A climatological view of the vertical stratification of RH, O3 and CO within the PBL and at the interface with free troposphere as seen by IAGOS aircraft and ozonesondes at northern mid-latitudes over 1994–2016
Authors: Petetin Hervé , Sauvage Bastien , Smit Herman G. J. , Gheusi François , Lohou Fabienne , Blot Romain , Clark Hannah , Athier Gilles , Boulanger Damien , Cousin Jean-Marc , Nedelec Philippe , Neis Patrick , Rohs Susanne , Thouret Valérie 
Published: July 2018

Abstract. This paper investigates in an innovative way the climatological vertical
stratification of relative humidity (RH), ozone (O3) and carbon monoxide (CO)
mixing ratios within the planetary boundary layer (PBL) and at the
interface with the free troposphere (FT). The climatology includes all
vertical profiles available at northern mid-latitudes over the period
1994–2016 in both the IAGOS (In-service Aircraft for a Global Observing System)
and WOUDC (World Ozone and Ultraviolet Radiation Data Centre) databases,
which represents more than 90 000 vertical profiles. For all individual
profiles, apart from the specific case of surface-based temperature
inversions (SBIs), the PBL height is estimated following the elevated
temperature inversion (EI) method. Several features of both SBIs and EIs are
analysed, including their diurnal and seasonal variations. Based on these PBL
height estimates (denoted h), the novel approach introduced in this paper
consists of building a so-called PBL-referenced vertical distribution of
O3, CO and RH by averaging all individual profiles beforehand
expressed as a function of z∕h rather than z (with z the altitude).
Using this vertical coordinate system allows us to highlight the features
existing at the PBL–FT interface that would have been smoothed otherwise. Results demonstrate that the frequently assumed well-mixed PBL remains an
exception for both chemical species. Within the PBL, CO profiles are
characterized by a mean vertical stratification (here defined as the
standard deviation of the CO profile between the surface and the PBL top,
normalized by the mean) of 11 %, with moderate seasonal and diurnal
variations. A higher vertical stratification is observed for O3 mixing
ratios (18 %), with stronger seasonal and diurnal variability (from
∼ 10 % in spring–summer midday–afternoon to ∼ 25 % in winter–fall night). This vertical stratification is distributed
heterogeneously in the PBL with stronger vertical gradients observed at both
the surface (due to dry deposition and titration by NO for O3 and due
to surface emissions for CO) and the PBL–FT interface. These gradients vary
with the season from the lowest values in summer to the highest ones in winter. In
contrast to CO, the O3 vertical stratification was found to vary with
the surface potential temperature following an interesting bell shape with
the weakest stratification for both the lowest (typically negative) and highest
temperatures, which could be due to much lower O3 dry deposition in
the presence of snow. Therefore, results demonstrate that EIs act as a geophysical interface
separating air masses of distinct chemical composition and/or chemical
regime. This is further supported by the analysis of the correlation of
O3 and CO mixing ratios between the different altitude levels in the
PBL and FT (the so-called vertical autocorrelation). Results indeed
highlight lower correlations apart from the PBL–FT interface and higher
correlations within each of the two atmospheric compartments (PBL and FT). The mean climatological O3 and CO PBL-referenced profiles analysed in
this study are freely available on the IAGOS portal for all seasons and
times of day (https://doi.org/10.25326/4).