CALIPSO detection of an Asian tropopause aerosol layer

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 38,, doi: /2010gl046614, 2011 CALIPSO detection of an Asian tropopause aerosol layer J. P. Vernier, 1 L. W. Thomason, 1 and J. Kar 2 Received 27 December 2010; revised 22 February 2011; accepted 28 February 2011; published 15 April [1] The first four years of the CALIPSO lidar measurements have revealed the existence of an aerosol layer at the tropopause level associated with the Asian monsoon season in June, July and August. This Asian Tropopause Aerosol Layer (ATAL) extends geographically from Eastern Mediterranean (down to North Africa) to Western China (down to Thailand), and vertically from 13 to 18 km. The Scattering Ratio inferred from CALIPSO shows values between on average with associated depolarization ratio of less than 5%. The Gaussian distribution of the points indicates that the mean value is statistically driven by an enhancement of the background aerosol level and not by episodic events such as a volcanic eruption or cloud contamination. Further satellite observations of aerosols and gases as well as field campaigns are urgently needed to characterize this layer, which is likely to be a significant source of non volcanic aerosols for the global upper troposphere with a potential impact on its radiative and chemical balance. Citation: Vernier, J. P., L. W. Thomason, and J. Kar (2011), CALIPSO detection of an Asian tropopause aerosol layer, Geophys.Res.Lett., 38,, doi: /2010gl Introduction [2] The Tropical Tropopause Layer (TTL) is generally recognized to control the entry of tropospheric air into the stratosphere [Holton et al., 1995; Fueglistaler et al., 2009]. However, the Asian monsoon circulation offers an alternative pathway that bypasses the TTL region. [Gettelman et al., 2004; Randel et al., 2010]. Since Asia is currently one of the highest atmospheric sulfur producing regions in the world, the transport of this material to higher altitudes by the monsoon could affect the chemical balance of the stratosphere and the climate. The Asian summer monsoon system is characterized by fast deep convective transport of boundary layer air into the upper troposphere over South East Asia which is horizontally and vertically advected by an anticyclone over a very large area into the lower stratosphere [Randel and Park, 2006]. In addition, the direct incursion of tropospheric air into the lower stratosphere during fast convective overshooting updrafts has been observed over the Tibetan plateau [Fu et al., 2006]. Model simulations had suggested that deep convection could efficiently lift human derived species from strong source regions over Asia [Lawrence et al., 2003], from which they could be trapped and transported to the Mediterranean Sea along the southern edge of the anticyclone circulation (see Lawrence and Lelieveld [2010] for a review). Park et al. [2004] 1 NASA Langley Research Center, Hampton, Virginia, USA. 2 Science Systems and Applications Inc., Hampton, Virginia, USA. Copyright 2011 by the American Geophysical Union /11/2010GL reported enhancements of CH 4, H 2 O and NO x near the tropopause over Asian monsoon region using HALOE observations and model simulation. Satellite data on carbon monoxide (CO) also confirmed strong plumes of CO in the upper troposphere [Kar et al., 2004]. Subsequently, similar plumes were also observed over this area in CO, HCN, C 2 H 6,C 2 H 2,CH 4,H 2 O from other satellite observations [Li et al., 2005; Randel and Park, 2006; Park et al., 2007, 2008; Xiong et al., 2009; Randel et al., 2010]. [3] An interesting question in this context is whether aerosols or their gas precursors are also transported to the upper troposphere by deep convection over the Asian monsoon region. Li et al. [2005] in their model simulation had found an aerosol plume over the Tibetan plateau that was confined to the anticyclone at 150 hpa. Unlike the gas species, aerosol observations in the upper troposphere either from satellites or other platforms have been very sparse. More recently, the spaceborne lidar CALIPSO has suggested the possible presence of an aerosol layer over South Asia during the monsoon season [Vernier et al., 2009]. This work will present a more comprehensive analysis, where CALIPSO data and retrieval algorithms used to detect the layer will be discussed in section 2. Section 3 will be dedicated to accurately locate (spatially and temporally) this layer, while the optical properties deduced from CALIPSO will be used to demonstrate in section 4 that these particles are aerosols and not a misclassification of ice crystals. The nature and origin of the layer will be discussed in section 5 and a brief conclusion will be given in section CALIPSO Measurements of Aerosols in the Upper Troposphere/Lower Stratosphere (UT/LS) [4] The CALIPSO mission is dedicated to the study of clouds and aerosols from the troposphere to the stratosphere [Winker et al., 2009]. CALIPSO is flying at 705 km in polar orbit with several other satellites forming a constellation called the A train and providing measurements at 01h30 LT and 13h30 LT with a repeat cycle of 16 days. CALIPSO has operated since June 2006, measuring through its lidar CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization), the total attenuated backscatter at 532 nm and 1064 nm, and the depolarization at 532 nm. With its vertical resolution of 60 m in the upper troposphere (<20.3 km) and 180 m in the stratosphere [Hostetler et al., 2006], CALIPSO samples the atmosphere at global scale with an unprecedented resolution. However, because of the low level of aerosols in the UT/LS region during non volcanic periods [Vernier et al., 2009], features could not be clearly detected by the level 2 layer products, even at the maximum averaging resolution (80 km) of CALIPSO level 2 layer algorithm. The procedure used here is based on the averaging of 532 nm level 1 V3.01 nighttime profiles in one degree latitude bands ( 111 km, 300 profiles) and further rearranged into a regular grid of 1of6

2 Figure 1. Maps of the mean Scattering Ratio (SR) from CALIPSO at 532 nm between km in Jul Aug (a) 2006, (b) 2007, (c) 2008 and (d) Superimposed are the wind vectors from ECMWF at 100 hpa integrated during the same periods. The data in the South Atlantic Anomaly, represented by the white lozenge are discarded. Ice crystals are removed when the volume depolarization ratio within a pixel is greater than 5%. 1 latitude 2 longitude 200 m. Then, the Scattering Ratio (SR) (the ratio of aerosol plus molecular backscatter to molecular alone) is computed using the total backscatter (aerosol+molecular) from CALIPSO and the molecular alone from European Center Medium Weather Forecast (ECMWF) model density [Vernier et al., 2009]. However, preliminary results have shown that the CALIPSO calibration needs to be adjusted in the tropics, due to the presence of aerosols in the calibration zone between km. To rectify it, all CALIOP data were corrected using the recalibration method developed by Vernier et al. [2009]. Below 20 km, cloudy pixels with a mean volume depolarization ratio (d) greater than 5% are removed. 3. An Asian Tropopause Aerosol Layer (ATAL) [5] Figure 1 represents maps of the mean Scattering Ratio (SR) between km for Jul Aug 2006 (Figure 1a), 2007 (Figure 1b), 2008 (Figure 1c) and 2009 (Figure 1d), on which the mean wind fields from ECMWF have been superimposed. The three consecutive summertime maps, from 2006 to 2008, depict the presence of an Asian Tropopause Aerosol Layer (ATAL), extending from Eastern Mediterranean (down to North Africa) to Western China (down to Thailand) with a SR between These aerosols are confined in the anticyclone circulation as shown by the wind fields, with a maximum more pronounced on its western part. Since this feature appears every year over the same region, a volcanic origin hypothesis can be excluded at least for the first three years. However, this is not the case in Jul Aug 2009, when a few months earlier, a large plume of 1 Tg of SO2 was injected above the tropopause after the eruption of Sarychev volcano (Kamchatka, Russia) on June 7th, 2009 [Haywood et al., 2010]. It induced a large scale volcanic plume circumnavigating the northern hemisphere for 6 months that was consequently observed in Jul Aug 2009 by CALIPSO with a SR greater than 1.2 (Figure 1d). During this period, the anticyclone pattern is revealed by the volcanic tracers bypassing northern Asia, and subsequently transported equatorward along its eastern branch. Since the density of aerosol in Jul Aug 2009 inside the anticyclone, compared to the background, remains relatively low, it also shows that air inside the anticyclone is well isolated from the rest of the world, a result in agreement with the behavior of other trace gases such as CO, O3 and H2O [Park et al., 2008]. However and in contrast to what has been deduced from model, the northern part of the anticyclone seems to offer an effective dynamical barrier for troposphere to stratosphere exchange [Dunkerton, 1995; Chen, 1995]. We notice also a similar aerosol feature, though less pronounced, over Western United States associated with the North American monsoon. [6] In order to study the seasonal variation of the Asian layer, we plotted in Figure 2 the mean SR time series at 3 levels (15, 16, 17 km) within the box roughly bounding the anticyclone [15 45oN; 5 105oE]. The maximum of Scattering Ratio, when not disturbed by volcanic input, peaks in phase at each level every year between July and August. This is not the case in 2008, when a time shift of 1 month can be observed at 17 km due to the volcanic aerosols from Okmok which erupted on 7 August 2008 and subsequently were transported toward Asia. The next year, the strong 2 of 6

3 Figure 2. Time series at 15, 16 and 17 km of the mean CALIPSO SR within [15 45N; 5 105E] The geographic boundaries of the average are delimited by the red rectangle in Figure 1a. Figure 3. Mean SR longitudinal cross section between N for Jul Aug (a) 2006, (b) 2007, (c) 2008 and (d) The mean potential temperature levels at 380 and 420 K are plotted in white. 3 of 6

4 Figure 4. Probability Density Functions of Scattering Ratio values with d < 5% for all the pixels within the box [5 105 E, N, km] for Jul Aug 2006, 2007, 2008, 2009 and Dec Feb peak at 17 km is due to the volcanic debris from the Sarychev eruption which leads to SR values greater than 1.2 and shifted by two months relative to 2006 or In summary, the seasonal variation of the non volcanic aerosol above Asia can be differentiated from the episodic volcanic input on a time series basis by analyzing the time shift of the maxima at different levels. [7] To better characterize these aerosols, we plotted in Figure 3 longitudinal cross sections of SR between o N during the same periods as displayed in Figure 1. In Jul Aug 2006, an aerosol layer can be seen between km ( K) from 20W 100E, below the Soufriere Hills volcanic plume at km [Vernier et al., 2009]. During the following two years, the position and intensity of the same layer have not significantly changed while the stratospheric aerosol layer is located at higher levels consistent with its vertical transport by the Brewer Dobson circulation. The last period (Jul Aug 2009) is highly perturbed by the Sarychev volcanic plume located between km and bypassing the Asian anticyclone region on its upper part, consistent with a vertical isolation of the anticyclone from the stratosphere at 18 km. [8] To summarize, an aerosol layer (ATAL) is observed every year in Jul Aug from 2006 to 2008, extending vertically between km ( K) and geographically from Eastern Mediterranean to Western China confined by the Asian anticyclone. ATAL is likely to be a primary source of non volcanic aerosols for the global upper troposphere. 4. Optical Properties of ATAL [9] The optical properties of ATAL are discussed based on the Probability Density Functions (PDF) of SR for d < 5% in Figure 4, calculated with all non cloudy pixels within the box [5 105 o E, o N, km] in Jul Aug every year between 2006 and 2009 and in Dec Jan The PDF is very similar for Jul Aug 2006, 2007 and 2008 where the maximum of the Gaussian function is located close to a SR of 1.1. The long tail of positive SR values with higher density in Jul Aug 2009 is due to the volcanic aerosols from Sarychev, contrasting with the low aerosol level during the period Dec Jan 2008 where the maximum density is located below The periods 2006, 2007 and 2008 are remarkably similar displaying an enhancement of SR to values close to 1.1 compared to the reference period Dec Jan 2008, demonstrating that the average calculated in Figures 1 3 are not driven by a few points due to possibly some cloudy pixels slipping through the screening process but rather by an increase of the aerosol background. 5. Discussion [10] CALIPSO observations of ATAL are consistent with two independent optical measurements. One was carried out with a Ground based lidar in Lhasa (Tibet, China), indicating an aerosol layer between km with SR = and d <5%[Kim et al., 2003], similar to the optical characteristics deduced from the CALIPSO lidar. Moreover, in situ balloon measurements obtained with an Optical Particles Counter at the same location in August 1999 have shown an enhancement of small particles of effective radius <0.6mm at the tropopause level [Tobo et al., 2007]. [11] The nature and the origin of ATAL is a fundamental question for understanding its radiative and chemical impact on the climate. First of all, these aerosols are certainly non volcanic, at least from 2006 to 2008, since these seem to be occurring seasonally over the same region with a peak in July August. Besides, these are very likely not a misclassification of ice crystals since: i) their geographical extension is westward and northward compared to the cirrus clouds cover related to the Southeast Asian monsoon 4of6

5 [Sassen et al., 2009], ii) their optical properties are very different from the typical high depolarization ratio values (>30%) of ice crystals, iii) There is no similar feature observed over the Western Pacific during the NH winter (not shown), known to be a region where the frequency of occurrence of cirrus clouds is very high [Sassen et al., 2009]. The optical characteristics of ATAL would indicate that these particles are spherical, but we cannot completely exclude according to the Mie theory and T matrix calculation that they could be very small aerosols (effective radius < 0.1mm) with an irregular shape. In this case, they could be very small dust particles since this region is an important source of mineral dust from Taklamakan and Gobi deserts in the middle troposphere [Liu et al., 2008], that could be subsequently transported over the Indian Sub continents or/and Tibetan Plateau by deep convection associated with the Asian Monsoon [Li et al., 2005; Randel and Park, 2006; Fu et al., 2006]. This idea is supported by recent observations of non volatile particles (r 10 nm) at the tropopause level carried out at the edge of ATAL in Ouagadougou (Burkina Faso) during the SCOUT AMMA campaign in August 2006 [Borrmann et al., 2010]. [12] The upward transport of primary aerosols (dust, soot, salt) by convection could result in the formation of an aerosol layer at higher levels depending on their solubility with water. The Southeast Asian monsoon offers then a possible vertical pathway for the transport of particles from the boundary layer to the upper troposphere. However, there is no clear connection in Figure 3 between lower levels ( 12 km) and ATAL, suggesting either that the transport of particles by convection is masked by clouds since we cannot distinguish aerosols when mixed with ice particles or that ATAL is the result of secondary aerosol formation and growth influenced by low temperature at the tropopause level. [13] CALIPSO is not sensitive to particles of a few nm that are produced by nucleation events that are associated with the outflow level of convective systems and triggered by very low temperatures [Brock et al., 1995; Hamill et al., 1997; Frey et al., 2011]. On the other hand, microphysical processes could lead to the growth of these nuclei to detectable levels by: [14] 1. Aerosol swelling due to relative high humidity levels associated with the Asian monsoon at 100 hpa. Using theoretical formulations of Steele and Hamill [1981] and gas measurements of H 2 O and HNO 3 from MLS/Aura at 100 hpa, we calculated that an increase of water vapor mixing ratios from 3 ppmv to 6 ppmv could result in an enhancement of scattering ratio between 10 and 50%. [15] 2. Coagulation of aerosols along their transport. Since air parcels detrained in the upper troposphere by convection are believed to be slowly lofted to the tropopause level within one or two months [Park et al., 2007], there could be sufficient time for newly nucleated particles to coagulate to larger sizes that result in a measureable enhancement of the CALIPSO backscatter signal. [16] The Southeast Asian region is an increasingly important source of pollution that could possibly lead to the formation of new particles at higher levels. Sulfur emissions increase from Chinese s coal mines were suggested by Hofmann et al. [2009] to be the main component of the 4 8% trend in stratospheric aerosol observed by ground based lidars in Mauna Loa and Boulder during the last decade. However, Vernier et al. [2009, also Volcanic origin of the recent stratospheric aerosol trend, submitted to Geophysical Research Letters, 2010] showed that multiple small tropical volcanic eruptions had a major contribution on the stratospheric aerosol variation. After isolating the volcanic plumes from the rest of UTLS aerosols, CALIPSO observations suggest that ATAL could be made of sulfur and/or organics associated with the transport of Asian pollution [Randel et al., 2010]. 6. Conclusion [17] We report the presence of an aerosol layer at the tropopause level above Asia during the monsoon that we have named the Asian Tropopause Aerosol Layer (ATAL). Extending from Western China to Eastern Mediterranean, the aerosol density peaks in July August every year excluding a possible volcanic origin. The optical and geometrical properties of the layer, made of very low depolarizing and small particles is confirmed by ground based lidar measurements and Balloon Optical Particles Counter from Lhasa (Tibet, China). Very small dust particles, soot, in situ formation of new particles, growing particles on pre existing aerosols could in principle populate this layer. The nature of the ATAL remains unclear but its presence during the monsoon emphasizes the role of deep convection associated with the monsoon in transporting primary aerosols or their precursors in the upper troposphere. Information on aerosol size distribution and composition are needed for a better understanding of the nature and origin of ATAL. [18] Acknowledgments. This study was conducted through the NASA Postdoctoral Program at Langley Research Center, administrated by Oak Ridge Associated Universities. The authors acknowledge J.P Pommereau, A. Garnier, J. Pelon, F. Cairo, M. Vaughan and C. Trepte for helpful discussions. The CALIOP and ECMWF data were made available at the ICARE data center ( icare.univ lille1.fr) and CALIOP data were processed at NASA LaRC. ECMWF data used for CALIPSO stratospheric aerosol retrieval were made available at NILU ( nilu.no/index.cfm?lan_id=3). The authors want to thank J. Jumelet and P. Lucker for their respective help in processing ECMWF and CALIOP data. The authors acknowledge M. Pitts and L. Pool for their helpful discussions about theoretical calculation of scattering ratio. [19] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper. References Borrmann, S., et al. (2010), Aerosols in the tropical and subtropical UT/LS: In situ measurements of submicron particle abundance and volatility, Atmos. Chem. Phys., 10, , doi: /acp Brock, C. A., P. Hamill, J. C. Wilson, H. H. Jonsson, and K. R. 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Kar, Science Systems and Applications Inc., 1 Enterprise Pkwy., Ste. 200, Hampton, VA 23666, USA. L. W. Thomason and J. P. Vernier, NASA Langley Research Center, Mail Stop 475, Hampton, VA , USA. (jean paul.vernier@ nasa.gov) 6of6