Direct observations of daytime NO3: Implications for urban boundary layer chemistry

The nitrate radical (NO 3 ) is the dominant atmospheric oxidant during the night in most environments. During the day, however, NO 3 has thus far been considered insignificant. Here we present daytime measurements of NO 3 by Differential Optical Absorption Spectroscopy near Houston, Texas, during the Texas Air Quality Study 2000. On 3 consecutive days in August/September 2000, NO 3 reached levels from ∼5 ppt 3 hours before sunset to 31 ppt around sunset. Daytime NO 3 had a negligible effect on the photostationary state (PSS) between O 3 and NO x , with the exception of the last hour before sunset, when it significantly accelerated NO‐to‐NO 2 conversion. On August 31, chemical reactions involving NO 3 destroyed 8 (±4) ppb O x (= O 3 + NO 2 ) during the day and 27 (±6) ppb at night. NO 3 chemistry contributed 10 (±7)% to the total O x loss during the daytime, and 28% (±18%) integrated over a 24‐hour period. It therefore played an important role in the O x budget. NO 3 also contributed significantly to the daytime oxidation of hydrocarbons such as monoterpenes and phenol in Houston. The observed daytime NO 3 mixing ratios can be described as a function of O 3 and NO x . Above NO x /O 3 ratios of 3%, daytime NO 3 becomes independent of NO x and proportional to the square of O 3 . Our calculations indicate that elevated (>1 ppt) NO 3 levels can be present whenever ozone mixing ratios exceed typical urban smog levels of 100 ppb.