radiative fluxes and stellar winds of solar-like stars with different ages as well as lunar and meteorite fossil records, which indicate that the early Sun underwent a highly active phase resulting in up to about 100 times higher X-ray and extreme ultraviolet (XUV) radiation fluxes and much higher solar wind mass fluxes 100–500 Myr after it arrived at the zero-age-main-sequence. Because the evolution of Titan’s atmosphere can only be understood within the context of the evolving solar particle and radiation flux, we estimated the solar wind parameters and the XUV flux over Titan’s history. Our study indicate that Saturn’s magnetosphere, due to a higher solar wind pressure, was highly compressed during most of its lifetime to distances smaller than Titan’s orbital radius of about 20 Saturn radii. Because the early Sun had a much denser solar wind mass flux up to 1000 times higher than today, we estimate the ion pick up and sputter loss rates on early Titan in the order of about 1026 and 8 × 1028 s−1, respectively, and found that both non-thermal atmospheric loss processes can not explain the observed enrichment in 15N isotopes. Therefore, atmospheric loss during hydrodynamic conditions is investigated, since it can be shown that the exospheric temperature may have been above the “blow-off” temperature of molecular nitrogen of about 8000 K during the early evolutionary stages of Titan’s atmosphere. We found that due to the high XUV radiation input of the young Sun, Titan’s nitrogen atmosphere could have been expanded up to about 3 Titan radii and a massive atmospheric evaporation with N2 loss rates in the order of about 5 × 1030 s−1 may have occurred. The results of our study indicate that the 15N/14N isotope anomaly can be explained if Titan’s upper atmosphere experienced hydrodynamic conditions during a time period of about 50 Myr after the satellites origin. In situ measurements by the Gas Chromatograph and Mass Spectrometer (GCMS) instrument on board of ESA’s Huygens entry probe can be used as a confirmation of the Earth-based isotope anomaly observations and will be of great importance for understanding the formation and evolution of atmospheres around bodies in the solar system.

Estimating UV Erythemal Irradiance by Means of Neural Networks¶

ABSTRACTIn recent years, there has been a substantial increase in attempts to model the flux of ultraviolet radiation (UV). UV irradiance at surface level is a result of the combined effects of solar zenith angle, surface elevation, cloud cover, aerosol load and optical properties, surface albedo and the vertical profile of ozone. In this study, we present the development of an artificial neural network (ANN) model that can be used to estimate solar UV irradiance on the basis of optical air mass, ozone columnar content, latitude, horizontal visibility data and cloud information such as type, coverage and height. ANN are widely accepted as a technology offering an alternative way to tackle complex and ill‐defined problems. They can learn from examples, are fault tolerant in the sense that they are able to handle noisy and incomplete data, are able to deal with nonlinear problems and, once trained, can perform prediction and generalization at high speed. In this study, a multilayer perceptron network (MLP) consisting of an input layer, an output layer and one hidden layer was used. Training of the neural network was done using the Bayesian regulation back propagation algorithm. The study was developed using data from three stations on the Iberian Peninsula: Madrid and Murcia during the period 2000–2001 and Zaragoza in 2001. To train and validate the MPL neural networks, independent subsets of data were extracted from the complete database at each station. The results suggest that a MLP neural network using optical air mass, ozone columnar content, latitude and total cloud coverage provides the best estimates, with mean bias deviation and root mean square deviation of −0.1% and 18.0%, 1.6% and 19.6%, 0.1% and 14.6% at Madrid, Murcia and Zaragoza, respectively. Despite the dependence of the cloud radiative effect on cloud type, the use of additional information such as cloud type or cloud elevation did not improve these results. The performance of the developed ANN has been checked regarding its ability to estimate the UV index (UVI); results indicate that in more than 95% of the cases, the difference between estimated and measured values does not exceed one unit of UVI.

Estimating UV erythemal irradiance by means of neural networks.

In recent years, there has been a substantial increase in attempts to model the flux of ultraviolet radiation (UV). UV irradiance at surface level is a result of the combined effects of solar zenith angle, surface elevation, cloud cover, aerosol load and optical properties, surface albedo and the vertical profile of ozone. In this study, we present the development of an artificial neural network (ANN) model that can be used to estimate solar UV irradiance on the basis of optical air mass, ozone columnar content, latitude, horizontal visibility data and cloud information such as type, coverage and height. ANN are widely accepted as a technology offering an alternative way to tackle complex and ill-defined problems. They can learn from examples, are fault tolerant in the sense that they are able to handle noisy and incomplete data, are able to deal with nonlinear problems and, once trained, can perform prediction and generalization at high speed. In this study, a multilayer perceptron network (MLP) consisting of an input layer, an output layer and one hidden layer was used. Training of the neural network was done using the Bayesian regulation back propagation algorithm. The study was developed using data from three stations on the Iberian Peninsula: Madrid and Murcia during the period 2000-2001 and Zaragoza in 2001. To train and validate the MPL neural networks, independent subsets of data were extracted from the complete database at each station. The results suggest that a MLP neural network using optical air mass, ozone columnar content, latitude and total cloud coverage provides the best estimates, with mean bias deviation and root mean square deviation of -0.1% and 18.0%, 1.6% and 19.6%, 0.1% and 14.6% at Madrid, Murcia and Zaragoza, respectively. Despite the dependence of the cloud radiative effect on cloud type, the use of additional information such as cloud type or cloud elevation did not improve these results. The performance of the developed ANN has been checked regarding its ability to estimate the UV index (UVI); results indicate that in more than 95% of the cases, the difference between estimated and measured values does not exceed one unit of UVI.

A simple device for the evaluation of the UV radiation index

The solar ultraviolet radiation (UV) flux density at the earth's surface depends on the incoming solar energy and the transmission properties of the atmosphere. UV radiation is strongly absorbed by ozone in the spectral range 200–310 nm, while the attenuation is increasingly weaker at longer wavelengths. Following the discovery of the Antarctic ozone hole in 1985, the risk of a possible UV increase at ground level, due to the observed stratospheric ozone depletion, has heightened the interest within the scientific community given the potentially harmful effects on terrestrial and aquatic ecosystems. Spectroradiometers, broad-band meters and dosimeters may be used for measurements of solar UV. In addition, radiation transfer models can be used to quantify UV irradiances at various times and locations, provided that the extraterrestrial solar radiation and the state of the atmosphere are known. Information about UV radiation at the earth's surface is given by the ultraviolet index ‘UVI’, which is defined as the effective integrated irradiance (280–400 nm) weighted by the erythemal action spectrum. The UV Index is widely used by many international weather services as an indicator of UV levels at the earth's surface providing public awareness of the effects of prolonged exposure to the sun's rays. The aim of this paper is to present a device capable of estimating the UV Index. This device is a compact disc, used as a sundial, and is based on modelled UV irradiances derived from the STAR radiative transfer model (System for Transfer of Atmospheric Radiation). The device was tested in an urban setting under clear sky conditions. Copyright © 2003 Royal Meteorological Society

The role of solar radiation in structuring the shallow benthic communities of boreal forest lakes

We have attempted to quantify the roles of physical, chemical, and biological interactions structuring the attached algal and invertebrate communities in eight boreal lakes at the Experimental Lakes Area, northwestern Ontario, varying in dissolved organic carbon (DOC) concentrations (3–9 mg L−1). Attached benthic communities on rocky, south‐facing shores were sampled at 0.1, 0.3, 0.7, and 1.5 m depths. Multivariate redundancy analysis explained 58% of the variance in algal communities and 75% of the variance in invertebrate communities. Algal composition was determined most by photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) fluxes, whereas the composition of associated invertebrates was driven by a combination of exposure to solar radiation and food quality and quantity. Communities on rock surfaces exposed to high solar fluxes in these lakes had very high concentrations of a photoprotective scytonemin‐like pigment (SLP‐A), and were dominated by filamentous green algae. Conversely, deeper, more shielded communities were dominated by diatoms and had low concentrations of SLP‐A. This suggests that previous paleolimnological evidence of SLP‐A increases might reflect changes in littoral attached algal communities rather than planktonic ones. Decreases in DOC concentrations as a result of climate warming will result in increased penetration of solar radiation in boreal lakes. As a result, in the event of decreased precipitation, we predict that the dominance of UVR‐resistant epilithic algae in littoral zones will occur in deeper waters than before, with coincident reductions in densities of many invertebrate taxa in algal biofilms increasingly exposed to high solar fluxes.

Potassium ferrate [Fe(VI)] does not mediate self-sterilization of a surrogate mars soil

BackgroundMartian soil is thought to be enriched with strong oxidants such as peroxides and/or iron in high oxidation states that might destroy biological materials. There is also a high flux of ultraviolet radiation at the surface of Mars. Thus, Mars may be inhospitable to life as we know it on Earth. We examined the hypothesis that if the soil of Mars contains ferrates [Fe(VI)], the strongest of the proposed oxidizing species, and also is exposed to high fluxes of UV radiation, it will be self-sterilizing.ResultsUnder ambient conditions (25°C, oxygen and water present) K2FeO4 mixed into sand mineralized some reactive organic molecules to CO2, while less reactive compounds were not degraded. Dried endospores of Bacillus subtilis incubated in a Mars surrogate soil comprised of dry silica sand containing 20% by weight K2FeO4 and under conditions similar to those now on Mars (extreme desiccation, cold, and a CO2-dominated atmosphere) were resistant to killing by the ferrate-enriched sand. Similar results were observed with permanganate. Spores in oxidant-enriched sand exposed to high fluxes of UV light were protected from the sporocidal activity of the radiation below about 5 mm depths.ConclusionBased on our data and previously published descriptions of ancient but dormant life forms on Earth, we suggest that if entities resembling bacterial endospores were produced at some point by life forms on Mars, they might still be present and viable, given appropriate germination conditions. Endospores delivered to Mars on spacecraft would possibly survive and potentially compromise life detection experiments.

Mars is an awful place to live

Some believe that the planet Mars holds promise as a new home for humankind and that it could become the focus of a large scale colonisation effort at some undefined point in the future. In this paper I support the assertion that Mars holds promise as a site for human scientific, and possibly commercial, exploration, but I question the idea that Mars will be colonised in a manner akin to the New World. The surface of Mars is physically extreme. Mean annual temperature is—60°C, the ultraviolet radiation flux is a thousand times more damaging to DNA than that found on the surface of the earth, and there is little or no liquid surface water. The atmosphere is unbreathable and the soil may be toxic. Although Mars is less awful than the most awful places in the solar system (such as the radiation bombarded surfaces of the Jovian moons), it is considerably more awful than the most extreme places on earth, such as the continental interior of Antarctica and the High Arctic. I suggest that the polar model of human settlement is the most accurate from which to extrapolate the future of human Mars exploration, but even this model is optimistic. Using the most hopeful assessments of colonisation prospects, the human population of Mars would be a maximum of about three million people, and would most probably be substantially less. Understanding the most likely social trajectory of human Mars exploration is not only sociologically interesting, but it is practically important for determining how Mars exploration programmes should be presented to the public.

Quantification of the ultraviolet radiation (UVR) field in the human eye in vivo using novel instrumentation and the potential benefits of UVR blocking hydrogel contact lens

BACKGROUND/AIMSCertain degenerative eye conditions occur predominantly nasally, at the limbal region, and are associated with solar ultraviolet radiation (UVR) induced damage. The relative contribution to the in vivo ocular flux...

Physical Conditions in the Foreground Gas of Reflection Nebulae: NGC 2023, vdB 102, and NGC 7023

High-resolution optical spectra of HD 37903 and HD 147009, which illuminate the reflection nebulae NGC 2023 and vdB 102, were obtained for comparison with our results for HD 200775 and NGC 7023. Ground-based measurements of the molecules CH, C2, and CN and the atoms Na I and K I were analyzed to extract physical conditions in the foreground cloud. Estimates of the gas density, gas temperature, and flux of ultraviolet radiation were derived and were compared with the results from infrared and radio studies of the main molecular cloud. The conditions are similar to those found in studies of diffuse clouds. The foreground material is less dense than the gas in the molecular cloud behind the star(s). The gas temperature was set at 40 K, the temperature determined for the foreground gas in NGC 7023. The flux of ultraviolet radiation was found to be less intense than in the molecular material behind the star(s). The column densities of Na I and K I were reproduced reasonably well when the extinction curve for the specific line of sight was adopted. We obtained NEWSIPS data from the IUE archive for HD 37903 and HD 200775. The ultraviolet data on C I and CO allow extraction of the physical conditions by alternate methods. General agreement among the various diagnostics was found, leading to self-consistent pictures of the foreground photodissociation regions. An appendix describes checks on the usefulness of IUE NEWSIPS data for interstellar studies. Equivalent widths are compared with a previous analysis of IUE observations for interstellar gas toward 20 Aql. Excellent agreement is found with NEWSIPS results having smaller errors. A comparison of NEWSIPS data for C I toward X Per with data acquired with the Hubble Space Telescope shows similar levels of agreement in equivalent widths and that the derived column densities from IUE results are accurate to better than a factor of 2 for absorption lines of moderate strength.

An Ultralight Aircraft as Platform for Research in the Lower Troposphere: System Performance and First Results from Radiation Transfer Studies in Stratiform Aerosol Layers and Broken Cloud Conditions

The ultraviolet actinic radiation flux governing the photochemical reactions in the atmosphere is dependent on the optical properties of atmospheric aerosols and reflective surfaces of ground and clouds. Theoretical models exist for horizontal stratified atmospheres without clouds and currently becoming available are the first three-dimensional models that include broken cloud conditions. The experimental database for model verification is scarce for both ground and vertical distribution. To close this experimental gap an airborne system was developed and is described in this paper. An ultralight aircraft was used as an instrument platform to provide detailed measurements of radiation transfer in the ultraviolet and complimentary parameters that can be used for comparison with model calculations. The measurement system consists of standard, but in size and power consumption strongly modified, instruments. It produces a dataset covering upwelling and downwelling radiation as well as absorbing gaseous compounds and aerosol optical properties, their size, number, and scattering coefficients. The system is designed to be used on a variety of aircraft, from medium size to microlight. Measurements in an altitude range of up to 4500 m above sea level are presented using the ultralight aircraft under conditions with clouds, haze, and dust layers of natural and anthropogenic origin within a visibility range of 120 to about 1 km. Selected vertical distributions are presented, measured in remote and polluted environments. The results presented clearly demonstrate the capability of the ultralight aircraft as an instrument platform for lower-tropospheric research missions.

Sources, Sinks, and the Distribution of OH in the Lower Stratosphere

Extensive measurement campaigns by the NASA ER-2 research aircraft have obtained a nearly pole-to-pole database of the species that control HO_x (OH + HO_2) chemistry. The wide dynamic range of these in situ measurements provides an opportunity to demonstrate empirically the mechanisms that control the HO_x system. Measurements in the lower stratosphere show a remarkably tight correlation of OH concentration with the solar zenith angle (SZA). This correlation is nearly invariant over latitudes ranging from 70° S to 90° N and all seasons. An analysis of the production and loss of HO_x in terms of the rate determining steps of reaction sequences developed by Johnston and Podolske and Johnston and Kinnison is used to clarify the behavior of the system and to directly test our understanding of the system with observations. Calculations using in situ measurements show that the production rate of HOx is proportional to O3 and ultraviolet radiation flux. The loss rate is proportional to the concentration and the partitioning of NO_y (reactive nitrogen) and the concentration of HO_2. In the absence of heterogeneous reactions, the partitioning of NO_y is controlled by O_3 and HO_x and the concentration of HO2 is controlled by NO_y and O_3, so that the removal rate of OH is buffered against changes in the correlation of O_3 and NO_y. The heterogeneous conversion of NO_2 to HNO_3 is not an important net source of HO_x because production and removal sequences are nearly balanced. Changes in NOy partitioning resulting from heterogeneous chemistry have a large effect on the loss rates of HO_x, but little or no impact on the measured abundance of OH. The enhanced loss rates at high NO_2/HNO_3 are offset in the data set examined here by enhanced production rates resulting from increased photolysis rates resulting from the decreased O_3 column above the ER-2.