Zhiwei Han

Model study on particle size segregation and deposition during Asian dust events in March 2002

[1] A size-segregated aerosol model that includes most of the major physical processes (generation, transport, and dry and wet deposition) is developed. This model is coupled with a Regional Air Quality Model (RAQM) and is applied to simulate Asian dust storms during the 10-day period of 15-24 March 2002. A nonhydrostatic mesoscale model (MM5) is used to provide meteorological fields. Model results are verified by available observational data including surface weather observations and size-segregated particle concentrations. The validation demonstrates a good capability of this model system in capturing most of the key features of dust evolution and reproducing the particle mass size distribution along the transport pathway of soil dust. An apparent feature has been both observed and reproduced by the model, showing a shift of size range with peak mass concentration from coarse mode to finer mode on the pathway from source regions to distant downwind areas. The maximum dust concentration averaged over 10 days is simulated to be 3000 μg m over the southern China-Mongolia border. Total dry deposition of soil dust for 10 days is up to 30 g m -2 in the Gobi desert along the southern China-Mongolia border. Distribution and magnitude of particle deposition are strongly dependent on both concentration and size-segregated dry deposition velocity and scavenging rate. While dry deposition dominates the removal of dust particles in or in the vicinity of source regions, the influence of wet deposition increases along the transport pathway of soil dust, with high removal efficiency for coarser particles (>2 μm) and very low efficiency for particles in the accumulation mode. Of the total dust emission (43.2 megatons), about 71% is redeposited onto the underlying surface by the dry deposition process, 6% is removed by the wet deposition process, and the remaining 23% is suspended in the atmosphere or subject to long-range transport.

Inorganic chemical composition and source signature of PM2.5 in Beijing during ACE-Asia period

Aerosol samples for PM2.5 were collected in Beijing for 38 consecutive days from March to April 2001 using an IMPROVE Sampler. Concentrations of w0 elements in PM2.5 were determined using a PIXE method. Results show that he average mineral dust concentration of PM2.5 was 14.6 μg/m3 during the observation period. On the sanddust event days of March 21 and April 10, dust PM2.5 mass concentrations were 62.4 and 54.1 μg/m3, respectively. These demonstrate that fine particle pollution by dust event in Beijing was very severe. The enrichment factors of S and Cu reached minimums on the dusty days and were high on the non-dusty days. It is considered that enrichment factors of elements in PM2.5, which are associated with human activities, can probably provide an effective method to distinguish local sources from external sources of dust. Factor analysis on the chemical composition in PM2.5 shows that sources of crustal matters, anthropogenic emission, and oil combustion contributed to PM2.5 levels in air in the springtime of 2001 in Beijing.

Impacts of biogenic emissions of VOC and NOx on tropospheric ozone during summertime in eastern China.

This study is intended to understand and quantify the impacts of biogenic emissions of volatile organic compounds (VOC) and nitrogen oxides (NO(x)) on the formation of tropospheric ozone during summertime in eastern China. The model system consists of the non-hydrostatic mesoscale meteorological model (MM5) and a tropospheric chemical and transport model (TCTM) with the updated carbon-bond chemical reaction mechanism (CBM-IV). The spatial resolution of the system domain is 30 km x 30 km. The impacts of biogenic emissions are investigated by performing simulations (36 h) with and without biogenic emissions, while anthropogenic emissions are constant. The results indicate that biogenic emissions have remarkable impacts on surface ozone in eastern China. In big cities and their surrounding areas, surface ozone formation tends to be VOC-limited. The increase in ozone concentration by biogenic VOC is generally 5 ppbv or less, but could be more than 10 ppbv or even 30 ppbv in some local places. The impacts of biogenic NO(x) are different or even contrary in different regions, depending on the relative availability of NO(x) and VOC. The surface ozone concentrations reduced or increased by the biogenic NO(x) could be as much as 10 ppbv or 20 ppbv, respectively. The impacts of biogenic emissions on ozone aloft are generally restricted to the boundary layer and generally more obvious during the daytime than during the nighttime. This study is useful for understanding the role of biogenic emissions and for planning strategies for surface ozone abatement in eastern China. Due to limitations of the emission inventories used and the highly non-linear nature of zone formation, however, some uncertainties remain in the results.

Black carbon in a continental semi‐arid area of Northeast China and its possible sources of fire emission

[1] Surface continuous measurements on black carbon (BC) were performed at Tongyu (44.42°N, 122.87°E) from March to December 2008. Tongyu is an international reference site of semi-arid climate and environment researches, located in a continental semi-arid area of northeastern China. Mass concentrations and potential contributing sources of BC are discussed for the sampling site. Hourly BC concentrations ranged from 77.61 ng m-3 to 59.55 μg m -3 with an average of 2.52 μg m -3 , showing a clear seasonal pattern with high values in spring, fall, and winter and low values in summer. Events of high BC concentrations occurred frequently in spring as a result of long-range transport of polluted air masses from the north, northwest, and south in the atmosphere over the Asian continent. The Yangtse River Delta (YRD) and China Anhui Province, the North China Plain (NCP), Northeast China, the Mongolian boundary near China and Russia, Middle Asia, and some areas of Russian Far East were major potential contribution sources for the observed pollutants. The region between Jiangsu Province and Anhui Province of China and the boundary regions between China and Russian Primorskiy and Khabarovskiy were possible source areas of fire emissions having great contributions to high BC levels at Tongyu. The region between Jiangsu Province and Anhui Province of China was the most important anthropogenic fire source of agricultural residue burning outflows.

Model study of the impact of biogenic emission on regional ozone and the effectiveness of emission reduction scenarios over eastern China

The impact of biogenic emission on regional ozone and emission control scenarios has been numerically studied through a series of sensitivity model simulations. A typical episode with elevated ozone over eastern China from 12 to 16 August 2001 was investigated by using a tropospheric chemistry and transport model (TCTM), driven by a non-hydrostatic mesoscale model MM5. The meteorological conditions during this period were characterized by high-pressure systems associated with low wind speeds, high temperatures and clear skies. Afternoon ozone concentrations exceeding 80 parts per billion (ppb) occurred over broad areas of eastern China. There is a generally good agreement between simulation and observation, indicating that the TCTM is able to represent major physical and chemical processes of tropospheric ozone and well reproduce the diurnal and day-to-day variability associated with synoptic conditions. The sensitivity analysis reveals a significant influence of biogenic hydrocarbons on regional ozone. Ozone levels are apparently enhanced by biogenic emission over large areas of eastern China. The largest increase up to 30 ppb in daytime average concentration is found in portions of the middle reaches of the Yangtze River, Yangtze Delta and northeast China. However, the response of ozone to biogenic emission varies spatially, showing more sensitivity in polluted areas than that in clean rural areas. The regimes limited by nitrogen oxides (NOx) and volatile organic carbon (VOC) in eastern China are further investigated with respect to biogenic emission. Ozone shows a clear tendency to shift from VOC limitation to NOx limitation as it moves from urban and industrial areas to rural areas. Most of the rural areas in southern China tend to be NOx limited, whereas most of the northern parts of China appear to be VOC limited. By considering biogenic emission, ozone tends to become more NOx limited and less VOC limited, both in extent and intensity, over eastern china. Furthermore, some regions have completely shifted from being VOC limited to being NOx limited.

Direct radiative effect of aerosols over East Asia with a Regional coupled Climate/Chemistry model

Tropospheric chemistry and aerosol processes have recently been incorporated into a Regional Integrated Environmental Model System (RIEMS) to study direct radiative effect of aerosols (DRE) over East Asia where intense human activity and continuous economic growth occur. Five aerosol components (sulfate, black carbon, organic carbon, soil dust and sea salt) and their relevant processes affecting distribution (emission, transport, diffusion, deposition, chemistry etc.) are added into RIEMS, with gas phase chemistry represented by CB-IV mechanism. The study periods are March, July, October and December 2006, generally representing 4 typical seasons in East Asia. The modeled monthly average aerosol optical depth (AOD) is generally consistent with MISR retrievals, although the model tends to underpredict AOD in some regions. The AOD distribution was characterized by high values in eastern China and deserts of western China, and AOD in winter is apparently lower than that in other seasons. The DRE show apparent seasonal variation, with maximum surface cooling in March and minimum surface cooling in winter, but maximum Top Of the Atmosphere (TOA) cooling in July and minimum TOA cooling in winter. The light TOA heating in March under all-sky condition is due to both the abundant absorbing aerosol and the enhanced heating effect by cloud. The DRE averaged over the study domain and the four periods under all-sky condition are -6.2 W m ―2 at the surface and -0.2 W m ―2 at TOA, respectively, over East Asia.

Roles of regional transport and heterogeneous reactions in the PM 2.5 increase during winter haze episodes in Beijing

Regional transport and chemical conversions are two major processes that lead to the severe haze pollution in China. Our observations during five haze episodes in Beijing between February 19 and March 12 of 2014 show that the two processes played different roles as PM2.5 increased from the clean (<75μgm-3) to the light-medium pollution level (75-150μg m-3) and to levels of heavy (150-250μgm-3) and severe (>250μgm-3) pollution. In the initial twelve hours of each episode, the PM2.5 reached the light-medium level with an increase of approximately 120μgm-3. At the same time, the particle (~10-700nm) number concentration also showed a distinct increase accompanied by a rapid increase in the mean diameter. A light-medium PM2.5 occurred in the south areas prior to the haze occurrence in Beijing and the southerly winds were predominant, indicating the rapid increase of PM2.5 in the initial stage was caused by the regional transport from the south. Subsequently, PM2.5 elevated to the heavy and severe levels when the wind was weak, relative humidity was high and ozone concentration was low. The increase of PM2.5 in the elevated stages was characterized by a high percentage (45% for the heavy level and 55% for the severe level) of secondary inorganic components, indicating the substantial contribution of the formation of secondary aerosols. In addition, the increases of the mean diameter (from 108nm to 120nm) and the total volume concentration (by 67%) are regarded as a consequence of heterogeneous reactions on the surfaces of aerosol particles because the particle number concentration remained nearly constant in these two stages. Our results indicate that, during the five winter haze episodes, the regional transport from the south was the major reason for the initial-stage PM2.5 increase, while heterogeneous reactions dominated the later elevation.

Model analysis of long-term trends of aerosol burdens and direct radiative forcings over East Asia

Long-term trends of aerosol concentrations and direct radiative forcings in East Asia have been investigated by adopting an online-coupled regional climate-chemistry-aerosol model (RIEMS-Chemaero) and the IPCC AR5 (the Intergovernmental Panel on Climate Change Fifth Assessment Report) emission inventory with the focus on eastern China. Model evaluation against surface observations of aerosol components demonstrates that the model can reproduce the spatial distribution and temporal variation of aerosol components generally well but tends to under-predict aerosol magnitudes especially for carbonaceous aerosols. Model comparison with aerosol optical depth (AOD) from both satellite retrieval and surface measurement shows that the model is capable of reproducing the distribution feature of AOD reasonably well but tends to under-predict magnitude. North of eastern China and the Sichuan basin are high concentration centres for near-surface anthropogenic aerosols. The domain averaged column burden of inorganic aerosol over East Asia increased from 1850 to 2010 with the peak in 1990 (18.7mg m−2), whereas that in eastern China peaked in 2010 (32.9 mg m−2). The burden of carbonaceous aerosol peaked in 1980 (5.2 mg m−2) in East Asia, but it peaked in 1990 (7.1 mg m−2) in eastern China and then decreased thereafter. Both inorganic and carbonaceous aerosol burdens showed little change during 1990–2000 due to effective emission control in China. AOD values over eastern China increased almost continuously from 1850 to 2010 with the Sichuan basin the high AOD centre. All-sky aerosol direct radiative forcing at the surface (ADRFSRF) is negative over all of East Asia, while radiative forcing at the top of the atmosphere (ADRFTOA) is negative over eastern China but positive over west China. Domain and annual mean ADRFSRF in East Asia peaked in 2000 (−11.9 W m−2), but that in eastern China peaked in 2010 (−17.5 W m−2). Mean ADRFTOA became negative since 1970 and showed the strongest forcing in 1990 (−2.9 W m−2) for East Asia and in 2010 (−4.8 W m−2) for eastern China. Sensitivity simulation shows that anthropogenic emission change plays the dominant role in the variation of aerosol concentration and direct radiative forcing in this region.

A numerical simulation of aerosols’ direct effects on tropopause height

The direct effects of sulfate aerosol, dust aerosol, carbonaceous aerosol, and total combined aerosols on the tropopause height are simulated with the Community Atmospheric Model version 3.1 (CAM3.1). A decrease of global mean tropopause height induced by sulfate, carbonaceous aerosol, and total combined aerosols is found, and a tropopause height increase is induced by dust aerosol. Sulfate aerosol decreases the tropospheric temperature and increases the stratospheric temperature. These effects cause a decrease in the height of the tropopause. In contrast, carbonaceous and total combined aerosols increase both the tropospheric and the stratospheric temperatures, and they also cause a decrease in the height of the tropopause. The changes in the tropopause height show highly statistically significant correlations with the changes in the tropospheric and stratospheric temperatures. The changes in the tropospheric and stratospheric temperatures are related to the changes in the radiative heat rate, cloud cover, and latent heat, but none of these factors absolutely dominate the temperature change.

Regional integrated environmental modeling system: development and application

The demand for high-confidence regional climate change scenarios is increasing. It is therefore vitally important to better understand the behavior of Earth’s climate system on regional scale and advance the knowledge of regional responses to global climate. With their ability to represent meso-scale forcings, such as coastline, complex topography, anthropogenic aerosols and land cover/use changes, Regional Climate Models (RCMs) are developed and used worldwide to investigate the effects of the above-mentioned meso-scale forcings on the local circulations that regulate the regional distribution of climatic variables. Considering the complexity of Asian Monsoon system, which is not only a physical process but also modulated by the interaction among physical, biological, chemical and social processes, a modeling framework Regional Integrated Environmental Modeling System (RIEMS) was proposed, developed and well tested before it was widely used in regional climate studies in the East Asia monsoon region.