Improvements to the treatment of organic nitrogen chemistry and deposition in CMAQ

Reactive nitrogen plays an important role in the chemistry of the atmosphere and recent studies have pointed to the importance of organic nitrates in atmospheric chemistry. The way we represent organic nitrogen chemistry can have implications for ozone and PM predictions because it can act as a reservoir for NOx by either removing it from the system or releasing it at another location downwind, where chemical or meteorological conditions might be different. Organic nitrogen comprises thousands of different types of molecules, with a corresponding large range of physical and chemical properties. The reaction rate of common organic nitrates, which controls the chemical lifetime in the atmosphere, for example, can vary from 15 minutes to 5 days depending on the structure of the compound. Even more dramatically, the Henry's Law constant of different organic nitrogen compounds can vary over 4 orders of magnitude depending on the presence of polar functional groups, affecting its lifetime in the presence of clouds, rain and fog. In current chemical mechanisms, organic nitrates are assigned to only one or two “representative” compounds, but the huge variation in physical and chemical properties raises questions about whether these models adequately represent the behavior of the entire family of organic nitrates.

In this study, we use the Community Multiscale Air Quality Model (CMAQ) to look at the sensitivity of ozone concentrations to changes that are made to the assignment of both biogenic and anthropogenic organic nitrates to additional model species and the physical properties assigned to these new modeled nitrates. We use the Carbon Bond (CB05) as our base mechanism and classify secondary nitrates by features that have the largest effect on nitrate decay and removal, such as the presence of alkenyl, hydroxy, oxo and hydroperoxy groups. Since air quality models must be optimized to consider as few species as possible, we examine how to find a balance between detail and computational efficiency. We detail assumptions and uncertainties in the modifications.

In addition to the importance in atmospheric chemistry, organic nitrates are an important contributor to nitrogen deposition, contributing 10-30% of the total nitrogen deposition. The current version of the CMAQ model greatly underestimates this contribution. Improvements to the speciation as well the solubility of the organic nitrate species are important to correctly predicting the dry and wet deposition. Changes to the deposition approaches are coordinated with changes in the CB05 chemical mechanism. In addition to changes in the Henry's Law constants which affect wet deposition, we defined additional deposition velocity surrogates to account for the effect of variations in diffusivity and solubility among the different organic nitrates. The diffusivity affects the stomatal uptake while the solubility affects the dry deposition to wetted surfaces such as plant cuticles or the ground as well as dry deposition to water. We examine the sensitivity of model predictions of wet and dry deposition to the changes in the speciation of organic nitrates and the use of alternate deposition surrogates.