PhD Research Project: Isoprene secondary organic aerosol in a changing world
Isoprene is a reactive volatile organic compound (VOC) emitted in large quantities by dense broadleaf trees in the tropics and Southeast US. Isoprene makes a large contribution to organic aerosol (OA) and so impacts public health and climate. Isoprene secondary OA (SOA) is from reactive uptake of oxidation products to pre-existing aqueous aerosol. 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). This is in direct conflict with laboratory studies that obtain peak isoprene SOA yields under very dry conditions atypical of humid isoprene-rich forests. There are large regional changes in SO2 emissions due to rapid development and air quality policy that will impact isoprene SOA.
Figure 1. Anthropogenic enhancement in isoprene secondary organic aerosol (SOA) formation.
Isoprene SOA yields are challenging to quantify, as SOA precursors include multiple oxidation products that react further in the aerosol phase. 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).
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?
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.
In addition to completing an online application form, you will also need to complete and submit the CENTA studentship application form available from www.centa.org.uk.
CENTA studentships are for 3.5 years and are funded by the Natural Environment Research Council (NERC). In addition to the full payment of their tuition fees, successful candidates will receive the following financial support.
Annual stipend, set at £14,296 for 2016/17
Research training support grant (RTSG) of £8,000
CENTA students are required to undertake from 45 days training throughout their PhD including a 10 day placement.
 Marais et al., Atmos. Chem. Phys., 16, 1603-1618, 2016.
 Guenther et al., Geosci. Model Dev., 5, 1471-1492, 2012.
 Carlton et al., Atmos. Chem. Phys., 9, 4987-5005, 2009.
 Surratt et al., PNAS, 107, 6640-6645, 2010.
 Klimont et al., Environ. Res. Lett., 8, 2013.
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