Observational evidence of particle condensational 1 growth in the UTLS over Tibetan Plateau

26 We measured the vertical profiles of aerosol backscattering ratio (BSR) with a 27 balloon-borne lightweight COBALD at Linzhi, located in the southeastern Tibetan 28 Plateau, in the summer of 2014. An enhanced aerosol layer in the upper 29 troposphere/lower stratosphere (UTLS), with BSR (455 nm)>1.1 and BSR (940 30 nm)>1.4, was observed. The Color Index (CI) of the enhanced aerosol layer, defined as 31 the ratio of aerosol backscatter ratios at wavelengths of 940 nm and 455 nm, varied 32 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2019-6 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 28 January 2019 c © Author(s) 2019. CC BY 4.0 License.


Introduction
The Asian Tropopause Aerosol Layer (ATAL) extends over a large area within the Asian summer monsoon circulation and may significantly influence ozone, cirrus clouds and global climate by chemical, micro-physical and radiative processes [Gettelman et al., 2011;Vernier et al., 2011;Fadnavis et al., 2013;Thomason and Vernier, 2013;Vernier et al., 2015].Particles in the ATAL are likely to be lifted to the lower stratosphere by the large-scale upward circulation within the south Asian anticyclone, and then influence the aerosol amount in the global stratosphere significantly [Park et al., 2007].Solomon et al. [2011] found that the radiative forcing of increased aerosols in the global stratosphere from 2000 to 2010 is -0.1W• m -2 , which weakened the global warm effect from greenhouse gases.
In addition to the maximum concentration of aerosols found in the ATAL as mentioned above, the concentrations of tropospheric trace gases (i.e., water vapor, CO, CH4 and HCN) are higher within the Asian summer monsoon anticyclone than in surrounding regions, while the stratospheric trace gases (i.e, O3, HNO3 and HCl) are lower [Park et al., 2004;Randel et al., 2010].Actually, the maximum aerosol concentration near the tropopause over the Tibetan Plateau has also been observed by lidar and balloon borne measurements [Kim et al., 2003;Tobo et al., 2007;He et al., Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-6Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 28 January 2019 c Author(s) 2019.CC BY 4.0 License.

2014]
. Li [2005] showed that the aerosol plume is detectable in the anticyclone around the altitude of 150 hPa over the Tibetan Plateau through satellite observations and model study.
The formation mechanism of ATAL has not been fully understood mainly due to sparse in situ measurements.[Frey et al., 2011] proposed that nucleation events at very low temperatures accompanied by the outflow of convective systems could be dominant process in the production of ATAL.Vernier et al. [2015] found that there is a onemonth phase lag of the aerosol scattering ratio from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) after the relative humidity with respect to ice (RHi) from the Microwave Limb Sounding (MLS) at the beginning of the convective period (May/June), possibly due to the growth of the nanometric particles to the larger particles that can be detected by satellites.
Both condensation and coagulation contribute to the particle growth, even though these two processes are triggered by different mechanism.Model studies have shown that the coagulation is more important than the nucleation in the control of the number concentration of fine particles (with diameter larger than 10 nm) in the UTLS [English et al., 2011;Pierce and Adams, 2009;Timmreck et al., 2010].Except for coagulation, the effect of condensation on particle growth is less documented in previous studies.Weigel et al. [2011] found that supersaturated gases, which can nucleate to form neutral and charged molecular clusters, also condense onto pre-existing aerosol particles and cloud droplets.Earlier studies demonstrated that the stratospheric aqueous H2SO4 aerosol can absorb a large amount of gaseous HNO3 and H2O at temperatures (about 200K) between the nitric acid trihydrate (NAT) and ice frost points [Carslaw et al., 1994;Tabazadeh et al., 1994], leading to a steep increase in particle volume.
Heterogeneous reactions are active on the extreme cold stratospheric aerosols and polar stratospheric clouds (PSCs) over the winter poles.These aerosols and PSCs are composed either of supercooled ternary solution (STS) droplets (HNO3H2OH2SO4), ice particles or solid hydrates (most likely NAT) and can grow to larger particles that are easy to sediment [Voigt et al., 2008;Engel, 2013].However, unlike the studies about PSCs, the growth mechanism of the particles in the ATAL is still poorly described In-depth investigations on the aerosol size distribution, chemical composition and growth process are needed for a better understanding of the characteristics and formation mechanism of ATAL.It is difficult to obtain much more information merely by means of remote sensing measurements, such as satellite and lidar, because those sensors are not sensitive to ultra-fine particles.In such case, balloon and/or air borne in situ measurement provide an additional and even better tool for exploring the ATAL.
Using a balloon-borne optical particle counter at Lhasa, China, Tobo et al. (2007) measured the vertical profiles of aerosols and found occurrences of relatively high number concentrations of sub-micron size aerosols near the tropopause region during the Asian summer monsoon period.They considered that the enhanced aerosol layer in the UTLS connected closely with the transportation of water vapor from the Asian summer monsoon.An increased amount of water vapor was found in the UTLS within the Asian summer monsoon anticyclone (Bian et al., 2012;Li et al., 2017).Recently, a climate model simulation demonstrated that the abundant anthropogenic aerosol precursor emissions from Asia coupled with rapid vertical transport associated with monsoon convection could lead to significant particle formation in the upper troposphere within the ASM anticyclone (Yu et al., 2017).More Recently, a series of balloon borne activities over India and Saudi Arabia during the Balloon Measurements of the Asian Tropopause Aerosol Layer (BATAL) campaigns revealed that the ATAL is composed of mostly small (r < 0.25 μm) liquid (~80%-95%) aerosols with the dominant composition of nitrate (Vernier et al., 2017).

As part of the project Tibetan Ozone, Aerosol and Radiation (TOAR) [see More
Information on ACP Special Issue, available at: http://www.atmos-chemphys.net/special_issue331.html], the vertical profiles of aerosols over the southeastern Tibetan Plateau were measured in June and July of 2014.In this paper, we present the results from balloon borne radiosonde measurements, and investigate the contribution of condensational growth by gas-to-particle conversion processes to the observed high concentrations of fine particles in the UTLS over the Tibetan Plateau.

Experiment
The field experiment was carried out at the Linzhi Meteorological Bureau (29.67ºN, 94.33ºE; 2992 m MSL), located in the southeastern Tibetan Plateau, from June 6 to July 31, 2014.During the field campaign, seven balloon sondes were launched, with each sounding taking place at about 16:00 UTC on June 18 (case 1), June 24 (case 2), July 6 (case 3), July 15 (case 4), July 21 (case 5), July 25 (case 6) and July 30 (case 7), respectively.The balloon sonde payload was composed of the Compact Optical Backscatter AerosoL Detector (COBALD) particle backscatter sonde, the IMet and RS92 radiosonde, and the cryogenic frost-point hygrometer (CFH).The payload was lifted by a 1600 g latex balloon, which flew at an ascent rate of 5-7 m s −1 .Data were obtained from the lunching point until an altitude between 30 km to 35 km where the balloon generally burst.In this study, only the ascending data are analyzed.

COBALD particle backscatter sonde
The lightweight COBALD, developed by Prof. Thomas Peter's group at ETH Zurich, uses two high power light emitting diodes (LEDs) operating at 455nm (blue) and 940nm (infrared) with a silicon detector averaging the light scattered back from molecules or aerosols at angles centered near 173º for typically one second time periods [Rosen and Kjome, 1991;Wienhold, 2012;Cirisan et al., 2014].COBALD measurements are only carried out at local nighttime as daylight saturates the sensitive detector.
Backscatter ratios (BSR) at two wavelengths are retrieved from COBALD measurement, which is defined as follow, where  denotes backscatter coefficient, N the number concentration, and σ the backscatter cross section.The subscripts a and m indicate contributions from aerosol particles and air molecules, respectively.The backscatter cross section for air molecules can be calculated from Rayleigh scatter theory and the number concentration for air molecules is derived from atmospheric pressure and temperature measured by the radiosonde.From the COBALD raw data the blue and infrared backscatter ratio of each individual flight profile was derived with an accuracy of 5 % and the precision in an order of 1% [Vernier et al., 2015].The backscattering cross section for aerosol particles can be calculated from Mie scatter theory for a specified effective radius.The aerosol backscatter ratio (ABSR) is defined as, The ABSR values at two wavelengths are used to calculate the Color Index [CI, Rosen et al., 1997], which is defined as the ABSR at 940 nm divided by the ABSR at 455 nm.The CI is proportional to the ratio of the backscatter cross sections at 940 and 455 nm, and hence it can provide an estimate of the particle size.Assuming an index of refraction of 1.45 with 75% sulfate and a typical lognormal size distribution of the stratospheric aerosols [Rosen and Kjome, 1991], the backscatter cross sections σa at the wavelengths used by COBALD are calculated by Mie theory, and further the CI as a function of the mean radius of total aerosol particles is derived.Because no information on standard deviation of the lognormal distribution is available, the possible lower and upper limits the standard deviation are assumed to be 1.8 and 2.2 [Deshler et al., 2003].
By comparing the observed CI with the calculated one for different standard deviations, the range of possible mean radius can be obtained, and the number concentration and further volume concentration for aerosol particles can be retrieved from the observed ABSR according to the Equation (1).

Radiosonde observations
In this study we use the air temperature profiles from the RS92 radiosondes with an uncertainty of ±0.2℃ below 100 hPa and ±0.3℃ between 100 and 20 hPa.The profiles of water vapor are obtained from CFH measurements.The CFH is a microprocessor-controlled instrument with a lightweight of 400 g, and it uses a cryogenic liquid as cooling agent and operates based on the chilled-mirror principle [Vömel et al., 2007a].The uncertainty of frost point or dew point measured by the CFH is smaller than 0.2 K. Correspondingly, the uncertainty in relative humidity is estimated to be 2 % for measurement in the lower troposphere and 5 % in the tropical tropopause region [Vömel et al., 2016].As a standard for water vapor measurements, CFH has Microwave Limb Sounder (MLS) water vapor products, globally [Vömel et al., 2007b] and specifically over the Tibetan Plateau [Yan et al., 2016].

Results and discussion
Figure 1 shows the BSR profiles at two wavelengths and calculated CI profiles from COBALD measurement, as well as the profiles of temperature and RH over ice respectively from RS92 and CFH measurement for three typical cases on June 18, July 15 and 25, 2014.The COBALD measurements suggest an enhanced aerosol layer (BSR (455 nm)>1.1 and BSR (940 nm)>1.4)extending from 200 hPa (~12 km) to 10hPa (~28 km) with a maximum above the tropopause (90 hPa, ~17 km).The RHi near the maximum of the enhanced aerosol layer varies from 30% to 40%, indicating that it is impossibly caused by cirrus cloud, which cannot persist at these dry conditions.It should be noted that the volcanic eruption might have negligible influence on the observed aerosol layers as no volcanic eruption occurred during the experiment period.
The calculated CI of the enhanced aerosol layer is around 5 (4-8), far below CI of cirrus cloud (being around 10 with the maximum value exceeding 20) at 250 hPa [Vernier et al., 2015].Pinnick et al. [1975] adopted a lognormal distribution with a mode radius of 0.0725 μm and standard deviation (σ) of 1.86 to parameterize the background aerosols in the stratosphere.Rosen and Kjome [1991] suggested a mode radius between 0.04 and 0.06 μm and σ value of ~2.0-2.2 for the 20-km stratospheric aerosol background layer.In this study, the CI as a function of mode radius was derived from Mie calculation using a lognormal distribution for different size of aerosols with standard deviations (σ) of 1.8 and 2.2 respectively and the result is shown in Fig. 2. The CI increases monotonously from 1 to 15 with mode radius growing from 1 nm to 1μm.The CI of the enhanced aerosol layer from COBALD measurement usually varied from 4 to 8 as indicated in this figure.With the assumed lognormal widths, the measured CI imposes an upper limit of 100 nm on the particle radius.Therefore, we conclude that the enhanced aerosol layer is composed of a large number of fine particles with radius less than 0.1 μm.It has been documented that aerosols in the UTLS are mainly composed of liquid inorganics with typical mode radii smaller than 0.1 μm [Tobo et al., 2007].
Our observations in Linzhi are consistent with previous findings.The middle troposphere over the Tibetan Plateau is likely to act as a pipe for the transport of water vapor from the marine boundary layer (i.e., Indian Ocean and South China Sea) to the UTLS, leading to an increase of H2O mixing ratio near the tropopause [Fu et al., 2006;Lelieveld et al., 2007].Figure 3 increase by 10% to 50% with a water vapor mixing ratio enhancement from 3 ppmv to 6 ppmv [Vernier et al., 2011].Fig. 3(b) presents the variation of CI with RHi for all cases between 50 hPa and 150 hPa, the typical altitude range for the ATAL.The dependency of CI on RHi can be classified into three types according to the CI of dry aerosols, i.e. the aerosols existing at very low relative humidity (e.g., RHi < 20%): (1) When the CI of dry aerosol is larger than about 6, CI of the enhanced aerosol layer shows an exponential growth with increasing RHi; (2) When the CI of dry aerosol is smaller than about 6, CI of the enhanced aerosol layer decreases with increasing RHi in a slope of -0.03; (3) When the CI of dry aerosol is close to 6, it keeps almost constant with variation of RHi.
As the CI can be regarded as an indicator of aerosol particle size, it can be inferred that for those aerosol particles with large dry sizes (Type 1, i.e., CI> 6), increasing RHi facilitates water vapor and other gaseous precursors to condense onto pre-existing aerosol particles and then contribute to the particle growth.For those with small dry sizes (Type 2 and Type 3, i.e., CI <= 6), the situation appears to be more completed and cannot be fully understood without more detailed information about aerosol chemical composition and their gas precursors.Since all these aerosol particles were observed at very low RHi, well below 40% deliquescence relative humidity of most of the salts (e.g., 40% for NH4HSO4) [Benson et al., 2009], the condensation of water vapor should have negligible effect on the growth of these particles.New particle formation through the gas-to-particle conversion process, which tends to become faster with increasing RH [Fountoukis and Nenes, 2007], increases the number concentration, resulting in decrease of mode radius of bulk aerosols.Therefore, the decrease of CI with RHi (Type 2) indicates that new particle formation might play an important role in the formation and prevalence of fine particles in the UTLS over the Tibetan Plateau.

Conclusions
The vertical profiles of aerosol BSR measured over the Tibetan Plateau during summertime demonstrate an enhanced aerosol layer, consisting predominantly of fine particles with mode radius smaller than 0.1 m, in the UTLS.The size of particles in the enhanced aerosol layer shows an exponential increase with increasing RHi when the CI of dry aerosols is larger than 6 (corresponding mode radius larger than 0.04 m).
It can be inferred that for increasing RHi leads to the condensation of water vapor and other gaseous precursor onto pre-existing aerosol particles and contributes to the particle growth.For the CI of dry aerosols smaller than about 6 (i.e., mode radius smaller than 0.04 m), the size of particles in the enhanced aerosol layer decreases with Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-6Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 28 January 2019 c Author(s) 2019.CC BY 4.0 License.due to the lack of sufficient observations.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-6Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 28 January 2019 c Author(s) 2019.CC BY 4.0 License.been used in numerous intercomparison experiments, such as the validation of Aura Fig. 1.(a) Three cases of the backscattering ratio profile from COBALD and MPL measurements on June 18 (top), July 15 (middle) and July 25 (bottom), 2014.(b) The calculated CI profiles from the ABSR at two wavelengths.(c) Temperature and RHi profiles measured by the RS92 radiosonde and CFH, respectively.

Fig. 2 .
Fig. 2. CI as a function of mode radius from Mie calculation assuming an index of refraction of 1.45 and a lognormal size distribution with the indicated standard deviations (σ) of 1.8 and 2.2.The dotted and dashed lines represent the minimum (~4) and maximum (~8) CI of the enhanced aerosol layer from COBALD measurement for all cases.
(a) presents the CFH H2O profiles from 110 hPa (~16 km ASL) to 90 hPa (~17.5 km ASL).It is noticed that H2O concentration changes greatly in the vertical direction (3~12ppmv) for some cases.The dehydration process results in minimum H2O concentration just above the altitude of each lowest temperature.Pronounced decrease of the H2O concentration from 110 hPa to 90 hPa are attributed to convection transport of moist air parcels just occurring during the balloon flying periods.The three relatively uniform H2O profiles (on June 18, July 25 & 30) correspond to the well mixed status of strong upward transport prior to the balloon-based measurements.The water vapor cycle driven by synoptic-scale .Phys.Discuss., https://doi.org/10.5194/acp-2019-6Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 28 January 2019 c Author(s) 2019.CC BY 4.0 License.convection increases the possibility of the condensational growth of aerosols near the tropopause over the Tibetan plateau.It has been estimated that the scattering ratio could

Fig. 3 .
Fig. 3. (a) H2O concentration from CFH measurements, and (b) the variation of CI with 12 BSR and CI at the UTLS altitudes (50-150 hPa) from COBALD, we 16 calculated the aerosol volume concentration in the enhanced aerosol layer for the two 17 typical CI variation trend according to an assumption of lognormal size distribution 18 with standard deviation of 1.8.The variation of aerosol volume concentration 19 distributions with RHi is shown in Fig. 4. It can be seen from Fig 4a that when RHi is 20 less than 60%, aerosol mode radius ranges mostly between 0.04 and 0.07 m, and it .Phys.Discuss., https://doi.org/10.5194/acp-2019-6Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 28 January 2019 c Author(s) 2019.CC BY 4.0 License.increases steeply to 0.2 m when RHi is more than 60%.The aerosol volume concentrations are obviously high compared with those in dry condition, especially for those particles with a mode radius of 0.1 µm.For those aerosols with small initial dry particle size (as shown in Fig 4b), accompanied by a mode radius decrease from 0.04 to 0.03 m, the aerosol volume concentration increases by 4-5 times when RHi rises from nearly zero to 40%, indicating that the number concentrations experience an explosive growth due to the formation of new particles.

Fig. 4 .
Fig. 4. The variation of aerosol volume concentration distributions in the enhanced aerosol layer with RHi for (a) case 5 (July 21), and (b) the other cases corresponding to the black dots in Fig 3b.The color of each distribution represents RHi labeled on the color bar.