APRIL 2002: Sources and Chemistry of Nitrogen Oxides over the Pacific

Shown in the top row are concentrations of NO, HNO₃, and PAN observed during the PEM-Tropics B aircraft mission conducted in March-April 1999 over the tropical Pacific. The bottom row shows simulated concentrations from a global 3-dimensional model analysis of the same time period using assimilated meteorological observations from ECMWF reanalyzed by T.N. Krishnamurti's group at Florida State University. The observed and simulated concentrations are averaged over 165-100W longitude and plotted as a function of aitude and latitude. Elevated concentrations of NO in the upper troposphere reflect input from lightning emissions, which provided the largest source of NO_x over the equatorial and Suth Pacific. Simulated concentrations of HNO₃ are sensitive to the model representation of deep convection and associated HNO₃ scavenging, as illustrated by the depletion in the tropical upper troposphere. Chemical losses of NO_x during PEM-Tropics B exceed chemical sources by a factor of 2 in the South Pacific upper troposphere. The chemical imbalance, also apparent in the low observed HNO₃/NO_x ratio, is explained by NO_x injection from lightning and by frequent convective overturning, depleting HNO₃. The simulation of PAN is sensitive to the treatment of acetone and acetaldehyde, which were observed at mean concentrations of 431 pptv and 78 pptv, respectively, during PEM-Tropics B [Singh et al., 2001]. The model results presented here account for an oceanic source of acetone, which improves the simulation of PAN and NO_x. However, when the model is constrained to reproduce the high acetaldehyde concentrations measured throughout the tropospheric column, simulated PAN concentrations greatly exceed those observed, indicating a problem either in the acetaldehyde measurements or in our understanding of PAN chemistry. This work was led by Amanda Staudt and a full account is given in Staudt et al [2002].