La distinción entre igualdad ante la ley y la igualdad en la ley

Satellite-borne measurements provide datasets with a global coverage. However, those data have to be evaluated with in situ measured data. This section compares data from satellites with the OPC data to illustrate a further possible application. As here data from two very different measurement techniques (remote sensing wide area scan vs. in situ single point measurement) are compared, the focus lies on qualitative rather than on quantitative comparison.

Several aerosol optical parameters were measured with a seven channel sun photometer onboard the Earth Radiation Budget Satellite (ERBS) within the Stratospheric Aerosol and Gas Experiment (SAGE II; www-sage2.larc.nasa.gov; last access 20.07.2014). The pole orbiting ERBS provided data from October 1984 to August 2005. Bauman et al., [2003a, b] published climatologies for the particle extinction coefficient at different wavelength, the particle surface area concentration, and the effective particle radius. The data are presented as function of latitude, geometric altitude and time between October 1984 and August 1999. On request the raw data for all figures in Bauman et al [2003b] were courtesy provided by Jill J.

Bauman (NASA). Within this section the above mentioned aerosol parameters are compared to the corresponding parameters derived from the CARIBIC measurements.

The extinction coefficient for = 525 nm was calculated from the particle number size distribution using a custom Mie-scattering program [Schladitz et al., 2011], which is based on a Mie-code by Bohren and Huffman [1983]. The particle surface area concentration was directly obtained from the OPC analysis algorithm (Sec. 5.3). The effective particle radius was calculated from the integrals of particle volume and the particle surface according to Eq. 6.1:

117

reff effective particle radius [µm]

nOPC number of size channels in the OPC particle size distribution [1]

,

dp i mean particle diameter of a certain size channel (i) [µm]

,

dp i

 

  measured particle concentration at a certain size channel with d_{p i,} [1/cm³]

As the SAGE II data were provided only for altitudes 2 km above the tropopause or higher, only stratospheric CARIBIC measurements with potential vorticity > 5 PVU were used for this comparison. 2036 measurement points in the northern hemisphere (north of 30°N) and none in the southern hemisphere fulfill this requirement.

Since the datasets originate from different periods, only a comparison of long-term averages of background values is expedient. To obtain representative background values, the SAGE II data were averaged over time from January 1989 to May 1991 and January 1996 to August 1999. Between June 1991 and December 1995 the Pinatubo eruption⁴⁰ strongly increased the particle extinction, surface, and effective radius.

Before January 1989 the background concentrations were influenced by the Nevado del Ruiz eruption in November 1985 (cf. Fig. 2 to 5, 9, and 16 in Bauman et al., [2003b]).

Comparing data from different periods is difficult, because Solomon et al [2011]

showed that from 1996 to 2010 the global stratospheric aerosol optical depth increased by about 34%. On the contrary SAGE II data were measured at ambient conditions i.e.

ambient humidity, whereas the CARIBIC data are representative for dried aerosol. With Tab. 6.3 and the associated discussion it was shown that the particles shrink inside the CARIBIC aerosol inlet system by about 25%. These effects of increasing stratospheric optical depth (i.e. increasing aerosol loading) on the one hand and the comparison of aerosol optical parameters from ambient and dry measurements on the other hand are opposed and therefore should compensate. Hence, the SATGE II and the CARIBIC data should be on the same order of magnitude.

Figure 6.26 shows the meridionally, zonally, and temporally averaged vertical profiles of the particle extinction coefficient at a wavelength of = 525 nm, the ambient particle surface area concentration, and the effective particle radius. The stratospheric CARIBIC data were measured up to 12.5 km and were sorted into 0.5 km altitude bins. SAGE II data start at 11 km⁴¹ and have a vertical resolution of 1 km. For both datasets the solid line give the median value and the dashed lines in the same color       

40 The explosive phase of the Pinatubo eruption started in June 1991.

41 As the given altitude is the middle of the individual altitude layer, the lines in Fig. 6.26 start in the middle (e.g. at 11.5 km). However, the given value is representative for the whole layer (i.e. from 11 km to 12 km).

6.6 Comparison of aerosol optical parameters with data from SAGE II

indicate the 25% and 75% percentiles, respectively. Considering the above mentioned uncertainties, the good agreement of the two datasets within the crossover area (11 km to 12.5 km) for particle extinction coefficient (Fig. 6.26a) and particle surface area concentration (Fig. 6.26b) is remarkable. With 1.27310^-6 1/m the median SAGE II particle extinction coefficient is only 8.6% higher than the CARIBIC median⁴². The averaged SAGE II median for the particle surface area concentration of 3.64 µm²/cm³ is only 4.6% lower than the CARIBIC median (3.82 µm²/cm³). In addition, both techniques show the same slope with decreasing values for increasing altitude.

42 For both datasets the median was averaged between 11.0 km and 12.5 km, giving a value of 1.27310^-6 1/m for SAGE II and 1.17310^-6 1/m for CARIBIC. 

Figure 6.26: Vertical gradients of the particle extinction coefficient at λ = 525 nm (a), particle surface area concentration (b), and effective particle radius (c). The stratospheric CARIBIC measurements (PV > 5 PVU) were sorted into 0.5 km altitude bins. SAGE II data have a vertical resolution of 1 km.

Both data sets were meridionally and zonally averaged. CARIBIC data were measured between June 2010 and February 2011, SAGE II data were averaged from 1989 to May 1991 and January 1996 to August 1999. While solid lines represent the median concentrations, dashed lines indicate the 25% and 75% percentiles. The CARIBIC particle surface area concentrations are given at ambient conditions to be comparable to SAGE II data.

0.10 0.15 0.20 0.25 0.30 0.35

10 particle extinction coeficient at =525 nm

altitude [km]

119  

On the contrary, hardly any vertical gradient but a significant offset between the datasets was found for the effective particle radius (Fig. 6.26c). With 0.259 µm the averaged SAGE II median is 74% higher than the averaged CARIBIC median (0.149 µm). This is surprising after good agreements for particle extinction coefficient and surface area were found. The different measurement conditions (ambient humidity vs. dried aerosol) would explain the difference only partly. According to Eq. 6.1, the effective particle radius is calculated from the particle surface area and the particle volume concentration. Increasing the OPC measured particle size by 25% to match the ambient humidity particle size (cf. Tab. 6.3) would increase the particle surface by 56%

and the particle volume by 95%. The resulting effective radius would be 25% larger. A methodical error explaining the remaining discrepancy of 50% is not apparent. As the particle surface area concentration was found to be in good agreement, the difference might be caused by an even higher SAGE II particle volume concentration.

Figure 6.26 shows that the CARIBIC observations have generally a much higher variability than the SAGE II measurements⁴³. This is probably caused by the much shorter CARIBIC measurement period of one year compared to nearly six years for SAGE II. Please note, the different slopes in the vertical representation of the CARIBIC OPC data in figures 6.20b, 6.24a and 6.26b arise by the use of the different vertical coordinates (geometric altitude vs. PV) and different conditions (ambient vs. STP).

In summary, considering that large scale remote sensing data are compared with averages of in situ single particle measurements recorded several years later, the agreement is better than expected. As the increase of the stratospheric aerosol loading from 1996 to 2010 [Solomon et al., 2011] is compensated by the comparison of measurements at ambient humidity (SAGE II) with dry conditions (CARIBIC), the good agreement for the particle extinction coefficient and the particle surface area concentration is reasonable. However, the reason for the discrepancy of the effective particle radius was not found yet, but should be a focus of further studies. Nevertheless, this comparison clearly demonstrates that the increasing CARIBIC OPC dataset can be used to validate data from satellite and ground based remote sensing (e.g. LIDARs).

43 The CARIBIC 25% and 75% percentiles were found to be about 40% (particle extinction), 30%

(surface area), and 11% (effective radius) around the medians. The corresponding SAGE II variability was calculated to be 13% (particle extinction), 10% (surface area), and 5% (effective radius), respectively.

6.6 Comparison of aerosol optical parameters with data from SAGE II