Sixteen months of continuous measurements and the analysis of atmospheric mercury (gaseous elemental mercury GEM, gaseous oxidized mercury GOM, and particulate bound mercury PBM) under urban and coastal settings were conducted in Southeastern Texas. At the urban site, the GEM mean mixing ratio was 185 ppqv, 5%–10% higher than the Northern Hemisphere GEM background level. GOM and PBM mixing ratios were as much as six times higher than their background level. The coastal site GEM mean mixing ratio was 165 ppqv, higher than other coastal sites located in the Northern Hemisphere. GOM and PBM mean mixing ratios at the coastal site were 0.75 ppqv and 0.58 ppqv. The urban site had a higher frequency of high mercury events (>300 ppqv) compared to the coastal site. The diurnal patterns were found for both sites: In the urban environment, GEM accumulated to the maximum mixing ratio just after sunrise and decreased to the minimum mixing ratio in late afternoon. In the coastal environment, GEM decreased at night reaching its minimum mixing ratio before sunrise. The relationship between atmospheric mercury species and meteorological parameters was investigated. An examination of the relationship between atmospheric mercury species and key trace gases was conducted as well, showing that the concurrence of GEM, CO2, CO, CH4, and SO2 maximum mixing ratios was notable and provided evidence they may originate from the same emission source. The coastal site was at times influenced by polluted air from urban Houston and the cleaner Gulf of Mexico marine air at other times.

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Abstract. The Tracking Aerosol Convection Experiment Air Quality (TRACER-AQ) campaign probed Houston air quality with a comprehensive suite of ground-based and airborne remote sensing measurements during the intensive operating period in September 2021. Two post-frontal high-ozone episodes (September 6–11 and 23–26) were recorded during the said period. In this study, we evaluated the simulation of the planetary boundary layer (PBL) height and the vertical ozone profile by a high-resolution (1.33 km) 3-D photochemical model, Weather Research and Forecasting (WRF)-driven GEOS-Chem (WRF-GC). We contrasted the model performance between ozone-episode days and non-episode days. The model captures the diurnal variations of the PBL during ozone episodes (R = 0.72–0.77; normal mean bias (NMB) = 3 %–22 %) and non-episode days (R = 0.88; NMB = -21 %), compared with the ceilometer at La Porte. Land-water differences in PBL heights are captured better during non-episode days than episode days, compared with the airborne High Spectral Resolution Lidar-2 (HSRL-2). During ozone episodes, the simulated land-water differences are 50–60 m (morning), 320–520 m (noon), and 440–560 m (afternoon) in comparison with the observed values of 190 m, 130 m, and 260 m, respectively. During non-episode days, the simulated land-water differences are 140–220 m (morning) and 360–760 m (noon) in comparison with the observed values of 210 m and 420 m, respectively. For vertical ozone distributions, the model was evaluated against vertical profile measurements from the Tropospheric Ozone lidar (TROPOZ), the HSRL-2, and ozonesondes, as well as at the surface from a model 49i ozone analyzer and a site from the Continuous Ambient Monitoring Stations (CAMS) at La Porte. The model underestimates free tropospheric ozone (2–3 km aloft) by 9 %–22 % but overestimates near-ground ozone (< 50 m aloft) by 6 %–39 % during the two ozone episodes. Boundary layer ozone (0.5–1 km aloft) is underestimated by 1 %–11 % during September 8–11 but overestimated by 0 %–7 % during September 23–26. Based on these evaluations, we identified two model limitations: the single-layer PBL representation and free tropospheric ozone underestimation. These limitations have implications for the predictivity of ozone’s vertical mixing and distribution in other models.

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