Xin Long

Effect of heavy haze and aerosol pollution on rice and wheat productions in China.

In China, regional haze pollution is a serious environmental problem. The impact on ecosystem, however, is not clearly understood. This study investigates the effect of regional haze pollution on the yields of rice and wheat in China. The spatial and temporal distributions of aerosol optical depth (AOD) show high particulate pollution in the North China Plain region, Yangtze River Delta region, the central eastern China, and the Si Chuan Basin, coexisted largely with crop growth in time and space. The solar irradiance reaching these regions is estimated to reduce by up to 28–49%, calculated using the AOD distributions and tropospheric ultraviolet-visible (TUV) model. Reduction of solar irradiance in these regions can depress optimal yields of about 45% of rice and 75% of wheat growth in China, leading to 2% reduction in total rice production and 8% reduction in total wheat production in China. However, there are large uncertainties of the estimate related to the diffuse solar radiation. For high diffuse radiation case, the estimate reductions of rice and wheat decrease to 1% and 4.5%, respectively. A further detailed study is needed to clearly understand this effect to meet the growing food demand in the nation in the coming decades.

Effect of hydrolysis of N2O5 on nitrate and ammonium formation in Beijing China: WRF-Chem model simulation

Beijing, the capital of China, is a mega city with a population of >20 million. In recent years, the city has experienced heavy air pollution, with particulate matter (PM) being one of its top pollutants. In the last decade, extensive efforts have been made to characterize the sources, properties, and processes of PM in Beijing. Despite progress made by previous studies, there are still some important questions to be answered and addressed. The focus of this research is to study the impact of the heterogeneous hydrolysis of N2O5 on the formation of nitrate (NO3-) and ammonium (NH4+) in Beijing. The results show that during heavy pollution days (e.g., during 14-17 September 2015, with PM2.5 concentration over 100μg/m3), the concentrations of NO2 and O3 were high, with maxima of 90 and 240μg/m3, respectively, providing high precursors for the formation of N2O5. In addition, the aerosol and sulfate concentrations were also high, with maxima of 201μg/m3 and 23μg/m3 respectively, providing reacting surface for the heterogeneous reaction. As a result, the hydrolysis of N2O5 led to 21.0% enhancement of nitrate (NO3-) and 7.5% enhancement of ammonium (NH4+). It is worth to note that this important effect only occurred in high pollution days (PM2.5 concentration over 100μg/m3). During low-pollution periods (PM2.5 concentration <100μg/m3), the effect of hydrolysis of N2O5 on the formation of nitrate and ammonium was insignificant (variation rate <5%). This study suggests that during heavy pollution periods, the hydrolysis of N2O5 enhances the level of aerosol pollution in Beijing, and needs to be further studied in order to perform efficient air pollution control and mitigation strategies.

Urban dust in the Guanzhong basin of China, part II: A case study of urban dust pollution using the WRF-Dust model

We developed a regional dust dynamical model (WRF-Dust) to simulate surface dust concentrations in the Guanzhong (GZ) basin of China during two typical dust cases (19th Aug. and 26th Nov., 2013), and compared model results with the surface measurements at 17 urban and rural sites. The important improvement of the model is to employ multiple high-resolution (0.5-500 m) remote sensing data to construct dust sources. The new data include the geographic information of constructions, croplands, and barrens over the GZ basin in summer and winter of 2013. For the first time, detailed construction dust emissions have been introduced in a regional dust model in large cities of China. Our results show that by including the detailed dust sources, model performance at simulating dust pollutions in the GZ basin is significantly improved. For example, the simulated dust concentration average for the 17 sites increases from 28 μg m(-3) to 59 μg m(-3), closing to the measured concentration of 66 μg m(-3). In addition, the correlation coefficient (r) between the calculated and measured dust concentrations is also improved from 0.17 to 0.57, suggesting that our model better presents the spatial variation. Further analysis shows that urban construction activities are the crucial source in controlling urban dust pollutions. It should be considered by policy makers for mitigating particulate air pollution in many Chinese cities.

Impacts of Himalayas on black carbon over the Tibetan Plateau during summer monsoon

The Tibetan Plateau (TP) plays important roles in global climate and environment. This study combines in-situ BC measurements in the Himalayas and the Indo-Gangetic Plain (IGP) with a regional dynamical and chemical model (WRF-Chem model) to investigate the effect of the trans-Himalayas on black carbon (BC) from the IGP to the TP during Indian summer monsoon. To determine topographic effects of the trans-Himalayas on BC concentrations over the TP, sensitive experiments were conducted by applying the WRF-Chem model. The results showed that the reduction of the altitude of the Himalayas had an important effect on the trans-Himalayas transport of BC. There was an obvious increase in BC concentration over the trans-Himalayas region, but no significant increase over the TP because the TP (a.m.s.l ~4km) always acted as a wall to prevent BC transport from the IGP to the TP. The trans-Himalayas transport of BC was strongly dependent upon meteorological conditions over the IGP. During summer monsoon, there were three types of cyclones at different locations and one kind of convergent circulation in the IGP. Under the condition of convergent airflows, a strong northeastward wind produced the trans-Himalayas transport of BC. As a result, BC concentrations in the southeastern TP significantly increased to 0.6-0.8μgm-3. When the cyclone located in the eastern IGP, high BC concentrations over the IGP were transported along the foothill of the Himalayas, resulting in a significant reduction of the trans-Himalayas transport. When the cyclone moved to the west, the dynamical perturbations for the trans-Himalayas transport were weaker than the eastern cyclone, and the trans-Himalayas transport were enhanced in the middle and eastern Himalayas. This study will be helpful to assess the impacts of BC particles emitted from South Asia on regional climate change and ecological environment over the TP in the future.

Black carbon aerosol and its radiative impact at a high-altitude remote site on the southeastern Tibet Plateau

Aerosol black carbon (BC) was measured with an Aethalometer™ at Lulang, a high-altitude station in southeastern Tibetan Plateau (TP), from July 2008 to August 2009. Daily mean BC loadings varied from 57.7 to 5368.9 ng m−3 (grand average ± standard deviation = 496.5 ± 521.2 ng m−3), indicating a significant BC burden even at free tropospheric altitudes. BC loadings were highest during the pre-monsoon and lowest during the monsoon, and peaks in BC were coincident with high atmospheric boundary layers. Daily peaks in BC occurred from 08:00–10:00 local time with minor fluctuations at other times. The BC mass absorption efficiency (MAE) was calculated from elemental carbon concentrations obtained from a thermal/optical reflectance method and absorption coefficients from the Aethalometer™, and values ranged from 6.1–31.7 m2 g−1 (average = 16.6 ± 5.7 m2 g−1). Strong variations in the MAEs during the monsoon can be ascribed to large uncertainties due to low BC and babs, and possibly coatings on the BC. High MAEs during pre-monsoon pollution events were likely due to internal mixing during transport. The mean direct surface radiative forcing (DRF) estimated from a radiation model was −19.9 (±7.4) for the full aerosol population and −3.9 (±1.8) W m−2 for a BC only scenario. The BC DRF during a case study (−36.0 W m−2) was much stronger than the typical, and the BC contribution to the forcing was higher (~50%) than usual (~20%). These results show that BC can at times account for a relatively large fraction of the aerosol surface heating over the southeast TP, which may affect both climate and hydrological cycles.

Effect of biomass burning on black carbon (BC) in South Asia and Tibetan Plateau: The analysis of WRF-Chem modeling

The focus of this study is to evaluate the impact of biomass burning (BB) from South Asia and Southeast Asia on the glaciers over the Tibetan Plateau. The seasonality and long-term trend of biomass fires measured by Terra and Aqua satellite data from 2010 to 2016 are used in this study. The analysis shows that the biomass burnings were widely dispersed in the continental of Indian and Southeast Asia and existed a strong seasonal variation. The biomass burnings in winter (January) were relatively weak and scattered and were significantly enhanced in spring (April). The highest biomass burnings located in two regions. One was along the foothill of Himalayas, where is a dense population area, and the second located in Southeast Asia. Because these two high biomass burning regions are close to the Tibetan Plateau, they could have important effects on the BC deposition over the glaciers of the Tibetan Plateau. In order to study the effect of BB emissions on the deposition over the glaciers in the Tibetan Plateau, a regional chemical model (WRF-Chem; Weather Research and Forecasting Chemical model) was applied to simulate the BC distributions and the transport from BB emission regions to the glaciers in Tibetan Plateau. The result shows that in winter (January), due to the relatively weak BB emissions, the effect of BB emissions on BC concentrations was not significant. The BC concentrations resulted from BB emissions ranged from 0.1 to 2.0 μg/m3, with high concentrations distributed along the foothill of Himalayas and the southeastern Asia region. Due to the relative low BC concentrations, there was insignificant effect of BB emissions on the deposition over the glaciers in the Tibetan Plateau in winter. However, the BB emissions were highest in spring (April), producing high BC concentrations. For example, along the Himalayas Mountain and in the southeastern Asia region, The BC concentrations ranged from 2.0 to 6.0 μg/m3. In addition to the high BC concentrations, there were also west and south prevailing winds in these regions. As a result, the BC particles were transported to the glaciers in the Tibetan Plateau, causing significant deposition of BC particles on the snow surface of the glaciers. This study suggests that the biomass burning emissions have important effects on the BC deposition over the glaciers in the Tibetan Plateau, and the contaminations of glaciers could have significant impact on the melting of snow in the Tibetan Plateau, causing some severe environmental problems, such as the water resources.

Surface PM 2.5 , Satellite Distribution of Atmospheric Optical Depth and Related Effects on Crop Production in China

The surface concentrations of PM2.5 measured by the Chinese National Environmental Monitoring Center (CNEMC) and the aerosol optical depth (AOD) observed by the Moderate-Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite are used to study the spatial and temporal variations of aerosol pollution in eastern China. A solar radiation transfer model (Tropospheric Ultraviolet-Visible - TUV) developed at National Center of Atmospheric Research (NCAR) is applied to study the reduction of solar radiation by aerosol pollution, and the effect of this air pollution on crop production in the major farmlands of eastern China. The study shows that the correlation between the surface PM2.5 concentrations and the AOD values is complicated. In addition to PM2.5 concentrations, the vertical integration of the aerosol particle concentration, the hygroscopic growth of the particles, and the aerosol optical properties affect the AOD values. As a result, AOD is a better parameter to characterize the effect of aerosol particles on solar radiation in the atmosphere. Because regions with high values of AOD are collocated in many cases with major crop production regions, aerosol pollution has an important impact on the crop production in eastern China. Heavy aerosol load strongly reduces the sunlight reaching the surface in the major farmlands of eastern China, and, as a consequence, is responsible for a substantial decrease in crop production. However, the uncertainty in the estimated reduction of crop yield is large. One of the important uncertainties is associated with the calculation of the partitioning between direct and diffusive sunlight. The study suggests that, under the high diffusive case, the estimated reduction in crop yield is significantly lower than under the low diffusive case. For example, the rice reduction ranges from ~2% in the low diffusive case to ~1% in the high diffusive case. The reduction in the wheat yield ranges from ~6% in the low diffusive case, to ~4% in the high diffusive case. Future studies will have to be performed to reduce the uncertainty in these estimates.