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Yonghoon Choi (Chemistry and Dynamics Branch/Contractor)

Title: Research Scientist
Technical Focus Areas: Atmospheric Composition, Chemistry & Dynamics, Flight Projects, Air Quality & Weather, Airborne Science
Study Topics: In situ airborne measurements of trace gases/water vapor
Mission/Project/Groups: DISCOVER-AQ, KORUS-AQ, ACT-America, ACTIVATE, FIREX-AQ, ACCLIP

About:

My research is focused on the instrumentation/measurement and data analysis of trace gases (CO, CO2, CH4), and water vapor on aircraft.

Education/Professional Experience:

  • SSAI, NASA Langley Research Center, 2012 – Present
  • National Institute of Aerospace, 2004-2012
  • National Research Council Post-doc, NASA Langley Research Center, 2001 – 2003
  • Ph.D., Geochemistry, Florida State University, 2001

January 24, 2022

Four joint flights were conducted this past Tuesday and Wednesday (Jan 18-19) to capitalize on another cold air outbreak event, similar to the previous week. We observed significant temperature variations in the various vertical profiles conducted by the low-flying Falcon, with evidence of significant precipitation near the transition from overcast to open-cell cloud conditions. A significant decreasing gradient in cloud drop number concentrations was observed with distance offshore especially during the January 18 flights.

June 20, 2022

ACTIVATE’s final flight deployment ended this past week with Research Flight 179 (Saturday June 18) transiting back from Bermuda to Virginia. A number of flights in the past week continued to build on the dataset for aerosol-cloud-meteorology interactions surrounding the Bermuda area, including on Tuesday June 14 a “process study flight” where the coordinated aircraft characterized a building cumulus cloud system. The Falcon conducted its traditional “wall” pattern used during process study flights with ~20 stacked legs going from below to above the cloud. Meanwhile the UC-12 flew overhead conducting remote sensing measurements of the same system while launching numerous dropsondes. A day earlier (June 13), the joint research flight conducted was synchronized with a CALIPSO overpass in conditions that are ideal for intercomparison of data including cloud-free air with significant aerosol concentrations and a diversity of aerosol types including in particular African dust. Now the ACTIVATE team focuses on processing and data archival of the 2022 flight deployments.

January 24, 2022

Four joint flights were conducted this past Tuesday and Wednesday (Jan 18-19) to capitalize on another cold air outbreak event, similar to the previous week. We observed significant temperature variations in the various vertical profiles conducted by the low-flying Falcon, with evidence of significant precipitation near the transition from overcast to open-cell cloud conditions. A significant decreasing gradient in cloud drop number concentrations was observed with distance offshore especially during the January 18 flights.

June 14, 2021

This past week included two double-flight days on Monday-Tuesday (June 7-8). June 7 was notable in that the second flight (RF 80) was a “process study” flight, which accounts for approximately 10% of ACTIVATE flights. We targeted an area with a cluster of clouds and conducted a total of 10 Falcon legs in cloud at different altitudes ranging from ~2 to ~13 kft. These legs and a subsequent downward spiral resulted in 10 cloud water samples for a single cloud system. Simultaneously, the King Air conducted a ‘wheel and spoke” pattern far above to allow the remote sensors to characterize the environment and cloud that the Falcon was directly sampling. A total of 14 dropsondes were launched by the King Air in the ~3 hr flight. This flight and the other “process study” flight in this summer campaign (RF77 on June 2) will provide a remarkable dataset to investigate aerosol-cloud-meteorology interactions with very detailed measurements for single evolving cloud systems.

March 15, 2021

ACTIVATE conducted four more successful joint flights (Research Flights 51-54) this past week. We characterized a variety of cloud conditions including post-frontal clouds associated with another cold air outbreak on Monday (March 8) in contrast to the following day (Tuesday March 9) where there was a sharp inversion with uniform cloud top heights and generally thin clouds. Flights this past week were marked by influence from local and regional burning emissions. The second of two flights on Friday (March 12) was coordinated with a CALIPSO overpass.

Febraury 5, 2021

ACTIVATE’s had its first joint flight of the winter 2021 campaign on February 3. We were successful to sample a transition from overcast stratocumulus clouds to broken cumulus clouds near our farthest southeast point of the flight track. There was extensive mixed-phase precipitation in areas closer to shore but pure liquid clouds farther offshore coinciding with the open cell cloud field. Although at low optical depth, an interesting aerosol layer was observed above 6 km that most likely was dust due to its depolarizing nature.

January 30, 2020

This past week ACTIVATE took to the skies again to begin our 2021 winter campaign. In contrast to last year, we started a bit earlier in the month of January to capitalize on a higher frequency of cold air outbreak events. Friday’s flights (January 29) were particularly ideal with both aircraft sampling along cloud streets aligned with the predominant wind direction coming from the north/northwest. We observed a transition from supercooled droplets to mixed phase precipitation with distance away from shore.

June 13, 2022

The past week coincided with a string of excellent weather conditions leading to eight joint flights between June 7-11 (RF166-173). There was evidence of African dust in the region that the aircraft sampled, in addition to coordinated efforts with glider platforms operated by the Bermuda Institute of Ocean Sciences to study the upper parts of the ocean surface that may affect the ACTIVATE measurements via sea-air interactive processes. Research flight 166 on 7 June was somewhat unique in that we sampled distinct cloud streets that we more commonly flew in during the winter season associated with cold air outbreaks. The ACTIVATE team also hosted a successful outreach event at the Longtail Aviation hangar featuring 40 students from three local grade schools.

June 6, 2022

On 31 May, the ACTIVATE team conducted a joint plane transit flight from Langley Research Center to Bermuda to base operations there until June 18. A series of flights (Research Flights 161-165) up through Sunday 5 June helped obtain statistics of atmospheric conditions around Bermuda. Many of the local Bermuda flights ended with a spiral sounding just offshore the Tudor Hill facility to obtain important vertical data for trace gases, aerosol, and weather parameters that will complement extensive surface monitoring work going on in coordination with the NSF-funded BLEACH project going on focused on halogen chemistry. Flights have already gathered important statistics associated with shallow “popcorn” cumulus cloud fields.

May 23, 2022

Four graduate students from the University of Arizona visited Langley Research Center to learn about and participate in the operational side of ACTIVATE. They took part in a very active flight week, with a total of eight joint flights deployed (Flights 153 - 160). Flights 156 and 157 on Wednesday, May 18th were special because these were the first flights to and from Bermuda that included a CALIPSO underflight. The CALIPSO track was clear of clouds and various aerosol layers such as smoke and dust were present. Another set of joint flights to and from Bermuda was conducted on Saturday, marking a successful end to the May flights. The next update will be in a couple weeks as the coming week will be used to prepare to fly out to Bermuda to base operations there from 1-18 June.

May 16, 2022

The previous week was marked by a persistent low pressure system positioned off the mid-Atlantic coast that impacted flight operations. Only one joint flight was conducted as a result on Tuesday (10 May; Research Flight 152), which featured strong northeasterly winds and warm air advection over the coastal cold waters created stratiform clouds near the surface. During parts of the flight there were several layers of decoupled stratiform cloud in the lower (free) troposphere.  There was evidence of strong sea salt influence on this day with a high volume of cloud water samples collected that will be helpful for continued characterization of the cloud chemistry in the study region. This week was marked by some visitors to Langley Research Center from the science team including Hailong Wang (PNNL) and Minnie Park (BNL), along with Simon Kirschler who is visiting from DLR in Germany.

May 09, 2022

ACTIVATE’s sixth and final deployment began this past week with three successful joint flights (Flights 149-151). In contrast to the winter deployment, aerosol optical depths increased this past week with dust and smoke signatures, with the latter possibly stemming from plumes advected from the western United States. These data will be helpful to learn more about the impacts of these aerosol types on clouds even if they reside above cloud tops. On Thursday (5 May 2022) we conducted a successful refueling trip to Providence, Rhode Island marked by extensive cloud characterization and upwards of 20 cloud water samples helpful for cloud composition studies.

March 30, 2022

We wrapped up Deployment 5 on Tuesday after finishing a couple joint flights (Research Flights 146-148). Monday’s flight was intriguing owing to the diversity of aerosol types sampled ranging from the usual marine aerosol types such as sea salt to also smoke, dust, and pollen. Tuesday’s flights were excellent for cold air outbreak characterization including upwind clear air sampling and then also the transition from overcast cloud conditions to an open cloud field. We will begin Deployment 6 in the first week of May and conduct flights through the end of June.

March 28, 2022

After considerable effort and patience due to pandemic-related barriers, ACTIVATE was able to successfully execute its first flight to Bermuda this past week. Research flights 142-143 on Tuesday March 22nd involved out-and-back flights from Hampton, Virginia to Bermuda. Flights to Bermuda are important for a number of reasons including the ability to extend the spatial range of data off the U.S. East Coast to be farther removed from continental and Gulf Stream influence and closer to more “background marine” conditions. Flights 144-145 on Saturday March 26th were special in that a wide range of aerosol types were sampled including dust, smoke, sea salt, and biological particles especially in the form of pollen near the coast.

March 21, 2022

ACTIVATE had a golden flight day on 13 March 2022 (Sunday) with a cold air outbreak and two joint flights in morning and afternoon. In the morning flight we sampled an overcast cloud field that began to transition into a more broken field. We conducted 3 “walls” with the low flyer (Falcon) involving level legs below and in cloud stacked vertically on top of each other for better vertical characterization of the ‘aerosol-cloud system’. We launched 11 dropsondes with the high flyer (King Air). Data suggest significant new particle formation above cloud tops offshore during the cold air outbreak event. The two flights that day provide excellent data for model intercomparison to understand boundary layer cloud evolution. Later in the week (Monday March 14) was marked by smoke conditions offshore that the Falcon was able to characterize with its suite of instruments. Two graduate students and a research scientist from the University of Arizona visited NASA Langley Research Center this past week to learn about and participate in the operational side of ACTIVATE.

March 14, 2022

This week was dominated by a stalled cold front over the ACTIVATE flight domain, which prevented the team from executing flights most of the week owing to complex conditions that would affect data quality (e.g., mid and high level clouds impacting remote sensors on the King Air) and sampling of well-defined boundary layer clouds. We were successful though with flights at the beginning of the week (Research flights 135-136) on Monday March 7th, including both clear air and cloud characterization to the southern part of our usual sampling domain. The following week appears to be very promising with cold air outbreak conditions setting up as soon as this Sunday March 13th.

March 7, 2022

The past week of ACTIVATE flights (research flights 130-134) including more clear air characterization than past weeks, with both dust and smoke influence over the northwest Atlantic. Two of the flights consisted of a vertical spiral sounding in cloud-free and polluted conditions with the HU-25 Falcon with the King Air flying overhead, which will be helpful for a number of types of analyses, including intercomparison between aerosol remote sensing products from the HSRL-2/RSP (on the King Air) and in situ aerosol observations from the Falcon. The two flights on Friday March 4th in particular were excellent as there was high cloud fraction across most of our sampling region which afforded a chance to sample clouds impacted by potential dust and smoke plumes.

March 1, 2022

After standing down for a week to swap the B200 with the UC-12 King Air, flights resumed this past week (research flights 120-125) with three days of double-flights (Feb. 15, 16, 19). The statistical database representative of typical wintertime conditions continued to expand with these flights that all included cloud sampling and similar characteristics as recent weeks. For instance, gradients of decreasing cloud drop concentration with distance east of the shore continued to be observed, along with both warm and mixed-phase precipitation, and situations where cumulus clouds connected to overlying stratiform clouds.

February 22, 2022

After standing down for a week to swap the B200 with the UC-12 King Air, flights resumed this past week (research flights 120-125) with three days of double-flights (Feb. 15, 16, 19). The statistical database representative of typical wintertime conditions continued to expand with these flights that all included cloud sampling and similar characteristics as recent weeks. For instance, gradients of decreasing cloud drop concentration with distance east of the shore continued to be observed, along with both warm and mixed-phase precipitation, and situations where cumulus clouds connected to overlying stratiform clouds.

February 7, 2022

Research flights 115-119 in the past week continued the extensive characterization of the northwest Atlantic in during typical wintertime conditions. Notable features this week included gradients offshore such as how in flight 115 (Tuesday, Feb 1) clouds were initially scattered by the coast and then rapidly started to deepen and fill in forming an overcast deck on the outbound leg. Towards the northeast part of the flight path, clouds took on a distinctly decoupled appearance with cumulus clouds feeding an upper stratiform deck. Aerosol gradients were evident too with regard to number concentration and composition. These distinct differences in the study region on individual flights present a critical opportunity for data analysis to better understand the aerosol-cloud-meteorology system.

January 31, 2022

Six joint flights were conducted this past week, including three double-flight days between January 24 and 27. The two flights on January 24th included more sampling towards the southern part of our operation domain to get more diversity in conditions with regard to weather and aerosol conditions. The two flights on Thursday (Jan 27) included a refueling stop at Providence, Rhode Island to allow us to extend our spatial range of sampling. That day included complex cloud structure with wave characteristics (i.e., variable base and top heights) and decoupling of cloud layers. There was an abundance of ice nuclei during the two flights on this day.

January 18, 2022

ACTIVATE returned with flights this past week by executing Research Flights 100-104, including consecutive double-flight days on Tuesday and Wednesday (January 11-12, 2022). The two flights on January 11th were used to sampled upwind and into a region of clouds during a cold air outbreak event; the second flight was used to keep tracking the evolution of the cold air outbreak farther downwind to the southeast of where the first flight left off. Intriguing features were observed on the two flights on Tuesday including steam fog, funnel clouds, and waterspouts. Both warm and mixed-phase precipitation were observed, along with new particle formation above cloud tops.

December 13, 2021

Four joint flights were conducted this past week in ACTIVATE’s final week of science flights for December before resuming flights in January 2022. Notable was the back-to-back flight day on Thursday (9 Dec 2021) when the two aircraft flew north for a refueling stop at Quonset State Airport (Rhode Island). This marks the first refueling stop at a secondary base in the ACTIVATE project. Extending our typical spatial range was helpful for a more extensive characterization of the complex cloud scene  including solid and broken boundary layer cloud structure with distinctly different cloud types including both warm and mixed-phase precipitation. ACTIVATE measurements during these two flights will be very helpful to understand gradients in the aerosol-cloud system during the transitions between cloud types (e.g., stratocumulus, fair weather cumulus) and the solid versus broken cloud fields.

ACTIVATE Logo
The ACTIVATE team hosted an open data workshop with 70+ participants over two days on October 20-21, 2021. Discussion centered around how to access and use the data, in addition to walking through two detailed case study flights. Participants from the international audience presented some slides of their own to stimulate ideas and brainstorming around research into aerosol-cloud-meteorology interactions. Material from the workshop, including recordings of the two days can be found at: https://www-air.larc.nasa.gov/missions/activate/docs/data_workshop/Oct2021.html

December 6, 2021

The 5th ACTIVATE deployment started this past week with two joint flights having similar headings going southeast from the base of operations at NASA Langley Research Center. These flights allowed for unique sampling of trace gases, aerosols, and marine boundary layer clouds in the month of December, which has yet to be done during ACTIVATE’s first 93 flights leading up to these two flights. More flights are planned in the coming week before a break and then resumption of flights in January.

July 1, 2021

We finished our summer campaign this past week with four more ACTIVATE flights (Research Flights 90-93) between June 28 and 30. These flights focused on extensive data collection in typical summertime shallow cumulus clouds. A notable feature in these flights was sampling behind ship vessels near the coast that yielded especially large enhancements in particle concentration parameters.

June 28, 2021

Four flights were conducted last week, with two single flight days on June 22 and 24, and a double flight day on June 26. Saturday’s conditions (June 26) were in particular very good for ACTIVATE with a scattered shallow cumulus cloud scene throughout the day that both planes were able to jointly characterize. The past week also was linked to high variability in aerosol conditions with the northward advancement of African dust into our study region.

June 21, 2021

This past week included three single-flight days on Tuesday-Thursday (June 15-17). The first flight of this week (June 15) was a statistical cloud survey but proved to be a challenging flight to execute as the King Air encountered pervasive cirrus along the track and the Falcon dealt with low clouds at varying altitude ranges. The June 16 flight targeted mostly clear skies with observations of moderate aerosol loading. This flight also included an overflight of Langley Research Center at the end to intercompare with the AERONET site and the High Altitude Lidar Observatory (HALO) HSRL/water vapor lidar that was conducting upward looking ground tests. The last flight of the week (June 17) included a coordinated run along the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite overpass and then two reverse headings to capture in cloud data in vicinity of the ASTER overpass for additional contextual data. The flights on June 16-17 both saw non-spherical particles near the coast and drizzle over the ocean was observed on June 17.

June 7, 2021

Four successful joint flights occurred last week. The double flight day on Wednesday June 2 was particularly noteworthy. Our morning flight conducted our typical statistical survey flight plan to an area south of the Virginia coast where there was a cumulus cloud field, with some regions evolving into deeper, more organized, convection. Based on that flight and satellite imagery, we set up the second flight to execute a “process study” pattern where the Falcon conducted a series of transects through a selected cloud cluster to characterize the vertical microphysical properties of the developing cluster immediately followed by an environmental profile in the surrounding cloud-free region. Simultaneously, the King Air conducted a “wheel and spoke” pattern centered around the cloud system, with multiple dropsondes launched above, and on the periphery of the cloud cluster alongside remote sensing transects to characterize the cloud and aerosol system underneath. Data from both planes will be used to characterize the range of cloud types observed on that day, with a focus on understanding the processes that drive shallow cumulus organization.

June 1, 2021

The last two weeks were busy with 9 joint flights, including three separate double-sortie days. The May 21 morning flight in particular was intriguing with a mixture of different conditions offshore with the two aircraft flying mostly straight to the east and then returning on the same track to NASA LaRC. Closer to shore, the aircraft observed a stratus deck with a prominent aerosol layer just above cloud as observed by the HSRL-2. These clouds then transitioned progressively into a more scattered cumulus cloud field to the east. At the far eastern end of the track there was a cold pool that we sampled within and just outside. Throughout this and the other flights this past week, there was evidence both either (or both) smoke and dust in the free troposphere. Measurement data will help unravel how these various aerosol types interact with the different types of clouds such as in the May 21 flights. On May 19, we also coordinated the flight along the CALIPSO satellite track where both aircraft and the satellite had successful made measurements.

May 17, 2021

After a short break after the Winter 2021 campaign, ACTIVATE took back to the skies this past week to start the Summer 2021 campaign. We conducted 4 successful joint flights between May 13-15 with interesting cloud conditions in each flight. The lower-flying Falcon characterized multiple layers of clouds and observed both warm and mixed-phase precipitation. Remote sensing observations on the higher-flying King Air detected aerosol layers aloft in the free troposphere potentially from dust and smoke on separate flights.

April 5, 2021

ACTIVATE wrapped up its winter 2021 flight campaign with five joint research flights this past week (RF 57-61) capped off by a double-flight day on Friday (4/2) to capitalize on another cold air outbreak event. Those two flights included an increased number of dropsondes (~10 per flight) to get extensive temporal and spatial characterization of the vertical atmospheric structure as the cold air outbreak cloud field evolved during the day. Notable in the other flights last week was successful coordination with ASTER and CALIPSO overpasses in our flight region.

March 29, 2021

We executed a joint flight (RF 56) on Tuesday March 23rd on a day marked by fairly ‘clean’ conditions in terms of very low aerosol and cloud drop number concentrations in the marine boundary layer. Cloud fraction on this day was markedly lower than a typical cold air outbreak type of day, which is helpful for ACTIVATE which is aiming to generate statistics in a wide range of conditions associated with aerosols, clouds, and meteorology.

March 22, 2021

The previous week posed significant weather challenges but Saturday (March 20, 2020) did finally provide low clouds evolving in a cold air outbreak. Interesting features in that joint flight (Research Flight 55) were Asian dust residing aloft above the boundary layer clouds, in addition to an interesting layer of depolarizing aerosol right above clouds near the end of flight as observed by the HSRL-2; it is unclear what the source of that layer was, but data analysis with the Falcon data will help unravel those details.

March 8, 2021

ACTIVATE executed three successful joint flights (Research Flights 48-50) this past week. On Thursday March 4th we coordinated our flight with a NASA A-Train overpass over an area with some scattered marine boundary layer clouds. The back-to-back flights on Friday March 5th served two objectives to capitalize on an excellent cold air outbreak event: (i) characterize the aerosol and meteorological characteristics upwind of the cloud field farther downwind; and (ii) characterize the evolution of the cloud field with the desire to capture the transition from overcast cloudy conditions to open cell structure. Noteworthy features in these flights were dust layers from long-range transport and significant new particle formation.