A global climatology of stratospheric aerosol size distribution parameters derived from SAGE II data over the period 1984–2000: 2. Reference data
Christine Bingen, Didier Fussen, and Filip Vanhellemont
Belgian Institute for Space Aeronomy, Brussels, Belgium
Received 19 February 2003; revised 26 June 2003; accepted 23 October 2003; published 19 March 2004.
[1] In a companion paper, a global climatology was presented for different parameters characterizing the size distribution of stratospheric aerosols; the particle number density, median radius, and mode width of the particle size distribution were assumed to be lognormal. Those aerosol parameters were retrieved by optical inversion of Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction profiles. The data cover the time period 1984– 2000. In this second part of the study, we present the climatology in a more quantitative way, and starting from the case of the Mount Pinatubo eruption in June 1991, we try to derive reference data for the three
considered aerosol parameters. Using four time intervals representative of the different phases of the aerosol relaxation after a major eruption, we investigate the data set spanning the period June 1991 to mid-1999 and present a spatiotemporal description of the aerosol distribution for the whole relaxation period. The range of the data dispersion due to seasonal effects and random variations is also estimated. Finally, a study of the error on the three parameters is presented for the whole data set, as well as an overview of our validation efforts. INDEXTERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0370 Atmospheric Composition and Structure: Volcanic effects (8409); KEYWORDS: stratospheric aerosols, aerosol climatology, aerosol size distribution
Citation: Bingen, C., D. Fussen, and F. Vanhellemont (2004), A global climatology of stratospheric aerosol size distribution parameters derived from SAGE II data over the period 1984 – 2000: 2. Reference data,J. Geophys. Res.,109, D06202, doi:10.1029/2003JD003511.
1. Introduction
[2] During the last decades, a lot of effort has been devoted to a better understanding and characterization of the aerosol evolution in the stratosphere. Thanks to the development of instrumental techniques, it became possi- ble to carry out aerosol measurements on a large spatial and/or temporal scale. Concerning in situ measurements, a 30 year aerosol measurement series has been performed by optical particle counting above Laramie, Wyoming (41N) [Hofmann and Rosen, 1981; Hofmann, 1988;
Deshler et al., 1997, 2003; Hervig and Deshler, 2002].
Also, long time series of lidar measurements are available over at least one decade or more [Ja¨ger and Hofmann, 1991; Hofmann, 1988]. Finally, the development of remote sensing with spatial missions such as SAM [McCormick and Trepte, 1987], Stratospheric Aerosol and Gas Experiment (SAGE) I and II [Brogniez et al., 1996], POAM [Randall et al., 2000] and several experi- ments aboard UARS [Lambert et al., 1997; Hervig and Deshler, 2002] have widely contributed to supply valu- able information about aerosol over a large spatiotemporal
range. In this respect, the SAGE II mission has brought a unique set of extinction profiles covering a wide range of situations from very low to very high volcanic aerosol load. It provided valuable material to study the influence of microphysics and transport in the aerosol dispersal, and to build up reference models for different aerosol param- eters of interest.
[3] Hitchman et al.[1994] built a climatology of aerosols using extinction profiles measured by SAGE I, SAGE II and SAM II. Using those measurements spread over nearly a decade, they proposed a reference model of mean extinction profile covering the whole latitudinal range for altitudes from 8 to 24 km. They also analyzed the main influences affecting this profile: seasonal effects and QBO.Anderson and Saxena[1996] studied the evolution of aerosol effective radius, surface area density and mass loading consecutively to the Pinatubo eruption, from 1991 to 1994. Later, Thomason et al.[1997] proposed a climatology of aerosol surface area density for the period 1984 – 1994, at four altitudes from 15.5 to 30.5 km. This climatology, based on a previous version of SAGE II extinction profiles, provides a refined overview of aerosol properties over a wide temporal and spatial range. In other respects, our research group developed a climatology of aerosol extinc-
Copyright 2004 by the American Geophysical Union.
0148-0227/04/2003JD003511
D06202
tions based on the SAGE II data [Fussen and Bingen, 1999], and studied the features of the aerosol layer in function of spatial parameters and of the volcanism that we quantified by using the optical thickness of the atmosphere [Bingen and Fussen, 2000]. Lambert et al.[1997] used extinction profiles in the infrared region near 12 mm to retrieve information about the aerosol composition and about some integrated parameters describing the size distribution: vol- ume density, surface area density and effective radius.
Those profiles cover the whole latitudinal range and alti- tudes from 15 to 33 km, during the period October 1991 to March 1993.
[4] The aim of the present work is to contribute to this global characterization of stratospheric aerosol, by propos- ing a climatology on three aerosol parameters for which, so far, no climatological data have been proposed on a global scale: the particle number density, the median radius and the mode width of the particle size distribution. The basic data set was retrieved by optical inversion of SAGE II extinction data, using a method described in a companion paper [Bingen et al., 2004a].
[5] In a first step, we present the applied methodology and the climatological data, by trying to discern the respec- tive contributions of aerosol relaxation related to volcanism, and seasonal effects. Afterward, we attempt to quantify the validity domain of our results, and we present in this aim detailed error estimation. Finally, we extend the validation work that we already presented in a previous paper [Bingen et al., 2002]. This allows us to give more insight about the reliability of the climatology and to illustrate the limitations of remote sounding techniques in the aerosol parameter retrieval.
2. Climatological Data
[6] Figure 1 shows the evolution of the particle number densityN, the median radiusrand the modal widthsduring the whole SAGE II mission at the equator. The contours were obtained by binning all SAGE II retrieved profiles using 1 month windows and 10 latitude intervals. In the tables reporting the climatological data, one latitude interval will be designated by its middle value.
[7] The influence of major volcanic eruptions such as El Chichon (17N, November 1982), Ruiz (5N, November 1985), Kelut (8S, February 1990) and Pinatubo (15N, June 1991) is visible, with a sudden increase of the aerosol density and a simultaneous change in the size distribution characteristics due to the formation of a huge aerosol mass up to the middle stratosphere. Afterward, a gradual subsi- dence of the aerosol cloud is observed during several months or years, depending on the eruption.
[8] The time evolution of the aerosol load after a major volcanic event follows different phases where nucleation, coagulation, removal by sedimentation or by transport become in turn dominating processes, influencing in differ- ent ways the characteristic relaxation time of each aerosol parameter.
[9] The aim of this section is to quantify this temporal evolution during the whole post-eruption period from the transient characterized by a rapid growth of the volcanic aerosol content to the long term evolution toward a more or less stationary state. Therefore we focused on the case of the
Pinatubo eruption in June 1991. In order to circumvent the difficulty to model this complex and changing combination of influences and to present an operational data set in a suffi- ciently concise way, we chose to restrict ourselves to a simple model, by selecting time periods representative for the vari- ous phases of the aerosol relaxation and by fitting, at each latitude and altitude, a linear temporal dependence on the various aerosol parameters on each time interval. For the particle number density, we made use of the more appropriate logarithm ofN, giving
log10ðN tð ÞÞ ¼log10ðN0Þ t tN
ð1Þ
whereas ther,sdependence was written as
rð Þ ¼t r0þvr:t ð2Þ
sð Þ ¼t s0þvs:t ð3Þ [10] For all three aerosol parameters, the time evolution following the Pinatubo eruption was found to be reasonably described by splitting the post-eruption period into four time intervals, as depicted in Figure 2. The results of the corresponding fits are presented in Tables 1 – 12.
[11] After a rapid increase of the volcanic loading (phase I in Figure 2; Tables 1, 5, and 9) the aerosol mass loading decreases significantly during a period extending roughly to the 48th month after the eruption. During this period, a change is observed in most cases in the relaxation curves around 2 years after the eruption. This finds expression in a slope break of the curve describing the number density decay and in a marked change in the growth ratesvrandvs. The median radius shows in most cases a clear slowdown in the relaxation process, or even a switch from a phase of size reduction to a growing period. At the same time, the distribution width is characterized, at least above 20 km, by a transition from a marked constriction to a much slower decrease ofs. In the lower southern latitudes, one even observes a new broadening of the size distribution. This two-step evolution is indicated by phases II and III in Figure 2 and described in Tables 2 and 3, 6 and 7, and 10 and 11 for the 3 aerosol parametersN,rand s, respectively. Finally, after about 4 years, the aerosol parameters reach a phase where the influence of volcanic decay becomes weak compared to seasonal influences (phase IV in Figure 2; Tables 4, 8, and 12).
[12] Although the different temporal phases described above are generally quite well defined at all latitudes, our choice of the time periods, identical for all altitudes and latitudes, is somewhat arbitrary, because the aerosol evolu- tion differs following the latitude because of time delays and local variations of aerosol density caused by transport.
The time periods and the available data set must also be large enough to allow discerning relaxation effects through seasonal variations. Our choice was made in such a way that it takes into account all these aspects, and offers a good trade-off between all of them. For the same reasons, the time intervals are often not contiguous although they correspond to similar stages in the aerosol evolution.
[13] The root mean square of the fit residuals, reported for all fits and referred to asdlog10(N),dranddsfor the fits (1),
Figure 1. Mean dependence in time and altitude of the three aerosol parameters of interest, representative for the latitude range (0, 10): (a) particle number density illustrated by log10(N), (b) median radius, and (c) modal width. See color version of this figure at back of this issue.
(2) and (3), respectively, is mostly influenced by the seasonal variations and, in a much less extent, by the uncertainty on the aerosol parameters (See next section).
Therefore it allows one to get good insight into the respective influences of volcanism and seasonal effects on the aerosol evolution.
[14] In Tables 1, 5, and 9, the valuesN0(1), r0(1) ands0(1) of the initial valuesN0,r0ands0in equations (1), (2), and (3) correspond to the aerosol parameters estimated just before the Pinatubo eruption. They are thus representative for a period of very low volcanism.
[15] As shown in Table 1, the characteristic time of the transienttN(1) reflects a rapid increase ofNat all latitudes.
This phase of aerosol load increasing leads to the aerosol number densityN0(2)reported in Table 2. It can be seen that Figure 2. Temporal evolution of the median radius atz = 22.5 km representative for the latitudinal
ranges (60S, 50S) and (40S, 30S), respectively. The solid curve represents the SAGE-II-derived evolution of r, and dashed curves denote the corresponding fit. For the sake of clarity, the curves corresponding to the (40S, 30S) interval have been shifted by a constant value of 0.2mm, as indicated in the legend.
Table 1. Temporal Evolution of the Particle Number Density During the First Year Following the Pinatubo Eruption, From May 1991 to June 1992 (‘‘Phase I’’)a
tN (1)
55S 45S 35S 15S 5N 25N 45N 55N 65N
29.5 km 7.9 4.0 6.0 6.5 10.5 5.8 5.2 7.1 6.4 27.5 km 4.6 4.4 5.4 10.2 13.5 8.3 5.6 7.0 8.8 25.5 km 5.5 4.4 6.2 12.5 8.5 8.0 5.5 4.8 6.1 23.5 km 5.2 6.5 8.2 10.7 7.5 8.0 6.3 6.0 6.3 21.5 km 6.1 7.0 10.3 12.2 – 8.3 7.2 7.6 7.0
19.5 km 8.3 8.4 10.8 4.2 – 5.2 7.0 7.9 9.5
17.5 km 13.0 12.1 10.7 3.6 – 3.5 10.1 11.2 9.6 N0(1)
55S 45S 35S 15S 5N 25N 45N 55N 65N
29.5 km 0.01 0.00 0.01 0.03 0.23 0.01 0.00 0.00 0.00 27.5 km 0.01 0.00 0.02 0.35 0.69 0.08 0.00 0.00 0.00 25.5 km 0.02 0.01 0.05 0.76 0.49 0.21 0.00 0.00 0.00 23.5 km 0.03 0.09 0.22 0.51 0.35 0.27 0.04 0.02 0.00 21.5 km 0.06 0.13 0.44 0.84 – 0.35 0.13 0.11 0.04 19.5 km 0.12 0.24 0.43 0.13 – 0.07 0.16 0.22 0.13 17.5 km 0.26 0.36 0.23 0.05 – 0.02 0.36 0.36 0.22
dlog10(N)(1)
55S 45S 35S 15S 5N 25N 45N 55N 65N
29.5 km 0.49 0.74 0.60 0.74 0.49 0.40 0.48 0.56 0.21 27.5 km 0.34 0.62 0.48 0.53 0.48 0.27 0.48 0.62 0.25 25.5 km 0.60 0.61 0.51 0.36 0.39 0.50 0.46 0.50 0.25 23.5 km 0.58 0.49 0.44 0.29 0.32 0.45 0.35 0.40 0.58 21.5 km 0.68 0.41 0.34 0.31 – 0.35 0.28 0.35 0.25 19.5 km 0.76 0.35 0.26 0.22 – 0.75 0.29 0.22 0.18 17.5 km 0.53 0.17 0.24 0.62 – 0.82 0.26 0.18 0.17
aThe parameters are obtained by linear regression on log10(N) (see equation (1)). Here,tN
(1), relaxation timetNin months;N0
(1), typical value of the particle number densityN0 in cm3 at the beginning of the time interval;dlog10(N)(1), standard deviation on log10(N).
Table 2. Same as Table 1 for the Time Period July 1992 to August 1993 Covering the 14th to 27th Months Following the Pinatubo Eruption (‘‘Phase II’’)
t(2)N
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 31 40 13 71 13 11 13 14 19
27.5 km 72 14 10 20 14 11 13 12 12
25.5 km 27 20 18 18 16 9 15 15 13
23.5 km 37 14 11 13 13 9 13 12 13
21.5 km 20 18 14 17 16 16 13 13 17
19.5 km 20 18 19 43 33 28 19 16 17
17.5 km 31 31 57 47 42 21 25 24 26
N0(2)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.01 0.01 0.06 0.09 1.28 0.34 0.05 0.04 0.03 27.5 km 0.02 0.23 0.40 0.85 2.19 0.86 0.09 0.12 0.07 25.5 km 0.20 0.42 0.56 2.13 3.19 2.66 0.37 0.33 0.26 23.5 km 0.51 1.70 3.73 5.40 5.54 4.82 1.48 0.86 0.83 21.5 km 1.91 3.62 5.95 6.46 7.05 4.96 4.47 2.71 1.27 19.5 km 4.77 6.47 6.30 3.44 3.36 4.59 5.77 5.10 4.40 17.5 km 5.23 5.15 3.30 1.36 1.06 3.01 5.54 5.82 4.69
dlog10(N)(2)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.64 0.68 0.35 0.51 0.21 0.69 0.56 0.31 0.31 27.5 km 0.42 0.31 0.31 0.36 0.15 0.48 0.48 0.27 0.47 25.5 km 0.43 0.30 0.49 0.26 0.14 0.39 0.30 0.30 0.23 23.5 km 0.31 0.23 0.16 0.17 0.09 0.28 0.24 0.41 0.25 21.5 km 0.23 0.16 0.12 0.07 0.10 0.12 0.16 0.26 0.40 19.5 km 0.08 0.10 0.06 0.05 0.07 0.07 0.13 0.19 0.19 17.5 km 0.06 0.07 0.10 0.12 0.08 0.07 0.11 0.10 0.12
the highest values of N0(2), ranging between 1 and 7 particles per cm3, mainly gather in the intertropical region up to 27.5 km. They spread down to about 21.5 km at midlatitudes. Contours of similar magnitude follow lines which correspond to the typical pattern governing the global circulation. The relaxation times tN(2) characterizing the first part of the decay period (see Table 2) are clearly smaller than the correspondingtN(3)values of the second part of the decay phase (see Table 3), except around 50– 60latitude where the presence of the polar jet favors subsidence during the winter. At those latitudes, the occurrence time of the winter within the considered time interval (January to December 1994) significantly influences the values found fort(3)N . This leads to negative values oft(3)N in the northern lower stratosphere, whereasNis found to further decrease in the southern hemisphere (positive values of t(3)N ). Finally, relaxation times tN(4) in Table 4 show that the influence of volcanic relaxation is very limited after four years above 22.5 km. The particle number density further decreases at lower altitudes, where the values ofNare higher (about 101part./cm3, compared to typical values of 103 to 102part./cm3at high altitudes, out of the intertropical region). Between the tropics, the particle number density still takes relatively high values (about 101 part./cm3) to higher altitudes (about 27.5 km), because of the presence of the tropical aerosol reservoir [Trepte and Hitchman, 1992;
Hitchman et al., 1994].
[16] Parallel to the evolution of N, the median radius, which ranges in May 1991 between about 0.2 mm at high
altitudes to about 0.4mm at low altitudes in the intertropical region, undergoes a rapid increase during the first months after the Pinatubo eruption (see Table 5). The growth ratevr is particularly high around 55S below 22.5 km, where the polar night jet forms an efficient barrier for transport during the southern winter. This results in higher values ofrin the accumulation region, at the end of the transient period (See values of r(2)0 in Table 6). In the same region, s takes similarly higher values than in the northern corresponding latitudes, contrary tovs, as can be seen in Table 9.
[17] After this rapid growing phase of aerosol particles,r reaches high values of 0.3 mm at high altitude to about 0.6mm in the equatorial lowermost stratosphere and starts to decrease in most cases (see Tables 6 and 7). During this first decay phase (described by the time interval December 1991 to May 1993, see Table 10),sdecreases rapidly, especially in the region surrounding the aerosol layer along its upper limit.
[18] The second part of the aerosol relaxation, described in Tables 7 and 11, is characterized by a much slower evolution of the particle size distribution. The median radius decreases further between the tropics and in the lowermost stratosphere, which roughly corresponds to the regions of rather big particles (r0.3mm). Elsewhere,rstabilizes, or increases. At this time, the mode width tends to become smaller, except in the tropical region where a further increase is observed.
[19] Finally, the evolution ofrandsbecomes very slow after about 4 years (Tables 8 and 12). Values reached byr Table 3. Same as Table 1 for the Time Period January to
December 1994 Covering the 32nd to 43th Months Following the Pinatubo Eruption (‘‘Phase III’’)a
tN(3)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 22 32 49 23 10 13 99 224 –
27.5 km 19 41 13 57 19 56 50 +1 –
25.5 km 19 90 20 1 72 47 107 17 36
23.5 km 13 1 107 98 126 111 53 51 –
21.5 km 7 41 51 41 62 231 111 21 16
19.5 km 7 46 65 71 31 96 1 18 23
17.5 km 6 133 1 44 127 32 +1 30 104
N0(3)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.00 0.00 0.01 0.04 0.17 0.04 0.00 0.00 – 27.5 km 0.01 0.01 0.05 0.14 0.40 0.05 0.01 0.00 – 25.5 km 0.02 0.04 0.11 0.25 0.45 0.08 0.03 0.01 0.01 23.5 km 0.10 0.12 0.22 0.41 0.69 0.18 0.09 0.05 – 21.5 km 0.39 0.38 0.48 0.52 0.94 0.33 0.18 0.08 0.12 19.5 km 0.81 0.62 0.74 0.94 1.02 0.56 0.34 0.16 0.35 17.5 km 1.78 0.77 0.75 0.24 0.36 0.47 0.52 0.35 0.43
dlog10(N)(3)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.52 0.33 0.39 0.30 0.17 0.44 0.41 0.23 – 27.5 km 0.44 0.41 0.17 0.37 0.13 0.59 0.46 0.18 – 25.5 km 0.54 0.35 0.24 0.25 0.12 0.43 0.28 0.34 0.31 23.5 km 0.42 0.27 0.15 0.20 0.09 0.34 0.30 0.24 – 21.5 km 0.21 0.25 0.15 0.09 0.06 0.39 0.28 0.24 0.14 19.5 km 0.42 0.21 0.15 0.06 0.03 0.20 0.20 0.17 0.27 17.5 km 0.66 0.18 0.11 0.20 0.11 0.19 0.12 0.11 0.14
aWhen the relaxation time is greater than 240 or lower than240, its value is noted as +1or1, respectively.
Table 4. Same as Table 1 for the Time Period July 1995 to June 1999 Covering the 50th to 97th Months Following the Pinatubo Eruption (‘‘Phase IV’’)a
tN (4)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 1 108 125 1 1 1 157 1 149
27.5 km 94 +1 1 +1 +1 1 +1 +1 1
25.5 km 66 +1 132 1 220 1 +1 1 234
23.5 km 56 1 182 181 126 +1 219 114 1
21.5 km 1 147 167 88 67 142 +1 208 1
19.5 km 116 123 72 61 59 99 142 +1 144
17.5 km 62 69 55 83 194 111 119 70 116
N0(4)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.002 0.004 0.005 0.011 0.029 0.006 0.002 0.001 0.000 27.5 km 0.006 0.004 0.009 0.050 0.074 0.014 0.003 0.002 0.001 25.5 km 0.012 0.011 0.037 0.081 0.152 0.029 0.006 0.004 0.002 23.5 km 0.031 0.030 0.121 0.193 0.249 0.103 0.011 0.006 0.007 21.5 km 0.033 0.153 0.240 0.302 0.313 0.214 0.043 0.021 0.028 19.5 km 0.095 0.272 0.411 0.294 0.250 0.245 0.146 0.084 0.078 17.5 km 0.288 0.329 0.280 0.124 0.101 0.109 0.220 0.237 0.203
dlog10(N)(4)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.398 0.285 0.474 0.445 0.434 0.488 0.389 0.308 0.303 27.5 km 0.503 0.369 0.418 0.375 0.334 0.428 0.515 0.351 0.231 25.5 km 0.439 0.370 0.363 0.287 0.238 0.345 0.376 0.312 0.352 23.5 km 0.513 0.457 0.322 0.203 0.149 0.278 0.357 0.264 0.278 21.5 km 0.485 0.267 0.211 0.159 0.205 0.256 0.382 0.346 0.181 19.5 km 0.460 0.222 0.199 0.183 0.148 0.204 0.303 0.386 0.298 17.5 km 0.280 0.260 0.304 0.296 0.291 0.276 0.227 0.262 0.247
aWhen the relaxation time is greater than 240 or lower than240, its value is noted as +1or1, respectively.
range between about 0.25mm at high altitude to 0.47mm in the lowermost equatorial stratosphere.
3. Error Budget
[20] The error range has been estimated for the three considered aerosol parameters. The uncertainty on the median radius and width depends on the experimental uncertainty on the extinction profiles, but is also affected by the optical inversion process. Indeed, the correctness of the parameter retrieval depends on the features of the merit function M [see Bingen et al., 2004a, equation (4)]
to be minimized during the least squares optimization. In our case, as already described in the literature [Echle et al., 1998; Fussen et al., 2001], the behavior of M tends to favor the instability of the solution in s, so that the error on this parameter is expected to be quite high. In other respects, because of the insensitivity of the extinc- tion cross section Q(r; l) to r in the Rayleigh limit, the quality of the size distribution retrieval is rather poor in the case of small particles with respect to the wavelength range, so that the error on all aerosol parameters is expected to increase where the amount of very thin particles is high. Finally, since the particle number density N is derived in a latter stage of the inversion process from the values of the median radius and mode width, we expect a correlation between the error on N and the error on r and s.
[21] The error estimate onrcorresponding to the median radius field illustrated in Figure 1b is given in Figure 3.
Roughly, the same general features of this profile are found at all latitudes. The error varies in the most cases between 5 and 20% in an altitude domain ranging from about 12 to 22 km at high latitudes and midlatitudes and about 18 to 30 km at equatorial latitudes. As expected, an increase of the error is found in periods of reduced aerosol load.
Especially at southern midlatitudes, it reaches 50% in the 12 – 22 km altitude region after the middle of 1997, and about 35% in the lower tropical stratosphere (z < 20 km) during the period 1989 to mid-1991.
[22] Above 25 km altitude, the error ranges between 5 and 50%.
[23] The error onscorresponding to the mode width map in Figure 1c is given in Figure 4. This case concerns the equatorial latitudes, but the same general features are observed at all latitudes. The error is found to be the smallest (smaller than 50%) at periods and regions where the aerosol load is high. Before mid-1985, the error profile above 30at low altitudes (z< 22 km) is similar to the error profile obtained during the period mid-1986 to begin 1988 at central latitudes and altitudes lower than 25 km. After the Pinatubo eruption, the same typical error range is found within a 10 km thick layer subsiding from about 25 km to 20 km during a relaxation period of 40 months (high latitudes) to 55 months (central latitudes), as shown in the figure. Afterward, the error increases at the same altitudes, whereas the aerosol loading progressively diminishes.
Concerning periods of low volcanism, the error reaches 50 – 100% at the middle of 1990. After the Pinatubo eruption, this feature is observed during the year 1996.
Later, the low aerosol concentration results in a higher error range fors, reaching 100 to 150% at the beginning of 1998, and 100 to 250% during the period of very low volcanism 1998 – 2000. Above an altitude of 25 km, the error on s varies most of the time between 100 and 250% during the whole SAGE II mission.
[24] Because of the way of retrievingNfromrandsand the possible high range of the error on these parameters, the error on the number density can take, in some cases, quite high values. Nevertheless, the error mostly does not exceed 200% in an altitude interval ranging from 12 – 20 km to 25 – 30 km, depending on the latitude. The error profile obtained at equatorial latitudes is shown in Figure 5. As in the case of s, the smallest values are found when the aerosol mass loading is high. During the first 2 years after the Pinatubo eruption, the error on N is mostly smaller than 25%, and can reach locally 50 to 75%. Afterward, the error range increases to about 50 to 100%, and reach locally 150 to 175%. During the relaxation period following the El Chi- chon eruption, the error values vary generally between 25 and 75%.
4. Comparison With Other Data Sets
[25] The validation by means of both remote and in situ measurements has been presented elsewhere [Bingen et al., 2002, 2003, 2004b]. Concerning remote sounding measure- ments, we found a quite good agreement with results obtained from the ORA experiment [Fussen et al., 2001].
The comparison ofN,randsprofiles between our results Table 5. Temporal Evolution of the Median Radius Between May
and December 1991 (1 Month Before to 7 Months After the Pinatubo Eruption; Referred to as ‘‘Phase I’’)a
vr (1)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km0.057 0.086 0.138 0.145 0.045 0.076 – – – 27.5 km 0.034 0.173 0.142 0.369 0.323 0.0870.0040.009 – 25.5 km 0.065 0.173 0.149 0.327 0.560 0.111 0.0310.135 – 23.5 km 0.195 0.351 0.249 0.299 – 0.139 0.237 0.253 – 21.5 km 0.294 0.455 0.3380.100 – 0.306 0.294 0.247 – 19.5 km 0.434 0.328 0.3010.113 – 0.061 0.371 0.201 – 17.5 km 0.450 0.208 0.1270.228 – 0.040 0.173 0.208 –
r0 (1)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.297 0.228 0.234 0.223 0.248 0.229 – – – 27.5 km 0.298 0.244 0.252 0.255 0.254 0.235 0.284 0.335 – 25.5 km 0.291 0.281 0.255 0.307 0.267 0.261 0.289 0.322 – 23.5 km 0.271 0.246 0.266 0.281 – 0.325 0.253 0.261 – 21.5 km 0.278 0.236 0.252 0.346 – 0.279 0.239 0.245 – 19.5 km 0.220 0.258 0.271 0.375 – 0.317 0.228 0.244 – 17.5 km 0.215 0.279 0.313 0.418 – 0.321 0.269 0.263 –
dr(1)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.019 0.019 0.034 0.042 0.034 0.019 – – – 27.5 km 0.024 0.032 0.043 0.057 0.027 0.021 0.030 0.087 – 25.5 km 0.021 0.048 0.040 0.093 0.041 0.039 0.040 0.020 – 23.5 km 0.023 0.064 0.062 0.074 – 0.050 0.034 0.023 – 21.5 km 0.045 0.035 0.037 0.069 – 0.040 0.015 0.029 – 19.5 km 0.042 0.036 0.042 0.050 – 0.049 0.036 0.020 – 17.5 km 0.049 0.051 0.067 0.060 – 0.051 0.025 0.015 –
aThe parameters are obtained by the linear regression onr, described by equation (2). Here,v(1)r, growth ratevrinmm/year;r0(1), value ofrinmm at the beginning of the time interval;dr(1), standard deviation onr.
Table 6. Same as Table 5 for the Time Period January to December 1992 Covering the 8th to 19th Months Following the Pinatubo Eruption (‘‘Phase II’’)
vr(2)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.033 0.033 0.064 0.106 0.006 0.028 0.059 0.032 0.157
27.5 km 0.051 0.049 0.036 0.108 0.001 0.020 0.017 0.017 0.202
25.5 km 0.064 0.071 0.029 0.142 0.069 0.013 0.011 0.010 0.141
23.5 km 0.096 0.035 0.083 0.068 0.070 0.024 0.022 0.026 0.028
21.5 km 0.089 0.087 0.123 0.064 – 0.075 0.056 0.031 0.015
19.5 km 0.116 0.130 0.051 – – 0.067 0.021 0.024 0.122
17.5 km 0.113 0.108 0.035 – – 0.061 0.028 0.061 0.225
r0 (2)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.244 0.272 0.299 0.339 0.306 0.310 0.310 0.242 0.235
27.5 km 0.296 0.304 0.314 0.380 0.330 0.337 0.311 0.299 0.337
25.5 km 0.341 0.351 0.338 0.443 0.397 0.359 0.341 0.313 0.356
23.5 km 0.387 0.365 0.398 0.453 0.442 0.369 0.337 0.320 0.336
21.5 km 0.413 0.421 0.452 0.446 – 0.379 0.324 0.314 0.323
19.5 km 0.459 0.504 0.468 – – 0.408 0.408 0.379 0.394
17.5 km 0.506 0.535 0.495 – – 0.450 0.433 0.382 0.392
dr(2)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.027 0.029 0.024 0.025 0.004 0.015 0.020 0.025 0.019
27.5 km 0.021 0.024 0.018 0.018 0.010 0.017 0.023 0.018 0.008
25.5 km 0.025 0.018 0.016 0.023 0.016 0.014 0.017 0.013 0.023
23.5 km 0.038 0.015 0.022 0.022 0.053 0.029 0.025 0.007 0.007
21.5 km 0.029 0.020 0.020 0.038 – 0.020 0.016 0.011 0.006
19.5 km 0.021 0.024 0.024 – – 0.055 0.017 0.019 0.009
17.5 km 0.028 0.022 0.031 – – 0.041 0.030 0.052 0.028
Table 7. Same as Table 5 for the Time Period January 1993 to May 1995 Covering the 20th to 48th Months Following the Pinatubo Eruption (‘‘Phase III’’)
vr(3)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.008 0.000 0.007 0.006 0.007 0.007 0.027 0.002 0.024
27.5 km 0.013 0.007 0.019 0.003 0.002 0.012 0.039 0.015 0.016
25.5 km 0.025 0.022 0.012 0.001 0.013 0.007 0.022 0.037 0.040
23.5 km 0.018 0.020 0.005 0.024 0.036 0.005 0.020 0.028 0.035
21.5 km 0.001 0.004 0.021 0.032 0.050 0.032 0.012 0.008 0.029
19.5 km 0.015 0.012 0.034 0.042 0.064 0.050 0.003 0.004 0.012
17.5 km 0.033 0.033 0.054 0.051 0.052 0.049 0.024 0.010 0.010
r0 (3)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.269 0.303 0.280 0.290 0.309 0.280 0.270 0.296 0.276
27.5 km 0.289 0.296 0.290 0.324 0.331 0.294 0.282 0.302 0.300
25.5 km 0.288 0.295 0.308 0.354 0.375 0.319 0.300 0.287 0.293
23.5 km 0.308 0.311 0.338 0.408 0.439 0.361 0.313 0.300 0.298
21.5 km 0.338 0.340 0.380 0.453 0.479 0.432 0.340 0.339 0.310
19.5 km 0.368 0.376 0.434 0.494 0.542 0.489 0.382 0.376 0.339
17.5 km 0.422 0.449 0.497 0.545 0.570 0.532 0.443 0.409 0.395
dr(3)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.028 0.061 0.038 0.013 0.026 0.021 0.046 0.034 0.018
27.5 km 0.039 0.026 0.043 0.018 0.023 0.028 0.053 0.047 0.020
25.5 km 0.029 0.025 0.019 0.017 0.027 0.027 0.026 0.029 0.027
23.5 km 0.022 0.023 0.022 0.024 0.026 0.028 0.018 0.017 0.018
21.5 km 0.017 0.013 0.019 0.031 0.058 0.035 0.021 0.017 0.017
19.5 km 0.022 0.022 0.026 0.028 0.040 0.033 0.038 0.032 0.013
17.5 km 0.031 0.026 0.027 0.030 0.034 0.023 0.029 0.025 0.015
and various in situ measurements [Bingen et al., 2002]
shows a moderate agreement with data derived byDeshler et al.[1992, 1993] from measurements using optical particle counters (OPC), and a fairly good agreement with data from Pueschel et al. [1994] and Sugita et al.[1999]. The latter comparison shows an underestimation of the particle num- ber density retrieved from SAGE II with respect to in situ data, for very small particles. A refined analysis of these comparisons shows that the discrepancies found between our data and the various in situ data sets are due to the inability of SAGE II to discriminate very thin particles in the Rayleigh limit of scattering. This hypothesis was veri- fied by comparing surface area density time series obtained from the present climatology with similar data derived from OPC measurements [Deshler et al., 2003], and by analyzing the contribution of the different particle classes through the partial surface area density and the differential surface area density [Bingen et al., 2004b]. Out of the regions and periods where tiny particles are found to be dominant, the agreement between OPC and our SAGE II derived surface area times series was very good.
[26] In another work, we derived aerosol mixing ratio time series over the whole post-Pinatubo period from both the Wyoming size distribution [Deshler et al., 2003] and from the present data set. It showed, for the considered particle class withr0.25mm, a good agreement between both time series, included during the periods of very low volcanism [Bingen et al., 2003].
[27] Another way to assess the performances of our data set is to perform intercomparisons of the partial number densities N[rr*](z) corresponding to the different particle classes detected by the OPC. In the case of a lognormal size
Table 8. Same as Table 5 for the Time Period July 1995 to June 1999 Covering the 50th to 97th Months Following the Pinatubo Eruption (‘‘Phase IV’’)
vr(4)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.0175 0.0016 0.0018 0.0139 0.0187 0.0088 0.0058 0.0108 0.0105
27.5 km 0.0129 0.0055 0.0015 0.0157 0.0219 0.0135 0.0004 0.0155 0.0245
25.5 km 0.0047 0.0005 0.0102 0.0189 0.0216 0.0173 0.0114 0.0020 0.0011
23.5 km 0.0000 0.0091 0.0130 0.0191 0.0217 0.0170 0.0084 0.0095 0.0092
21.5 km 0.0063 0.0125 0.0151 0.0189 0.0227 0.0237 0.0141 0.0194 0.0109
19.5 km 0.0166 0.0155 0.0149 0.0122 0.0152 0.0158 0.0180 0.0167 0.0149
17.5 km 0.0155 0.0146 0.0146 0.0095 0.0111 0.0100 0.0164 0.0133 0.0157
r0 (4)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.2493 0.2680 0.2694 0.2994 0.3122 0.3024 0.2997 0.2705 0.3292
27.5 km 0.2804 0.2838 0.2889 0.3141 0.3323 0.3123 0.3122 0.3384 0.2862
25.5 km 0.3041 0.2940 0.3093 0.3337 0.3485 0.3346 0.3382 0.3366 0.3486
23.5 km 0.3092 0.3133 0.3152 0.3473 0.3758 0.3439 0.3451 0.3605 0.3664
21.5 km 0.3097 0.3198 0.3281 0.3847 0.4113 0.3688 0.3519 0.3785 0.3573
19.5 km 0.3309 0.3380 0.3565 0.4163 0.4401 0.3842 0.3616 0.3604 0.3476
17.5 km 0.3423 0.3689 0.4080 0.4643 0.4670 0.4301 0.3721 0.3592 0.3517
dr(4)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.0276 0.0297 0.0457 0.0279 0.0368 0.0379 0.0504 0.0536 0.0177
27.5 km 0.0391 0.0426 0.0411 0.0301 0.0264 0.0256 0.0586 0.0390 0.0349
25.5 km 0.0361 0.0353 0.0315 0.0217 0.0199 0.0283 0.0426 0.0442 0.0370
23.5 km 0.0301 0.0246 0.0228 0.0190 0.0249 0.0312 0.0401 0.0362 0.0367
21.5 km 0.0161 0.0161 0.0152 0.0252 0.0270 0.0279 0.0305 0.0331 0.0269
19.5 km 0.0212 0.0122 0.0181 0.0306 0.0307 0.0298 0.0254 0.0246 0.0175
17.5 km 0.0216 0.0240 0.0313 0.0293 0.0459 0.0352 0.0235 0.0241 0.0143
Table 9. Temporal Evolution of the Mode Width Between May and December 1991 (1 Month Before and 7 Months After the Pinatubo Eruption; Referred to as ‘‘Phase I’’)a
vs(1)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.10 0.07 0.15 0.38 0.38 0.36 – – – 27.5 km 0.52 0.42 0.61 0.32 0.29 0.45 0.18 0.09 – 25.5 km 0.50 0.39 0.69 0.360.01 0.48 0.34 0.31 – 23.5 km 0.54 0.36 0.78 0.39 – 0.35 0.63 0.61 – 21.5 km 0.52 0.40 0.56 1.00 – 0.52 0.58 0.76 – 19.5 km 0.32 0.49 0.43 0.35 – 0.41 0.56 0.75 – 17.5 km 0.13 0.46 0.410.03 – 0.09 0.35 0.54 –
s0 (1)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.04 0.03 0.03 0.05 0.04 0.00 – – – 27.5 km 0.00 0.00 0.00 0.07 0.15 0.01 0.02 0.01 – 25.5 km 0.09 0.08 0.03 0.06 0.23 0.09 0.02 0.00 – 23.5 km 0.10 0.12 0.01 0.17 – 0.14 0.02 0.00 – 21.5 km 0.11 0.11 0.05 0.07 – 0.09 0.05 0.00 – 19.5 km 0.16 0.09 0.10 0.10 – 0.07 0.03 0.00 – 17.5 km 0.16 0.06 0.03 0.16 – 0.11 0.03 0.02 –
ds(1)
55S 45S 35S 15S 5 25N 45N 55N 65N
29.5 km 0.06 0.03 0.03 0.05 0.05 0.04 – – – 27.5 km 0.08 0.07 0.06 0.07 0.09 0.03 0.03 0.02 – 25.5 km 0.07 0.10 0.08 0.12 0.13 0.04 0.07 0.06 – 23.5 km 0.08 0.12 0.06 0.14 – 0.09 0.05 0.04 – 21.5 km 0.04 0.10 0.06 0.04 – 0.09 0.10 0.05 – 19.5 km 0.11 0.09 0.08 0.07 – 0.04 0.05 0.04 – 17.5 km 0.12 0.07 0.05 0.08 – 0.09 0.04 0.03 –
aThe parameters are obtained by the linear regression ons, described by equation (3). Here,vs(1), growth ratevs;s(1)0 , value ofsat the beginning of the time interval;ds(1), standard deviation ons.
