Abstract
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).
Highlights
As the region of the atmosphere where exchanges of momentum, water and trace chemical species occur with the Earth’s surface, the planetary boundary layer (PBL) is of fundamental importance for atmospheric studies
During the development of the daytime convective PBL, air from the free troposphere (FT) or the residual layer (RL) is entrained into the PBL, which modifies the budget of the chemical species
The mean PBL height is 1253 m, with values ranging from 1132 m during the night to 1483 m in the afternoon. This corresponds to diurnal variability of 28 %, the diurnal variability here being calculated as the maximum minus minimum PBL height normalized by the mean PBL height based on the values available during the different time slots shown in Fig. 5 (for this example (1483−1132) × 100/1253 = 28 %)
Summary
As the region of the atmosphere where exchanges of momentum, water and trace chemical species occur with the Earth’s surface, the planetary boundary layer (PBL) is of fundamental importance for atmospheric studies. A recent diagnostic evaluation of the WRF-Chem model focusing (for the first time) on the O3 entrainment highlighted deficiencies in the model, including an overestimation of the O3 entrainment and a too-efficient vertical mixing in the lower PBL (Kaser et al, 2017) These deficiencies were found to originate mainly from errors in the entrainment rate and PBL height during the morning and an erroneous representation of the O3 gradient at the PBL–FT interface during the rest of the day. This paper focuses on these two pollutants, but some results will be (more briefly) discussed for relative humidity (RH) and potential temperature (θ ) For this purpose, we benefit from the two main sources of in situ vertical profiles in the troposphere: (i) the In-service Aircraft for a Global Observing System (IAGOS) database and (ii) the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) ozonesondes database.
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