Where: La Réunion, Indian Ocean

When: March 2016

Among natural sources, sea spray emissions play a critical role in our understanding of climate. It has been shown recently that sea spray emission depends on sea surface temperature (SST) in addition to wind speed and waves. Climate models predict an SST increase of 2 to 4°C in the Tropics and Subtropics by 2100, with the largest increase expected in the Indian and Pacific Oceans. Sea spray emission would increase by 14 to 40%, resulting in a negative feedback in clear sky on a warming climate which is not taken into account in climate models due to biased emission parameterizations.

Recently, marine aerosols have also been highlighted as an important source of ice nuclei (IN) in pristine oceanic environments [Wilson et al., 2015]. Differences in tropical tropopause layer (TTL) cirrus properties and water vapor concentrations could exist for air masses with low and high aerosol loadings. However the lack of data from pristine environments prevents us from making firm conclusions on the possible influence of anthropogenic emissions on IN distribution in the TTL and heterogeneous cirrus cloud formation. Studies are mostly based on northern hemisphere cirrus measurements, while hemispheric differences in ice supersaturation, aerosol content and concentration could lead to different ice microphysics for the southern hemisphere TTL [Ovarlez et al., 2002].

Recent literature has shown that the Southern Indian Ocean is potentially a hot spot for sea spray emission due to strong trade winds and high sea surface temperature. Hence, measurements taken from La Réunion are relevant for several topics related to marine aerosols and pristine oceanic regions.

The POPS (Printed Optical Particle Spectrometer) instrument, developed at CSD by NOAA and CIRES (Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder) researchers, deployed at La Réunion during a 2 week field campaign in March 2016. The European ACTRIS-2 Trans-National Access funded the AeroMarine project.

The goal of the AeroMarine project was to:

  1. Test the POPS instrument with a drone, a light plane, and a balloon launch
  2. Acquire datasets on marine aerosol distribution and atmospheric thermodynamic state to further our understanding on marine aerosols and marine boundary layer
  3. Estimate the exchange of aerosols between the MBL (marine boundary layer) and free troposphere

The AeroMarine project was also envisioned as an international collaborative effort between NOAA (USA) and OSU-RĂ©union and LACy laboratory (France). By bringing in expertise from NOAA, it will help design new strategies at Maïdo Observatory on innovative measurement techniques for future monitoring of atmospheric composition.

POPS profiles from UAL
Unmanned Aerical Vehicle (UAV) deployment strategy: The ACInnov company located in La Réunion is specialized in long-duration UAV flights. Between March and June 2016, it is planned to integrate the POPS on board a UAV that can carry a payload of 2kg over a distance of up to 300km. Difficulties were encountered during the takeoff stage and prevented us from doing POPS measurements on board the UAV. The alternative plan was to use a light plane. Two profiles were performed over the Cambaie Bay area and 2km offshore with POPS installed on an ultra light aircraft.
POPS aerosol profile from baloon
Balloon launches: 2 balloon sondes were launched at Maïdo Observatory at nighttime. The payload consisted of a POPS and a COBALD lightweight aerosol backscatter sonde, permitting the detection, profiling and optical properties of aerosol particles. Communication was lost at 10km altitude for the first profile, but the aerosol concentration profile was completed up to 30km during the second profile.
POPS total particle concentration indicates that the aerosol content in the TTL over La Réunion is low (~10 partices/cm3) and is dominated by aerosol sizes corresponding to the accumulation mode (particle sizes between 0.1 and 2.5µm, not shown). This is probably reasonable as inter-hemispheric mixing is weak and the TTL over the SH is cleaner than the NH.
ULA pilot Jerome Jerome and ULA pilot ULA takeoff UAV launch
POPS installation on an ultra light aircraft (ULA), flown from Le Port, La Réunion (in the Indian Ocean); with the flight directed by Jerome Brioude. Photos: K. Rosenlof, NOAA
Jerome Brioude, Stephanie Evan, Karen Rosenlof, Ru-Shan Gao Jerome Brioude, Stephanie Evan, Ru-Shan Gao
balloon payload with POPS instrument balloon cryogenics preparing the balloon payload preparing the balloon payload
balloon launch at night preparing the balloon launch
POPS installation on a balloon, launched from Maïdo Observatory at nighttime. Photos: K. Rosenlof and R. Gao, NOAA