Assessing the Parameterization of Nitric Oxide Emissions By Lightning in a Chemical Transport Model with Nitric Acid Columns from the IASI Satellite Instrument

Abstract:

Nitrogen oxides (NO_x ≡ NO + NO₂) in the free troposphere largely control the production of ozone (O₃), an important greenhouse gas and atmospheric oxidant. As HNO₃ is the dominant sink of tropospheric NO_x, improved understanding of its production and loss mechanisms can help to better constrain NO_x emissions, and in turn improve understanding of ozone production and its effect on climate. However, this understanding is inhibited by the scarcity of direct measurements of free tropospheric HNO₃, particularly in the tropics. We interpret tropical tropospheric nitric acid columns from the IASI satellite instrument with a global chemical transport model (GEOS-Chem). Overall GEOS-Chem generally agrees with IASI, however we find that the simulation underestimates IASI nitric acid over Southeast Asia by a factor of two. The bias is confirmed by comparing the GEOS-Chem simulation with additional satellite (HIRDLS, ACE-FTS) and aircraft (PEM-Tropics A and PEM-West B) observations of the middle and upper troposphere. We show that this bias can be explained by the parameterization of lightning NO_x emissions, primarily from the misrepresentation of concentrated subgrid lightning NO_x plumes. We tested a subgrid lightning plume parameterization and found that an additional 0.5 Tg N with an ozone production efficiency of 15 mol/mol would reduce the regional nitric acid bias from 92% to 6% without perturbing the rest of the tropics. Other sensitivity studies such as modified NO_x yield per flash, increased altitude of lightning NO_x emissions, or changes to convective mass flux or wet deposition of nitric acid required unrealistic changes to reduce the bias. This work demonstrates the importance of a comprehensive lightning parameterization to constraining NO_x emissions.