A new-generation sand and dust storm forecasting system GRAPES_CUACE/Dust: Model development, verification and numerical simulation

Development of GRAPES-CUACE adjoint model version 2.0 and its application in sensitivity analysis of ozone pollution in north China

We presented the development of the gaseous chemistry adjoint module of the meteorological-chemical model system GRAPES-CUACE (Global/Regional Assimilation and PrEdiction System coupled with CMA Unified Atmospheric Chemistry Environmental Forecasting System) on the basis of the previously constructed aerosol adjoint module. The latest version of the GRAPES-CUACE adjoint model mainly includes the adjoint of the physical and chemical processes, the adjoint of the transport processes, and the adjoint of interface programs, of both gas and aerosol. The adjoint implementation was validated for the full model, and adjoint results showed good agreement with brute force sensitivities. We also applied the newly developed adjoint model to the sensitivity analysis of an ozone episode occurred in Beijing on July 2, 2017, as well as the design of emission-reduction strategies for this episode. The relationships between the ozone concentration and precursor emissions were well captured by the adjoint model. It is indicated that for a case used here, the Beijing peak ozone concentration was influenced mostly by local emissions (6.2-24.3%), as well as by surrounding emissions, including Hebei (4.4-16.8%), Tianjin (1.8-6.6%), Shandong (1.8-2.6%), and Shanxi (<1%). In addition, reduction of NO_x, VOCs, and CO emissions in these regions would effectively decrease the Beijing peak ozone concentration. This study highlights the capability of GRAPES-CUACE adjoint model in quantifying "emission-concentration" relationship and in providing guidance for environmental control policy.

The Different Impacts of Emissions and Meteorology on PM2.5 Changes in Various Regions in China: A Case Study

Emissions and meteorology are significant factors affecting aerosol pollution, but it is not sufficient to understand their relative contributions to aerosol pollution changes. In this study, the observational data and the chemical model (GRAPES_CUACE) are combined to estimate the drivers of PM2.5 changes in various regions (the Beijing–Tianjin–Hebei (BTH), the Central China (CC), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD)) between the first month after COVID-19 (FMC_2020) (i.e., from 23 January to 23 February 2020) and the corresponding period in 2019 (FMC_2019). The results show that PM2.5 mass concentration increased by 26% (from 61 to 77 µg m−3) in the BTH, while it decreased by 26% (from 94 to 70 µg m−3) in the CC, 29% (from 52 to 37 µg m−3) in the YRD, and 32% (from 34 to 23 µg m−3) in the PRD in FMC_2020 comparing with FMC_2019, respectively. In the BTH, although emissions reductions partly improved PM2.5 pollution (−5%, i.e., PM2.5 mass concentration decreased by 5% due to emissions) in FMC_2020 compared with that of FMC_2019, the total increase in PM2.5 mass concentration was dominated by more unfavorable meteorological conditions (+31%, i.e., PM2.5 mass concentration increased by 31% due to meteorology). In the CC and the YRD, emissions reductions (−33 and −36%) played a dominating role in the total decrease in PM2.5 in FMC_2020, while the changed meteorological conditions partly worsened PM2.5 pollution (+7 and +7%). In the PRD, emissions reductions (−23%) and more favorable meteorological conditions (−9%) led to a total decrease in PM2.5 mass concentration. This study reminds us that the uncertainties of relative contributions of meteorological conditions and emissions on PM2.5 changes in various regions are large, which is conducive to policymaking scientifically in China.

Evaluating the contributions of changed meteorological conditions and emission to substantial reductions of PM2.5 concentration from winter 2016 to 2017 in Central and Eastern China

The monthly average PM_2.5 concentration decreased from 127.15 μg m^-3 in December 2016 to 85.54 μg m^-3 in December 2017 (approximately 33%) in Central and Eastern China (33°N-41°N, 113°E-118°E). This decrease is attributed to the combined impacts of meteorology and emission sources changes, though the question of which is more important has raised great concerns. Four sensitivity experiments based on the Global-Regional Assimilation and Prediction System coupled with the Chinese Unified Atmospheric Chemistry Environment (GRAPES-CUACE) model, together with comparative analysis of the observed meteorological conditions and emission inventory between 2016 and 2017, are used to evaluate the relative contributions of meteorology and emission to the substantial reductions of PM_2.5 concentration from December 2016 to December 2017. The results show that the meteorological conditions and emission in December 2017 were both beneficial to the PM_2.5 decrease in Central and Eastern China. Regarding the entire region, 21.9% of the PM_2.5 decrease was a result of the favorable meteorological conditions, and 78.1% of the decrease was a result of emission reductions, showing the distinct contributions of emission reductions on the air quality. The relative contributions of meteorology varied from 12.2% to 50.9% to the PM_2.5 decrease from December 2016 to December 2017, while the emission contributed 49.1% to 87.8%, in different cities depending on geographical location and topography. Meteorology showed the largest contributions to the PM_2.5 decrease from 2016 to 2017 in Beijing (BJ), which caused the greatest total decrease of PM_2.5 compared to that of other cities. In addition, in Central and Eastern China, the dominant factors of the decrease of PM_2.5 were favorable meteorological conditions (accounting for 98.2%) during clear periods and emission reductions (accounting for 72.5-81.2%) during pollution periods.

Insights into a dust event transported through Beijing in spring 2012: Morphology, chemical composition and impact on surface aerosols

Multiple approaches were used to investigate the evolution of surface aerosols in Beijing during the passage of a dust event at high altitude, which was from the Gobi areas of southern Mongolia and covered a wide range of North China. Single particle analysis with electron microscope showed that the majority of coarse particles were mineral ones, and most of them were in the size range of 1-7μm with a peak of number concentration at about 3.5μm. Based on elemental composition and morphology, the mineral particles could be classified into several groups, including Si-rich (71%), Ca-rich (15%), Fe-rich (6%), and halite-rich (2%), etc., and they were the main contributors to the aerosol optical depth as the dust occurred. The size distributions of surface aerosols were significantly affected by the dust intrusion. The average number concentration of accumulation mode particles during the event was about 400cm(-3), which was much lower than that in heavily polluted days (6300cm(-3)). At the stage of floating dust, the number concentration of accumulation mode particles decreased, and coarse particles contributed to total volume concentration of particulate matter as much as 90%. The accumulation mode particles collected in this stage were mostly in the size range of 0.2-0.5μm, and were rectangular or spherical. They were considered to be particles consisting of ammonium sulfate. New particle formation (NPF) was observed around noon in the three days during the dust event, indicating that the passage of the dust was probably favorable for NPF.