Global Trends in Satellite-Derived Fine Particulate Matter & Developments to Reactive Nitrogen in a Global Chemical Transport Model

Abstract

At present, air pollution is the leading global environmental risk factor for premature mortality. Highly respirable fine particulate matter (PM2.5) dominates this global burden of disease, while ozone makes smaller but noticeable contributions. Nitrogen oxides (NOx ≡ NO + NO2) modulate oxidant fields and influence air quality. This thesis is composed of three research chapters which make use of, and developments to, a global atmospheric chemical transport model (CTM) for the purposes of (i) monitoring and understanding global trends in satellite-derived PM2.5 and (ii) improving the simulation of NO2 reaction on ground surfaces.

First, aerosol optical depth (AOD) retrieved from two satellite instruments, MISR and SeaWiFS, is used in conjunction with the GEOS-Chem CTM to produce a unified 15-yr global time series (1998–2012) of ground-level PM2.5. Four regional areas with significant and spatially coherent trends are examined in detail: eastern U.S., Arabian Peninsula, South Asia, and East Asia. The linear tendency over the eastern U.S. (-0.37 ± 0.13 µg m-3 yr-1) agrees well with that from ground-level monitors (-0.38 ± 0.06 µg m-3 yr-1).

Next, the trace gas dry deposition parameterization from GEOS-Chem is reimplemented to run in single-point-mode to facilitate direct evaluation of isolated components against above-canopy fluxes of nitric acid (HNO3), NO2, and total oxidized reactive nitrogen (NOy) observed by the method of eddy covariance. A low bias of -80% in simulated nocturnal NO2 deposition velocity was eliminated by representing a reaction pathway for NO2 heterogeneous hydrolysis on deposition surfaces, paying attention to canopy surface area effects and interferences from soil NOx emissions.

Finally, we develop a parameterization to represent the process of subgrid dry deposition of near-surface emitted NOx and implement into the GEOS-Chem CTM along with aforementioned updates to NO2 dry deposition. Resulting reductions in ground-level NO2 are on the order of 5–20% with commensurate reduction in regional concentrations of total nitrate (HNO3 + particulate nitrate). Large increases (>100%) in simulated surface concentrations of nitrous acid (HONO)—an important precursor of the hydroxyl radical (OH)—stem from improved representation of NO2 surface processes and help to alleviate a large low bias compared to aircraft observations.