Laser induced fluorescence instrument for NO2 measurements: Observations at a central Italy background site

A laser induced fluorescence (LIF) instrument has been developed to measure tropospheric NO2 with low detection limit. The instrument design, development and first measurements are reported. There are also details of the temporal gate system built for the fluorescence acquisition. The instrument is able to make fast measurements (up to 4 Hz) and shows a limit of detection of 10 pptv/60 s. Continuous observations (2 weeks in summer 2007) in a small town in central Italy were used to test the performance of the instrument and to study the photochemistry of ozone in a background site. LIF and a commercial chemiluminescence (CL) instrument simultaneous observations of NO2 show a good linearity (LIF = 1.02 CL + 0.6 (ppb), R2 = 0.98) but there is a bias of the commercial instrument of about 0.60 ppbv on average. The overestimation of the CL system is probably due to conversion of NOy species into NO by the molybdenum converter used in the CL instrument to detect NO2. Analysis of 1 s data is used to test the instrument response and the coupling between nitrogen oxides and ozone.