Radiation-transfer modeling of snow-pack photochemical processes during ALERT 2000

The delta-Eddington radiation transfer model is used to calculate actinic fluxes and photolysis rates within the snow pack during the ALERT 2000 field campaign. Actinic fluxes are enhanced within the snow pack due to the high albedo of snow and conversion of direct light to diffuse light. The conversion of direct to diffuse light is highly dependent on the solar zenith angle, as demonstrated by model calculations. The optical properties of Alert snow are modeled as 100 μm radius ice spheres with impurity added to increase the absorption coefficient over that of pure water ice. Using these optical properties, the model achieves good agreement with observations of irradiance within the snow pack. The model is used to calculate the total actinic flux as a function of solar zenith angle and depth for either clear sky or cloudy conditions. The actinic flux is then used to calculate photochemical production of nitrogen oxides from nitrate photolysis assuming that nitrate in snow has the same absorption cross section and quantum yield in snow as in aqueous solution. Assuming all photo-produced nitrogen oxides are released to the gas phase, we derive a maximal flux of nitrogen oxides (NO_x+HONO and possibly other products) from the snow pack. The value of this maximal flux depends critically on the assumed quantum yield for production of NO₂, which is unknown in ice. Depending on the assumed quantum yield, the calculated maximal flux varies between values four times smaller than the observed NO_x+HONO flux to five times larger than the NO_x+HONO flux. Therefore, it appears that the calculated flux is in approximate agreement with the observations with a great need for improved understanding of nitrogen photochemistry in snow.     

Variability in ultraviolet total optical depth during the Southern California Ozone Study (SCOS97)

Formation of photochemical air pollution is governed in part by the solar ultraviolet actinic radiation flux, but wavelength-resolved measurements of UV radiation in polluted urban atmospheres are rarely available. As part of the 1997 Southern California Ozone Study, cosine weighted solar irradiance was measured continuously at seven UV wavelengths (300, 306, 312, 318, 326, 333 and 368 nm) at two sites during the period 1 July to 1 November 1997. The first site was at Riverside (260 m a.s.l.) in the Los Angeles metropolitan area, which frequently experiences severe air pollution episodes. The second site was at Mt Wilson (1725 m a.s.l.), approximately 70 km northwest of Riverside, and located above much of the urban haze layer. Measurements of direct (i.e., total minus diffuse) solar irradiance were used to compute total atmospheric optical depths. At 300 nm, optical depths (mean±1 S.D.) measured over the entire study period were 4.3±0.3 at Riverside and 3.7±0.2 at Mt Wilson. Optical depth decreased with increasing wavelength, falling at 368 nm to values of 0.8±0.2 at Riverside and 0.5±0.1 at Mt Wilson. At all wavelengths, both the mean and the relative standard deviation of optical depths were larger at Riverside than at Mt Wilson. At 300 nm, the difference between the smallest and largest observed optical depths corresponds to over a factor 2 increase in the direct beam irradiance for overhead sun, and over a factor 7 increase for a solar zenith angle of 60°. Principal component analysis was used to reveal underlying factors contributing to variability in optical depths. PCA showed that a single factor (component) was responsible for the major part of the variability. At Riverside, the first component was responsible for 97% of the variability and the second component for 2%. At Mt Wilson, 89% of the variability could be attributed to the first component and 10% to the second. Dependence of the component contributions on wavelength allowed identification of probable physical causes: the first component is linked to light scattering and absorption by atmospheric aerosols, and the second component is linked to light absorption by ozone. These factors are expected to contribute to temporal and spatial variability in solar actinic flux and photodissociation rates of species including ozone, nitrogen dioxide, and formaldehyde.     

Spectral attenuation of solar ultraviolet radiation in humic lakes in Central Finland

The attenuation of solar ultraviolet (UV) radiation in five lakes in Central Finland was evaluated through field measurements and/or by determining the optical properties of the lake water during summer 1999. Spectral UV irradiance in the air and at several depths underwater was measured in three lakes (Lake Palosj?rvi, Konnevesi, and Jyv?sj?rvi) with dissolved organic carbon (DOC) ranging from 4.9 to 8.7 mg l(-1) and chlorophyll a ranging from 1.6 to 16 g l(-1). According to the field measurements, 99% of the UV-B radiation was attenuated in approximately a half meter water column in the clearest lake. In the UV-A region at 380 nm, the corresponding attenuation occurred in the upper one meter. In a small humic lake (DOC 13.2-14.9 mg l(-1)) UV-B radiation was attenuated to 1% of the subsurface irradiance within the top 10 cm water column, whereas UV-A radiation (at 380 nm) penetrated more than twice as deeply (maximum 25 cm), as predicted from the absorption coefficients. These results suggest the importance of the dissolved fraction of lake water in governing the UV attenuation in lakes. This was seen from the significant relationship between the vertical attenuation coefficients (Kd) based on field measurements and the absorption coefficients (ad) derived from spectrophotometric laboratory scannings, as well as between Kd and DOC.     

Ultraviolet Irradiance over Mexico City

Abstract

This article compiles and analyzes information on the interaction of both ground-level ultraviolet radiation and anthropogenic ozone in the extremely polluted atmosphere of Mexico City. Simultaneous anthropogenic ozone and global flux (sun and sky) measurements were recorded in near ultraviolet radiation (295-385 nm) for a 30-month period (March 1990 to August 1992). Results reveal that under clear sky conditions, high concentrations of ozone (and presumably of other pollutants) impede UV-fluxes from reaching their maximum values. Furthermore, heavy cloud cover causes ozone concentrations to reach their minimum values together with UV-minimum fluxes. Seasonal variation in UV-irradiance is also discussed. Findings indicate that maximum UV-fluxes occur in spring and summer and that minimum values occur in autumn and winter. Significant daily local reductions in UV-fluxes (up to 50%) during afternoon hours in Mexico City can be attributed to air pollution. Finally, the growing incidence of rickets in children residing in the Mexico City metropolitan area may be related both to dietary deficiencies and to the UV-flux depletion on the ground.     

A quantitative comparison of α-Å turbidity parameter retrieved in different spectral ranges based on spectroradiometer solar radiation measurements

Spectroradiometric direct irradiance measurements in the 300–1100 nm wavelength range with a spectral resolution of 6.2 nm have been used in a study of the variation in the Ångström turbidity parameter α and its dependence on the spectral range used in its determination. The measurements have been carried out under clear sky conditions at two different climate stations in Spain. Least-square fits of the experimental spectral aerosol optical depth (AOD) to the Ångström formula in different spectral ranges, selected for convenience depending on the objective or application (e.g., UV–VIS (350–400 nm), VIS (400–670 nm), VIS–NIR (370–870 nm), etc.), result in different sets for the α parameter. Due to this dependence on the spectral range, where the α-values are determined, a quantitative comparative analysis is carried out in order to assess the differences for a given data-base covering very different atmospheric conditions. The study reveals the necessity of a ‘standard spectral range’ to achieve confident data comparisons. We show some applications that are relevant for aerosol studies, from UV absorption by aerosols to satellite remote sensing.     

Comparison of modelled and measured ozone concentrations and meteorology for a site in south-west Sweden: implications for ozone uptake calculations

Measurements of ground-level ozone concentrations and meteorology (temperature, vapour pressure deficit (VPD), solar radiation) at the monitoring site Ostad (south-west Sweden) were compared to data from the corresponding grid in the EMEP photo-oxidant model for 1997, 1999 and 2000. The influence of synoptic weather on the agreement between model and measurements was studied. Implications of differences between modelled and observed inputs for ozone flux calculations for wheat and potato were investigated. The EMEP model output of ozone, temperature and VPD correlated well with measurements during daytime. Deviations were larger during the night, especially in calm conditions, attributed to local climatological conditions at the monitoring site deviating from average conditions of the grid. These differences did not lead to significant differences in calculated ozone uptake, which was reproduced remarkably well. The uptake calculations were sensitive to errors in the ozone and temperature input data, especially when including a flux threshold.     

Attenuation of UV-B radiation in the atmosphere: Clouds effect,at Qena (Egypt)

The effects of clouds (amount, type and height) on the surface UV-B radiation have been investigated at Qena, Egypt (26°17′, 32°10′, 96 m asl) using 2 years data (2004–2005) carried out by South Valley University (SVU)-meteorological research station. Thus, the characteristics of cloud's statistical property during the study period were employed to evaluate the general feature of the region of this study. However, ≈86% of all the observations were ⩽2 octas and the overcast conditions (8 octas) were very rarely over the study region (only 0.2% of all cases). These observations included 10% low-level clouds, 3.16% mid-level clouds and 7.59% high-level clouds. The dominated types of these clouds are stratocumulus (8.9%) and cirrus (5.8%).The hourly values of cloudless sky UV-B radiation (UV-B₀) and consequently the cloud modification factor (CMF) were estimated. An empirical model was developed for CMF as a function of the amount of cloud at low- and mid-level and high-level clouds. The correlation coefficients were equal to 0.985 and 0.987, respectively. In addition, a general expression of the CMF for situations those are considered as the effect of different clouds was found. The efficiency of this model has been tested in combination with a cloudless sky empirical model using independent data set. For this purpose, the hourly values of UV-B at selected cloudless and cloudy days were estimated. A good agreement was observed between the measured and the predicted values of our model. The mean value of the correlation coefficients of these selected days was 0.98.In addition, the attenuation of UV-B radiation could be determined by considering low- and mid-level and high-level clouds. The reduction of UV-B radiation as a function of cloud amount was non-linear for the both cases. At cloud amount of 100%, UV-B radiation was reduced by 83% on average by the high-level clouds.     

Impact of clouds and aerosols on ozone production in Southeast Texas

A radiative transfer model and photochemical box model are used to examine the effects of clouds and aerosols on actinic flux and photolysis rates, and the impacts of changes in photolysis rates on ozone production and destruction rates in a polluted urban environment like Houston, Texas. During the TexAQS-II Radical and Aerosol Measurement Project the combined cloud and aerosol effects reduced j(NO₂) photolysis frequencies by nominally 17%, while aerosols reduced j(NO₂) by 3% on six clear sky days. Reductions in actinic flux due to attenuation by clouds and aerosols correspond to reduced net ozone formation rates with a nearly one-to-one relationship. The overall reduction in the net ozone production rate due to reductions in photolysis rates by clouds and aerosols was approximately 8 ppbv h^?1.     

Ozone uptake to the polar snowpack at Summit,Greenland

The uptake of atmospheric ozone to the polar, year-round snowpack on glacial ice was studied at Summit, Greenland during three experiments in 2003, 2004, and 2005. Ozone was measured at up to three depths in the snowpack, on the surface, and above the surface at three heights on a tower along with supporting meteorological parameters. Ozone in interstitial air decreased with depth, albeit ozone gradients showed a high variation depending on environmental conditions of solar radiation and wind speed. Under low irradiance levels, up to 90% of ozone was preserved up to 1 m depth in the snowpack. Ozone depletion rates increased significantly with the seasonal and diurnal cycle of solar irradiance, resulting in only 10% of ozone remaining in the snowpack following solar noon during summertime. Faster snowpack air exchange from wind pumping resulted in smaller above-surface-to-within snowpack ozone gradients. These data indicate that the uptake of ozone to polar snowpack is strongly dependent on solar irradiance and wind pumping. Ozone deposition fluxes to the polar snowpack are consequently expected to follow incoming solar radiation levels and to exhibit diurnal and seasonal cycles. The Summit observations are in stark contrast to recent findings in the seasonal, midlatitude snowpack [Bocquet, F., Helmig, D., Oltmans, S.J., 2007. Ozone in the mid-latitude snowpack at Niwot Ridge, Colorado. Arctic, Antarctic and Alpine Research, in press], where mostly light-independent ozone behavior was observed. These contrasting results imply different ozone chemistry and snowpack–atmosphere gas exchange in the snow-covered polar, glacial conditions compared to the temperate, mid-latitude environment.     

Shortwave radiative forcing efficiency of urban aerosols--a case study using ground based measurements

Aerosols reduce the surface reaching solar flux by scattering the incoming solar radiation out to space. Various model studies on climate change suggest that surface cooling induced by aerosol scattering is the largest source of uncertainty in predicting the future climate. In the present study measurements of aerosol optical depth (AOD) and its direct radiative forcing efficiency has been presented over a typical tropical urban environment namely Hyderabad during December, 2003. Measurements of AOD have been carried out using MICROTOPS-II sunphotometer, black carbon aerosol mass concentration using Aethalometer, total aerosol mass concentration using channel Quartz Crystal Microbalance (QCM) Impactor Particle analyser and direct normal solar irradiance using Multifilter Rotating Shadow Band Radiometer (MFRSR). Diurnal variation of AOD showed high values during afternoon hours. The fraction of BC estimated to be approximately 9% in the total aerosol mass concentration over the study area. Results of the study suggest -62.5 Wm(-2) reduction in the ground reaching shortwave flux for every 0.1 increase in aerosol optical depth. The results have been discussed in the paper.     

Long-Term Monitoring and Analysis of Hourly Solar UV Radiation in the 290-380 nm Range in the Middle Region of Saudi Arabia

ABSTRACT

The UV and global solar radiation on a horizontal surface at ground level in Riyadh City (latitude 24° 34' N, longitude 46° 43' E) have been measured and analyzed. Measurements of UV radiation (295-385 nm) were recorded every 10 min for five years (January 1983-December 1987). The maximum recorded hourly mean irradiance UV radiation was 28 W/m² and occurred in July, while the minimum was 14 W/m² in December.     

Evaluation of nitrogen dioxide photolysis rates in an urban area using data from the 1997 Southern California Ozone Study

The photolysis of nitrogen dioxide and formaldehyde are two of the most influential reactions in the formation of photochemical air pollution, and their rates are computed using actinic flux determined from a radiative transfer model. In this study, we compare predicted and measured nitrogen dioxide photolysis rate coefficients (j_NO2). We used the Tropospheric Ultraviolet-Visible (TUV) radiation transfer model to predict j_NO2 values corresponding to measurements performed in Riverside, California as part of the 1997 Southern California Ozone Study (SCOS’97). Spectrally resolved irradiance measured at the same site allowed us to determine atmospheric optical properties, such as aerosol optical depth and total ozone column, that are needed as inputs for the radiative transfer model. Matching measurements of aerosol optical depth, ozone column, and j_NO2 were obtained for 14 days during SCOS’97. By using collocated measurements of the light extinction caused by aerosols and ozone over the full height of the atmosphere as model input, it was possible to predict sudden changes in j_NO2 resulting from atmospheric variability. While the diurnal profile of the rate coefficient was readily reproduced, j_NO2 model predicted values were found to be consistently higher than measured values. The bias between measured and predicted values was 17–36%, depending on the assumed single scattering albedo. By statistical analysis, we restricted the most likely values of the single scattering albedo to a range that produced bias on the order of 20–25%. It is likely that measurement error is responsible for a significant part of the bias. The aerosol single scattering albedo was found to be a major source of uncertainty in radiative transfer model predictions. Our best estimate indicates its average value at UV-wavelengths for the period of interest is between 0.77 and 0.85.     

Analysis of the number of flux chamber samples and study area size on the accuracy of emission rate measurements

Monte Carlo simulations were conducted on a set of flux chamber measurements at a landfill to estimate the relationship between the number of flux chamber samples and study area size on the emission rate measurement accuracy. The spatial variability of flux was addressed in the study by utilizing an existing flux chamber measurement data set that is one of the most dense flux chamber sampling arrays published to date for a landfill. At a probability of 95%, the Monte Carlo simulations indicated that achieving an accuracy within 10% with the flux chamber method is highly unlikely. An accuracy within 20% was achieved for small areas of less than about 0.2 hectares using 220 flux chamber measurements, but achieving this level of accuracy for area emission sources, of similar or greater variability, that are larger than this is highly unlikely. An accuracy within 30% was achieved up to the Full Area of about 0.4 hectares if more than approximately 120 samples were obtained. Even for an accuracy within 50%, at least 40 flux chamber measurements were needed for the Full Area of about 0.4 hectares. Available methods of estimating the number of samples required were compared to the Monte Carlo simulation results. The Monte Carlo simulations indicate that, in general, more samples are required than determined from an existing statistical method, which is a function of the mean and standard deviation of the population. Specifying the number of samples based on a regulatory method results in very poor accuracy. A modification to the statistical method for estimating the number of samples, or for estimating an accuracy for a given probability and number of samples, is proposed.

Implications: The flux chamber method is the most widely used method of measuring fugitive emission rates from area sources. However, extrapolation of a set of individual flux chamber samples to a larger area results in area flux measurement values of unknown accuracy. Quantification of the accuracy of the extrapolation of a set of flux chamber measurements would be beneficial for understanding the confidence that can be placed on the measurement results. Guidance as to the appropriate number of flux chamber measurements to achieve a desired level of accuracy would benefit flux chamber method practitioners.     

Effects of ultraviolet-B radiation (UV-B) on growth and physiology of the dune grassland species Calamagrostis epigeios

Seedlings of Calamagrostis epigeios were exposed to four levels of UV-B radiation (280-320 nm), simulating up to 44% reduction of stratospheric ozone concentration during summertime in The Netherlands, to determine the response of this plant species to UV-B irradiation. After six weeks of UV-B treatment, total biomass of all UV-B treated plants was higher, compared to plants that had received no UV-B radiation. The increase of biomass did not appear to be the result of a stimulation of net photosynthesis. Also, transpiration rate and water use efficiency were not altered by UV-B at any exposure level. Pigment analysis of leaf extracts showed no effect of enhanced UV-B radiation on chlorophyll content and accumulation of UV absorbing pigments. UV-B irradiance, however, did reduce the transmittance of visible light (400-700 nm) of intact attached leaves, suggesting a change in anatomical characteristics of the leaves. Additionally, the importance of including an ambient UV-B treatment in indoor experiments is discussed.     

Radiation modeling of a photo-reactor using a backward ray-tracing method: an insight into indoor photocatalytic oxidation

This article aims to understand the radiation behavior within a photo-reactor, following the ISO 22197-1:2007 standard. The RADIANCE lighting simulation tool, based on the backward ray-tracing modeling method, is employed for a numerical computation of the radiation field. The reflection of the glass cover in the photo-reactor and the test sample influence the amount of irradiance received by the test-sample surface in the photo-reactor setup. The reflection of a white sample limits the irradiance reduction by the glass cover to 1.4 %, but darker samples can lead to an overestimation up to 9.8 % when used in the same setup. This overestimation could introduce considerable error into the interpretation of experiments. Furthermore, this method demonstrates that the kinetics for indoor photocatalytic pollutant degradation can be refined through radiation modeling of the reactor setup. In addition, RADIANCE may aid in future modeling of the more complex indoor environment where radiation affects significantly photocatalytic activity.     

Volatile organic compounds from Italian vegetation and their interaction with ozone

Volatile Organic Compounds (VOCs) emitted from vegetation (particularly isoprenoids) represent an important source of atmospheric hydrocarbons almost double the anthropogenic source. When biogenic VOC mix with NO_x in the presence of UV radiation, ozone (O₃) is formed. In Italy, optimal conditions for O₃ formation in terms of VOC/NO_x ratios and abundance of UV radiation occur for long periods of the year. Moreover, Italian vegetation includes several species that are strong and evergreen isoprenoid emitters, and high temperatures for part of the year further stimulate these temperature-dependent emissions. We review emission of isoprenoids from Italian vegetation, current knowledge on the impact of rising O₃ levels on isoprenoid emission, and evidence showing that isoprenoids can increase both the O₃ flux to the plant and protection against oxidative stress because of their antioxidant functions. This trait not only influences plant tolerance to O₃ but also may substantially alter the flux of O₃ between atmosphere and biosphere.     

Intercalibration of a passive wind-vane flux sampler against a continuous-flow denuder for the measurements of atmospheric ammonia concentrations and surface exchange fluxes

A passive wind-vane flux sampler is a simple low-cost device used to estimate long-term vertical fluxes of ammonia in the atmospheric surface boundary layer. The passive flux sampler measures the horizontal flux of ammonia. A vertical gradient of the horizontal flux, combined with micro-meteorological measurements of wind speed and temperature, is used to estimated the vertical flux of ammonia using a modified aerodynamic gradient technique. The passive wind-vane flux sampler gradient was calibrated against a gradient measured with fast response (6 min) continuous-flow denuders. The measurements were carried out at a heathland located in an intensive farming area in the centre of the Netherlands. A field campaign took place over 70 day period in the summer of 1996, during which the sampling periods of the passive wind-vane flux sampler varied between 3 and 9 days. The comparison clearly showed that the long-term measurements with the passive wind-vane flux samplers gave accurate average ammonia deposition values for the field campaign as a whole which deviated by only 18% from the reference flux. However, there was no significant correlation between the fluxes from the passive samplers and the reference method for the individual 10 periods which were compared. Possible explanations found for the lacking correlation were (I) a high percentage number of half-hour emission events within each period resulted in a significant large relative deviation between the fluxes, and (II) uncertainties in the reference method might also explain the lacking correlation. The passive wind-vane flux samplers proved to be a stable method for long-term measurements (months to years) due to a close to 100% optimal functioning during the field campaign.     

The Rossby Centre Regional Atmospheric Climate Model part II: application to the Arctic climate

The Rossby Centre regional climate model (RCA2) has been integrated over the Arctic Ocean as part of the international ARCMIP project. Results have been compared to observations derived from the SHEBA data set. The standard RCA2 model overpredicts cloud cover and downwelling longwave radiation, during the Arctic winter. This error was improved by introducing a new cloud parameterization, which significantly improves the annual cycle of cloud cover. Compensating biases between clear sky downwelling longwave radiation and longwave radiation emitted from cloud base were identified. Modifications have been introduced to the model radiation scheme that more accurately treat solar radiation interaction with ice crystals. This leads to a more realistic representation of cloud-solar radiation interaction. The clear sky portion of the model radiation code transmits too much solar radiation through the atmosphere, producing a positive bias at the top of the frequent boundary layer clouds. A realistic treatment of the temporally evolving albedo, of both sea-ice and snow, appears crucial for an accurate simulation of the net surface energy budget. Likewise, inclusion of a prognostic snow-surface temperature seems necessary, to accurately simulate near-surface thermodynamic processes in the Arctic.     

Effect of UV radiation and temperature on mineralization and volatilization of coumaphos in water

This study was undertaken to determine the dissipation and degradation of coumaphos [O-(3-chloro-4-methyl-2-oxo-2H-1-benzopyran-7-yl) O,O-diethyl phosphorothioate] under different sunlight conditions and at different temperatures. The effect of the ultra violet (UV) component of solar radiation was also studied using quartz tubes in addition to other radiation in the visible range using glass tubes and the results were compared with those obtained under the dark light conditions. Water suspensions of coumaphos were incubated at three temperatures viz. 22 degrees C, 37 degrees C and 53 degrees C in closed systems to study the effect of temperature. Volatilization, mineralization and degradation of coumaphos increased with an increase in temperature and exposure to solar radiation, particularly under the UV component of the solar radiation. Major loss of the pesticide occurred through volatilization. The optimum temperature for the degradation of coumaphos was found to be at 37 degrees C. The data obtained from the mineralization and degradation studies indicated that 53 degrees C crosses the biological range for suitable growth of microorganism. UV radiation exposure along with maintaining temperature at 37 degrees C may prove useful in the dissipation and/or degradation of coumaphos prior to its disposal as waste from cattle dipping vats.