First direct observations of daytime NO3 during TEXAQS 2000

The importance of nitrate radical (NO₃) chemistry at night is well known. NO₃ has not been directly observed during the day thus far, and has therefore been considered insignificant. Here we present the first direct daytime detection of NO₃ by Differential Optical Absorption Spectroscopy at LaPorte, Texas during the Texas Air Quality Study 2000. On three consecutive days NO₃ mixing ratios reached ¯ 5 ppt three hours before sunset, increasing to 31 ppt around sunset. These elevated daytime NO3 levels have important implications for the atmospheric chemistry: 1. Although daytime NO₃ has no significant effect on the photostationary state between O3 and NO_x, it drastically accelerates NO-to- NO₂ conversion during the last hour before sunset. 2. Daytime NO₃ plays an important role in the O_x (= O₃ + NO₂) budget: On August 31, NO₃ chemistry contributed 10 +/- 7 % to the total O_x loss during the day (destroying 8 (+/- 4) ppb), and 28 +/- 18 % integrated over a 24-hour period (destroying 27 (+/- 6) ppb). 3. NO₃ contributes significantly to the daytime oxidation of hydrocarbons such as monoterpenes and phenol in Houston. The observed daytime NO₃ concentrations can be described as a function of O₃ and NO_x. Above NO_x/ O₃ ratios of 3 %, daytime NO₃ becomes independent of NO_x and proportional to the square of O₃. Our calculations show that elevated (> 1ppt) NO₃ levels are present whenever ozone mixing ratios exceed 100 ppb.