A Targeted Approach to Delineating the Source(s) and Optical Properties of CDOM

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A Targeted Approach to Delineating the Source(s) and Optical Properties of Chromophoric Dissolved Organic MatterThe colored components of dissolved organic matter (DOM) absorb light to varying degrees in all natural waters. This material, known as colored or chromophoric dissolved organic matter (CDOM), plays an essential role in determining the color of natural waters and influences the amount of ultraviolet and visible light that penetrates into water bodies. Because CDOM absorbs strongly in the ultraviolet, it can protect aquatic organisms from damaging radiation, whereas its absorption in the visible can reduce the amount of light available for photosynthesis, thus potentially affecting aquatic ecosystems. The absorption of light by CDOM also initiates a series of photochemical reactions, which produce reactive oxygen species that can transform CDOM and other compounds, and affect the availability of essential trace metals to aquatic organisms. Despite its established importance in aquatic chemistry, the compounds and structures that give rise to light absorption and emission and produce the photochemical reactions are still poorly understood. This work will use a series of laboratory tests to identify chemical groups within the CDOM that influence its optical and photochemical properties and how these vary with source.These tests (and the information they can provide) include: 1) the effect of sodium borohydride reduction on the optical absorption and emission properties (contribution of ketones/aldehydes to optical properties); 2) pH dependence of the spectral absorption and emission before and after borohydride reduction (identify contributions of carboxylic acids and phenols and possible electronic interactions between these groups and ketones/aldehydes); 3) the effect of anoxic dithionite reduction on the optical properties (content and impact of quinones to optical properties); 4) molecular size dependence of the optical properties of CDOM from differing sources/locales and how this may be affected by borohydride reduction and pH. This work will focus on laboratory studies of humic substances and of archived samples of CDOM acquired in past field work from diverse oceanic and freshwaters sites, but will be augmented by samples collected from the Chesapeake Bay through a complementary collaboration with a NASA project that aims to examine water quality in shellfish beds through direct sampling and optical remote sensing.This proposal will further support plans to initiate a program for talented and gifted students at Kenmoor Middle School in the Prince George’s County Public Schools in Maryland. Plans for this program include presentations/demonstrations to the students and faculty at the middle school that are related to the goals of this project, followed by a visit of an interested, select group of students to the PI’s laboratory where they will be allowed to see and operate some of the scientific instruments employed in this project and be involved in additional hands-on demonstrations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.