GOAMAZON-LASERLUM

Observations and Modeling of the Green Ocean Amazon: Laser Luminescence

1 January 2014 - 31 December 2014

Lead Scientist: Frank Keutsch

Observatory: AMF, MAO

The formation of secondary organic aerosol (SOA) and ozone (O3), two secondary pollutants that impact human health, the biosphere, and climate, is directly coupled to the oxidation of volatile organic compounds (VOCs). The primary objective of the proposed work was to utilize a suite of measurements, especially of glyoxal and formaldehyde obtained by Keutsch group instrumentation based on laser-induced luminescence techniques, in the Amazon to (a) test and improve our mechanistic understanding of processes controlling the oxidative capacity in a region of high biogenic VOC (BVOC) concentrations and (b) investigate the details of anthropogenic influence on processes transforming BVOC emissions into secondary pollutants, especially SOA and O3. Isoprene is the most abundant VOC, contributing to all non-methane VOC emissions. Similarly, the OH radical is the dominant daytime tropospheric oxidant. Therefore, accurate understanding of all processes controlling the chemistry and fate of isoprene and the OH radical is critical for understanding the formation of SOA and O3. The objectives were pursued as a field study in the central Amazon downwind of the industrial city of Manaus (population 2 million), Brazil, during two intensive operating periods (IOPs) (Feb/Mar and Aug/Oct 2014) of the GoAmazon 2014/15 campaign. The Keutsch Group obtained high time resolution data sets of formaldehyde and glyoxal, two important tracers of the oxidative processes forming SOA and O3, and the only two for which satellite retrievals exist. The measurement site downwind of Manaus was ideal for studies of anthropogenic influence as the fetch oscillates between the extremes of (a) a pristine atmosphere when the Manaus pollution plume does not affect the site and (b) heavy pollution and the interactions of that pollution with the natural environment when the plume conforms to its mean flow. The timing of the IOPs in the wet, clean season and the dry, biomass burning season, provided for an additional important scientific contrast. The Amazon was a particularly effective region for the proposed work as intense sunlight, high BVOC emissions dominated by isoprene, and sustained, high OH concentrations result in rapid photochemical processing and SOA formation. The mechanistic details underlying these findings remain poorly understood, for both clean conditions and with anthropogenic influence. The following questions merit further investigation, and were the objectives of our proposed study: (i) How is high oxidative capacity maintained at high BVOC concentrations, and what are the implications of recent findings that question previous OH measurements? (ii) How is the gas-phase processing of BVOCs, especially isoprene, affected by anthropogenic influence? Specifically, how do anthropogenic NOx (NOx = NO+NO2) and sulfate affect: (a) the degree of functionalization versus fragmentation of the BVOC oxidation product distribution, affecting product volatility? (b) radical chain termination versus propagation, affecting oxidative capacity and thus oxidation rates? (c) SOA formation via (non-traditional) aqueous pathways? A common theme throughout the proposal was the attempt to develop tracers of or metrics for representing oxidative capacity and anthropogenic influence from a product-based perspective, i.e., the effect these processes have on tropospheric composition. We focused on testing consistency between measurements of OH, models of OH, and measurements and models of the resulting oxidation product distribution as function of anthropogenic influence. These factors determine the relative ozone productions rates and mass and properties of reactive carbon in the gas and condensed phases. We also addressed profound uncertainties concerning glyoxal in regions such as the Amazon that call into question our understanding and models of VOC oxidation and SOA formation. In addition to the formaldehyde and glyoxal datasets multiple publications are expected as a result of the field campaign.