Publication Date:
2016-01-28
Description:
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 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 NO x shows a large negative gradient with increasing altitude in the BL. Our analysis suggests that the magnitude of the NO x 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 (ACM2, YSU, MYJ) and two land-surface schemes (Noah and RUC). The model reasonably reproduces the median vertical profiles of NO x under different BL stability conditions within 30% of observations, classified based on potential temperature gradient and BL height. Comparisons with NO x 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 WRF have a small impact on the NO x gradient. Using model simulations, we analyze the impact of BL NO x gradient on the calculation of the ozone production rate and satellite NO 2 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 NO 2 vertical profiles is responsible for 5~10% variability in the retrieved NO 2 tropospheric vertical columns.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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