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

Abstract

Abstract. Three years of multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements (2011)(2012)(2013) have been used for estimating the NO 2 mixing ratio along a horizontal line of sight from the high mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3 • N, 16.5 • W). The method is based on horizontal path calculation from the O 2 -O 2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO 2 column density, and is applicable under low aerosol-loading conditions.The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO 2 contamination resulting from the arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during middle hours of the day. Comparisons with in situ observations show that during most of the measuring period, the MAXDOAS is insensitive or very slightly sensitive to the upwelling breeze. Exceptions are found for pollution events during southern wind conditions. On these occasions, evidence of fast, efficient and irreversible transport from the surface to the free troposphere is found.Background NO 2 volume mixing ratio (vmr), representative of the remote free troposphere, is in the range of 20-45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry-climate CAM-Chem model are in good agreement with the NO 2 measurements, and are used to further investigate the possible drivers of the NO 2 seasonality observed at Izaña.

Estimates of free-tropospheric NO<sub>2</sub> and HCHO mixing ratios derived from high-altitude mountain MAX-DOAS observations in the mid-latitudes and tropics

“…The y intercept of the linear least-squares fit (excluding June) in the order of 20 ppt (not shown) provides some information on the background levels of X NO 2 in this region and at this altitude. In comparison, Gomez et al (2014) and Gil-Ojeda et al (2015) have reported NO 2 mixing ratios between 20 and 45 ppt for the clean subtropical FT at a similar altitude, which is in good agreement with the background value obtained for Zugspitze, which is located more than 2000 km north of the Izaña station. The multi-year averaged values of X NO 2 for the higher elevated station Pico Espejo are somewhat smaller when compared to X NO 2 values at Zugspitze (Table 3).…”

“…A statistically significant linear relationship between aerosol optical depth (AOD) and X NO 2 is obtained for the measurements from both measurement sites, suggesting that both NO 2 and aerosols are closely connected to air pollution in these regions and at the respective altitude level. The results of a linear least-squares fit performed on the averaged data show that upper limits for the background free troposphere NO 2 mixing ratios are about 20 and 4.5 ppt for Zugspitze and Pico Espejo, respectively, where the former value is in good agreement with background NO 2 mixing ratios reported by Gomez et al (2014) and Gil-Ojeda et al (2015) l.). The latter site on Tenerife is relatively remote, whereas Zugspitze is farther north but both are probably probing on average similar free tropospheric air masses transported from Tenerife towards Europe.…”

“…The aim of the present study is to extend these two studies and provide estimates of NO 2 and HCHO mixing ratios in the FT. For this purpose, two long-term MAX-DOAS data sets from two high-altitude sites located in the mid-latitudes and tropics are analyzed. In contrast to Gomez et al (2014) and Gil-Ojeda et al (2015), the retrieval of O 4 , NO 2 , and HCHO DSCDs in our study is performed in the ultraviolet range of the electromagnetic spectrum. Here, we obtain monthly means of NO 2 and HCHO mixing ratios during clear sky conditions at the two measurement sites, focus on the seasonal variability, and compare our values with values reported in the literature.…”

Laser-induced fluorescence-based detection of atmospheric nitrogen dioxide and comparison of different techniques during the PARADE 2011 field campaign

“…Uncertainties in the NO x budget have recently been highlighted (Stavrakou et al, 2013). These include the uncertainty in the estimate of the rate coefficient for NO 2 + OH under tropospheric conditions (Mollner et al, 2010), a lack of proper representation in chemical mechanisms for the loss of NO x via organic nitrate formation (Browne and Cohen, 2012), and the formation of HNO 3 in a minor branch of the reaction between NO and HO 2 (Butkovskaya et al, 2007), which showed significant impacts on the concentrations of NO x , OH, HNO 3 , and related chemistry (Cariolle et al, 2008;Gottschaldt et al, 2013). Additionally, a lack of agreement between modelled and measured OH concentrations over forests (Lelieveld et al, 2008;Kubistin et al, 2010) and urban regions (Hofzumahaus et al, 2009) contribute to the uncertainty in NO x chemistry.…”

Intercomparison of stratospheric nitrogen dioxide columns retrieved from ground-based DOAS and FTIR and satellite DOAS instruments over the subtropical Izana station

“…Hendrick et al (2012) also found negative trends over Jungfraujoch for the period 1990-2009. Gil-Ojeda et al (2015) found a hemispherical and latitude dependence on the sign and magnitude of the trend based on four GB DOAS stations. Northern latitudes display a positive trend whereas trends are negative in the southern latitudes.…”