Southern Oxidant & Aerosol Study
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ABOUT THE SOAS WORKSHOP

Workshop Purpose: A workshop to discern the most urgent and critical open science questions regarding atmosphere/biosphere interactions. There will be a special emphasis on the Southeastern U.S. due to anomalous temperature trends compared to the rest of the U.S. (the Southeastern U.S. has actually been cooling over the past 100 years). A small workshop will facilitate interactive discussion, often lost in big meeting settings, to identify which critical questions are the most important and which we, as a community are best suited to tackle with our varied and multi-disciplinary resources. Often, substantial leaps in progress require community-based efforts that rely on simultaneous co-located measurements, controlled laboratory studies, and openly distributed models. During the workshop we will engage all participants to investigate how the community imagines what simultaneous information would be needed to gain insight and answer the most critical questions? A major goal of the workshop is to bring together established researchers with proven track records and early career scientists with new and different perspectives to provide opportunities for professional interactions in a focused and productive forum.

Workshop Overview: Emission of volatile organic compounds (VOCs) by terrestrial vegetation to the atmosphere affects the oxidative capacity, contributes to the aerosol burden and influences the global carbon cycle. The degree to which the atmosphere, in particular man-made pollution, affects biogenic emissions and their fate in the atmosphere remains poorly understood. Conventional wisdom regarding biogenic emissions (BVOCs) has been that compounds, most namely isoprene, react in the atmosphere to increase O3 and CH4, while decreasing OH[1]. However, current models cannot adequately describe oxidant levels in biogenically-dominated areas[2] and the pathways describing oxidation are still uncertain and debated[3-5]. Biogenic contributions to the atmospheric particulate matter (PM) burden in terrestrial environments were thought to arise largely from primary emissions related to plant debris (e.g., cuticular waxes)[6]. However, biogenic emissions can form secondary organic aerosol (SOA) from many precursors[7, 8], and their chemical tracers have been measured in a variety of environments[9-13], including the Southeast U.S.[14]. There is evidence for terrestrial biogenic contribution to particulate organics in the free troposphere[15]. The Southeastern U.S. has not warmed like other parts of the U.S. in response to global climate change[16, 17]. The temperature anomaly may be related to aerosols derived from BVOC precursors[16], largely overlooked in earlier studies (e.g., SOS[18]) of this biogenically-dominated area.

CURRENT CRITICAL SCIENCE QUESTIONS:

  1. Are biogenic emission models sufficient to describe BVOC and oVOC emissions in forested areas, suburban areas, urban areas? Biogenic VOC (BVOC) emissions dominate over anthropogenic sources[19]. Despite the abundance and importance of BVOCs, emission algorithms are poorly constrained[20], in part due to a lack of consistent and widespread measurements. Isoprene and oxygenated-VOC emissions for the same location can differ substantially[21, 22].
     
  2. Why does adequate description of the gas-phase chemistry of biogenic volatile organic carbon remain elusive? Several BVOC oxidation pathways have recently been elucidated in laboratory and theoretical studies: the formation of alkyl nitrates[23, 24], epoxides[25], HOx reformation[4, 5, 26], and organic aerosol[24, 27, 28]. The relative contribution from each pathway impacts the ozone concentration, the fate of reactive nitrogen, and the formation of secondary organic aerosol (SOA).
     
  3. Are there anthropogenic influences on biogenic SOA formation? There is evidence to suggest that anthropogenic pollution can affect the formation of SOA from BVOC precursors. For example, laboratory evidence indicates a difference in the isoprene SOA yield depending on NOx conditions[29]. Field measurements of chemically-characterized products suggest evidence of the low NOx pathway with observation of epoxydiols[30] and the high NOx pathway with observation of methyltetrols[14]. Increased aerosol acidity, often a consequence of anthropogenic SO2 emissions, may also affect SOA production from BVOCs[31, 32].
     
  4. To what extent is there aqueous/cloud processing of BVOCs and related aerosols? The vertical profile of the short-lived climate forcer (SLCF) particulate organic carbon is not well simulated in atmospheric models and this contributes to uncertainty in climate projections because radiative scattering is altitude dependent[33]. Changes in emissions, SOA partitioning parameters, (among other efforts) do not improve model-predicted vertical profiles[34], but inclusion of aqueous phase organic chemistry (e.g., cloud processing of VOCs) does[35]. Many of the aqueous phase products of cloud processing are known to produce light-absorbing "brown carbon"[36].
     
  5. What are the effect(s) of biogenic aerosol (VOC of biogenic origin) on local climate? Global temperatures have increased over the past 100 years, yet these increases have not been uniform. The southeastern US has, in fact, cooled over this period. This could be a result of changing global circulation patterns, increased forest cover, or trends in aerosol radiative forcing[16] and clouds. Isoprene-derived compounds contribute to organic aerosol in the free troposphere[15].

References

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