On recent ( 2008 – 2012 ) stratospheric aerosols observed by lidar over Japan

An increase in stratospheric aerosols caused by the volcanic eruption of Mt. Nabro (13.37 ◦ N, 41.70 E) on 12 June 2011 was detected by lidar at Tsukuba (36.05 ◦ N, 140.13 E) and Saga (33.24 ◦ N, 130.29 E) in Japan. The maximum backscattering ratios at a wavelength of 532 nm were 2.0 at 17.0 km on 10 July 2011 at Tsukuba and 3.6 at 18.2 km on 23 June 2011 at Saga. The maximum integrated backscattering coefficients (IBCs) at 532 nm above the first tropopause height were 4.18 × 10−4 sr−1 on 11 February 2012 at Tsukuba and 4.19 × 10−4 sr−1 on 23 June 2011 at Saga, respectively. A time series of lidar observational results at Tsukuba have also been reported from January 2008 through May 2012. Increases in stratospheric aerosols were observed after the volcanic eruptions of Mt. Kasatochi (52.18 ◦ N, 175.51 E) in August 2008 and Mt. Sarychev Peak (48.09 ◦ N, 153.20 E) in June 2009. The yearly averaged IBCs at Tsukuba were 2.54× 10−4 sr−1, 2.48× 10−4 sr−1, 2.45× 10−4 sr−1, and 2.20× 10−4 sr−1 for 2008, 2009, 2010, and 2011, respectively. These values were about twice the IBC background level (1.21× 10−4 sr−1) from 1997 to 2001 at Tsukuba. We briefly discuss the influence of the increased aerosols on climate and the implications for analysis of satellite data.


Introduction
Stratospheric aerosols play important roles in climate regulation and atmospheric chemistry.The effect of the aerosols produced by the Pinatubo eruption is a good example.The volcanic eruption of Mt.Pinatubo (15.14 • N, 120.35 • E) on 15 June 1991 injected huge amounts of SO 2 and ash into the stratosphere.The Volcanic Explosivity Index (VEI) was 6 (Smithsonian Institution, 2012).The eruption injected into the stratosphere an amount of SO 2 estimated to be about 20 Tg, almost three times the input from the 1982 El Chichón eruption (Bluth et al., 1992).The injected SO 2 was oxidized to sulfuric acid particles through homogeneous nucleation (Wu et al., 1994).Read et al. (1993) estimated the e-folding decay time of SO 2 to be 33 days.The Pinatubo aerosol particles were effectively transported from tropical regions into northern mid-latitudes during fall through spring with planetary wave activity.The maximum backscattering ratio observed at a wavelength of 532 nm was 14.1 at 22.7 km over Tsukuba (36.05 • N, 140.13 • E) on 29 November 1991.The maximum value of the integrated backscattering coefficient (IBC) above the first tropopause height was 7.1 × 10 −3 sr −1 over Tsukuba on 22 February 1992 (Uchino et al., 1995).
The stratospheric aerosol surface area increased after the Pinatubo eruption (Jäger et al., 1995;Uchino, 1996), and severe ozone loss occurred in 1992 and 1993 because of heterogeneous chemical reactions on aerosol surfaces in the presence of high concentrations of anthropogenic chlorine and bromine (Hofmann et al., 1994;Kondo et al., 1995;WMO, 1995;Solomon et al., 1996).
The maximum net (thermal minus solar) radiative forcing from the 1991 Pinatubo eruption was about −3 W m −2 (Hansen et al., 2005).Global lower stratospheric (30-100 hPa) temperature anomalies increased after the eruption, and global tropospheric (300-850 hPa) temperature anomalies decreased after the eruption in spite of the warm ENSO episode in 1991/1992 (Kawamata et al., 1992).Global tropospheric temperatures generally increase after a warm ENSO episode.For two years following major volcanic eruptions, global mean surface temperatures decrease by 0.1-0.2• C, and mean surface temperatures in the latitude band 30-60 • N by 0.3 • C during the summer (Robock and Mao, 1995).A model simulation of the effects of the 1991 Pinatubo eruption predicted a decrease in the global surface temperature by about 0.5 • C in September, October, and November 1992, in agreement with observations during that time (Hansen et al., 1996).In contrast, warm surface temperatures were recorded over Europe, Siberia, and North America, while cooling occurred over western Asia in the winters after the three major volcanic eruptions of Mt. Agung in 1963, Mt. El Chichón in 1982, and Mt. Pinatubo in 1991(Kodera, 1994).
The IBC of the Pinatubo aerosols decayed with e-folding times of 1.14, 1.29, and 1.37 years over Tsukuba and Naha (26.21 • N, 127.69 • E) in Japan and over Lauder (45.04 • S, 169.68 • E) in New Zealand, respectively.The IBC over Tsukuba varied in a clearly seasonal manner, with a maximum in winter and early spring and a minimum in summer.The IBC over Tsukuba reached the background level in October 1997 (Nagai et al., 2010).
Since about 2000, an increase of 4-7 % per year in the IBC has been detected within the 20-30 km altitude range at both Mauna Loa, Hawaii (19 • N), and Boulder, Colorado (40 • N), and the increase could be caused by anthropogenic emission of SO 2 (Hofmann et al., 2009).Likewise, after the IBC over Lauder reached a minimum between 1997 and 2000, it increased 3.8 % per year from 2000 to 2009 (Nagai et al., 2010).Based on some satellite data, the stratospheric aerosol optical thickness (AOT) increased after 2000 as the result of a series of moderate but increasingly intense volcanic eruptions (Vernier et al., 2011).In fact, increases in stratospheric aerosols were reported from lidar observations after the volcanic eruptions of Mt.Kasatochi (52.18 • N, 175.51 • E) in August 2008 (Bitar et al., 2010) and Mt.Sarychev Peak (48.09 • N, 153.20 • E) in June 2009 (Uchino et al., 2010;O'Neill et al., 2012).
In this paper we report observations of stratospheric aerosols in the year following the volcanic eruption of Mt.Nabro (13.37 • N, 41.70 • E) in June 2011 at two lidar sites in Tsukuba and Saga (33.24 • N, 130.29 • E), Japan.These two lidar sites are prioritized validation sites for studying the influence of aerosols and thin cirrus clouds on columnaveraged dry air mole fractions of carbon dioxide (XCO 2 ) and methane (XCH 4 ) derived from data collected by the Greenhouse gases Observing SATellite (GOSAT) (Yoshida et al., 2011;Morino et al., 2011;Uchino et al., 2012).GOSAT was launched on 23 January 2009.At Saga, lidar observations started in March 2010.Next, we present lidar observational results from January 2008 to May 2012 over Tsukuba.Finally we discuss briefly the influence of the recent increase in stratospheric aerosols on GOSAT products and compare their impact on climate to the 1991 Pinatubo eruption.

Lidar instruments and data analysis
The compact lidars installed at Tsukuba and Saga were twowavelength polarization lidar systems (Table 1), the fundamental and second harmonic having wavelengths of 1064 nm (λ 1 ) and 532 nm (λ 2 ), respectively.Backscattered photons from the atmosphere were collected by one or two Schmidt Cassegrain type telescopes.A polarizer divided photons at λ 2 into components parallel (P) and perpendicular (S) to the transmitted laser polarization plane.The received photons were converted to electrical signals by an avalanche photodiode (APD, C30956EH) at λ 1 .At λ 2 , three or five photomultiplier tubes (PMTs, R3234-01) were used to simultaneously obtain high-dynamic-range signals from near the surface to an altitude of ∼40 km.Transient recorders used a 12bit analog-to-digital (A/D) converter and a photon counter  to process the output signals of the APD and PMTs.Because the APD signals were noisy above altitudes of about 20-25 km, we used only lidar data at λ 2 for stratospheric aerosols.
The backscattering ratio R is defined as where BR and BA are the molecular and aerosol backscattering coefficients, respectively.We derived backscattering ratio profiles with an inversion method (Fernald, 1984).The lidar ratio S (particle extinction to backscatter ratio) is dependent on the stratospheric aerosol size distribution and refractive index, and equalled 20-60 sr at 532 nm during 1979-1999 (Jäger andDeshler, 2002, 2003).The lidar ratio was small just after the major volcanic eruptions of El  , 1976).The lidar backscattered signal was interactively normalized to unity around 25-33 km, where aerosol-free conditions could be assumed.
We obtained IBCs by summing up BAs from the first tropopause height to an altitude of 33 km.When cirrus clouds appeared above the tropopause, we set the lower limit of the integration to just above the altitude of the cirrus clouds.If the signal-to-noise ratio at higher altitudes was not good enough, the upper limit of the integration was decreased to a lower altitude where the signal-to-noise ratio was acceptable (Nagai et al., 2010).The IBC varies approximately by ± 10 % for change of S by 50 ± 20.The total linear depolarization ratio (δ) is defined as where P and S are the parallel and perpendicular components of the backscattered signals.The particle depolarization δ p is obtained from the equation where δ m is the depolarization ratio of atmospheric molecules (Sakai et al., 2003).We adopted a vertical resolution of 150 m in the following analysis.

Observational results over Tsukuba and Saga after the 2011 Nabro eruption
The Nabro volcano erupted in Eritrea on 12 June 2011.The volcanic ash was detected at 10:45 UTC on 13 June by the Moderate Resolution Imaging Spectrometer (MODIS) on the Aqua satellite (NASA, 2012).The first SO 2 associated with the eruption was measured on 12 June by the Infrared Atmospheric Sounding Interferometer (IASI), and continued emissions were observed for weeks.The total mass of SO 2 measured by IASI was on the order of 1.5 Tg (Clarisse et al., 2012).Over Tsukuba, new aerosol layers with double peaks were observed on 20 June 2011 about 8 days after the eruption (Fig. 1).The peak values of R were 1.58 and 1.32 at 16.0 and 16.4 km, respectively.The values of δ and δ p were 1.25 % and 3.4 % at 16.0 km, respectively and 1.94 % and 7.9 % at 16.4 km, respectively.Non-spherical ash particles were probably included in the layers with sulfuric acid particles that were produced from SO 2 through chemical reactions.Non-spherical particles were also present in the lower region of the aerosol layer on 12 September (δ p = 4.7 % at 17.0 km).The maximum backscattering ratio (R max ) of 2.0 was observed at 17.0 km on 10 July 2011.Over Saga, new stratospheric aerosols with double peaks were detected on 23 June 2011 (Fig. 2).Peak values of R were 2.27 and 3.68 at 17.2 and 18.2 km, respectively.The values of δ p were 0.2 % and 0.8 % at 17.2 and 18.2 km, respectively.In this case, aerosols were probably composed of spherical particles because δ p was very small.However, some non-spherical particles were also seen in the lower regions of the layers on 29 August (δ p = 3.6 % at 16.6 km) and 24 September (δ p = 4.0 % at 16.6 km).In the 1991 Pinatubo eruption, non-spherical particles were present in the lower stratosphere for at least six months (Nagai et al., 1993) because the Pinatubo ash particles were injected into higher altitudes than the Nabro ash particles.
We used the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data (Kalnay et al., 1996) and the Meteorological Data Explorer (METEX), developed by Dr. Jiye Zeng at the Centre for Global Environmental Research (CGER) in the National Institute for Environmental Studies (NIES) to calculate isentropic forward trajectories of 36 air parcels that originated from a square of ± 1 degree surrounding the Nabro volcano at altitudes of 16, 17, and 18 km.The calculation simulated the trajectories of the air parcels for ten days beginning at 23:00 UTC on 12 June 2011 (Fig. 3).Only some of the parcels that originated at 17 km (potential temperature of 384.3 K) over Mt.Nabro were transported to ∼16 km over Tsukuba on 20 June, a result that was consistent with lidar observations as shown in Fig. 1.The air parcels moved eastward around the northern part of the Tibetan highpressure ridge (Fig. 4).The composite image of maximum observed SO 2 columns in Fig. 12 of Clarisse et al. (2012) also shows this feature.We confirmed that the backward trajectory of an air parcel from Tsukuba (16 km, 13:00 UT on 20 June 2011) arrived at a point (16.7 km, 14.62 • N, 33.42 • E) near Mt.Nabro on 23:00 UTC on 12 June.Therefore, new aerosol layers observed over Japan in late June 2011 could have originated from the Nabro eruption on 12 June.The Nabro particles were also observed by the Optical Spectrograph and InfraRed Imaging System (OSIRIS) and those particles were transported to the middle and higher latitudes (Bourassa et al., 2012).2010).Over Tsukuba, stratospheric aerosols produced from those SO 2 gases were detected at 17.3 km and 16.0 km on 2 September and at 18.7 km and 17.3 km on 16 September, about one month after the eruption (Fig. 6).Clear peaks of R were also seen on 4 and 21 October, but subsequent peaks were ambiguous.Obvious stratospheric aerosols from the Kasatochi eruption were also observed from 10 September to 13 October over Ryori (39.03 • N, 141.82 • E) (Sakashita et al., 2009).
Mt. Sarychev Peak erupted on 12 June 2009 with a VEI of 4 (Smithsonian Institution, 2012).A new aerosol layer was observed at 20.6 km on 25 June over Tsukuba (Fig. 7).The peak value of R was 3.5.Because δ p was 7 %, some non-spherical ash particles were probably included in the layer.Backward trajectory analysis revealed that aerosols in the layer were transported to Tsukuba by easterly winds.Aerosols observed around 14-15 km on 5 July were transported by westerly winds.Enhanced aerosol layers were also observed over three other lidar sites in Japan (Uchino et al., 2010).Mt.Merapi (7.54 • S, 110.44 • E), one of Indonesia's most active volcanoes, erupted on 26 October 2010 with a VEI of 4 (Smithsonian, 2012).Shortly thereafter we did not observe enhanced stratospheric aerosols that originated from the Merapi eruption, because noticeable peaks of R were not detected.However, the enhancement of IBC in winter 2010 and spring 2011 could be partly due to the Merapi eruption.
The temporal variation of the IBC over Tsukuba from January 2008 through May 2012 is shown in Fig. 8, with the exception of about two months in 2011 after the Tohoku earthquake off the Pacific Coast of Japan, when lidar data were not obtained.The earthquake occurred in the northern part of Japan on 11 March 2011.After the decay of the Pinatubo aerosols, stratospheric aerosols were at background levels from October 1997 to September 2001 at Tsukuba (Nagai et al., 2010).The annual mean of the IBC for the background aerosols was 1.21 × 10 −4 sr −1 .Based on the fit of a sinusoidal function to the data, the amplitude of the seasonal variation was 6.84 × 10 −5 sr −1 , with a maximum in February and minimum in August (Fig. 8).According to Deshler et al. (2006), no long-term change in the background concentration of stratospheric aerosols has occurred over the period 1972-2004, and therefore the background level of the IBC observed over Tsukuba from October 1997 to September 2001 might be similar to the background levels during the period 1972-2004.The total masses of SO 2 from the Kasatochi, Sarychev Peak, and Nabro eruptions were estimated to be 1.6 Tg, 0.9 Tg, and 1.5 Tg, respectively (Clarisse et al., 2012).However, the production rate of stratospheric aerosols depends on the amounts of SO 2 that are injected into the stratosphere.Before the Kasatochi eruption, the IBC was larger than the background level, an observation consistent with that of Vernier et al. (2011) and possibly due to some other volcanic eruptions in the tropics, including Tavurvur (4.27  2008, 2009, 2010, and 2011, respectively.Therefore the elevations of the IBCs above background level were 1.33 × 10 −4 sr −1 , 1.27 × 10 −4 sr −1 , 1.24 × 10 −4 sr −1 , and 0.99 × 10 −4 sr −1 , respectively.The corresponding elevations of the AOTs above background levels were 0.0067, 0.0064, 0.0062, and 0.0050, respectively, for an assumed lidar ratio of 50 sr.The corresponding increases of negative radiative forcing (cooling) were roughly 0.17 W m −2 , 0.16 W m −2 , 0.16 W m −2 , and 0.13 W m −2 , respectively, based on a conversion factor of 25 W m −2 from AOT to radiative forcing (Hansen et al., 2005;Solomon et al., 2011).These values are not small compared to the positive radiative forcing (heating) caused by increases in atmospheric CO 2 , which has averaged about 0.28 W m −2 over the decade since 2000 (Solomon et al., 2011;NOAA, 2012).The average AOT for the 12 months following Pinatubo was 0.13 over Tsukuba, and the Pinatubo aerosol cooling was 3.1 W m −2 .Recent stratospheric aerosol radiative cooling is about one-twentieth of that caused by the Pinatubo aerosols.
The surface temperature could be lowered by about 0.015-0.025• C during the summer if we divide 0.3-0.5 • C by 20.It is very difficult to detect such a small change of surface temperature during one year.However, it is noteworthy that increased stratospheric aerosol radiative cooling continued for at least four years, from January 2008 to May 2012.Climate models have been used to simulate climate for a year after volcanic eruptions (Haywood et al., 2010;Kravitz et al., 2011), but multi-year simulations will be necessary to understand the effects of longer term increases in stratospheric aerosols, because, for example, the ocean integrates volcanic radiative cooling and responds over a wide range of time scales (Stenchikov et al., 2009).
We next estimated the influence of the increase in stratospheric aerosols after volcanic eruptions on the XCO 2 determined by GOSAT.When the GOSAT XCO 2 is retrieved by using the 1.6-µm band without taking account of sulfuric acid particles in the stratosphere, the negative bias of XCO 2 is estimated to be 0.3 % (∼1 ppm) for an AOT of 0.02 at 550 nm and surface albedo at 0.1 (Ota et al., 2008).It is noteworthy that the largest values of AOT at 532 nm after the volcanic eruptions of Mt.Sarychev and Mt.Nabro were O. Uchino et al.: On recent (2008-2012) stratospheric aerosols observed by lidar over Japan equal to or larger than 0.02.A regional and time-dependent bias of 1 ppm is not small for surface CO 2 flux estimation (Rayner and O'Brien, 2001;Takagi et al., 2011).Therefore, it is necessary to take into account the effects of increased stratospheric aerosols for GOSAT XCO 2 retrieval (Uchino et al., 2012).

Concluding remarks
An increase in stratospheric aerosols caused by the volcanic eruption of Mt.Nabro on 12 June 2011 was observed by lidar at Tsukuba and Saga in Japan.The maximum backscattering ratios at 532 nm were 2.0 at 17.0 km on 10 July over Tsukuba and 3.6 at 18.2 km on 23 June over Saga.The maximum integrated backscattering coefficients above the first tropopause height to 33 km were 4.18 × 10 −4 sr −1 on 11 February 2012 over Tsukuba and 4.19 × 10 −4 sr −1 on 23 June 2011 over Saga.
Lidar observational results at Tsukuba from January 2008 through May 2012 revealed increases in stratospheric aerosols after the volcanic eruptions of Mt.Kasatochi in August 2008 and Mt.Sarychev Peak in June 2009.The yearly averaged IBCs at Tsukuba were 2.54 × 10 −4 sr −1 , 2.48 × 10 −4 sr −1 , 2.45 × 10 −4 sr −1 , and 2.20 × 10 −4 sr −1 for 2008, 2009, 2010, and 2011, respectively.These values were about twice the IBC of the background level (1.21 × 10 −4 sr −1 ) during the period from 1997 to 2001 at Tsukuba.The elevations of annual average AOT above background levels were about 0.0050-0.0067from 2008 to 2011 based on an assumed lidar ratio of 50 sr.The negative radiative forcing (cooling) was then roughly 0.13-0.17W m −2 for the same period based on a conversion factor of 25 W m −2 from AOT to radiative forcing.These values are not small compared to the radiative heating associated with increases in CO 2 , about 0.28 W m −2 over the decade since 2000 (Solomon et al., 2011;NOAA, 2012).However, because the concentrations of these volcanic aerosols are not always spatially homogeneous, their radiative forcing might be overestimated.The influence of the increase in stratospheric aerosols caused by volcanic eruptions on GOSAT XCO 2 retrieval is non-negligible.In the following paper, lidar data over Tsukuba during 2002 through 2007 will be analyzed carefully to find out when the volcanic effect starts for the increase of stratospheric aerosols and to study whether or not the anthropogenic emissions have an impact on the increase.

Table 1 .
Characteristics of two-wavelength polarization lidar systems at Tsukuba and Saga.