Overview

Project Highlights:

  • Novelty: First extensive evaluation of isoprene secondary organic aerosol (SOA) mass yields in isoprene-rich environments. Past studies have focused on the Southeast US. This work will quantify yields in isoprene-rich forests in Borneo, West Africa, Amazonia, and Central America and relate these to local anthropogenic activity, in particular sulfur dioxide (SO2) sources that enhance isoprene SOA formation (Figure 1).
  • Feasibility: The methodology expands on an existing approach.[1] That is, quantify isoprene SOA yields by using GEOS-Chem to interpret relationships between organic aerosol (OA) and formaldehyde (HCHO) concentrations from measurements onboard aircraft (for OA and HCHO) and satellites (HCHO only). In this work the analysis will extend to other parts of the world by utilizing measurements from multiple aircraft campaigns.
  • Impact: This project will provide new information on the impact of rapid population growth, industrialization, and landcover change on isoprene SOA yields and in so doing provide vital information to regulate biogenic OA where people and forests coexist.

Isoprene is a reactive volatile organic compound (VOC) emitted in large quantities by dense broadleaf trees in the tropics and Southeast US.[2] Isoprene makes a large contribution to organic aerosol (OA) and so impacts public health and climate.[3] Isoprene secondary OA (SOA) is from reactive uptake of oxidation products to pre-existing aqueous aerosol.[4] Sulfate aerosol, formed from oxidation of sulfur dioxide (SO2), is a dominant aqueous aerosol component, so that isoprene-rich environments close to SO2 sources have enhanced isoprene SOA mass yields (Figure 1).[1] This is in direct conflict with laboratory studies that obtain peak isoprene SOA yields under very dry conditions atypical of humid isoprene-rich forests.[1] There are large regional changes in SO2 emissions due to rapid development and air quality policy that will impact isoprene SOA.[5]

Isoprene SOA yields are challenging to quantify, as SOA precursors include multiple oxidation products that react further in the aerosol phase.[4] Indirect estimate of isoprene SOA yields is possible by interpreting the linear relationship between observations of total OA and formaldehyde (HCHO) with the GEOS-Chem CTM. In isoprene-rich regions HCHO variability is driven by isoprene emissions and isoprene should be a substantial OA component, so that the relationship between OA and HCHO is sensitive to the underlying isoprene SOA yields. The isoprene SOA yield is then the value that in GEOS-Chem gives the same OA-HCHO slope as in the observations. This approach has been used to quantify isoprene SOA yields of 3.3% in the Southeast US in summer 2013 (~500 Gg SOA).[1]

Here we propose to extend this approach to other years in the Southeast US and other isoprene-rich locations. Research questions that will be addressed include: (i) what are isoprene SOA yields in forests in Borneo, West Africa, Amazonia, central America, and in other years in the Southeast US, and how do these compare to reported estimates?, (ii) How are yields of isoprene SOA impacted by rapid changes in anthropogenic activity?, (iii) what SO2 sources contribute to isoprene SOA formation that air quality policy should be target?, (iv) what is the global annual isoprene SOA budget?

Anthropogenic enhancement in isoprene secondary organic aerosol (SOA) formation.

Methodology

Isoprene SOA yields will be obtained from the relationship between total OA mass concentrations and HCHO mixing ratios from measurements onboard aircraft during the following campaigns: DC3 (2012) and INTEX-A (2004) in the Southeast US, INTEX-B/MILAGRO (2006) in Central America, OP3 (2008) in Borneo, AMMA (2006) and DACCIWA (2016) in West Africa, and AMAZE (2008) and GoAmazon (2014/15) in Amazonia. Where there are no aircraft measurements of HCHO (e.g., OP3) we will instead use satellite observations from the Ozone Monitoring Instrument (OMI). GEOS-Chem will be used to interpret the OA-HCHO slopes and determine aerosol composition.

Training and Skills

CENTA students are required to complete 45 days training throughout their PhD including a 10-day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes. 

The student will be trained by a University of Birmingham IT specialist to interface with the local servers to access large satellite datasets and use the GEOS-Chem CTM. Training to use GEOS-Chem and satellite HCHO observations will be provided by Dr. Marais. Additional GEOS-Chem training will be from workshops conducted by the GEOS-Chem Support Team at the biannual User’s Meeting (Cambridge, MA).

Timeline

Year 1:

  • Conduct literature review of isoprene SOA, atmospheric formaldehyde, Earth observations, and GEOS-Chem
  • Skills training in global 3D modelling (GEOS-Chem model) and the use of satellite observations of formaldehyde
  • CENTA generic skills training
  • Data acquisition (satellite formaldehyde, and aircraft/surface organic aerosol measurements)
  • Initial analysis of observations

Year 2:

  • Formulate introduction and methodology of journal article
  • First GEOS-Chem simulation and analysis of output
  • Attend GEOS-Chem User’s Meeting
  • Advanced CENTA training
  • Model updates and reruns
  • Formulate results and discussion sections of journal article
  • Present preliminary results at conference

Year 3+:

  • Final model simulations and analysis
  • Final edits and submit journal article
  • Compile thesis

Partners and collaboration (including CASE)

CASE: A potential CASE partner is the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA (contact: Kelly Chance; satellite formaldehyde retrieval expert).

Other Partners: Likely collaborators include Dr James Allan (Senior Research Fellow, School of Earth and Environmental Sciences, University of Manchester) and Dr James Levine (Research Fellow, School of Geography, Earth and Environmental Sciences, University of Birmingham). Dr. Allan has measured OA at numerous field campaigns. Dr. Levine’s research interests in biosphere-atmosphere interactions in Amazonia are complementary.

Further Details

For further details please contact:

Dr Eloïse A. Marais: marais.eloise@gmail.com or emarais@seas.harvard.edu (until 1/12/2016).

Environmental Health Fellow at GEES, University of Birmingham (from 1/12/2016).