In-situ measurements It is necessary to make measurements of aerosols in order to determine their properties and their impact on the environment. The first aerosol measurements were ground-based in-situ measurements. This type of measurements gives very detailed and accurate information about the size distribution, the chemical composition, the optical properties, etc. They are performed by collecting aerosols, filtering them according to their size and analysing the collected representative sample. Sampled aerosols can be treated with many different methods and instruments to acquire their properties. But ground based in-situ measurements are sparse and limited in locations and time.
It is quite expensive to build an extensive network, demanding a continuous effort and funds in order to acquire data for extended periods of time.
In-situ measurements nowadays are not only limited to ground based measurements. The ambient aerosol properties can be obtained by instrument on balloons or aircrafts. These flights usually have a good horizontal and vertical coverage, but they are limited to short periods of time during extensive observation campaigns (e.g. Dulac and Chazette, 2003; Heese and Wiegner, 2008).
Remote sensing measurements From the ground, aerosols can be also mea-sured by remote-sensing methods. These measurements, obtained by instru-ments like photometers or lidars, can provide a variety of aerosol properties.
They can be performed many times per day, and give a detailed information about aerosols on the local scale.
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The remote-sensing methods show their full potential for aerosol mea-surements from space (Gras,2003). Satellites provide routine measurements on a global scale. The time resolution ranges from almost continuously to at worst once every 2 to 3 days. Observations done by a single instrument cover all continents and for at least several years ensure the uniformity in the observation technique. This allows us to compare aerosols on the planet for longer periods of time and to establish the aerosol climatology and long-term trends. Aerosol satellite observations have been continuously performed for more than 30 years. Satellites instruments can be nadir-viewing which measure column-integrated quantities, or limb-viewing which provide aerosol profile information and much longer path through the atmosphere (Kaufman and Tanré,2003). Nowadays, nadir-viewing instruments are a lot more common.
1.9.1 AOD satellite retrieval
Aerosol optical depth is the most frequently retrieved aerosol quantity from space. It was also the first one (Griggs, 1975). The radiation reflected and emitted by Earth also picks the effects of the surface and the atmosphere. The AOD retrieval is based on measuring the radiances at the top of the atmosphere and extracting the aerosol effect out of it. Aerosols can change the intensity of light, its polarization or angular properties (Kaufman and Tanré, 2003).
Radiances are measured by spectrometers or radiometers that can cover a wide range of the spectrum. The retrieval is usually done for the visible and near-infrared parts of the spectrum because in these ranges the scattering is the dominant process of interaction of radiation with matter in the atmosphere.
Still, for bigger aerosols particles the retrieval can be done in the infrared part of the spectrum.
Algorithms Aerosol properties can be retrieved only when there are no clouds in the instrument line of sight. When there are clouds in the field of view, their radiance dominates the signal that arrives to the instrument and the aerosol retrieval cannot be performed. For nadir-viewing instruments, besides clouds, the surface reflectance also has an effect on the signal that has to be removed.
To retrieve AOD over the ocean and over the land, it is usually necessary to use two different algorithms. Over the ocean, surface reflectance characteristics are relatively well known. But over the land, the surface reflectance can be very variable. It varies spatially and temporally: it depends on the part of day, seasons, surface properties and precipitation, etc. It is necessary to make assumptions on the characteristics of the surface, its reflectance and how it
changes for different wavelengths. If the instrument is capable to observe with different viewing angles or at short-wavelength infrared (SWIR) wavelengths where the aerosol effect is negligible, the surface reflectance can be retrieved directly (De Leeuw et al.,2011;Kaufman and Tanré,2003), and this information can be used in the aerosol retrieval. Observations over highly reflective surfaces like deserts, are particularly difficult for aerosol retrieval. In these regions it is necessary to make algorithms that use shorter wavelengths in which the reflectance is low (Hsu et al.,2004).
The next step is to calculate the top of the atmosphere radiances by a radiative transfer model (RTM) to include the effect of gases in the atmosphere.
The RTM calculations are done for the viewing geometry (viewing angle of instrument, positions of satellite and Sun) using different probable values of surface characteristics and a wide range of aerosol atmospheric compositions obtained by aerosol models. These results are compared to the observed atmospheric path radiance (radiance without cloud and surface effects) and, by minimizing the difference between them, the AOD is estimated (De Leeuw et al.,2011).
Advances in the retrieval techniques also enable the retrieval of other aerosol properties than AOD: aerosol types, partitioning between fine and coarse particles, single scattering albedo, angstrom exponent, effective radius of the dominant mode, etc. (Lee et al.,2009).
Nowadays, aerosol retrieving techniques and algorithms are very sophisti-cated and they give results of very good quality. However, there are sometimes still large differences between retrievals from different (or even the same) instru-ments. They come from differences and uncertainties in calibration, sampling, cloud screening, surface reflectivity algorithms, aerosol models, wavelengths, viewing geometries and different instrument characteristics (Li et al., 2009;
Kokhanovsky et al.,2010).
1.9.2 AOD observations
Ground based The aerosol optical depth from the ground is measured by the extensive AERONET (AErosol RObotic NETwork) network (Holben et al., 1998). It includes a few hundreds of stations, many of them permanently measuring aerosols by sunphotometers. It already provides data collected long enough for climatological aerosol studies on the local scale. The AOD measurements are very accurate: ∆AOD = ±0.01. They cover different wavelengths, and besides AOD stations provide other aerosol properties like the
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single scattering albedo, refractive indices and the size distribution. Besides AERONET, there other aerosol ground network like GAW (Collaud Coen et al., 2013), SKYNET (Sano et al.,2003), etc.
Spaceborne Some spaceborne instruments that measure aerosols were spe-cially designed for this purpose, while others are not directly designed for it, but have been used for the retrieval of aerosols. An instrument dedicated to the aerosol measurements would desirably have the spectral sensitivity from ultra-violet to thermal infra-red, with multiple views and polarization sensitiv-ity. With these characteristics it is possible to retrieve a wide range of aerosol properties.
The first operational aerosol products were from AVHRR (Advanced Very High Resolution Radiometer) (Stowe et al., 1997) and TOMS (Total Ozone Mapping Spectrometer) (Torres et al., 2002) launched at the end of 1970s and both primarily used for the retrieval of other parameters than aerosols. Instruments that are dedicated for the retrieval of aerosols are, for example, MISR (Multiangle Imaging Spectro-Radiometer), MODIS (Moderate Resolution Imaging Spectrometer), POLDER (Polarization and Directionality of the Earth’s Reflectance). MISR (Kahn et al.,2005) and MODIS (Remer et al., 2005) are both capable of retrieving aerosols over ocean, land and highly reflective surfaces. MODIS is a set of instruments on-board of two sun-synchronous satellites, Terra and Aqua, observing at multiple wavelengths.
MODIS AOD products are the most used aerosol observations from space (De Leeuw et al.,2011); over ocean it can separate the fine and coarse particles, and over the land the aerosol type. The AODs are retrieved with an error of
Figure 1.15:An example of aerosol optical depth measured by the satellite instru-ment MODIS.
±0.05±0.20AOD over the land (Chu et al.,2002) and±0.03±0.05AOD over the ocean (Remer et al.,2002).
MISR observes at multiple wavelengths and at multiple angles, while POLDER is the most complete instrument by observing also the polarization characteristics of the light. Of the limb-viewing instruments, we can mention SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) (Bovensmann et al.,1999) which was primarily designed for the trace gases observations and it was operational until 2012. At the geostationary orbit is SEVIRI (Spinning Enhanced Visible and Infrared Imager) which provides observations of whole planet’s disk every 15 minutes. Its shortest wavelength is 0.6 µm and it lacks SWIR channel useful for the surface reflectance determination over land.