Removal of atmospheric organic carbon via wet deposition: quantification and characterization of organic carbon in rainwater across the United States.

A complete description of the atmospheric organic carbon (OC) budget requires a thorough understanding of the amount and chemical composition of OC removed via deposition; however, measurements of such values, spanning a wide temporal and geographic range, remain limited. In order to better constrain wet deposition of OC, we present new measurements of dissolved organic compounds in rainwater throughout the United States for samples collected at 16 sites in the National Atmospheric Deposition Network (NADP), across all four seasons of 2015. Two techniques are used to quantify and characterize the deposited OC in rainwater: Total Organic Carbon Analysis (TOC) and Aerosol Mass Spectrometry (AMS). TOC is used to quantify total OC, which includes both volatile species (dissolved gases) and low-volatility compounds (dissolved aerosol components). Freeze drying the samples removes the volatile species, enabling the study of the low-volatility fraction only, using both the AMS and TOC. AMS analysis is carried out using a new method to introduce samples into the instrument using small volumes of solution (4 μL) via ultrasonic nebulization, providing information on chemical signatures and elemental ratios (O:C, H:C, and N:C) of the OC. Trends in the composition and concentration of rainwater OC will be compared across these samples, providing quantitative information on key chemical characteristics across locations and seasons.