Joris Pepijn Veefkind

Algorithm for NO/sub 2/ vertical column retrieval from the ozone monitoring instrument

We describe the operational algorithm for the retrieval of stratospheric, tropospheric, and total column densities of nitrogen dioxide (NO/sub 2/) from earthshine radiances measured by the Ozone Monitoring Instrument (OMI), aboard the EOS-Aura satellite. The algorithm uses the DOAS method for the retrieval of slant column NO/sub 2/ densities. Air mass factors (AMFs) calculated from a stratospheric NO/sub 2/ profile are used to make initial estimates of the vertical column density. Using data collected over a 24-h period, a smooth estimate of the global stratospheric field is constructed. Where the initial vertical column densities exceed the estimated stratospheric field, we infer the presence of tropospheric NO/sub 2/, and recalculate the vertical column density (VCD) using an AMF calculated from an assumed tropospheric NO/sub 2/ profile. The parameters that control the operational algorithm were selected with the aid of a set of data assembled from stratospheric and tropospheric chemical transport models. We apply the optimized algorithm to OMI data and present global maps of NO/sub 2/ VCDs for the first time.

Total ozone from the ozone monitoring instrument (OMI) using the DOAS technique

This paper describes the algorithm for deriving the total column ozone from spectral radiances and irradiances measured by the Ozone Monitoring Instrument (OMI) on the Earth Observing System Aura satellite. The algorithm is based on the differential optical absorption spectroscopy technique. The main characteristics of the algorithm as well as an error analysis are described. The algorithm has been successfully applied to the first available OMI data. First comparisons with ground-based instruments are very encouraging and clearly show the potential of the method.

Aerosol optical depth over Europe in August 1997 derived from ATSR‐2 data

Data from the Along Tract Scanning Radiometer 2 (ATSSR-2) on board the European ERS-2 satellite have been used to derive the spatial distribution of the aerosol optical depth (AOD) over Europe for August 1997. The AOD was retrieved in cloud free areas using the dual view algorithm. The results agree with co-located ground based sun-photometer data within 0.1. The AOD from ATSR-2 with a resolution of 1 x 1 km2 were averaged on a grid of 0.1° x 0.1°, to produce daily maps of the spatial aerosol distribution over Europe. A composite map of AOD over Europe was constructed by averaging all daily maps for August 1997. This composite map shows a large spatial AOD gradients with variations of a factor of 3 over a few hundreds of kilometers. The AOD at 0.555 μm for relatively clean areas is around 0.1 while in strong industrialised areas the AOD can be 0.5 or higher.

Simulation of the aerosol optical depth over Europe for August 1997 and a comparison with observations

A chemical transport model has been extended with an aerosol model describing processes which determine the mass distribution of sulfate, nitrate, ammonium, and aerosol associated water. A specific summer episode is simulated, and the results have been compared to surface concentration measurements and with the aerosol optical depth (AOD) retrieved from satellite measurements, with a focus on the European continent. This study is one of the first to use satellite retrievals over land for this purpose. An average difference in AOD between model and satellite measurements of 0.17-0.19 is calculated, and on average only 40-50% of the observed satellite signal can be explained by our modeled aerosol. In contrast, the observed patterns of optical thickness are well simulated by the model. Also, surface concentrations of simulated aerosol components are in close agreement with measurements. Errors in the vertical distribution of sulfate, ammonium, and nitrate, and hence in the vertical distribution of hygroscopic growth, and errors in modeled optical parameters may partly account for the observed differences in AOD. However, we argue that the most important reason for the large difference is due to the fact that organic and mineral aerosol are not taken into account in this model simulation. A sensitivity study with reduced SO2 emissions in Europe showed that reduction of the emissions of SO2 in the model leads to a better agreement with surface measurements of SO2; however, calculated sulfate was less strongly influenced. Copyright 2001 by the American Geophysical Union.

A new algorithm to determine the spectral aerosol optical depth from satellite radiometer measurements

A new aerosol retrieval algorithm is presented which computes the spectral optical depth over the ocean from spaceborne radiometers. It includes both multiple scattering and the bi-directional reflectance of the ocean surface. The previous termalgorithmnext term is applied to data from the Along Track Scanning Radiometer 2 (ATSR-2). This radiometer aboard the ERS-2 satellite has 4 bands in the visible and near-infrared. The ATSR-2 has a dual view capability: the reflectance is measured both at nadir and at a forward angle of approximately 55° along track. This feature is used to test the algorithm by comparing independent retrievals from the forward and the nadir view, applied to Southern Hemisphere data from 23 July 1995. The retrieved aerosol optical depths compare favorably. The retrieved aerosol optical depths and spectral behavior are in agreement with expected values in clean marine environments.

Impact of aerosols on the OMI tropospheric NO2 retrievals over industrialized regions: how accurate is the aerosol correction of cloud-free scenes via a simple cloud model? (discussion paper)

The Ozone Monitoring Instrument (OMI) instrument has provided daily global measurements of tropospheric NO2 for more than a decade. Numerous studies have drawn attention to the complexities related to measurements of tropospheric NO2 in the presence of aerosols. Fine particles affect the OMI spectral measurements and the length of the average light path followed by the photons. However, they are not explicitly taken into account in the current OMI tropospheric NO2 retrieval chain. Instead, the operational OMI O2-O2 cloud retrieval algorithm is applied both to cloudy scenes and to cloud free scenes with aerosols present. This paper describes in detail the complex interplay between the spectral effects of aerosols, the OMI O2-O2 cloud retrieval algorithm and the impact on the accuracy of the tropospheric NO2 retrievals through the computed Air Mass Factor (AMF) over cloud-free scenes. Collocated OMI NO2 and MODIS Aqua aerosol products are analysed over East China, in industrialized area. In addition, aerosol effects on the tropospheric NO2 AMF and the retrieval of OMI cloud parameters are simulated. Both the observation-based and the simulation-based approach demonstrate that the retrieved cloud fraction linearly increases with increasing Aerosol Optical Thickness (AOT), but the magnitude of this increase depends on the aerosol properties and surface albedo. This increase is induced by the additional scattering effects of aerosols which enhance the scene brightness. The decreasing effective cloud pressure with increasing AOT represents primarily the absorbing effects of aerosols. The study cases show that the actual aerosol correction based on the implemented OMI cloud model results in biases between ?20 and ?40 % for the DOMINO tropospheric NO2 product in cases of high aerosol pollution (AOT ? 0.6) and elevated particles. On the contrary, when aerosols are relatively close to the surface or mixed with NO2, aerosol correction based on the cloud model results in overestimation of the DOMINO tropospheric NO2 product, between 10 and 20 %. These numbers are in line with comparison studies between ground-based and OMI tropospheric NO2 measurements under conditions with high aerosol pollution and elevated particles. This highlights the need to implement an improved aerosol correction in the computation of tropospheric NO2 AMFs.

Aerosol optical depth over Europe: Satellite retrieval and modeling

Aerosol optical depth (AOD) and Angstrom coefficients over Europe retrieved from satellite data for August 1997 provide information on the spatial variations of these aerosol properties. The AOD results are compared with initial results from model calculations, showing the relative influences of sulphate and nitrate aerosol.

Satellite derived aerosol distributions over Europe

With the development of new satellite sensors with more spectral information and improved calibration, it is now also possible to retrieve over land surfaces. Results from two retrieval algorithms for use over land, different satellite instruments and different methods, are presented.