Cloud Water and Precipitation Chemistry in The Northeast United States: A New Chemical Regime Under Changing Emissions

Author

Christopher Lawrence, University at Albany, State University of New YorkFollow

ORCID

https://orcid.org/0000-0002-4845-340X

Date of Award

Fall 2024

Language

English

Embargo Period

9-30-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Atmospheric and Environmental Sciences

Program

Atmospheric Science

First Advisor

Sara Lance

Committee Members

Mary Barth, James Schwab, Justin Minder, Fangqun Yu

Keywords

Cloud Chemistry, Organic Carbon, Organic Acids, Precipitation, Carbon Cycle, Climate Change

Subject Categories

Atmospheric Sciences | Biogeochemistry | Climate | Environmental Chemistry

Abstract

This dissertation explores the changing chemical regime in cloud water collected at the summit of Whiteface Mountain (WFM), and more broadly the Northeast U.S. A long-term trend analysis of WFM cloud water indicates that annual median concentrations of SO₄² and NO₃^- have decreased by factors of 4 and 2 since 1994, respectively, which has led median cloud water pH to increase by up to 2 pH units. Warm season median total organic carbon (TOC) concentrations have doubled since measurements began in 2009. In addition to this increasing TOC trend, the average measured cation/anion ratio has been steadily increasing, which strongly correlates with TOC, suggesting that organic compounds (hypothesized to be organic acids) represent an increasingly important fraction of unmeasured anions. A new inferred “top down” pH (pH_TD) was introduced to properly represent the pH of the vast majority of cloud droplets as they exist in the atmosphere by accounting for the mixing state of ions like calcium and magnesium that typically exist primarily within coarse particles. pH_TD closely tracked measured pH in the first decade of cloud water monitoring but the discrepancy grew in the second half of the record as pH_TD has remained essentially flat for the past decade. Monocarboxylic and dicarboxylic organic acid measurements conducted on WFM cloud water from 2018 - 2022 showed that formic acid (HCOOH), acetic acid (CH₃COOH) and oxalic acid (OxAc) were the most abundant organic acids on average, though limited measurements suggest malonic acid may also be important. While ion balance was partially improved by the addition of these organic acids, a substantial fraction of unmeasured anions remain uncharacterized, which still correlate with TOC and represent a growing fraction of cloud water acidity. A combination of WRF-Chem and chemical box modeling simulations suggest that biogenic emissions of isoprene represent the major sources of HCOOH, CH₃COOH, and OxAc. Gas phase box modeling greatly underestimates HCOOH and CH3COOH mixing ratios measured in WFM cloud water and while the inclusion of aqueous chemistry exacerbates the model error of HCOOH, does not change CH₃COOH, it can explain measured OxAc concentrations. pH sensitivity tests with the gas+aqueous box model indicate for pH values > 6 the aqueous production of OxAc increases but HCOOH, CH₃COOH, and OxAc are rapidly removed, indicating that using bulk pH measurements without proper representation of aerosol mixing state and organic acidity can greatly change model predictions and is currently not well constrained. A regional trend analysis of 4 other sites that measure TOC or DOC in cloud water or precipitation indicates a positive regional trend of atmospheric aqueous organic carbon (AAqOC) in the Northeast U.S. The two bulk deposition sites that collect precipitation during both wet and dry periods exhibited AAqOC growth rates that are 3-7x greater during the growing season vs the non-growing season. Several hypotheses that may explain the AAqOC trend were briefly explored. Increased emissions of biogenic VOCs are thought to be the most likely explanation, though increased wildfire influence could contribute further to this trend if frequency or strength of wildfire smoke increase under anthropogenic climate change. However, the lack of observational data makes it challenging to investigate the controlling mechanisms in detail. Increased observational data of organic carbon in the gas, particle, and aqueous phases are required to better constrain the atmospheric organic carbon budget.

License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Recommended Citation

Lawrence, Christopher, "Cloud Water and Precipitation Chemistry in The Northeast United States: A New Chemical Regime Under Changing Emissions" (2024). Electronic Theses & Dissertations (2024 - present). 112.
https://scholarsarchive.library.albany.edu/etd/112