Antti Arola

Surface ultraviolet irradiance from OMI

The Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura spacecraft is a nadir-viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600-km-wide viewing swath, and it is capable of daily, global contiguous mapping. We developed and implemented a surface ultraviolet (UV) irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths (305, 310, 324, and 380 nm). Additionally, noontime erythemal dose rate and the erythemal daily dose are estimated. The OMI surface UV algorithm inherits from the surface UV algorithm developed by NASA Goddard Space Flight Center for the Total Ozone Mapping Spectrometer (TOMS). The OMI surface UV irradiance products are produced and archived in HDF5-EOS format by Finnish Meteorological Institute. The accuracy of the surface UV estimates depend on UV wavelength and atmospheric and other geolocation specific conditions ranging from 7% to 30%. A postprocessing aerosol correction can be applied at sites with additional ground-based measurements of the aerosol absorption optical thickness. The current OMI surface UV product validation plan is presented.

Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) – Part 2: Analysis of site aerosol influence

Several validation studies have shown a notable overestimation of the clear sky ultraviolet (UV) irradiance at the Earth’s surface derived from satellite sensors such as the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) with respect to groundbased UV data at many locations. Most of this positive bias is attributed to boundary layer aerosol absorption that is not accounted for in the TOMS/OMI operational UV algorithm. Therefore, the main objective of this study is to analyse the aerosol effect on the bias between OMI erythemal UV irradiance (UVER) and spectral UV (305 nm, 310 nm and 324 nm) surface irradiances and ground-based Brewer spectroradiometer measurements from October 2004 to December 2008 at El Arenosillo station (37.1 ◦ N, 6.7 W, 20 m a.s.l.), with meteorological conditions representative of the South-West of Spain. The effects of other factors as clouds, ozone and the solar elevation over this intercomparison were analysed in detail in a companion paper (Ant ón et al., 2010). In that paper the aerosol effects were studied making only a rough evaluation based on aerosol optical depth (AOD) information at 440 nm wavelength (visible range) without applying any correction. We have used the precise information given by single scattering albedo (SSA) from AERONET for the determination of Correspondence to: V. E. Cachorro (chiqui@goa.uva.es) absorbing aerosols which has allowed the correction of the OMI UV data. An aerosol correction expression was applied to the OMI operational UV data using two approaches to estimate the UV absorption aerosol optical depth, AAOD. The first approach was based on an assumption of constant SSA value of 0.91. This approach reduces the OMI UVER bias against the reference Brewer data from 13.4% to 8.4%. Second approach uses daily AERONET SSA values reducing the bias only to 11.6%. Therefore we have obtained a 37% and 12% of improvement respectively. For the spectral irradiance at 324 nm, the OMI bias is reduced from 10.5% to 6.98% for constant SSA and to 9.03% for variable SSA. Similar results were obtained for spectral irradiances at 305 nm, and 310 nm. Contrary to what was expected, the constant SSA approach has a greater bias reduction than variable SSA, but this is a reasonable result according to the discussion about the reliability of SSA values. Our results reflect the level of accuracy that may be reached at the present time in this type of comparison, which may be considered as satisfactory taking into account the remaining dependence on other factors. Nevertheless, improvements must be accomplished to determine reliable absorbing aerosol properties, which appear as a limiting factor for improving OMI retrievals. Published by Copernicus Publications on behalf of the European Geosciences Union. 11868 V. E. Cachorro et al.: Part 2: Analysis of site aerosol influence

Direct and indirect effects of sea spray geoengineering and the role of injected particle size

[1] Climate-aerosol model ECHAM5.5-HAM2 was used to investigate how geoengineering with artificial sea salt emissions would affect marine clouds and the Earth’s radiative balance. Prognostic cloud droplet number concentration and interaction of aerosol particles with clouds and radiation were calculated explicitly, thus making this the first time that aerosol direct effects of sea spray geoengineering are considered. When a wind speed dependent baseline geoengineering flux was applied over all oceans (total annual emissions 443.9 Tg), we predicted a radiative flux perturbation (RFP) of 5.1 W m , which is enough to counteract warming from doubled CO2 concentration. When the baseline flux was limited to three persistent stratocumulus regions (3.3% of Earth’s surface, total annual emissions 20.6 Tg), the RFP was 0.8 Wm 2 resulting mainly from a 74–80% increase in cloud droplet number concentration and a 2.5–4.4 percentage point increase in cloud cover. Multiplying the baseline mass flux by 5 or reducing the injected particle size from 250 to 100 nm had comparable effects on the geoengineering efficiency with RFPs 2.2 and 2.1 Wm , respectively. Within regions characterized with persistent stratocumulus decks, practically all of the radiative effect originated from aerosol indirect effects. However, when all oceanic regions were seeded, the direct effect with the baseline flux was globally about 29% of the total radiative effect. Together with previous studies, our results indicate that there are still large uncertainties associated with the sea spray geoengineering efficiency due to variations in e.g., background aerosol concentration, updraft velocity, cloud altitude and onset of precipitation.

Version 2 total ozone mapping spectrometer ultraviolet algorithm: problems and enhancements

Satellite instruments provide global maps of surface UV irradiance by combining backscattered radiance measurements with radiative transfer models. The accuracy of the models is limited by uncertainties in input parameters representing the atmosphere and the Earths surface. To reduce these uncertainties, we have made enhancements to the currently operational TOMS surface UV irradiance algorithm (Version 1) by including the effects of diurnal variations of cloudiness, an improved treatment of snow/ice, and a preliminary aerosol correction. We compare results of the version 1 TOMS UV algorithm and the proposed version. We evaluate different approaches for improved treatment for average cloud attenuation within a satellite pixel, with and without snow/ ice on the ground. In addition to treating cloud transmission based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate atmospheric UV irradiance transmission throughout the day. A new method is proposed to obtain a more realistic treatment of the effects from snow-covered terrain. The method is based on an empirical relation between UV reflectivity and measured snow depth. The new method reduces the bias between the TOMS UV estimations and ground-based UV measurements for snow periods. We also briefly discuss the complex problem of estimating surface UV radiation in presence of UV-absorbing aerosols. The improved (Version 2) algorithm can be applied to reprocess the existing TOMS UV irradiance and exposure estimates (since November 1978) and to future satellite sensors (e.g., GOME-2, OMI on EOS/Aura, and Triana/EPIC).

Observational signature of the direct radiative effect by natural boreal forest aerosols and its relation to the corresponding first indirect effect

By using a screened set of long-term aerosol measurement data, the contribution of natural boreal forest aerosols to the direct radiative effect (DRE) was observed at a remote continental site in n ...

Long-term measurements of cloud droplet concentrations and aerosol–cloud interactions in continental boundary layer clouds

The effects of aerosol on cloud droplet effective radius (R eff), cloud optical thickness and cloud droplet number concentration (N d) are analysed both from long-term direct ground-based in situ measurements conducted at the Puijo measurement station in Eastern Finland and from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Terra and Aqua satellites. The mean in situ N d during the period of study was 217 cm−3, while the MODIS-based N d was 171 cm−3. The absolute values, and the dependence of both N d observations on the measured aerosol number concentration in the accumulation mode (N acc), are quite similar. In both data sets N d is clearly dependent on N acc, for N acc values lower than approximately 450 cm−3. Also, the values of the aerosol–cloud-interaction parameter [ACI=(1/3)*d ln(N d)/d ln(N acc)] are quite similar for N acc<400 cm−3 with values of 0.16 and 0.14 from in situ and MODIS measurements, respectively. With higher N acc (>450 cm−3) N d increases only slowly. Similarly, the effect of aerosol on MODIS-retrieved R eff is visible only at low N acc values. In a sub set of data, the cloud and aerosol properties were measured simultaneously. For that data the comparison between MODIS-derived N d and directly measured N d, or the cloud droplet number concentration estimated from N acc values (N d,p), shows a correlation, which is greatly improved after careful screening using a ceilometer to make sure that only single cloud layers existed. This suggests that such determination of the number of cloud layers is very important when trying to match ground-based measurements to MODIS measurements.

Contribution of Brown Carbon to Direct Radiative Forcing over the Indo-Gangetic Plain

The Indo-Gangetic Plain is a region of known high aerosol loading with substantial amounts of carbonaceous aerosols from a variety of sources, often dominated by biomass burning. Although black carbon has been shown to play an important role in the absorption of solar energy and hence direct radiative forcing (DRF), little is known regarding the influence of light absorbing brown carbon (BrC) on the radiative balance in the region. With this in mind, a study was conducted for a one month period during the winter-spring season of 2013 in Kanpur, India that measured aerosol chemical and physical properties that were used to estimate the sources of carbonaceous aerosols, as well as parameters necessary to estimate direct forcing by aerosols and the contribution of BrC absorption to the atmospheric energy balance. Positive matrix factorization analyses, based on aerosol mass spectrometer measurements, resolved organic carbon into four factors including low-volatile oxygenated organic aerosols, semivolatile oxygenated organic aerosols, biomass burning, and hydrocarbon like organic aerosols. Three-wavelength absorption and scattering coefficient measurements from a Photo Acoustic Soot Spectrometer were used to estimate aerosol optical properties and estimate the relative contribution of BrC to atmospheric absorption. Mean ± standard deviation values of short-wave cloud free clear sky DRF exerted by total aerosols at the top of atmosphere, surface and within the atmospheric column are -6.1 ± 3.2, -31.6 ± 11, and 25.5 ± 10.2 W/m(2), respectively. During days dominated by biomass burning the absorption of solar energy by aerosols within the atmosphere increased by ∼35%, accompanied by a 25% increase in negative surface DRF. DRF at the top of atmosphere during biomass burning days decreased in negative magnitude by several W/m(2) due to enhanced atmospheric absorption by biomass aerosols, including BrC. The contribution of BrC to atmospheric absorption is estimated to range from on average 2.6 W/m(2) for typical ambient conditions to 3.6 W/m(2) during biomass burning days. This suggests that BrC accounts for 10-15% of the total aerosol absorption in the atmosphere, indicating that BrC likely plays an important role in surface and boundary temperature as well as climate.

Version 2 TOMS UV algorithm: problems and enhancements

We evaluate the effects of possible enhancements of the current (version 1) TOMS surface UV irradiance algorithm. The major enhancements include more detailed treatment of tropospheric aerosols, effects of diurnal variation of cloudiness and improved treatment of snow/ice. The emphasis is on the comparison between the results of the version 1 TOMS UV algorithm and each of the changes proposed. TOMS UV algorithm does not discriminate between nonabsorbing aerosols and clouds. Absorbing aerosols are corrected by using the TOMS aerosol index data. The treatment of aerosol attenuation might have been improved by using newly derived TOMS products: optical depths and the single-scattering albedo for dust, smoke, and sulfate aerosols. We evaluate different approaches for improved treatment of pixel average cloud attenuation, with and without snow/ice on the ground. In addition to treating clouds based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate attenuation of the UV irradiance throughout the day. The improved (version 2) algorithm will be applied to reprocess the existing TOMS UV data record (since 1978) and to the future satellite sensors (e.g., Quik/TOMS, GOME, OMI on EOS/Aura and Triana/EPIC).

Impacts of brown carbon from biomass burning on surface UV and ozone photochemistry in the Amazon Basin.

The spectral dependence of light absorption by atmospheric particulate matter has major implications for air quality and climate forcing, but remains uncertain especially in tropical areas with extensive biomass burning. In the September-October 2007 biomass-burning season in Santa Cruz, Bolivia, we studied light absorbing (chromophoric) organic or “brown” carbon (BrC) with surface and space-based remote sensing. We found that BrC has negligible absorption at visible wavelengths, but significant absorption and strong spectral dependence at UV wavelengths. Using the ground-based inversion of column effective imaginary refractive index in the range 305–368 nm, we quantified a strong spectral dependence of absorption by BrC in the UV and diminished ultraviolet B (UV-B) radiation reaching the surface. Reduced UV-B means less erythema, plant damage, and slower photolysis rates. We use a photochemical box model to show that relative to black carbon (BC) alone, the combined optical properties of BrC and BC slow the net rate of production of ozone by up to 18% and lead to reduced concentrations of radicals OH, HO2, and RO2 by up to 17%, 15%, and 14%, respectively. The optical properties of BrC aerosol change in subtle ways the generally adverse effects of smoke from biomass burning.

Use of the moving time-window technique to determine surface albedo from TOMS reflectivity data

The seasonal variation of the surface albedo, due to snow or ice, complicates satellite estimation of the high-latitude surface UV irradiance. The TOMS instrument, that measures the backscattered radiances from the Earths atmosphere and surface, does not distinguish cloud backscattering from surface backscattering. When the TOMS UV algorithm is used, false interpretation of the measured high reflectivity as thick cloudiness leads to substantial underestimation of the surface UV irradiance. While the largest UV irradiance is usually received during the summer, the spring exposure to UV radiation is the main concern in high-latitudes since the sensitivity of some biological organisms to UV radiation is more pronounced at low temperatures, and snowcover enhances the surface UV irradiance. This paper presents a new method for estimation of the surface reflectivity. The method is based on analysis of the TOMS Lambertian equivalent reflectivity data using the moving time-window technique. The new method treats the measured reflectivity data as samples from a distribution whose lower tail corresponds to surface albedo. The basic method assumes that the distribution is homogeneous, i.e. the surface albedo is constant within the window. Adequate statistics is achieved only by using a wide time-window which, unfortunately, leads to underestimation of the surface albedo during spring and autumn transitions. Therefore, the method was developed further to account for transitions. The feasibility of the new method has been studied globally for high-latitude regions, and it is expected to improve springtime UV irradiance estimates of polar regions.