Lidar Ice nuclei estimates and how they relate with airborne in-situ measurements

The Cloud-Aerosol Transport System (CATS) onboard the International Space Station (ISS), is a lidar system providing vertically resolved aerosol and cloud profiles since February 2015. In this study, the CATS aerosol product is validated against the aerosol profiles provided by the European Aerosol Research Lidar Network (EARLINET). This validation activity is based on collocated CATS-EARLINET measurements and the comparison of the particle backscatter coefficient at 1064nm. Ice nuclei estimates and how they relate with airborne in-situ measurements. The Cloud-Aerosol Transport (CATS) onboard the International Space Station (ISS), is a lidar system providing vertically resolved aerosol and cloud profiles since February 2015. In this study, the CATS aerosol product is validated against the aerosol profiles provided by the European Aerosol Research Lidar Network (EARLINET). This validation activity is based on collocated CATS-EARLINET measurements and the comparison of the particle backscatter coefficient at 1064nm.: 2017 (Vol. 176). Conferences; Vol. ABSTRACT By means of available ice nucleating particle (INP) parameterization schemes we compute profiles of dust INP number concentration utilizing Polly-XT and CALIPSO lidar observations during the INUIT-BACCHUS-ACTRIS 2016 campaign. The polarization-lidar photometer networking (POLIPHON) method is used to separate dust and non-dust aerosol backscatter, extinction, mass concentration, particle number concentration (for particles with radius > 250 nm) and surface area concentration. The INP final products are compared with aerosol samples collected from unmanned aircraft systems (UAS) and analyzed using the ice nucleus counter FRIDGE.


INTRODUCTION
Despite their low concentration in the atmosphere, ice nucleating particles (INP) are crucial for the evolution of ice in clouds, as they act as seedsurfaces for water in order to enable the development and growth of ice. Mineral dust is one of the most important IN aerosol types and has been identified in liquid-water, mixed phase and cirrus clouds in field and laboratory studies [1]. Nevertheless, there are several questions on ice nucleation processes such as the abundance and vertical profile of INP as a function of cloud nucleation conditions (temperature and supersaturation) and the aerosol particle spectrum. Furthermore, field data of INP are limited and the number of observations is still relatively small. Gaps in our knowledge of the detailed impact of aerosols on the evolution of liquid-water, mixed phase and cirrus clouds drive atmospheric research towards field studies of aerosol-clouddynamics interaction. Mediterranean. We use the data from this field campaign to compare remote sensing with airborne in-situ retrievals of INP concentrations (INPC).

CAMPAIGN MEASUREMENTS
During the campaign, continuous lidar observations were performed at Nicosia (35°08'26"N, 33°22'52"E, 181m a.s.l.) with the Polly-XT lidar of National Observatory of Athens. The system is a 12 channel system with nearrange capabilities (3 elastic, 2 Raman, 2 depolarization, 1 water-vapor, 2 near-range elastic, and 2 near-range Raman signals) [2]. Since the middle of the campaign, lidar observations were complemented by a SUN/SKY/LUNAR AERONET photometer, used to evaluate the horizontal variability of the atmosphere between Nicosia and 2 additional monitoring sites located 30 km westerly, where additional in-situ INP measurements were performed. UAS, operating from the Cyprus institute airfield (Orounda, 35°05'42"N, 33°04'53"E, 327 m a.s.l.) performed observations with the aim to measure the abundance of ice nucleating particles in the lower troposphere [3]. Seven km further west than the airfield, at the Cyprus Atmospheric Observatory (Agia Marina Xyliatou, 35°02'19"N, 33°03'28"E, 532m a.s.l.), an AERONET sun photometer, along with a suite of aerosol and INP in-situ instruments were located.

POLLY-XT OBSERVATIONS
A first snapshot of the aerosol time-height variability is given in Figure 1 (30/3/2016 -27/4/2016). During that period, the Eastern Mediterranean was affected by air mass advection from Europe, northern Africa and the Atlantic as indicated by the backward trajectories and DREAM8b dust model [4]. During April 4 to 7 several elevated structures were observed reaching heights up to 10km. The sun photometer observed aerosol optical depths (AOD, 550 nm) values up to 0.3 and Angström exponents (AE, 380 -870 nm) between 0.5 -1.0. Atmospheric conditions favored the transport of dust from the west parts of North Africa over the Continental Europe and towards Cyprus. Following this period, we observed five Saharan dust events. The first event (April 8 to 11) was very intense with AOD values of 0.6 to 1.0 and AE less than 0.2. The events of April 15 -16, 21, and 26 reach AOD values of 0.5 and AE of 0.1 -0.4. On April 24 minor dust event reached AODs of 0.3 and AE of 0.3 -0.5. During four out of the five events, ice nucleation was observed in the top of the dust plums, and we observed volume depolarization ratio values (at 532 nm) greater than 32%. In order to retrieve INPC profiles from the lidar measurements we follow the methodology described in [5]. In brief, we separate the backscatter profile in dust and non-dust component using POLYPHON technique assuming particle depolarization ratio for dust of 0.31. Then, we calculate the extinction coefficient for the dust and non-dust components from which we estimate mass concentration, particle number concentration (for particles with radius >250nm) and surface area concentration profiles using AERONET parameterizations. (S15) [5]. Figure 2 provides an example of the estimated products during 21/4/2016 at 1-2 UTC.
INP concentrations start above 5 km a.g.l., where the temperature is less than -10 °C (N12) and above 6 km a.g.l. where the temperature is less than -20 °C (D15, S15). There are a number of distinct pathways of dust ice nucleation: contact freezing, immersion freezing, condensation freezing and deposition nucleation. N12 and D15 quantifies the immersion freezing at or above water saturation.

UAS MEASUREMENTS
Two different types of UAS were used for INP sampling, namely Cruiser and Skywalker X8, able to carry a payload of up to 10 kg and 2 kg respectively. 42 flights were performed in total, which generated a total of 52 samples over 19 days. INPC were measured by electrostatic precipitation of aerosol particles onto silicon wafers onboard the UAS, followed by laboratory analysis of the samples in the ice nucleus counter FRIDGE [2]. FRIDGE usually addresses the deposition / condensation freezing mode(s). During the campaign, the UAS reached maximum altitudes of 2.5 km a.g.l. (2.85 km a.s.l), making these measurements ideal for comparison with the lidar data. and 21 April 2016. From these two cases, we use the case of the 21 st of April that coincided with one of the dust events, and CALIPSO L2 product gave quality assured retrievals. The atmospheric curtain of CALIPSO is shown in Figure 3. In this plot, the green dotted line highlights Nicosia station and the box shows the part we use for the INPC retrievals (±80 km for the station). The curtain shows the elevated dust plume and cloud formation at 80 km away from the station, on the top of the dust plume and in heights greater than 5km.

CASE OF 21 APRIL 2016
On that day, atmospheric conditions favored the transport of dust from both Sahara Desert and the Arabian Peninsula. The two plums merged over Cyprus during the time of the event, with the major contribution coming from Sahara (DREAM8b dust model). Figure 4 (top) shows the 24hr Polly-XT measurements of that day. Above the station there was formation of ice clouds between 2 and 10 UTC. We were able to derive level 2 cloud-free optical profiles and estimate INPC close to the cloud. These profiles are shown in Figure 4

CONCLUSIONS
In Figure 5 the in-situ (UAS) and lidar INPC estimations are compared for three different temperatures (-20°C, -25°C and -30°C). When we apply the N12 and D15 parameterizations for the same height as the UAS flight (2.55 km a.g.l) we get between one to two orders of magnitude higher values compared to the FRIDGE measurements. When we compare the UAS measurements with the lidar INPC values in the actual height of -20°C, -25°C and -30°C (6 km, 6.5 km and 7 km a.g.l.), the N12 parameterization provide INP concentration values closer to the measurements while D15 gives one order of magnitude lower values. We will investigate this topic more intensively in the next time.