Nitrogen Oxides in the Nocturnal Boundary Layer: Chemistry of Nitrous Acid (HONO) and the Nitrate Radical (NO₃)

Jochen Stutz
Atmospheric Sciences Division
University of California-Los Angeles
Los Angeles, CA 99095
310-825-5369
Fax 310-206-5219
jochen@atmos.ucla.edu

It has recently become obvious that the nighttime chemistry of the lower troposphere can have considerable influence on daytime processes, such as the formation of ozone. The chemistry of nitrogen oxides plays a central role during the night, being responsible for the removal of hydrocarbons, the formation of precursors for daytime radicals, and the formation of nitric acid and particulate nitrate. Our current understanding of these nighttime processes is limited for a number of reasons. The observation of some of the key species, such as nitrous acid (HONO), the nitrate radical (NO₃), and dinitrogen pentoxide (N₂O₅) is challenging. Heterogeneous chemistry plays an important role, and we are just beginning to understand these reactions. Finally, the composition of the troposphere at night is strongly dependent on vertical mixing. A shallow nocturnal boundary layer (NBL) often inhibits transport of trace gases from the ground to the air aloft.

This project investigates the nighttime chemistry of nitrogen oxides by measuring the vertical distribution from the ground up to 200-m height of NO₃, HONO, and their precursors NO₂ and O₃ by differential optical absorption spectroscopy (DOAS). The vertical trace gas profiles are then interpreted to answer the following open questions about the chemistry of nitrogen oxides:

• What are the heterogeneous processes responsible for HONO formation?
• What are the production rates of HONO?
• How does HONO production influence daytime radical levels?
• How much NO_x is lost by the heterogeneous uptake of N₂O₅?
• What is the nighttime oxidation capacity of NO₃ in the troposphere?

The measurements are performed in the downwind plume of an urban area during the Nighttime Aerosol/Oxidant Plume experiments (NAOPEX) within the DOE-ACP framework. Steady state calculations and chemical box models are used to analyze the data acquired during the NAOPEX campaigns and the Texas Air-Quality Study 2000 in Houston.

More information about this research can be found on the Web at http://www.atmos.ucla.edu/~jochen

An update is provided in the viewgraphs from a presentation at the Atmospheric Sciences Program Annual Meeting held in March 2002.