Get documents about "Lidar observations of Kasatochi volcano aerosols in the
Document Sample
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,
1 Lidar observations of Kasatochi volcano aerosols in
2 the troposphere and stratosphere
1 1 2 2 3
L. Bitar, T. J. Duck, N. I. Kristiansen, A. Stohl, and S. Beauchamp
S. Beauchamp, Meteorological Service of Canada, 16th Floor, Queen Square, 45 Alderney Dr.,
Dartmouth, Nova Scotia, Canada, B2Y 2N6 (steve.beauchamp@ec.gc.ca)
L. Bitar, Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova
Scotia, Canada, B3H 3J5 (lubna.m.bitar@gmail.com)
T. J. Duck, Department of Physics and Atmospheric Science, Dalhousie University, Halifax,
Nova Scotia, Canada, B3H 3J5 (tom.duck@dal.ca)
N. I. Kristiansen, Norwegian Institute for Air Research (NILU), P.O. Box 100, N-2027 Kjeller,
Norway (nik@nilu.no)
A. Stohl, Norwegian Institute for Air Research (NILU), P.O. Box 100, N-2027 Kjeller, Norway
(ast@nilu.no)
1
Department of Physics and Atmospheric
D R A F T December 2, 2009, 3:15pm D R A F T
X -2 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
3 Abstract. The eruption of Kasatochi volcano on 7–8 August 2008 injected
4 material into the troposphere and lower stratosphere of the northern mid-
5 latitudes during a period of low stratospheric aerosol background concentra-
6 tions. Aerosols from the volcanic plume were detected with a lidar in Hal-
7 ifax, Nova Scotia (44.64◦ N, 63.59◦ W) one week after the eruption and for the
8 next four months thereafter. The volcanic origin of the plume is established
9 using the FLEXPART Lagrangian particle transport model for both the strato-
10 sphere and troposphere. The stratospheric plume descended 47.1±2.8 m/day
11 on average as it dispersed, corresponding to a cooling rate of 0.60 ± 0.07
12 K/day. The descent rate was the same for the tropopause (within statisti-
13 cal uncertainties). The top of the plume remained steady at about 18 km al-
14 titude, and was likely sustained by vertical eddy diffusion from large-scale
15 horizontal mixing. The lower boundary of the plume descended with the tropopause.
16 The optical depth between 15–19 km altitude was relatively constant at 0.003
17 for 532 nm wavelength. Observations and modeling of Kasatochi aerosols in
Science, Dalhousie University, Halifax, Nova
Scotia, Canada.
2
Norwegian Institute for Air Research
(NILU), Kjeller, Norway.
3
Meteorological Service of Canada,
Dartmouth, Nova Scotia, Canada.
DRAFT December 2, 2009, 3:15pm DRAFT
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X-3
18 the middle and lower troposphere indicate they reached the ground. The vol-
19 canic contribution to surface PM2.5 did not exceed 5 µg/m3 at the measure-
20 ment site.
D R A F T December 2, 2009, 3:15pm D R A F T
X-4 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
1. Introduction
21 The Kasatochi volcano in the central Aleutian Islands of Alaska (52.17◦ N, 175.51◦ W)
22 erupted three times between 2201 UTC on 7 August and 0435 UTC on 8 August
23 2008, followed by a continuous ash-rich discharge for approximately 17 hours thereafter
24 [Waythomas et al., 2008]. The eruption injected a sulfur-rich plume into the lower strato-
25 sphere [Kristiansen et al., 2009; Prata et al., 2009; Karagulian et al., 2009], perturbing
26 its chemistry during a time of relatively low background aerosol content [Deshler , 2008].
27 Sulfur dioxide (SO2 ) in the eruption plume oxidized and condensed into sulfate aerosol,
28 which was distributed across the Northern Hemisphere together with other emissions that
29 were observed by many different measurement systems [Martinsson et al., 2009; Theys et
30 al., 2009; Prata et al., 2009; Karagulian et al., 2009; Bourassa et al., 2009; Hoffmann et
31 al., 2009]. A significant amount of SO2 was introduced into the troposphere [Kristiansen
32 et al., 2009], although this has received less attention.
33 The eruption plume was observed by the Dalhousie Raman Lidar in Halifax, Canada
34 (44.64◦ N, 63.59◦ W), 7445 km distance from the volcano, over a four-month period from
35 15 August – 4 December 2008. Anomalous increases of stratospheric aerosol content were
36 first detected on 15 August 2008 and observed thereafter whenever the meteorological
37 conditions permitted. Tropospheric aerosol layers measured by the lidar on 21–24 August
38 2008 were also from the Kasatochi eruption. The analysis and interpretation of measure-
39 ments from the lidar together with surface data and model simulations are presented in
40 this paper.
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X-5
41 FLEXPART, a Lagrangian particle transport model [Stohl et al., 2005], was used to
42 establish the volcanic origin of observed aerosols. Comparisons of the stratospheric mea-
43 surements with FLEXPART are presented in a separate paper [Kristiansen et al., 2009],
44 and a similar approach was used here to confirm the tropospheric detections. Although
45 the consequences of volcanic aerosols for climate have been investigated [Kravitz et al.,
46 2009; McCormick et al., 1995; Robock , 2000, 2002], an assessment for the potential impact
47 on air quality is still needed. The Kasatochi plume is shown to have likely reached the
48 surface near Halifax, causing PM2.5 increases of up to 5 µg/m3 . The EPA standard for
49 air quality is 65 µg/m3 over 24 hours [U.S. EPA, 2004], and so the overall impact on
50 surface air quality was small, although potentially widespread.
51 In the stratosphere, the plume descent and dispersion are investigated. The plume
52 descended at the same rate as the tropopause (within statistical uncertainties), and an es-
53 timate for the net stratospheric cooling rate is obtained. The plume maintained a presence
54 near 18 km altitude despite the descent, and this is attributed to vertical diffusion from
55 large-scale horizontal eddy mixing. The optical depth of the plume was fairly constant
56 over a four-month period, although a greater number of exceptional events were observed
57 in the first two months.
58 We begin by describing the lidar and measurement inversion process, and discuss the
59 specifics of the model simulations. The stratospheric measurements and interpretation
60 are given next, followed by the tropospheric analysis.
D R A F T December 2, 2009, 3:15pm D R A F T
X-6 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
2. Instrumentation and Modeling
2.1. Dalhousie Raman Lidar
61 The Dalhousie Raman Lidar measures vertical profiles of scattering from atmospheric
62 aerosols, clouds and molecules. It is a transportable instrument that has been used in
63 international measurement campaigns [Duck et al., 2007; McKendry et al., 2008]. Figure 1
64 gives a schematic diagram and Table 1 lists its specifications. The lidar employs a high-
65 energy Nd:YAG laser that emits pulses of 532 nm wavelength light at a repetition rate
66 of 20 Hz. The pulses are expanded and collimated to minimize beam divergence before
67 being transmitted upward into the atmosphere. The outgoing laser beam is pointed
68 into the fields-of-view of a 25 cm Newtonian telescope and a co-aligned 8 cm refractor
69 (usually limited to 3 cm aperture). The two telescopes allow for the backscattered light
70 to be collected separately from both the near and far ranges. This capability is used to
71 simultaneously obtain measurements in the boundary layer and free troposphere / lower
72 stratosphere.
73 Optical fibers guide the signals from each telescope into a polychromator, which uses
74 interference filters, collimating lenses, and dichroic mirrors to separate the returns into
75 elastic (532 nm) and nitrogen (N2 ) Raman-shifted (607 nm) wavelengths. To obtain high-
76 altitude elastic returns with low noise, a mechanical shutter (bow-tie chopper) is used to
77 block the intense low-altitude signals from reaching one detector.
78 Photomultiplier tubes are used for photon detection and the signals are recorded as a
79 function of altitude using fast counting computer electronics. Neutral density filters and
80 irises are placed in front of the photomultipliers to maintain signal intensities at reasonable
81 levels.
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X-7
82 A radar is used to protect aircraft flying over the measurement site [Duck et al., 2005]. A
83 circuit automatically interrupts the laser if an aircraft is detected during lidar operations,
84 which results in occasional measurement gaps.
85 The lidar is operated during intensive summertime measurement campaigns. Since the
86 eruption of Kasatochi volcano on 7–8 August 2008 occurred near the end of the 2008
87 campaign, measurements were continued thereafter, but with less time coverage, until 4
88 December 2008. The lidar signals were measured with a vertical resolution of 9.6 m and
89 a temporal resolution of 15 s. Spatial and time averaging are used in the data processing
90 to improve signal-to-noise ratios.
91 Profiles of the aerosol extinction at 532 nm wavelength are retrieved by inversion of the
92 elastic signals using the Klett technique [Klett, 1981]. The Raman technique [Ansmann et
93 al., 1990] was not used because signals are insufficiently strong for such an analysis. A lidar
94 (extinction-to-backscatter cross-section) ratio of 40 sr was assumed for the stratospheric
95 aerosols, which falls in the mid-range of values observed for background conditions and
96 major volcanic eruptions [Jaeger and Hofmann, 1991; Jaeger et al., 1995; Jaeger and
97 Deshler , 2002]. A lidar ratio of 71 sr was used for tropospheric aerosols, which is the
98 average observed for sulfate aerosols of urban/industrial origin [Cattrall et al., 2005]. A
99 similar value of the lidar ratio was measured in the upper troposphere of Ny-˚lesund,
A
100 Spitsbergen for the Kasatochi plume [Hoffmann et al., 2009].
101 The Klett inversion technique requires initialization at an altitude of known extinction.
102 We visually identify a range of contiguous initialization altitudes in each measurement
103 by an apparent lack of structure. This range is taken to be clear air, with zero aerosol
104 extinction. Initialization regions both below and above an aerosol plume are used as
D R A F T December 2, 2009, 3:15pm D R A F T
X-8 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
105 appropriate, and are mixed evenly in the data set presented here. The aerosol extinction
106 values given in the following sections represent the median of all possible retrievals, which
107 reduces noise and uncertainties in the results.
2.2. FLEXPART
108 The Lagrangian particle dispersion model FLEXPART simulates the long-range trans-
109 port and dispersion of many particles released from a defined source, while considering
110 wet and dry deposition, radioactive decay, and convection along the transport path. A
111 description of the model is given by [Stohl et al., 2005]. Both backward and forward trajec-
112 tories can be calculated using wind, temperature, and pressure fields from meteorological
113 analyses.
114 FLEXPART modeling of the eruption plume transport was conducted in forward mode
115 using SO2 as a tracer. Backward-mode simulations for the transport of anthropogenic
116 SO2 were also conducted for comparison. Dry deposition and oxidation by OH radicals
117 are parametrized as removal processes. The simulations employed meteorological data
118 from the European Center for Medium-Range Weather Forecasts (ECMWF), which have
119 91 vertical levels and 0.5◦ × 0.5◦ horizontal resolution for the eastern North Pacific region
120 (1◦ ×1◦ globally). The volcanic source term for the simulations was taken from Kristiansen
121 et al. [2009], who used FLEXPART, satellite observations of SO2 during the first few days
122 after the eruption, and an inversion algorithm to determine an optimal emission height
123 profile.
124 SO2 is gradually converted into the sulfate aerosol observed by the lidar, and so com-
125 parisons between the FLEXPART SO2 simulations and aerosol optical depth and surface
126 PM2.5 measurements are qualitative. The model was run at 500 m vertical and 1 hour
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X-9
127 temporal resolution in order to provide detailed structural comparisons with the lidar
128 measurements.
3. Stratosphere
3.1. Vertical distribution and descent
129 Figure 2 shows aerosol extinction contours measured by the lidar on 21–24 August
130 and selected periods between 4–12 September 2008 which reveal the presence of aerosol
131 layers high in the atmosphere above Halifax. On August 21–24, an aerosol layer persisted
132 throughout the 84-hour measurement period at a relatively stable altitude of about 18 km,
133 and decreased in intensity with time. A second layer appeared in the morning of August
134 23 near 17 km and was observed until the morning of August 24. One month following
135 the Kasatochi eruption, distinct high-altitude aerosol plumes were still observed during
136 September 4–12. On each day of the measurements presented in Figure 2, the aerosol
137 plumes varied in optical density with values of the aerosol extinction ranging from just
138 above background levels to a maximum of approximately 0.03 km−1 . The aerosol layers
139 were highly structured with vertical thickness less than 1 km, and remained confined
140 between 16–18 km altitude. Light aerosol loading can be seen just below the main plume
141 in each measurement, likely of the same origin.
142 Temperature profiles measured by radiosondes launched from the nearest weather sta-
143 tion (Yarmouth, approximately 300 km southwest of Halifax at 43.86◦ N, 66.10◦ W) were
144 used to gauge the tropopause height (Figure 3). During the measurement intervals of
145 Figure 2 the tropopause ranged from 12–16 km altitude, indicating that the aerosols were
146 within the lower stratosphere. Injection of aerosols past the tropopause is suggestive of a
147 powerful event such as a volcanic eruption. In a separate paper, the stratospheric aerosols
D R A F T December 2, 2009, 3:15pm D R A F T
X - 10 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
148 observed above Halifax were verified to be from the Kasatochi eruption by using com-
149 parisons with FLEXPART simulations [Kristiansen et al., 2009]. Here, we focus on the
150 evolution of the plume in the four months following the eruption, assuming no further
151 stratospheric aerosol injections.
152 Figure 3 displays the altitude of maximum aerosol extinction (hereafter referred to as
153 just the “plume altitude”) versus time, along with the height of the tropopause. The plume
154 altitude was determined by inspection of hourly-integrated aerosol extinction profiles from
155 August–October and three-hour integrated profiles from November, both with a vertical
156 resolution of 100 m. The measurements during November were integrated over three hours
157 in order to improve signal-to-noise ratios since the measured aerosol extinction dropped as
158 the aerosols dispersed over time. Only profiles with identifiable maxima above the aerosol
159 background were considered, and those that contained too much noise to distinguish
160 an aerosol layer were ignored. Gaps in the data were mostly a result of meteorological
161 conditions inappropriate for lidar operation.
162 As seen in Figure 3, prior to 4 September 2008 the plume altitude increased with
163 time. This is not evidence for ascent of the stratospheric air mass, but is instead due
164 to differential advection of the plume by the jet stream, which has maximum speed near
165 the tropopause. After September 4, the plume descended until the end of observations
166 in early December, and this is likely due to stratospheric subsidence from net radiative
167 cooling during the transition from summer to winter. Hourly fluctuations in the plume
168 altitude were evident, in some cases varying by as much as 1 km during a single day.
169 The plume descent rate was determined to be 47.1 ± 2.8 m/day using a linear least-
170 squares fit. Weekly average values of the plume altitude were used in the calculation
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 11
171 to remove bias due to the uneven distribution of data in the 16 weeks of measurements
172 considered. The fitting procedure was applied only to the data measured after September
173 4.
174 The tropopause altitude shown in Figure 3 was determined from radiosonde measure-
175 ments obtained at 0000 UTC and 1200 UTC. The tropopause altitude was comparatively
176 much more variable, but in general also descended with time. A linear least-squares fit
177 after September 4 gives a descent rate of 52.7 ± 6.5 m/day. Although more uncertain,
178 this value is statistically consistent with the descent rate for the plume. The correla-
179 tion between the decent of the aerosols and the tropopause is intriguing given that the
180 tropopause is a dynamical feature. In this case, it appears to have descended like a mate-
181 rial element as the stratosphere cooled, although there is no obvious physical mechanism
182 to explain this behavior.
183 The potential temperatures corresponding to the plume altitudes in Figure 3 were de-
184 termined by using daily radiosonde data. As shown in Figure 4, a linear least-squares fit
185 to the potential temperature data yields a lower-stratospheric cooling rate of 0.60 ± 0.07
186 K/day. This is somewhat greater than expected from global circulation model simulations
187 (e.g., Hamilton et al. [1995]), and this may be due to differences between the dynamics in
188 model climatologies and the actual atmospheric conditions during our measurements. In
189 any event, the direct radiative impact of the aerosols would be expected to have a minimal
190 impact, given that the Pinatubo volcano eruption caused 0.01–0.05 K/day heating of the
191 stratosphere at mid-latitudes [Robock , 2000] for much larger optical depths.
192 The vertical dispersion of the Kasatochi eruption plume with time is shown in Figure 5,
193 which provides daily-average aerosol extinction profiles. The figure contains all profiles
D R A F T December 2, 2009, 3:15pm D R A F T
X - 12 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
194 obtained during the campaign, regardless of whether or not the plume was clearly in
195 evidence. The aerosol extinction varied from day to day with maximum values observed
196 on August 21 and September 11 and 23, which are displayed in more detail in Figure 2.
197 The vertical extent of the aerosols was variable with some days having two distinct layers
198 present. The plume descent is apparent and is consistent with the results depicted in
199 Figure 3. The plume is evident throughout the four-month measurement period.
200 There was little trend in the maximum altitude of the layer, which remained relatively
201 stable around 18 km until near the end of the four-month period of observations. This
202 result is consistent with the aerosol measurements for 40◦ and 50◦ N from the OSIRIS
203 satellite instrument [Bourassa et al., 2009]. A steady upper altitude for the plume is
204 interesting given that air in the stratosphere slowly descended from radiative cooling
205 during the transition from summer to winter, and is due to vertical eddy diffusion. The
206 characteristic length scale LD in diffusion problems is given by
207 LD = 4Kz t
208 where Kz is the eddy diffusivity and t is time. Substituting 5 km ascent over 12 weeks
209 yields a diffusivity Kz ≈ 0.9 m2 /s, and estimates using shorter intervals (Kz is quadratic
210 in LD ) yield lower values. The results are consistent with expectations “of a few times
211 10−1 ” for large-scale horizontal eddy mixing [Holton et al., 1995].
212 The base of the plume followed the descent of the tropopause so that by November,
213 the initially vertically thin aerosol plumes were distributed from approximately 12 to 18
214 km altitude. The influence of tropopause variability on the aerosol layer structure is
215 particularly evident in the measurements obtained beyond October (i.e., after day 54).
216 Martinsson et al. [2009] identified elevated sulfur and carbon particle concentrations in
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 13
217 the upper troposphere / lower stratosphere over Europe for three to four months after
218 the eruption that they attributed to the Kasatochi eruption. Our measurements suggest
219 that the continued source of upper tropospheric Kasatochi aerosols may have been the
220 lower stratosphere. The presumed mechanisms for tropospheric-stratospheric exchange
221 are tropopause folding (e.g., [Holton et al., 1995]) and eddy diffusion from meso- and
222 small-scale turbulence at the tropopause (e.g. Duck and Whiteway [2005]). The gradual
223 addition of aerosols to the upper troposphere by way of the stratosphere might cause an
224 indirect seasonal impact on radiative transfer through the modification of cloud optical
225 properties.
226 The measurements obtained during August to early October showed considerable vari-
227 ability in the intensity and time variation of the aerosol plume on hourly time scales, char-
228 acterized by narrow and distinct extinction maxima (e.g. Figure 2). At times, aerosols
229 were not consistently present during a single measurement. This scenario gradually gave
230 way to the constant presence of aerosols in the latter half of October and November, but
231 with reduced and more uniform intensity. This evolution is consistent with gradual and
232 extensive horizontal mixing as seen in satellite measurements.
3.2. Optical depth
233 Figure 6 provides the temporal evolution of the daily aerosol optical depths at 532 nm
234 wavelength between 15–19 km altitude determined from the extinction data in Figure 5.
235 Three measurements before the plume’s arrival show near-zero aerosol optical depth, which
236 illustrates the overall sensitivity of our measurement technique when compared to the
237 plume data that follow.
D R A F T December 2, 2009, 3:15pm D R A F T
X - 14 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
238 The optical depth of Kasatochi aerosols between 15–19 km remained relatively constant
239 at about 0.003 throughout the observation period. This is surprising at first because the
240 intensity of the aerosol layers immediately following the eruption was much higher (e.g.,
241 Figure 2) than toward the end. However, as noted above there were initially extended
242 periods where the plume was absent, and these were included in the integrations. Thus,
243 Figure 6 indicates that the aerosol was reasonably conserved during the horizontal mixing
244 process.
245 Relatively stable stratospheric optical depth over the same period was also observed by
246 the OSIRIS satellite instrument at 45◦ N [Kravitz et al., 2009]. The optical depth measured
247 by OSIRIS was about 0.0055 at 750 nm wavelength, which is of similar magnitude to the
248 mean of 0.003 we observed at 532 nm wavelength. The OSIRIS optical depth measurement
249 used a lower bound in potential temperature of 380 K, which corresponds to about the
250 same 15 km lower altitude limit for our optical depth measurements. If the difference
251 in wavelengths is taken into account, a scaling factor of 0.8 results [Kravitz et al., 2009].
252 This leads to a satellite-derived aerosol optical depth of 0.0069 at 550 nm, which is a little
253 more than twice what we measured. The difference can be partly attributed to systematic
254 uncertainties in the lidar ratio used in the retrievals. Notwithstanding, Kasatochi induced
255 a much smaller perturbation to the stratosphere than the Pinatubo eruption, which yielded
256 optical depths in the stratosphere up to 0.2 [Ansmann et al., 1997].
257 The variability of the optical depth measurements given in Figure 6 changed consider-
258 ably with time. The first two months were characterized by high variability, with a greater
259 incidence of outliers with high optical depth. This is to be expected for a plume before it
260 is well-mixed with the environment. The persistence of high optical depth outliers up to
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 15
261 two months after the eruption follows from the e-folding time for the conversion of SO2 to
262 sulfate aerosol, which is about 30 days [Textor et al., 2004]. During the latter two months
263 the aerosol load is much more consistent, with outliers having low optical depth. This
264 is consistent with a well-mixed plume occasionally descending to below the optical depth
265 measurement altitudes (15–19 km).
266 Interpretation of the optical depth measurements is subject to our assumption of a con-
267 stant lidar ratio, which might have systematic error and change with time. Measurements
268 of Kasatochi aerosols in the upper troposphere immediately following the eruption indi-
269 cated variable size, which would lead to varying lidar ratios. We expect that horizontal
270 mixing should diminish this effect after a few weeks time. Jaeger and co-authors [Jaeger
271 and Hofmann, 1991; Jaeger et al., 1995; Jaeger and Deshler , 2002] showed that the li-
272 dar ratio for stratospheric aerosols varies on a seasonal time-scale and decreases in the
273 year following a major volcanic eruption. The retrievals presented here have been ana-
274 lyzed using the different lidar ratios measured by Jaeger and co-authors for stratospheric
275 aerosols, which range from approximately 20–60 sr. The maximum systematic error in the
276 retrieved extinction profile introduced by assuming an incorrect lidar ratio is 40%. Our
277 actual uncertainties are expected to be below that level given the differences in time-scale
278 from what Jaeger and co-authors considered.
4. Troposphere
279 Figure 7 shows the tropospheric measurement corresponding to the August 21–24 data
280 presented in Figure 2. Aerosol layers were observed on August 22 through August 24,
281 extending up to 7 km in altitude. The onset of an intense low-altitude aerosol event was
282 observed on August 22, which descended from about 3 km altitude at 0000 UTC down
D R A F T December 2, 2009, 3:15pm D R A F T
X - 16 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
283 to ground level reaching the surface just after 1300 UTC. In the middle troposphere an
284 optically-thin “halo” of aerosols with an area of clear air at the center is apparent. On
285 August 24, a decrease in the surface aerosol extinction was observed, gradually extending
286 up to 1 km in altitude until 1200 UTC while enhanced aerosol extinction remained in the
287 overlying layer between approximately 1–1.5 km altitude.
288 Figure 8 shows the FLEXPART-simulated SO2 emissions from the Kasatochi eruption
289 above Halifax for August 21–24. The simulated SO2 concentrations, used as a proxy for
290 aerosol formation, showed a similar overall vertical distribution as the aerosols observed by
291 the lidar. The “halo” of aerosols was evident and in good spatial and temporal agreement
292 with the lidar measurement, which provides strong evidence that this feature originated
293 from the Kasatochi eruption. The intense lower-altitude event at 3 km altitude was also
294 captured by the model as well as its descent to the surface. This indicates a contribu-
295 tion from Kasatochi emissions to the surface aerosol burden, although there were likely
296 contributions from anthropogenic and other natural sources as well.
297 The timing difference in the onset of the simulated and measured aerosol events can
298 be attributed to the coarseness of the SO2 emissions inventory. The emission profile
299 represents the mean for the first two eruptive events, and the third eruption occurred six
300 hours after the first. Higher temporal resolution in the inventory could not be realistically
301 obtained in the retrieval process (see Kristiansen et al. [2009]). Thus, timing errors of up
302 to six hours are not unexpected.
303 In comparison to the stratospheric aerosol plumes observed during the same lidar mea-
304 surement, much more variability was apparent in the vertical structure of the tropospheric
305 aerosols throughout the measurement period. The aerosol plumes in the troposphere ap-
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 17
306 peared more diffuse and were distributed over a wider altitude range than those observed
307 in the lower stratosphere. This is a consequence of the lower static stability of the tro-
308 posphere compared to the stratosphere, which leads to material being detrained from the
309 eruptive column over deeper layers than in the stratosphere where injection occurs in
310 discrete layers. The aerosol extinction ranged in value from about 0.002 km−1 for the
311 lightest aerosol loads to 0.12 km−1 in the regions of densest aerosol content. The intensity
312 of aerosol extinction in the troposphere was more than that observed in the stratosphere,
313 with the greatest values detected below 3 km altitude. Kristiansen et al. [2009] showed
314 that a larger portion of the SO2 (about 60%) was emitted into the stratosphere; however,
315 as observed from Halifax, the aerosol intensity was greatest in the troposphere. This is
316 likely due to the relatively rapid formation of sulfate aerosols by SO2 oxidation in the
317 lower troposphere, and contributions in the troposphere from other aerosol sources.
318 Figure 9 compares the average aerosol extinction profile measured between August 22 at
319 1200 UTC and August 23 at 1200 UTC with the average simulated SO2 profile during the
320 same time interval. The vertical resolution of the FLEXPART profile is 500 m whereas for
321 the lidar profile it is 50 m. The simulated SO2 profile reproduces the form of the measured
322 aerosol extinction profile, and captures both the surface aerosols as well as those in the free
323 troposphere. The fact that the upper layer appears relatively stronger in the FLEXPART
324 simulation can be explained by the fact that the model simulates SO2 whose removal rate
325 decreases with altitude. On the other hand, the aerosol extinction is a measure of the
326 secondary aerosol product whose formation rate decreases with altitude.
327 Additional tropospheric lidar detections of Kasatochi aerosols in August were confirmed
328 by FLEXPART (not shown). The model indicated that the first arrival of volcanic aerosols
D R A F T December 2, 2009, 3:15pm D R A F T
X - 18 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
329 over Halifax was between 0200–0600 UTC on 14 August, where two aerosol layers were
330 observed just below the tropopause at about 7 km and 8 km. Optically-thin aerosol
331 layers originating from the Kasatochi eruption were also observed in the free troposphere
332 between 4–7 km in altitude on 15 August 2008. On these days, the aerosols remained in
333 the free troposphere and did not reach the boundary layer. The tropospheric detections
334 of volcanic aerosols produced by Kasatochi were dispersed much more rapidly than those
335 observed in the lower stratosphere. This made their identification more difficult given
336 the abundance of aerosols in the troposphere from other sources and more rapid removal
337 processes, and thus later detections of tropospheric volcanic aerosols cannot be confirmed.
338 Figure 10 shows surface PM2.5 measurements during August 21–24 from Kejimkujik Na-
339 tional Park (44.4◦ N, 65.2◦ W) near Halifax together with the simulated SO2 from FLEX-
340 PART for both volcanic and anthropogenic sources. The PM2.5 increases during August
341 22, and dissipates on August 23, in agreement with the lowest altitude of the lidar’s aerosol
342 extinction measurement given in Figure 7. FLEXPART indicates that the aerosol content
343 during August 21 and 22 is due equally to the Kasatochi and anthropogenic emissions,
344 whereas the peak values observed on August 23 are dominated by anthropogenic sources.
345 The maximum value for PM2.5 during the period of potential Kasatochi surface influence
346 was 5 µg/m3 , which we take to be the maximum possible impact from the volcanic emis-
347 sions at Kejimkujik. Although this value is small compared to the EPA standard for air
348 quality (65 µg/m3 over 24 hours), there was potentially a very large area impacted with
349 varying intensities.
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 19
5. Summary and Conclusions
350 Lidar measurements obtained from Halifax for four months following the eruption of the
351 Kasatochi volcano on 7–8 August 2008 were used to characterize the vertical structure and
352 evolution of the resulting volcanic aerosols. Kasatochi aerosols were first detected above
353 Halifax one week following the eruption, and were observed both in the troposphere and
354 lower stratosphere until 4 December 2008. The stratospheric aerosols developed as a
355 thin structured plume near 18 km altitude and gradually dispersed over the four month
356 observation period. Tropospheric aerosols were only observed definitely in the second
357 week after the eruption.
358 The stratospheric aerosol maximum descended with time in correlation with the
359 tropopause altitude during the transition from summer to winter. The descent corre-
360 sponded to a lower stratospheric cooling rate of 0.60 ± 0.07 K/day. The top of the plume
361 persisted at 18 km altitude, and was likely sustained there by vertical diffusion from large-
362 scale horizontal eddy mixing. The bottom of the plume reached the tropopause, and likely
363 provided an aerosol source for the upper troposphere, leading to a possible impact on cloud
364 properties and radiation during the four months or more following the eruption. Even
365 though the plume was dispersed the aerosol optical depth remained relatively stable, an
366 observation that is in agreement with similar measurements by OSIRIS.
367 In comparison to the stratospheric observations, the tropospheric aerosols observed on
368 August 21–24 were much more variable. Mixing with aerosols from different sources
369 diluted the tropospheric aerosols and removal processes made it difficult to observe them
370 after a short time interval had passed. Tropospheric aerosols originating from Kasatochi
D R A F T December 2, 2009, 3:15pm D R A F T
X - 20 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
371 likely reached the surface near Halifax. The maximum contribution to surface PM2.5 was
372 5 µg/m3 , which is considered small.
373 Acknowledgments. This study was supported by the Canadian Foundation for Cli-
374 mate and Atmospheric Science (CFCAS) and the Natural Sciences and Engineering Re-
375 search Council (NSERC) of Canada using equipment funded by the Canadian Foundation
376 for Innovation (CFI) and the Nova Scotia Research and Innovation Trust (NSRIT). Rob
377 Harris, Ben Bougher, and Marshall Hawkins helped operate the lidar during the summer-
378 time 2008 measurement campaign.
References
379 Ansmann, A., M. Riebesell, and C. Weitkamp (1990), Measurement of atmospheric aerosol ex-
380 tinction profiles with a Raman lidar, Opt. Lett., 15, 746–748.
381 Ansmann, A., I. Mattis, U. Wandinger, F. Wagner, J. Reichardt, and T. Deshler (1997), Evolution
382 of the Pinatubo Aerosol: Raman lidar observations of particle optical depth, effective radius,
383 mass, and surface area over Central Europe at 53.4◦ N, J. Atmos. Sci., 54, 2630–2641.
384 Bourassa, A. E., D. A. Degenstein, B.J. Elash, and E. J. Llewellyn (2009), Evolution of the strato-
385 spheric aerosol enhancement following the Kasatochi eruption: Odin-OSIRIS measurements,
386 J. Geophys. Res., special issue, submitted 2009.
387 Cattrall, C., J. Reagan, K. Thome, and O. Dubovik (2005), Variability of aerosol and spec-
388 tral lidar and backscatter extinction ratios of key aerosol types derived from selected Aerosol
389 Robotic Network locations, J. Geophys. Res., 110, D10S11, doi:10.1029/2004JD005124.
390 Deshler, T. (2008), A review of global stratospheric aerosol: Measurements, importance, life
391 cycle, and local stratospheric aerosol, Atmos. Res., 90, 223–232.
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 21
392 Duck, T. J., B. Firanski, F. D. Lind, and D. Sipler (2005), Aircraft-protection radar for use with
393 atmospheric lidars, Appl. Opt., 44, 4937–4945.
394 Duck, T. J., and J. A. Whiteway (2005), The spectrum of waves and turbulence at the tropopause,
395 Geophys. Res. Lett., 32, L07801, doi: 10.1029/2004GL021189.
396 Duck, T. J., B. J. Firanski, D. B. Millet, A. H. Goldstein, J. Allan, R. Holzinger, D. R. Worsnop,
397 A. B. White, A. Stohl, C. S. Dickinson, and A. van Donkelaar (2007), Transport of forest
398 fire emissions from Alaska and the Yukon Territory to Nova Scotia during summer 2004, J.
399 Geophys. Res., 112, D10S44, doi: 10.1029/2006JD007716.
400 Hamilton, K., R. J. Wilson, J. D. Mahlman, and L. J. Umscheid (1995), Climatology of the
401 SKYHI troposphere-stratosphere-mesosphere general circulation model, J. Atmos. Sci., 52,
402 5–43.
403 Hoffmann, A., C. Ritter, M. Stock, M. Maturilli, and R. Neuber (2009), Lidar measurements of
404 the Kasatochi aerosol plume in August and September 2008 in Ny-˚lesund, Spitsbergen, J.
A
405 Geophys. Res., special issue, submitted 2009.
406 Holton, J. R., P. Haynes, M. E. McIntyre, A. R. Douglass, R. B. Rood, and L. Pfister (1995),
407 Stratosphere-Troposphere Exchange, Rev. Geophys., 33, 403–439.
408 Jaeger, H. and D. Hofmann (1991), Midlatitude lidar backscatter to mass, area, and extinction
409 conversion model based on in situ aerosol measurements from 1980 to 1987, Appl. Opt., 21,
410 127–138.
411 Jaeger, H., T. Deshler, and D. J. Hofmann (1995), Midlatitude lidar backscatter conversions
412 based on ballooneborne aerosol measurements, Geophys. Res. Lett., 22, 1729–1732.
413 Jaeger, H. and T. Deshler (2002), Lidar backscatter to extinction, mass and area conversions
414 for stratospheric aerosols based on midlatitude ballooneborne size distribution measurements,
D R A F T December 2, 2009, 3:15pm D R A F T
X - 22 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
415 Geophys. Res. Lett., 29, doi:10.1029/2002GL015609.
416 Karagulian, F., L. Clarisse, P. F. Coheur, A.J. Prata, D. Hurtmans, and C. Clerbaux (2009),
417 Detection of SO2 , ash and H2 SO4 using the IASI sounder, J. Geophys. Res., accepted.
418 Klett, J. D. (1981), Stable analytical inversion solution for processing lidar returns, Appl. Opt.,
419 20, 211–220.
420 Kravitz, B., A. Robock, A. Bourassa, and G. Stenchikov, Negligible climatic effects from the 2008
421 Okmok and Kasatochi volcanic eruptions, J. Geophys. Res., special issue, submitted 2009.
422 Kristiansen, N. I., A. Stohl, A. J. Prata, A. Richter, S. Eckhardt, P. Seibert, A. Hoffmann,
423 C. Ritter, L. Bitar, T. J. Duck, and K. Stebel (2009), Remote Sensing and inverse transport
424 modeling of the Kasatochi eruption SO2 cloud, J. Geophys. Res., special issue, submitted 2009.
425 Martinsson, B. G., C. A. M. Brenninkmeijer, S. A. Carn, M. Hermann, K.-P. Heue, P. F. J.
426 van Velthoven, and A. Zahn (2009), Influence of the 2008 Kasatochi volcanic eruption on
427 sulfurous and carbonaceous aerosol constituents in the lower stratosphere, Geophys. Res. Lett.,
428 36, L12813, doi:10.1029/2009GL038735.
429 McCormick, M. P., L. W. Thomason, and C. R. Trepte (1995), Atmospheric effects of the Mt
430 Pinatubo eruption, Nature, 373, 399–404.
431 McKendry, I. G., A. M. Macdonald, W. R. Leaitch, A. van Donkelaar, Q. Zhang, T. Duck, and
432 R. V. Martin (2008), Trans-Pacific dust events observed at Whistler, British Columbia during
433 INTEX-B, Atmos. Chem. Phys., 8, 6297–6307.
434 Prata, A. J., G. Gangale, L. Clarisse,and F. Karagulian (2009), Ash and sulphur dioxide in
435 the 2008 eruptions of Okmok and Kasatochi - insights from high spectral resolution satellite
436 measurements, J. Geophys. Res., submitted.
437 Robock, A. (2000), Volcanic eruptions and climate, Rev. Geophys., 38, 191–219.
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 23
438 Robock, A. (2002), Pinatubo eruption: The climatic aftermath, Science, 295, 1242–1244.
439 Stohl, A., C. Forster, A. Frank, P. Seibert, and G. Wotawa (2005), Technical note: The La-
440 grangian particle dispersion model FLEXPART version 2.6, Atmos. Chem. Phys., 5, 2461–
441 2474.
442 Textor, C., H.-F. Graf, C. Timmreck, and A. Robock (2004), Emissions from volcanoes, Chapter
443 7 of Emissions of Chemical Compounds and Aerosols in the Atmosphere, C. Granier, P. Artaxo,
444 and C. Reeves, 269–303.
445 Theys, N., M. van Roozendael, B. Dils, F. Hendrick, N. Hao, and M. De Maziere (2009), First
446 satellite detection of volcanic bromine monoxide emission after the Kasatochi eruption, Geo-
447 phys. Res. Lett., 36, L03809, doi:10.1029/2008GL036552.
448 U.S. EPA (2004), Air quality criteria for particulate matter (final report, Oct 2004), U.S. Envi-
449 ronmental Protection Agency, Washington, DC, EPA 600/P-99/002aF-bF.
450 Waythomas, C. F., S. G. Prejean, and D. J. Schneider (2008), Small volcano, big eruption,
451 scientists rescued just in time: US Department of the Interior online publication People, Land,
452 and Water preprint.
D R A F T December 2, 2009, 3:15pm D R A F T
X - 24 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
Figure 1. A schematic diagram of the Dalhousie Raman Lidar System. The laser transmitter
employs a diverging mirror (DM) and collimating mirror (CM) as a beam expander, and an
actuator-controlled steering mirror (SM) directs the beam upward. Two telescopes (the Newto-
nian T1 and and actuator-controlled refractor T2) are used in the receiver. Light is coupled into
optical fibers using mirrors (M) and lenses (L). The polychromator makes use of long-pass (LP)
filters and interference filters (IF) to perform wavelength selection, and neutral density (ND)
filters to limit the signal strength. A beam splitter (BS) channels 99% of the primary 532 nm
channel through a chopper into the high-altitude portion of the receiver. Photomultiplier tubes
(PMTs, with wavelengths marked in nm) and fast-counting computer electronics are used for
signal detection. A pulse generator governs the instrument timing, and a radar is used to protect
aircraft flying overhead via the master interlock.
Figure 2. Stratospheric aerosol extinction (532 nm wavelength) measured above Halifax on 21–
24 August, and 4, 8–12 September 2008. The data show the temporal evolution of high-altitude
aerosol plumes originating from the 7–8 August 2008 eruption of Kasatochi volcano. Gaps in the
measurements are mostly due to interference from clouds below, which can completely attenuate
the laser beam.
Figure 3. The altitude of maximum aerosol extinction (“plume altitude”) and that of the
local tropopause during August through November 2008. The black straight line marks the
mean descent of the Kasatochi plume altitude and the grey line marks the mean descent of the
tropopause.
Figure 4. The potential temperatures θ corresponding to the daily-mean plume altitudes from
the data in Figure 3.
D R A F T December 2, 2009, 3:15pm D R A F T
BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS X - 25
Figure 5. The daily-averaged aerosol extinction profiles (532 nm wavelength) showing the
vertical dispersion of the Kasatochi plume. The tropopause height is overlayed as a white line
for comparison. Note the nonlinear time scale, varying from days to weeks. Interference from
cirrus clouds in the upper troposphere is greyed out at the bottom of the profiles.
Figure 6. The daily aerosol optical depths (532 nm wavelength) measured between 15–19 km
altitude in the four months following the eruption of Kasatochi.
Figure 7. Tropospheric aerosol extinction (532 nm wavelength) measured above Halifax on
21–24 August 2008.
Figure 8. FLEXPART simulation of Kasatochi SO2 emissions appearing above Halifax on
21–24 August 2008.
Figure 9. Average profiles of observed aerosol extinction and simulated SO2 for Halifax
between 1200 UTC 22 August to 12 UTC 23 August 2008.
Figure 10. Measured PM2.5 and simulated SO2 for Kejimkujik National Park on 21–24 August
2008. The simulated SO2 is broken down into Kasatochi emissions and anthropogenic (other)
sources.
D R A F T December 2, 2009, 3:15pm D R A F T
X - 26 BITAR ET AL.: LIDAR OBSERVATIONS OF KASATOCHI VOLCANO AEROSOLS
Table 1. Dalhousie Raman Lidar system specifications.
Transmitter
Laser Model Continuum Powerlite
Precision II 8020
Transmitted Wavelength 532 nm
Pulse Repetition Frequency 20 Hz
Pulse Duration 8 ns
Pulse Energy 550 mJ
Receiver
Far Field Telescope Diameter 25 cm
Near Field Telescope Diameter 8 cm
Photomultiplier Tubes Hamamatsu R7400P
Counter Board Model FAST ComTec P7888
Counter Speed 1 GHz
Data Acquisition
Elastic Wavelength 532 nm
Molecular Wavelength 607 nm
Range Resolution 9.6 m
Temporal Resolution 15 s
D R A F T December 2, 2009, 3:15pm D R A F T
