Long-path averaged mixing ratios of O<sub>3</sub> and NO<sub>2</sub> in the free troposphere from mountain MAX-DOAS

Abstract

Abstract.A new approximation is proposed to estimate O 3 and NO 2 mixing ratios in the northern subtropical free troposphere (FT). The proposed method uses O 4 slant column densities (SCDs) at horizontal and near-zenith geometries to estimate a station-level differential path. The modified geometrical approach (MGA) is a simple method that takes advantage of a very long horizontal path to retrieve mixing ratios in the range of a few pptv. The methodology is presented, and the possible limitations are discussed. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) highmountain measurements recorded at the Izaña observatory (28 • 18 N, 16 • 29 W) are used in this study. The results show that under low aerosol loading, O 3 and NO 2 mixing ratios can be retrieved even at very low concentrations. The obtained mixing ratios are compared with those provided by in situ instrumentation at the observatory. The MGA reproduces the O 3 mixing ratio measured by the in situ instrumentation with a difference of 28 %. The different air masses scanned by each instrument are identified as a cause of the discrepancy between the O 3 observed by MAX-DOAS and the in situ measurements. The NO 2 is in the range of 20-40 ppt, which is below the detection limit of the in situ instrumentation, but it is in agreement with measurements from previous studies for similar conditions.

NO<sub>2</sub> seasonal evolution in the north subtropical free troposphere

“…The breeze layer has a limited vertical extension and its relative contribution to the MAXDOAS long path is small. On the contrary, Izaña in situ data around noon are strongly influenced by the underlying polluted MBL (Puentedura et al, 2012;Gomez et al, 2014). Secondly, due to the long light paths achieved by MAXDOAS in the FT, very low concentrations, of a few parts per trillion, can be measured.…”

“…The DSCD is obtained by subtracting the measurement obtained at the zenith (SZA = 90 • ) from the measurement in the horizontal path. In a first approximation, the slant paths' contributions of 0 and 90 • geometries are cancelled out, and only the signal of the tracers present in the horizontal path remains (see Gomez et al, 2014 for details). The method assumes a quasi-Rayleigh atmosphere, i.e., very low aerosol optical depth (AOD), and a single scattering before the photon reaches the detector.…”

“…NO 2 at 294 K temperature from Vandaele et al (1998) and O 4 from Hermans (2011) cross sections have been used. Details of the instrument, settings and operational mode can be found in Puentedura et al (2012) and Gomez et al (2014).…”

“…The horizontal path length is obtained from the oxygen collisional complex (O 4 , hereafter) simultaneously measured with the tracer under consideration (NO 2 and O 3 ). Gomez et al (2014) examined a short summer period to demonstrate the validity of the method. Here we apply the same technique to data covering 3 full years (2011)(2012)(2013) to analyze the seasonal evolution of the NO 2 concentrations in volume mixing ratio (vmr).…”

“…Recently, Gomez et al (2014) presented a simple method based on a modified geometrical approximation (MGA) to estimate concentrations of trace gases at the level of the Izaña observatory from multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements. The horizontal path length is obtained from the oxygen collisional complex (O 4 , hereafter) simultaneously measured with the tracer under consideration (NO 2 and O 3 ).…”