Large vertical gradient of reactive nitrogen oxides in the boundary layer: Modeling analysis of DISCOVER‐AQ 2011 observations

Abstract

An often used assumption in air pollution studies is a well‐mixed boundary layer (BL), where pollutants are evenly distributed. Because of the difficulty in obtaining vertically resolved measurements, the validity of the assumption has not been thoroughly evaluated. In this study, we use more than 200 vertical profiles observed in the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER‐AQ) aircraft campaign in July 2011 to examine the vertical distributions of pollutants over the Washington‐Baltimore area. While many long‐lived species are well mixed in daytime, the observed average vertical profile of NOx shows a large negative gradient with increasing altitude in the BL. Our analysis suggests that the magnitude of the NOx gradient is highly sensitive to atmospheric stability. We investigate how parameterizations of the BL and land‐surface processes impact vertical profiles in a 1‐D chemical transport model, using three BL schemes (Asymmetric Convective Model version 2 (ACM2), Yonsei University (YSU), and Mellor‐Yamada‐Janjic (MYJ)) and two land‐surface schemes (Noah and Rapid Update Cycle (RUC)). The model reasonably reproduces the median vertical profiles of NOx under different BL stability conditions within 30% of observations, classified based on potential temperature gradient and BL height. Comparisons with NOx observations for individual vertical profiles reveal that while YSU performs better in the turbulent and deep BL case, in general, ACM2 (RMSE = 2.0 ppbv) outperforms YSU (RMSE = 2.5 ppbv) and MYJ (RMSE = 2.2 ppbv). Results also indicate that the land‐surface schemes in the Weather Research and Forecasting (WRF) model have a small impact on the NOx gradient. Using model simulations, we analyze the impact of BL NOx gradient on the calculation of the ozone production rate and satellite NO2 retrieval. We show that using surface measurements and the well‐mixed BL assumption causes a ~45% high bias in the estimated BL ozone production rate and that the variability of NO2 vertical profiles is responsible for 5–10% variability in the retrieved NO2 tropospheric vertical columns.

Inverse modelling of NO<sub><i>x</i></sub> emissions over eastern China: uncertainties due to chemical non-linearity

“…For inversion of ship emissions, Vinken et al (2014b) computed averaged non-linear factors for selected regions with perturbations proportional to the model-observation column difference. For polluted regions, the effect of profile change is small (Zhang et al, 2016) and is not included in this study. In the previous studies, model sensitivities are computed with domain-wide emission perturbations.…”

Biogenic isoprene emissions driven by regional weather predictions using different initialization methods: case studies during the SEAC<sup>4</sup>RS and DISCOVER-AQ airborne campaigns

“…1 (Tao et al, 2013;Yu et al, 2012). The commonly used MellorYamada-Janjic PBL scheme (Janjic, 2002) and the matching Monin-Obukhov (Janjic Eta) surface layer scheme (Monin and Obukhov, 1954) were chosen, although these might lead to shallower, cooler PBL and less vertical mixing than other available schemes in WRF (e.g., Saide et al, 2011;Angevine et al, 2012;Huang et al, 2013;Y. Zhang et al, 2016).…”

Reconciling the differences between OMI-based and EPA AQS in situ NO<sub>2</sub> trends

“…As a result, the retrieved tropospheric NO2 VCD computation depends on the a priori NO2 vertical profile, the surface reflectance, the surface pressure, the temperature profile, and the viewing geometry (Boersma et al, 2011). Previous studies have addressed the sources of uncertainties in NO2 retrievals, including surface reflectance resolutions (Russell et al, 2011;Lin et al, 2014), lightning NOx (Choi et al, 2005a;Martin et 25 al., 2007;Bucsela et al, 2010), a priori CTM uncertainties (Russell et al, 2011;Heckel et al, 2011;Lin et al, 2012;Laughner et al, 2016), surface pressure and reflectance anisotropy in rugged terrain (Zhou et al, 2009), cloud and aerosol radiance (Lin et al, 2014(Lin et al, , 2015, and boundary layer dynamics (Zhang et al, 2016a). The NO2 VCD trend analysis is particularly sensitive to the first two factors and we will discuss these in the following sections.…”

“…REAM has widely been used in atmospheric NO2 studies, including vertical transport (Choi et al, 2005;Zhao et al, 2009a;Zhang et al, 2016a), emission inversions (Zhao et al, 2009b;Yang et al, 2011;Gu et al, 2013Gu et al, , 2014Gu et al, , 2016, and regional and seasonal variations (Choi et 20 al., 2008a(Choi et 20 al., , 2008b. The model has a horizontal resolution of 36 km with 30 vertical layers in the troposphere, 5 vertical layers in the stratosphere, and a model top of 10 hpa.…”

“…For recent model evaluations of REAM with observations, we refer readers to Zhang et al (2016aZhang et al ( , 2016b, Cheng et al (2017), and Zhang et al (2017).…”