3.2 Radiative transfer settings
3.2.2 Definition of aerosol properties and vertical distribution
The role of the aerosol in the atmospheric radiative processes was already explained in detail in chapter 2. The influence of aerosol on the retrieval of tropospheric NO2 from satellite measurements is not only relevant for urban scenes, which are major sources for those two components, but also for biomass burning events (although these cases are out of the scope of this study). Furthermore, it is important to consider, as well, cases of aerosol (like desert dust) that is often transported far away from its origin to source regions of NO2. These plumes are often found at high altitudes in the atmosphere but, sometimes, are also at lower altitudes mixed with boundary layer pollution.
For the radiative transfer calculations, information on different aerosol characteristics is required, namely, the size distribution parameters, optical properties, its amount and vertical distribution.
Currently, datasets of aerosol attributes are available from records from both ground-based measurements and space-borne instruments.
a) Phase functions
Within the radiative transfer model, angular distributions of scattered light are required to simulate the interaction of particles and light. The phase function varies with the aerosol composition, the size of the particles relative to the wavelength of the radiation, and it also depends on particle shape and internal structure (see section 2.4 for more details). The optical properties and size distributions, at 440 nm, these were mainly taken from records of 12 worldwide AERONET stations presented in Dubovik et al. (2002). This dataset is representative of the usual classification of four different aerosol types that have distinctive physicochemical, optical and radiative properties: urban/industrial, biomass burning, desert dust and oceanic. In Table 3.1 the parameters for the different size distributions considered are given, together with the refractive index. The phase functions were determined with the program spher.f (FORTRAN program) developed by Michael Mishchenko and freely available at http://www.giss.nasa.gov/staff/mmishchenko/brf/. For each of the aerosol types, from the 12 locations considered, the phase functions (see Figure 3.3) for both fine and coarse particles were determined and used separately in each of the scenarios. A clear distinction between phase functions of fine and coarse aerosol is found, with the bigger particles showing a stronger peak in forward direction. In reality, a mixture of both sizes is normally found, but for the sensitivity study performed here a clear separation of particle sizes facilitated the interpretation of results. Nevertheless, it is important to keep in mind that coarse aerosol is mostly found in desert dust scenario or from the ocean, e.g., sea-salt. On the other hand, the aerosol emitted in urban areas and from open vegetation fires are, on average, dominated by small particles (Seinfeld and Pandis, 2006).
Table 3.1 Size distribution parameters (radius and sigma) and refractive indices taken from 12 AERONET stations (Dubovik et al., 2002): Urban (Urb), Biomass Burning (BB), Desert Dust (DD) and Oceanic. Data from measurements performed at 12 AERONET stations (Dubovik et al., 2002): Paris/Creteil – France (Urb), GSFC/Maryland – USA (Urb), Maldives (Urb), Mexico city – Mexico (Urb), Amazonia forest – Brazil (BB), South American cerrado – Brazil (BB), African savannah – Zambia (BB), Boreal forest – USA and Canada (BB), Cape Verde (DD), Persian Gulf (DD), Saudi Arabia (DD) and Lanai (Oceanic).
Location (Urban) Paris Maryland Maldives Mexico city r (m), (fine) 0.06, 0.43 0.07, 0.38 0.09, 0.46 0.06, 0.43 r (m), coarse 0.31, 0.79 0.42, 0.75 0.35, 0.76 0.68, 0.63 Refractive index 1.4 – i0.009 1.41 – i0.003 1.44 – i0.011 1.47 – i0.014 Location (Biomass
burning)
Amazonia forest
S. American cerrado
African
savannah Boreal forest r (m), (fine) 0.08, 0.40 0.06, 0.40 0.07, 0.40 0.08, 0.43 r (m), coarse 0.37, 0.79 0.37, 0.79 0.50, 0.73 0.32, 0.81 Refractive index 1.47 – i9.30E-4 1.52 – i0.015 1.51 – i0.021 1.5 – i9.4E-3 Location (desert dust +
oceanic) Cape Verde Persian Gulf Saudi Arabia Lanai r (m), (fine) 0.05, 0.49 0.08, 0.42 0.07, 0.40 0.07, 0.48 r (m), coarse 0.47, 0.63 0.69, 0.61 0.66, 0.60 0.53, 0.68 Refractive index 1.48 – i2.5E-3 1.55 – i2.5E-3 1.56 – i2.9E-3 1.36 – i1.5E-3
b) Vertical distribution
For remote sensing applications, the total amount of aerosol present in the atmosphere is often specified by an aerosol optical depth (AOD) which is the vertical integral of the extinction by aerosol from the top of the atmosphere to the ground (see section 2.4 for more details). As mentioned above, different aerosol vertical distributions were considered for the several scenarios. In the first phase of the study, the aerosol profile was defined as a box shaped profile, i.e., layers with homogenously distributed aerosol, which had variable top height for each setup considered. Three cases were set with extinction coefficients representative for three aerosol loads: 0.1 (low pollution level), 0.5 (moderate pollution) and 0.9 (polluted scene) aerosol optical depths. Next, the aerosol profile was defined in different ways relative to the trace gas: following the NO2 box profile; starting at surface level and with the top of the layer lower or higher than that of the NO2 profile; and, discrete elevated aerosol layers above the NO2 layer (assumed to be in the BL). These scenarios (A to H in Table 3.2) were selected as
simplified representations of potential occurrences. Normally, the urban aerosol is assumed to be either in homogenous layers extending from the surface to the top of BL or, often, following an exponential decrease with height (e.g., scenario K). In general, one can assume that the majority of anthropogenic sources are the same for both NO2 and aerosol and, therefore, they would have similar spatial distributions (e.g., scenarios C and J). However, depending on the source location and transport processes, the aerosol layer can extend to a higher altitude (e.g., scenarios F and I), whereas NO2 will be in general more concentrated closer to the emission site and at lower levels, due to a shorter lifetime. In, addition, measurements have already shown that a residual layer of aerosol can remain at higher altitudes above the boundary layer after this one has reduced its extension (Hodzic et al., 2004; 2006). For that reason, the extension of each layer was also varied independently so that different scenarios could be analysed. The scenarios with elevated discrete aerosol layers (e.g., scenarios D and M) are mostly adequate to illustrate plumes of biomass burning smoke and desert dust that are transported several hundreds to thousands of kilometres away from the source and which
Figure 3.3 Phase functions, at 440 nm, for fine (blue) and coarse (grey) aerosol determined for 4 distinct aerosol types: Urban (Urb), Biomass Burning (BB), Desert Dust (DD) and Oceanic. Optical properties taken from 12 AERONET stations (Dubovik et al., 2002): Paris/Creteil – France (Urb), GSFC/Maryland – USA (Urb), Maldives (Urb), Mexico city – Mexico (Urb), Amazonia forest – Brazil (BB), South American cerrado – Brazil (BB), African savannah – Zambia (BB), Boreal forest – USA and Canada (BB), Cape Verde (DD), Persian Gulf (DD), Saudi Arabia (DD) and Lanai (Oceanic). Average of phase functions for each of the aerosol sizes considered is presented in thick lines (blue for fine and black for coarse aerosol).
can be lifted to higher altitudes during transport (Damoah et al., 2004). These events can happen not only on a continental scale (e.g., smoke from fires in the African savannah that is transported across the Atlantic Ocean as shown by Husar et al. (1997)), but also on a regional scale, as transport within Europe (Balis et al., 2003; Hodzic et al., 2006; Arola et al., 2007). These aerosol plumes often occur in the free troposphere, but they can also be part of the boundary layer either by intrusion processes or due to the low initial injection height (e.g., scenarios L and P). Good examples of this case are the dust outbreaks from deserts that often can mix with urban type aerosol emitted within European or Asian cities (e.g., Zhou et al., 2002).
On a second stage of the research, the vertical aerosol profiles used in the calculations were based on different studies resulting from lidar measurements made at ground-based stations, such as those that are part of the European Aerosol Research LIdar NETwork (EARLINET, Mattis et al., 2002) or the Asian Dust network (AD-net, Murayama et al., 2001) (see Table 3.3 and Table 3.4). In this way, specific situations, as those suggested above, could be simulated in more realistic scenarios. The studies considered here are from different locations around the world and representative of different times of the year. Figure 3.4 shows the vertical profiles of extinction coefficients for all the cases investigated. This extensive selection of scenarios was designed to assure that many different possible cases would be considered and the conclusions drawn from this study could be generalised. However, the detailed analysis of all these examples would be too extensive and often repetitive as many of the results led to similar conclusions. Therefore, in the current manuscript, focus is given only to a few scenarios: I to P (represented in the top graphs of Figure 3.4). Further details of assumptions made for scenarios I to P are presented in Table 3.3, and the literature source references for the remaining cases are provided in Table 3.4.
The size distribution and corresponding phase functions, for both fine and coarse aerosol, were maintained from the initial stage. It is important to mention that the profiles considered in this study are not the exact representation of the original ones. Often adjustments were required in order to obtain a profile from surface to the top of atmosphere. Moreover, as these are meant to be examples for case studies their accuracy is not a subject of this analysis and does not influence the conclusions drawn. Since lidar measurements (both satellite and ground-based) are usually performed at 355 nm and/or 532 nm, an Ångström exponent (Ångström, 1929), also taken from the literature, was necessary to convert these values to the corresponding ones at 440 nm (within the wavelength region where NO2 is retrieved).
Cases for oceanic aerosol type were not included at this stage because this aerosol is normally only observed in very low concentrations at polluted sites and is usually mixed with other types of aerosol.
Therefore, for simplicity of the analysis, it was assumed that its influence in the NO2 retrieval is similar to that of the other types considered.
Table 3.2 Scenarios considered for the SCIATRAN runs, defined by the combination of a NO2 and an aerosol layer, as in box profiles (e.g., Scen.B: NO2 layer 0.0 - 1.0 km and aerosol layer 0.0 - 0.6 km).
Scenario A B C D E F G H
NO2 layer
(km) 0.0 – 0.6 0.0 – 1.0 0.0 – 2.0
Aerosol
layer (km) 0.0 – 0.6 0.0 – 0.6 0.0 – 1.0 0.6 – 1.0 1.0 – 2.0 0.0 – 2.0 2.0 – 3.0 0.0 – 2.0
Figure 3.4 Aerosol extinction profiles from surface level to 10.0 km used in the SCIATRAN settings for the AMF calculations for: (left) rural (Rur) and urban (Urb) locations; and (right) desert dust (DD) events and biomass burning (BB) plumes. These profiles are based on measurements as it is explained in Table 3.2 for scenarios I to P (top) and in Table 3.3 for the extra ones (bottom).
Table 3.3 Aerosol parameters (single scattering albedo (), Ångström exponent () and aerosol optical depth (AOD)) taken from each of the references mentioned, these were used to define the aerosol vertical profile (with extinction coefficients) for the SCIATRAN scenarios. These are representative of different aerosol types: Urban (Urb), Desert Dust (DD), and Biomass Burning (BB) scenes.
Scenario and Reference for aerosol ext. profile
Aerosol
type AOD Further notes
I based on CALIPSO
recordsa Urb 0.93b 1.4c 0.07 Background
conditions J Chazette et al. (2005) Urb 0.87 2.1 0.40 19 July 2000 in Paris
(FR) K Amiridis et al. (2005) Urb 0.93b 1.4c 0.62 4yr average over
Thessaloniki (GR)
L Zhou et al.(2002) DD 0.92b 0.19 1.05 12 May 2000 in
Heifei (CN)
M Murayama et al. (2003) DD
Altitude dependent
values – 0.8 to 0.95
Altitude dependent
values – 0.01 to 1.1
0.66 23 April 2001 in Tokyo (JP)
N Pérez et al. (2006) DD 0.93 0.19d 0.16 18 June 2003 in
Barcelona (SP) O Balis et al. (2003) BB 0.92b 1.4e 1.05 9 August 2001 in
Thessaloniki (GR)
P Müller et al. (2005) BB 0.92b
Altitude dependent
values – 0.0 to 1.1
0.42 26 June 2003 in Leipzig (DE)
a) Data provided by Chieko Kittaka and David Winker from NASA - Goddard Space Flight Center.
b) Average of the respective aerosol type based on Dubovik et al. (2002).
c) Average for urban aerosol in Mattis et al. (2004).
d) Same as Zhou et al. (2002).
e) From Müller et al. (2005) and references therein.
c) Single Scattering Albedo (SSA)
The SSA differs according to the type and source of aerosol and, thus, is in part dependent on the location of measurement (see for example Hu et al., 2007). For the majority of the scenarios considered in this analysis, the impact of aerosol absorption was investigated by comparing the AMFs determined with runs where was set to 0.93 (average from all the SSA values given at 440 nm in
Dubovik et al. (2002)) and others where was assumed to be 1.0 (i.e., non-absorbing aerosol). This allowed determining the maximum effect on the results when reducing the absorbing ability of aerosol. However, for a limited number of cases, this variation was analysed with further detail by setting to 0.80 and 0.95. Furthermore, in the last stage of the analysis, when considering measurements of aerosol profiles, the values varied from scenario to scenario: either taken from the corresponding records or based on typical values for the aerosol types (see Table 3.3).
Table 3.4 Source references used of the extra aerosol vertical profiles and further required information.
Scenario/Aerosol type Reference for aerosol ext. profile Rur(a) based on CALIPSO recordsa Urb(a) based on CALIPSO recordsa Urb(b) based on CALIPSO recordsa Urb(c) He et al. (2008)
Urb(d) He et al. (2008) Urb(e) Mattis et al. (2004) DD(a) Müller et al. (2003) DD(b) Murayama et al. (2003) DD(c) Pérez et al. (2006)
DD(d) Müller et al. (2003) and Murayama et al. (2003) BB(a) Murayama et al. (2004)
a) Data provided by Chieko Kittaka and David Winker from NASA - Goddard Space Flight Center.