Actinic Flux Measurements Research Articles

Quantitative evidence for organic peroxy radical photochemistry at 254 nm.

Quantitative evidence is presented for the importance of alkyl peroxy photochemistry in the formation of secondary organic aerosol at 254 nm. Particles were generated by extensively oxidizing dodecanoic acid with photolytically generated hydroxyl radicals in a flow cell. The resulting particles were collected and analyzed for composition, which shows a lower contribution from multiply substituted parent molecules and much more decomposition product than expected from typical low-NOx oxidation mechanisms. Studies were performed at two separate reaction times, and kinetics modeling calculations were done using theoretical work from the combustion literature to estimate the branching of the photoexcited products. Extrapolation of the ethyl peroxy radical absorption spectrum compared to actinic flux measurements also shows that the alkyl peroxy radical absorption at ∼310 nm leads to photochemical lifetimes under pristine tropospheric conditions that are comparable to predicted lifetimes from peroxy-peroxy recombination reactions, particularly at higher altitudes.

Large aerosol effects on ozone photolysis in the Mediterranean

Measurements of spectral actinic flux were performed at the ENEA Station for Climate Observations on the island of Lampedusa, in the central Mediterranean, during several periods in 2003, 2004, 2006 and 2008 with a METCON diode array spectrometer. The ozone photolysis frequency, J(O1D), was calculated from the spectral actinic fluxes. Actinic fluxes determinations were complemented with measurements of total ozone and aerosol optical depth, made with a Brewer spectrophotometer and a multi-filter rotating shadowband radiometer, respectively. As expected, J(O1D) shows a strong dependency on total ozone. The behaviour of J(O1D) was examined at fixed values of total ozone and solar zenith angle; a significant dependency on the aerosol optical depth emerges. The dependency on the aerosol optical depth at 416 nm appears to be linear, and weakly dependent on the total ozone value. The photolysis frequency amplification factor for aerosol is between 0.09 and 0.17 at 30°, and between 0.12 and 0.22 at 60° solar zenith angle, depending on total ozone. Day-to-day changes in AOD produce larger effects on J(O1D) than day-to-day changes in total ozone in summer, and comparable effects in spring. Ozone changes are dominant in autumn and winter. To our knowledge, this is the first attempt to characterize the influence of aerosol optical depth on J(O1D) from a relatively large dataset.

Heating rates and surface dimming due to black carbon aerosol absorption associated with a major U.S. city

In situ airborne measurements of actinic flux and the mass and mixing state of individual refractory black carbon (BC) particles were made in the lower troposphere around a major U.S. metropolitan area. These data were used to estimate atmospheric heating rates and surface dimming due to absorption of solar radiation by refractory BC. Direct absorption by ozone is also calculated and averages 10% of that of BC below 4 km but becomes dominant above this altitude. Variability in BC absorption is large due to source variability and clouds. Average BC heating rates are much smaller than those associated with BC and other absorbing aerosol components in global hotspot regions. These results, which are specific to refractory BC absorption, will help determine the value of short‐term climate‐change mitigation strategies involving BC emission reductions. They may also be used to estimate refractory BC contributions to total aerosol absorption.

Estimation of photolysis frequencies from TOMS satellite measurements and routine meteorological observations

Abstract. A study on the estimation of J(O1D) and J(NO2) photolysis frequencies when limited ground based measurements (or even no measurements at all), are available is presented in this work. Photolysis frequencies can be directly measured by chemical actinometry and filter radiometry or can be calculated from actinic flux measurements. In several meteorological stations, none of the methods above are applicable due to the absence of sophisticated instruments such as actinometers, radiometers or spectroradiometers. In this case, it is possible to calculate photolysis frequencies with reasonable uncertainty using either a) standard meteorological observations, such as ozone, cloud coverage and horizontal visibility, available in various ground based stations, as input for a radiative transfer model or b) satellite observations of solar global irradiance available worldwide, in combination with an empirical method for the conversion of irradiance in photolysis frequencies. Both methods can provide photolysis frequencies with a standard deviation between 20% and 30%. The absolute level of agreement of the retrieved frequencies to those calculated from actual actinic flux measurements, for data from all meteorological conditions, is within ±5% for J(O1D) and less than 1% for J(NO2) for the first method, while for the second method it rises up to 25% for the case of J(O1D) and 12% for J(NO2), reflecting the overestimation of TOMS satellite irradiance when compared to ground based measurements of irradiance for the respective spectral regions. Due to the universality of the methods they can be practically applied to almost any station, thus overcoming problems concerning the availability of instruments measuring photolysis frequencies.

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 up to 25% smaller. Total ozone values derived from Total Ozone Mapping Spectrometer (TOMS), Dobson measurements by NIWA, spectral direct sun measurements by ATI, and spectral global irradiance measurements by NIWA agreed generally within 2%–3%.

Effects of total solar eclipse of 29 March 2006 on surface radiation

Abstract. Solar irradiance spectral measurements were performed during a total solar eclipse. The spectral effect of the limb darkening to the global, direct irradiance and actinic flux measurements was investigated. This effect leads to wavelength dependent changes in the measured solar spectra showing a much more pronounced decrease in the radiation at the lower wavelengths. Radiative transfer model results were used for the computation of a correction for the total ozone measurements due to the limb darkening. This correction was found too small to explain the large decrease in total ozone column derived from the standard Brewer measurements, which is an artifact in the measured irradiance due to the increasing contribution of diffuse radiation against the decreasing direct irradiance caused by the eclipse. Calculations of the Extraterrestrial spectrum and the effective sun's temperatures, as measured from ground based direct irradiance measurements, showed an artificial change in the calculations of both quantities due to the fact that radiation coming from the visible part of the sun during the eclipse phases differs from the black body radiation described by the Planck's law.

Algorithm for the charge-coupled-device scanning actinic flux spectroradiometer ozone retrieval in support of the Aura satellite validation

Stratospheric ozone column data was acquired during four recent aircraft-based validation missions for the Aura satellite flown in years 2004-2006. The data was retrieved by the spectrally-resolved actinic flux measurements of the charge-coupled-device scanning actinic flux spectroradiometer (CAFS) instrument carried on board the NASA WB-57 and DC-8 aircrafts. Each dataset contains information on temporal and spatial variability in the stratospheric ozone column. Analyses of the CAFS datasets provide guidance for assimilation of data from individual satellite orbits into the global maps of stratospheric ozone. Moreover, the 10-second samplings of the CAFS data supply information on spatial variability of stratospheric ozone column across the footprint of a satellite measurement. The CAFS data is available as a function of altitude and geo location of the aircraft. This paper describes the algorithm for the retrieval of an ozone column above the aircraft level, along with validation of the CAFS retrieved ozone product. A discussion of the retrieval uncertainty is provided with emphasis on the algorithm's assumptions and instrumental uncertainties. Sensitivity of the ozone retrieval to fundamental atmospheric parameters is discussed in detail, and the range of uncertainties is estimated under a variety of observational conditions. The characteristic model uncertainty of the CAFS partial ozone column retrieval is better than 3 %, whereas the CAFS measurement precision contributes less than 1 % to the retrieval uncertainty.

A comparison of photolysis rate parameters estimated from measured and simulated actinic flux for wintertime conditions at Storm Peak Laboratory, Colorado

Photolysis rate parameters depend upon solar actinic flux and chemical species dependent quantum yields and cross sections. Spectrally resolved measurements of actinic flux should be preferred over flux derived from models for the analysis of field observations. Actinic flux can be difficult to derive from the irradiance measurements of flat-plate radiometers. It is also difficult to estimate from models due to uncertainties in the ozone column, aerosol concentrations and distributions, cloud cover, optical depth and surface albedo. A series of actinic flux measurements were performed at Storm Peak Laboratory (3,210 m above sea level), Colorado, United States with spectroradiometers during the wintertime (January 07–10, 2004). The site is relatively remote with a clean atmosphere and during the wintertime the ground is generally covered by fresh snow with a high albedo. The actinic flux measurements were used to estimate the photolysis rate parameters of ozone, nitrogen dioxide and formaldehyde. The measured actinic flux and the photolysis rate parameters derived from the flux were compared to calculations using the Tropospheric Ultraviolet-Visible Model (TUV), version 4.2 (Madronich and Flocke, 1998). The TUV modeled actinic flux, the measured flux and the photolysis rate parameters derived from them had similar temporal patterns. However there were significant differences in their magnitude due to uncertainties in the data available to initialize the TUV model and the calibration of the spectroradiometer.

Light penetration in the snowpack at Summit, Greenland: Part 1: Nitrite and hydrogen peroxide photolysis

Photochemical rate constants ( j values) are crucial indicators for evaluating the importance of photochemical reactions in environmental systems. While measurement of aqueous j values via chemical actinometry is relatively straightforward under most conditions, problems arise with ambient conditions below freezing, such as at very high latitudes or altitudes. To address this problem, we have developed a new method for low temperature actinometry using solutions of acetonitrile (ACN) and water, which have freezing points down to - 44 ∘ C . In this method we measure the formation of phenol from the photolysis of an OH • -generating chromophore in the presence of benzene. Using results from laboratory tests we correct our phenol field results in ACN / H 2 O to rate constants for chromophore photolysis expected for water–ice (i.e., in the quasi-liquid layer of snow grains) under the same conditions. In part 1 of this study, we use this method at Summit, Greenland on the surface snow and to depths of ∼ 30 cm using hydrogen peroxide (HOOH) and nitrite ( NO 2 - ) as the chromophores. While the method works well for determining the rate constant for HOOH photolysis ( j(HOOH)), we encountered problems using the technique with nitrite. However, measured PhOH formation rate constants for nitrite in acetonitrile, j NO 2 - → PhOH ACN , still provide an excellent means for calculating snowpack e -folding depths for NO 2 - photolysis (i.e., the depth over which the rate constant decreases by a factor of e ). Values of j(HOOH) and j ( NO 2 - ) determined from measurements of actinic flux (above the snow) and irradiance (in snow) suggest that the value of j(HOOH) on the surface snow at midday was 8.6 × 10 - 7 s - 1 in mid-March and increased by 300% by the start of May, while j NO 2 - → PhOH ACN midday surface values were consistently ( 1 – 3 ) × 10 - 7 s - 1 throughout the season. Within the snowpack, average e -folding depths determined from chemical actinometry were 13.3 ( ± 0.88 ) cm for j(HOOH) and 16.3 ( ± 4.2 ) cm for j NO 2 - → PhOH ACN ; e -folding depths determined from in-snow spectral radiometer measurements of irradiance were similar. The larger e -folding depth for nitrite is because this chromophore absorbs at longer wavelengths where there is less light extinction in the snow.

Solar spectral actinic flux and photolysis frequency measurements in a deciduous forest

Within the Emission and Chemical Transformation of Biogenic Volatile Organic Compounds (ECHO) project the photochemistry of biogenic organic compounds in a forest was investigated. In this context, two spectroradiometers were used to determine solar spectral actinic flux and photolysis frequencies within and above a deciduous forest at Jülich, Germany. Locations in the forest and in a small clearing were examined 2 m above ground. Under overcast conditions, photolysis frequencies were within 1–3% and 6–8% of above canopy values at forest and clearing locations, respectively. In a spectral range below 500 nm the canopy was found to act as a gray filter that diminished spectral actinic flux independent of daytime and wavelength. Under clear‐sky conditions, diurnal variations within the forest also followed that of diffuse sky radiation above the forest except for sharp peaks from direct sunlight that was incident sporadically at the selected locations. The results under all conditions were in accord with the foliage being opaque toward UV and gaps in the canopy being responsible for the remaining actinic flux. Sky photographs showed that the distribution and size of gaps in the foliage were extremely heterogeneous with patterns changing rapidly upon changing location. Regarding daytime chemical reactions of biogenic compounds within the forest, it was estimated that degradation by OH was reduced by a factor similar to the photolysis frequencies, while potential degradation by NO3 was increased by a factor of ∼3 compared with above forest conditions. As a result, it was estimated that minor fractions of 0.8% of isoprene and 3.4% of monoterpenes were degraded by reactions with OH, NO3, and O3 prior to transport into the layer above the forest.

NO<sub>2</sub> and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece

Abstract. An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of actinic flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [J(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral actinic flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 12% (2σ) for J(NO2) and 6% (2σ) for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 12% (2σ) for the method that can be used with double Brewers and 20% for the method that can be used for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of actinic flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from actinic flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.

Actinometric measurements of NO<sub>2</sub> photolysis frequencies in the atmosphere simulation chamber SAPHIR

Abstract. The simulation chamber SAPHIR at Forschungszentrum Jülich has UV permeable teflon walls facilitating atmospheric photochemistry studies under the influence of natural sunlight. Because the internal radiation field is strongly affected by construction elements, we use external, radiometric measurements of spectral actinic flux and a model to calculate mean photolysis frequencies for the chamber volume Bohn04B. In this work we determine NO2 photolysis frequencies j(NO2) within SAPHIR using chemical actinometry by injecting NO2 and observing the chemical composition during illumination under various external conditions. In addition to a photo-stationary approach, a time-dependent method was developed to analyse the data. These measurements had two purposes. Firstly, to check the model predictions with respect to diurnal and seasonal variations in the presence of direct sunlight and secondly to obtain an absolute calibration factor for the combined radiometry-model approach. We obtain a linear correlation between calculated and actinometric j(NO2). A calibration factor of 1.34±0.10 is determined, independent of conditions in good approximation. This factor is in line with expectations and can be rationalised by internal reflections within the chamber. Taking into account the uncertainty of the actinometric j(NO2), an accuracy of 13% is estimated for the determination of j(NO2) in SAPHIR. In separate dark experiments a rate constant of (1.93±0.12)x10-14 cm3 s-1 was determined for the NO+O3 reaction at 298K using analytical and numerical methods of data analysis.

Model-aided radiometric determination of photolysis frequencies in a sunlit atmosphere simulation chamber

Abstract. In this work diurnal and seasonal variations of mean photolysis frequencies for the atmosphere simulation chamber SAPHIR at Forschungszentrum Jülich are calculated. SAPHIR has a complex construction with UV permeable teflon walls allowing natural sunlight to enter the reactor volume. The calculations are based on external measurements of solar spectral actinic flux and a model considering the time-dependent impact of shadows from construction elements as well as the influence of the teflon walls. Overcast and clear-sky conditions are treated in a consistent way and different assumptions concerning diffuse sky radiance distributions are tested. Radiometric measurements inside the chamber are used for an inspection of model predictions. Under overcast conditions we obtain fractions of 0.74 and 0.67 of external values for photolysis frequencies j(NO2) (NO2+hν→NO+O(3P)) and j(O1D) (O3+hν→O2+O(1D)), respectively. On a clear sky summer day these values are time-dependent within ranges 0.65-0.86 and 0.60-0.73, for j(NO2) and j(O1D), respectively. A succeeding paper (Bohn et al., 2004) is dealing with an on-road test of the model approach by comparison with photolysis frequencies from chemical actinometry experiments within SAPHIR.

Actinic flux and O<sup>1</sup>D photolysis frequencies retrieved from spectral measurements of irradiance at Thessaloniki, Greece

Abstract. The results of two methods retrieving actinic flux and ozone photolysis frequencies (JO1D), from measurements of irradiance with a Brewer MKIII spectroradiometer are investigated in this paper. The first method uses actinic flux retrieved from irradiance measurements by the use of known formulas while the second is an empirical method converting irradiance to JO1D through polynomials extracted from a study of synchronous actinic flux and irradiance measurements. When examining the actinic fluxes derived from the first method to those measured by an actinic flux spectrometer data agree within ±10% for solar zenith angles lower than 75° for the UV-B and the UV-A wavelength band. Also, the actinic to global irradiance ratio derived, deviates within ±6% for solar zenith angles lower than 70° compared with cloudless sky calculations of the TUV model. For both cases the deviations are in the order of the magnitude of the measurement or model uncertainties. Values of JO1D calculated by the second method show a mean ratio of 0.99±0.10 (1σ) and 0.98±0.06 for all data and for cloudless skies respectively when compared with values of JO1D derived by a Bentham actinic flux spectroradiometer. Finally, the agreement of the two methods is within ±5% comparing two years' data of JO1D retrieved from irradiance measurements at Thessaloniki, Greece. The use of such methods on extensive data sets of global irradiance can provide JO1D values with acceptable uncertainty, a parameter of particular importance for chemical process studies.

Measurement of actinic flux and the calculation of photolysis rate parameters for the Central California Ozone Study

The field measurement program of the Central California Ozone Study (CCOS) was conducted during the summer of 2000 with an overall goal of improving the understanding of ozone formation over central and northern California. Measurements of actinic flux were made as part of the study using spectroradiometers located at University of California, Davis; Sunol, California, and the Desert Research Institute in Reno, Nevada. The measured actinic flux was compared with the standard Peterson flux for a high ozone episode that occurred at the end of July. The standard Peterson flux was found to be greater than the measured flux across the spectral range. The measured actinic flux was used, along with standard quantum yields and absorption cross-section data to calculate the photolysis rate parameters for nitrogen dioxide, ozone and formaldehyde, and a radiative transfer model was used to simulate these same photolysis rate parameters. The simulated photolysis rate parameters for nitrogen dioxide could be up to 56% greater than the photolysis rate parameters derived from the measured actinic flux; for ozone the difference was as great as 160%; for the molecular reaction of formaldehyde the difference was as great as 89%; and for the radical producing channel the difference was as great as 126%. A simple atmospheric chemistry box-model was used to estimate the significance of these differences for air quality assessment. For a typical high ozone episode day 31 July, differences between modeled ozone concentrations based on photolysis rate parameters versus those derived from measured actinic flux were less than 7% change in concentration, while differences in HO concentrations were as high as 15%.

International Photolysis Frequency Measurement and Model Intercomparison (IPMMI): Spectral actinic solar flux measurements and modeling

The International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) took place in Boulder, Colorado, from 15 to 19 June 1998, aiming to investigate the level of accuracy of photolysis frequency and spectral downwelling actinic flux measurements and to explore the ability of radiative transfer models to reproduce the measurements. During this period, 2 days were selected to compare model calculations with measurements, one cloud‐free and one cloudy. A series of ancillary measurements were also performed and provided parameters required as input to the models. Both measurements and modeling were blind, in the sense that no exchanges of data or calculations were allowed among the participants, and the results were objectively analyzed and compared by two independent referees. The objective of this paper is, first, to present the results of comparisons made between measured and modeled downwelling actinic flux and irradiance spectra and, second, to investigate the reasons for which some of the models or measurements deviate from the others. For clear skies the relative agreement between the 16 models depends strongly on solar zenith angle (SZA) and wavelength as well as on the input parameters used, like the extraterrestrial (ET) solar flux and the absorption cross sections. The majority of the models (11) agreed to within about ±6% for solar zenith angles smaller than ∼60°. The agreement among the measured spectra depends on the optical characteristics of the instruments (e.g., slit function, stray light rejection, and sensitivity). After transforming the measurements to a common spectral resolution, two of the three participating spectroradiometers agree to within ∼10% for wavelengths longer than 310 nm and at all solar zenith angles, while their differences increase when moving to shorter wavelengths. Most models agree well with the measurements (both downwelling actinic flux and global irradiance), especially at local noon, where the agreement is within a few percent. A few models exhibit significant deviations with respect either to wavelength or to solar zenith angle. Models that use the Atmospheric Laboratory for Applications and Science 3 (ATLAS‐3) solar flux agree better with the measured spectra, suggesting that ATLAS‐3 is probably more appropriate for radiative transfer modeling in the ultraviolet.

Solar actinic radiation (280–420 nm) in the cloud‐free troposphere between ground and 12 km altitude: Measurements and model results

Airborne measurements of the spectrally resolved actinic flux (280–420 nm) between the ground and 12 km altitude have been made using a new calibrated dual‐channel spectroradiometer. The measurements were made as part of the Photochemical Activity and Ultraviolet Radiation/Altitude Dependence of the Tropospheric Ozone Photolysis (PAUR/ATOP) measurement campaign in Greece during June 1996. Flights were made over the Agean Sea under cloudless conditions for various aerosol loads and solar zenith angles. The spectral actinic flux measurements are compared with radiative transfer model simulations based on the multistream discrete ordinate radiative transfer (DISORT) algorithm. All input to the radiative transfer model was provided by independent measurements performed simultaneously on the nearby island of Agios Efstratios. For altitudes between 3000–12,000 m the agreement between the measurements and the model simulations is within ±5% for wavelengths larger than 310 nm and within ±10% at 295–310 nm. For the lowest flight altitude, 108 m, the model underestimates the measured actinic flux systematically by about 12%. This may be partly explained by uncertainties in the aerosol optical properties and the surface albedo. Flights on days with small and large amounts of aerosols showed that under otherwise identical conditions the actinic flux increased by up to 10% when the aerosol amount was larger.

An empirical method for the conversion of spectral UV irradiance measurements to actinic flux data

Routinely monitored radiation parameters, including those at UV wavelengths, most commonly refer to irradiance, that is the radiation incident on a flat horizontal plate. However, in the study of atmospheric chemistry, where UV radiation is a prime photochemical driver, the target molecules are approximately spherical and the actinic flux (radiation on the surface of a sphere) is a better measure of the effective radiation. Unfortunately actinic flux measurements (also known as scalar irradiance) are uncommon research measurements. The ability to convert irradiance data to actinic flux data within a reasonable degree of uncertainty would provide an actinic flux database mapped from existing irradiance databases, thus vastly increasing knowledge of actinic flux variability and climatology. Synchronised spectral UV irradiance and actinic flux measurements have been made during the ADMIRA project, and used to develop an empirical method for converting irradiance to actinic flux. With some prior knowledge of the sky conditions during the irradiance measurements the actinic flux can be estimated to within a few percent. If no knowledge of the sky conditions is available then the empirical method still returns actinic fluxes within 10% of those measured at the site for which the conversion was developed.

Aerosol direct radiative impact over the INDOEX area based on passive and active remote sensing

This study is dedicated to the direct radiative impact assessment of the pollution aerosol particles during the Indian Ocean Experiment (INDOEX). We use here an instrumental synergy based on active and passive ground‐based (Goa University, India), airborne (Mystere 20 research aircraft), and spaceborne (Meteosat‐5) devices. An original method based on ground‐based active actinic flux measurements is developed to assess the atmospheric, columnar aerosol single scattering albedo. This parameter has been found between 0.88 and 0.93 (at 440 nm) with an absolute uncertainty of 0.04 during the operating period from 11 to 23 March 1999. We have also assessed this parameter off the West Indian coast by comparing simultaneous airborne lidar and flux measurements with radiative transfer simulations. The value retrieved for the atmospheric column is close to 0.83 ± 0.05 (at 523 nm) in agreement with the coastal value. The horizontal and vertical extent of the aerosol plume is investigated using airborne lidar and Meteosat‐5 satellite. Using the Meteosat‐5‐derived aerosol optical thickness, we have estimated the regional extent of the shortwave aerosol direct radiative forcing. The vertical profile of the aerosol extinction coefficient derived from lidar sounding is used to assess the atmospheric shortwave heating rate induced by the aerosol layer. For an aerosol optical thickness of 0.6 (±0.12), as it has been observed in the aerosol plume in late March, the top of atmosphere direct aerosol forcing is −17 (±5) W m−2. The surface forcing is between 2.5 and 4.5 higher than the top of atmosphere forcing. This difference leads to a significant heating of the three lowest kilometers of the atmosphere by the aerosols. For an aerosol optical thickness of 0.6, the atmospheric heating rate induced by aerosols is between 0.8 and 1.2 Kd−1 depending on the aerosol single scattering albedo value.

Surface and tethered‐balloon observations of actinic flux: Effects of arctic stratus, surface albedo, and solar zenith angle

As part of the First ISCCP Regional Experiment (FIRE III) Arctic Cloud Experiment actinic flux measurements were made above the Arctic Sea ice during May 1998. The actinic flux, which is also referred to as the 4π radiative flux, is the relevant radiative parameter needed to determine photodissociation rates. It is shown that for a plane‐parallel cloud the change in the net irradiance as a function of the optical depth is proportional to the magnitude of the actinic flux. Continuous actinic flux measurements were made just above the snow‐covered ice surface by a UV‐A and a visible 4π radiometer (wavelengths ∼365 and ∼550 nm, respectively). In addition, vertical profiles of the actinic flux through low arctic stratus clouds were obtained by means of a visible 4π radiometer suspended under a tethered balloon. The cloud thermodynamic and microphysical structure was determined from observations made by the National Center for Atmospheric Research C‐130 aircraft. In addition, the phase and liquid water path of the cloud was assessed from microwave radiometer, lidar, and radar data. During clear‐sky conditions the diurnal variation of the magnitude of actinic flux was controlled mainly by Rayleigh scattering and surface reflection. Above a stratus cloud layer the actinic flux was found to be almost the same as during clear‐sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. In the arctic stratus clouds the actinic flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the actinic flux increased significantly with height. The vertical profiles that were observed in arctic stratus differed from those measured in Atlantic stratocumulus; in the latter the actinic flux was found to increase gradually from cloud base to cloud top. A delta‐Eddington model is utilized to illustrate that the exact shape of the vertical profile is very sensitive to the solar zenith angle. During the arctic experiments the solar zenith angle was generally much larger than during the observations in Atlantic stratocumulus.