Keywords:grating spectrometer, modified Langley method, back-trajectory, urban pollution
The first 24-hour measurement of total column NO2 has been made over the Jet Propulsion Laboratory's Table Mountain Facilities (TMF) located in a suburban area of Los Angeles, California, USA (2.286 km above mean sea level, 34.38°N, 117.68°W) during a 1-week campaign in October, 2018 using a grating spectrometer that measures the telluric NO2 absorptions in the direct solar and lunar spectra. The measured 24-hour cycle of total column NO2 on clean days agrees well with a 1-D photochemical model calculation, including the monotonic changes during daytime and nighttime due to the exchange with the N2O5 reservoir and the abrupt changes at sunrise and sunset due to the activation or deactivation of the NO2 photodissociation. In our photochemical model, we find that the Arrhenius constant A = 6.7×10-11 cm3 molecule-1 s-1 and the activation energy E/R = 0.0 for the bimolecular reaction N2O + O(1D) → 2O from previous recommendation are adequate to well simulate the observed 24-hour cycle of the total column NO2. The resulting rate constant is ~15% less than that obtained using more recent recommended values of A and E/R, and it lies in the lower end of the suggested uncertainty. While the model is able to reproduce the observed rate of increase of daytime total column NO2, it overestimates the rate of decrease of nighttime total column NO2, implying that a lower nighttime temperature is required for a more accurate simulation. The total column NO2 in one of the afternoons during the campaign was much higher than the model simulation, implying the influence of urban pollution from nearby cities. A 24-hour back-trajectory analysis shows that the wind first came from inland in the northeast and reached the southern Los Angeles before it turned northeast and finally arrived TMF, allowing it to pick up pollutants from Riverside County, Orange County, and Downtown Los Angeles.