Abstract
Abstract. Daily NO2 vertical column density (VCD) has been routinely measured by zenith sky spectroscopy at the subtropical station of Izaña (28° N, 16° W) since 1993 in the framework of the Network for the Detection of Atmospheric Composition Change (NDACC). Based on 14 years of data the first low latitude NO2 VCD climatology has been established and the main characteristics from short timescales of one day to interannual variability are presented. Instrumental descriptions and different sources of errors are described in detail. The observed diurnal cycle follows that expected by gas-phase NOx chemistry, as can be shown by the good agreement with a vertically integrated chemical box model, and is modulated by solar radiation. The seasonal evolution departs from the phase of the hours of daylight, indicating the signature of upper stratospheric temperature changes. From the data record (1993–2006) no significant long-term trends in NO2 VCD can be inferred. Comparison of the ground-based data sets with nadir-viewing satellite spectrometers shows excellent agreement for SCIAMACHY with differences between both datasets of 1.1%. GOME displays unrealistic features with the largest discrepancies during summer. The ground-based data are compared with long-term output of the SLIMCAT 3-D chemical transport model (CTM). The basic model, forced by ECMWF (ERA-40) analyses, captures the observed NO2 annual cycle but significantly underestimates the spring/summer maximum (by 12% at sunset and up to 25% at sunrise). In a model run which uses assimilation of satellite CH4 profiles to constrain the model long-lived tracers the agreement is significantly improved. This improvement in modelled column NO2 is due to better modelled NOy profiles and points to transport errors in the ECMWF ERA-40 reanalyses.
Highlights
NO2 plays an important role in the chemistry of ozone from the mid stratosphere to the mesosphere through catalytic reactions: NO + O3 → NO2 + O2 (1)NO2 + O → NO + O2 (2)Net: O3 + O → 2O2 (3)The NOx contribution to the overall O3 equilibrium depends on latitude, altitude and season
Gil et al.: NO2 climatology in the subtropics added to the Differential Slant Column Density (DSCD), and the vertical column conversion is performed by dividing by the Air Mass Factor (AMF) coefficients
Optimized for twilight observations, Differential optical absorption spectrometry (DOAS) visible spectroscopy can be used to study the diurnal evolution of the NO2 in unpolluted environments
Summary
NO2 plays an important role in the chemistry of ozone from the mid stratosphere to the mesosphere through catalytic reactions: NO + O3 → NO2 + O2. Arrival Heights (Antarctica, 78◦ S) data with a threedimensional coupled chemistry-climate model (CCM). They again found a greater rate of increase of NO2 compared with N2O both in the measurements and the model. These data have helped establish a better understanding of nitrogen oxides stratospheric behaviour in the extra-tropical region. While polar orbiting satellites offer the advantage of global coverage, no study of stratospheric diurnal variation is possible since they scan low latitude regions once per day at best. We report the behaviour of the NO2 column over the high altitude subtropical station of Izana (28◦ N, 16◦ W, Tenerife, Canary Islands, 2370 m a.s.l.) on different time scales, from diurnal to interannual, based on 14 years of data. Gil et al.: NO2 climatology in the subtropics els and satellite measurements for the same location, constituting a limited low-latitude NO2 climatology for the period 1993–2006
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