Nighttime Planetary Boundary Layer Research Articles

WRF-Chem quantification of transport events and emissions sensitivity in Korea during KORUS-AQ

To quantify the relative roles of long-range transport (LRT) versus locally emitted aerosol and ozone precursors during polluted periods in Korea, high-resolution (4 km) Weather Research and Forecasting with Chemistry model simulations were performed. The model was evaluated using surface and airborne observations collected during the KORea and United States Air Quality campaign. Ozone above 40 ppb had mean bias of −5.9 ppb. PM2.5 was biased high (8.2 µg/m3), with a relative bias of 30% given the mean observed value of 26.8 µg/m3. The absolute amounts and shifts between phases for all PM2.5 species except nitrate reasonably match observations across all 4 phases. Notable limitations include an underestimation of nighttime planetary boundary layer height. Transport versus domestic emissions influence was studied by model runs with perturbed emissions and by comparing east-west fluxes over the Yellow Sea to Korean emissions and other normalization metrics. Domestic anthropogenic emission contributions to surface air quality were quantified by location across Korea, segregated by synoptic meteorological phase. The largest contributions from Korean emissions were found under high-pressure stagnant conditions and the smallest for conditions with strong westerly winds. For example, at Seoul, domestic contributions of PM2.5 averaged 49% and 29% in the aforementioned meteorological phases, respectively. Surface concentrations of NOx and toluene in Seoul were over 85% due to domestic emissions. CO and black carbon had both local and remote contributions. Nitrate and ammonium contributions varied greatly by phases in Seoul, with 7%–51% nitrate and 42%–70% of ammonium from remote sources. Variation in direction (west-to-east vs. east-to-west) and magnitude of fluxes support the model sensitivity results. Analysis using fluxes facilitates the quantification of source contributions for secondary species and, in many cases, can be done using a single model run or reanalysis result. The analysis presented shows the importance of using models with high spatial resolution to capture pollutant transport and mixing around Korea. However, there remain uncertainties in secondary aerosol production mechanisms and indications that local production at times could be higher than those modeled in this analysis. Therefore, the results presented here should be viewed as an upper limit on the importance of LRT.

Improvements of Simulating Urban Atmospheric CO2 Concentration by Coupling with Emission Height and Dynamic Boundary Layer Variations in WRF-STILT Model

Although cities only account for 3% of the global land area, they have disproportionately contributed 70% of total anthropogenic CO2 emissions; the main issue in estimating urban anthropogenic CO2 emissions is their large uncertainty. Tower-based atmospheric CO2 observations and simulations in urban areas have been frequently used as an independent approach to constrain and evaluate greenhouse gas emissions from city to regional scales, where only daytime CO2 observations and simulations are used considering the consensus that the large bias in simulating nighttime planetary boundary layer heights (PBLH) and atmospheric CO2 concentration will cause overestimation/underestimation in CO2 emission inversions. The above strategy of only using daytime observations makes the numbers of available concentration observations largely decrease even with the fact that tower-based atmospheric CO2 observations are sparsely distributed and conducted. Here, to solve the issue of large bias in nighttime CO2 simulations, we conducted four months of atmospheric CO2 observations from January to April in 2019, and raised an approach by coupling emission heights with dynamic PBLH variations in a WRF-STILT model. We found (1) the overestimation of simulated nighttime CO2 concentration decreased by 5–10 ppm, especially between 0:00 and 7:00. (2) The statistics for nighttime simulations were largely improved by using a revised model and posteriori emissions. The regression slopes of daily averages were 0.93 and 0.81 for the default model using a priori emissions and the revised model using the same a priori emissions, and the slope largely improved to 0.97 for the revised model using posteriori emissions. Moreover, the correlation coefficient also increased from 0.29 and 0.37 to 0.53; these results indicate our revised model obviously calibrated the bias in both nighttime and daily CO2 concentration simulations. In general, it is strongly recommended to use the revised WRF-STILT model in future inversion studies, which can effectively reduce the overestimation of nighttime spikes and make full use of nighttime observations.

Winds at the Mars 2020 Landing Site: 1. Near‐Surface Wind Patterns at Jezero Crater

AbstractThis is the first part of a two‐part paper. NASA's Mars 2020 Perseverance rover measured winds on the Jezero crater floor close to the delta of an ancient river. A mostly repeatable diurnal cycle was observed and presented two regimes: (a) a convective regime, from dawn to sunset, with average easterly to southeasterly winds, during which maximum wind speeds were measured, and (b) a nighttime regime with westerly‐northwesterly winds followed by a relatively calm period with highly variable wind directions as a function of sol and time of night. The timing and magnitude of the observed regimes are consistent with primary control by regional and local slope flows. Data suggest that the surface circulation at Jezero region in northern spring and summer is highly unaffected by large‐scale circulation except during particular periods in the diurnal cycle or generally during dust storms, which is supported by MarsWRF model simulations. Consequently, the observed seasonal variability was weak. However, sol‐to‐sol and seasonal variability were measured, most of it during certain nighttime periods. Traveling waves consistent with baroclinic instability were clearly observed in surface winds at Ls ∼ 75°. The early MY36/2022A regional dust storm at Ls ∼ 153° disturbed the wind patterns with changes suggesting enhanced tidal flows. After sunset, the dust storm also produced detectable gravity wave activity, increasing the mixing in the nighttime planetary boundary layer during storm conditions. Inferred wind directions from dust devil movies strongly suggest that prevailing winds continued to be slope‐driven during the late summer, fall and early winter seasons.

Impacts of future urban expansion on summer climate and heat-related human health in eastern China

China is the largest and most rapidly urbanizing nation in the world, and is projected to add an additional 200 million city dwellers by the end of 2030. While this rapid urbanization will lead to vast expansion of built-up areas, the possible climate effect and associated human health impact remain poorly understood. Using a coupled urban-atmospheric model, we first examine potential effects of three urban expansion scenarios to 2030 on summer climate in eastern China. Our simulations indicate extensive warming up to 5°C, 3°C, and 2°C in regard to low- (>0%), high- (>75%), and 100% probability urban growth scenarios, respectively. The partitioning of available energy largely explains the changes in 2-m air temperatures, and increased sensible heat flux with higher roughness length of the underlying urban surface is responsible for the increase of nighttime planetary boundary layer height. In the extreme case (the low-probability expansion pathway), the agglomeration of impervious surfaces substantially reduces low-level atmospheric moisture, consequently resulting in large-scale precipitation reduction. However, the effect of near-surface warming far exceeds that of moisture reduction and imposes non-negligible thermal loads on urban residents. Our study, using a scenario-based approach that accounts for the full range of urban growth uncertainty by 2030, helps better evaluate possible regional climate effects and associated human health outcomes in the most rapidly urbanizing areas of China, and has practical implications for the development of sustainable urban regions that are resilient to changes in both mean and extreme conditions.

Analysis of ozone and VOCs measured in Shanghai: A case study

Shanghai Meteorological Administration has established a volatile organic compounds (VOCs) laboratory and an observational network for VOCs and ozone (O 3) measurements in the city of Shanghai. In this study, the measured VOCs and O 3 concentrations from 15 November (15-Nov) to 26 November (26-Nov) of 2005 in Shanghai show that there are strong day-to-day and diurnal variations. The measured O 3 and VOCs concentrations have very different characterizations between the two periods. During 15-Nov to 21-Nov (defined as the first period), VOCs and O 3 concentrations are lower than the values during 22-Nov to 28-Nov (defined as the second period). There is a strong diurnal variation of O 3 during the second period with maximum concentrations of 40–80 ppbv at noontime, and minimum concentrations at nighttime. By contrast, during the first period, the diurnal variation of O 3 is in an irregular pattern with maximum concentrations of only 20–30 ppbv. The VOC concentrations change rapidly from 30–50 ppbv during the first period to 80–100 ppbv during the second period. Two chemical models are applied to interpret the measurements. One model is a regional chemical/dynamical model (WRF-Chem) and another is a detailed chemical mechanism model (NCAR MM). Model analysis shows that the meteorological conditions are very different between the two periods, and are mainly responsible for the different chemical characterizations of O 3 and VOCs between the two periods. During the first period, meteorological conditions are characterized by cloudy sky and high-surface winds in Shanghai, resulting in a higher nighttime planetary boundary layer (PBL) and faster transport of air pollutants. By contrast, during the second period, the meteorological conditions are characterized by clear sky and weak surface winds, resulting in a lower nighttime PBL and slower transport of air pollutants. The chemical mechanism model calculation shows that different VOC species has very different contributions to the high-ozone concentrations during the second period. Alkane (40 ppbv) and aromatic (30 ppbv) are among the highest VOC concentrations observed in Shanghai. The analysis suggests that the aromatic is a main contributor for the O 3 chemical production in Shanghai, with approximately 79% of the O 3 being produced by aromatic. This analysis implies that future increase in VOC (especially aromatic) emissions could lead to significant increase in O 3 concentrations in Shanghai.

Multistatic lidar profiling of urban atmospheric aerosols

It has become increasingly important to obtain information on the vertical distribution, i.e., profile, of aerosols in the open atmosphere. To complement data obtained from a monostatic Raman lidar, a multistatic receiver configuration was designed to take measurements of the parallel and perpendicular polarization scattered components for altitudes from 10 m to 1000 m above the ground. The measurements, taken at night, were used to form a polarization ratio that was subsequently fitted using Mie theory and a trimodal lognormal aerosol distribution. Experimental results revealed the presence of strong variations in the aerosol distribution within the nighttime planetary boundary layer with some time sequences revealing rapid temporal and spatial changes. Subsequent modeling of the aerosol profile suggested that the observed aerosol variations may depend on a combination of factors, including altitude, composition, and number density. However, in instances when the atmosphere was found to be uniformly mixed, retrieval of aerosol parameters (number density, median radius, and geometric standard deviation) was obtained.