Model analysis of vertical exchange of boundary layer ozone and its impact on surface air quality over the North China Plain.

In addition to photochemical production and horizontal regional transport, surface O3 concentration can also likely be affected by vertical transport, which is not well known so far. The process analysis was conducted by using the Regional Atmospheric Modeling System Community Multiscale Air Quality (RAMS-CMAQ) model to investigate photochemical production and the vertical transport mechanism of boundary-layer O3 during a typical O3 pollution episode in the North China Plain (NCP), and further quantify the contribution of vertical transport to surface O3. The diurnal variations of vertical budgets of O3 and NO2 in the boundary layer at multiple sites showed that there were substantial differences in the vertical distribution of O3 production and transport between urban and suburban/rural areas. In urban areas, surface O3 is consumed by titration reaction to generate NO2, which is then transported to the upper boundary layer and produces O3 by photochemical reaction. With the development of the boundary layer, the upper-layer O3 stored in the residual layer at nighttime can be transported vertically to the surface as the turbulent diffusion intensifies the next morning. While in suburban and rural areas, the vertical transport is relatively weaker because the photochemical formation of O3 occurs in the whole boundary layer, although it decreases slightly with the altitude. Model simulation showed that 20.6-27.9% of urban surface O3 changes in the morning (09:00-10:00 LST) was attributable to the downward transport from the residual layer, while it is 15.0-22.1% at suburban site. The vertical transport from above the boundary layer contributed 24.0-63.6% to daytime urban surface O3 changes, which was weak in suburban areas. Differences and similarities in O3 formation and transport mechanism in urban and suburban regions revealed here highlight the importance of earlier control and regional collaboration.

Spatial and temporal distribution of the cloud optical depth over China based on MODIS satellite data during 2003-2016.

The cloud optical depth (COD) is one of the important parameters used to characterize atmospheric clouds. We analyzed the seasonal variations in the COD over East Asia in 2011 using cloud mode data from the AERONET (Aerosol Robotic Network) ground-based observational network. The applicability of the MODIS (Moderate Resolution Imaging Spectroradiometer) COD product was verified and compared with the AERONET cloud mode dataset. There was a good correlation between the AERONET and the MODIS. The spatial and temporal distribution and trends in the COD over China were then analyzed using MODIS satellite data from 2003 to 2016. The seasonal changes in the AERONET data and the time sequence variation of the satellite data suggest that the seasonal variations in the COD are significant. The result shows that the COD first decreases and then increases with the season in northern China, and reaches the maximum in summer and minimum in winter. However, the spatial distribution change is just the opposite in southern China. The spatial variation trend shows the COD in China decreases first with time and gradually increases after 2014. And the trend of COD in the western and central China is consistent with that in China. While the trend of COD shows a continuously increasing over time in northeast China and the Pearl River Delta.

[Observation study on aerosol optical properties and radiative forcing using the ground-based and satellite remote sensing at background station during the regional pollution episodes].

The significant effect of anthropogenic pollutants transportation on the physical and optical properties of regional background atmospheric aerosol was studied by using ground-based and satellite remote sensing data obtained at the atmospheric background station (Shangdianzi, Beijing) of North China during October 1 to 15 in 2011. The aerosol mass concentration and reactive gases concentration increased obviously during periods of October 4-5, October 7-9, and October 11-12. Comparing with the background period of October 1-3, volume concentration increased by a factor of 3-6 for reactive gases such as NO(x), and CO, and a factor of 10-20 for SO2. Mass concentration of PM2.5 was about 200 microg x m(-3) on October 9. During haze period, the AOD at 500 nm varied between 0.60 to 1.00. The single scattering albedo (SSA) was lower than 0.88. And the black carbon concentration increased 4-8 times, which suggested the aerosol absorption was very strong during this pollution episode. The absorption of aerosol particles could cause 100-400 W x m(-2) increase of atmospheric radiation. The surface radiation decreased by about 100-300 W x m(-2) due to the aerosol scattering and absorption. This could cause higher stability of atmosphere, which will significantly affect the cloud and precipitation, and thus the regional weather and climate.

Validation of MODIS aerosol retrievals and evaluation of potential cloud contamination in East Asia.

MODIS aerosol retrievals onboard Terra/Aqua and ground truth data obtained from AERONET (Aerosol Robtic Network) solar direct radiance measurements are collocated to evaluate the quality of the former in East Asia. AERONET stations in East Asia are separated into two groups according to their locations and the preliminary validation results for each station. The validation results showed that the accuracy of MODIS aerosol retrievals in East Asia is a little worse than that obtained in other regions such as Eastern U.S., Western Europe, Brazil and so on. The primary reason is due to the improper aerosol model used in MODIS aerosol retrieval algorithm, so it is of significance to characterize aerosol properties properly according to long-term ground-based remote sensing or other relevant in situ observations in order to improve MODIS retrievals in East Asia. Cloud contamination is proved to be one of large errors, which is demonstrated by the significant relation between MODIS aerosol retrievals versus cloud fraction, as well as notable improvement of linear relation between satellite and ground aerosol data after potential cloud contamination screened. Hence, it is suggested that more stringent clear sky condition be set in use of MODIS aerosol data. It should be pointed out that the improvement might be offset by other error sources in some cases because of complex relation between different errors. Large seasonal variation of surface reflection and uncertainties associated with it result in large intercepts and random error in MODIS aerosol retrievals in northern inland of East Asia. It remains to be a big problem to retrieve aerosols accurately in inland characterized by relatively larger surface reflection than the requirement in MODIS aerosol retrieval algorithm.

Evaluation of dust activity and climate effects in North China.

TOMS/AI data with nearly 20 years are utilized in the paper to evaluate dust activities in North China. Combined with simultaneous NCEP reanalysis climate data, climate effects on dust activities are assessed. The results showed that the whole North China suffers impact by dust aerosols, with three centers standing out in TOMS/AI spring average map that are western three basins, which are characterized by lower annual precipitation and elevation. Gobi deserts in Mongolia Plateau do not attain higher TOMS/AI value due to cloud contamination and relative higher elevation. Spring is the season with the highest TOMS dust aerosol index; within the western three basins, high dust aerosol index appears in both spring and summer, especially in Tarim Basin. Wind speed in spring and precipitation in previous rainy season play important roles in controlling dust activities, higher wind speed and less precipitation than the normal are in favor of dust activities in spring. Temperature in spring and previous winter also affect dust activity to a certain extent, but with contrary spatial distribution. Temperature in winter exert effect principally in west part, contrarily, temperature effect in spring is mainly shown in east part. Both of them have negative correlation with dust activity.

Automatic and continuous measurement of aerosol properties in Dunhuang, China.

Ground-based simultaneous observations of sun direct and scattering radiation were carried out in Dunhuang for nearly 2 years. Aerosol optical depth, Angstrom wavelength exponent and size distribution were obtained from solar extinction and sky radiation. Water vapor content was obtained from sun direct radiation measurement at 940 nm. Relationship between aerosol properties and water vapor was discussed. Results showed that distinct seasonality of aerosol optical depth and Angstrom wavelength exponent was corresponding to seasonal variation of dust activity. Aerosol relative size distribution kept stable and volume concentration change was the reason resulting in variation of aerosol optical depth. Water vapor had minor effects on aerosol optical and physical properties.

Chemical composition of aerosols in winter/spring in Beijing.

In 1999 aerosol samples were collected by cascade at Meteorological Tower in Beijing. The 12 group aerosol samples obtained were analyzed using PIXE method, which resulted in 20 elemental concentrations and size distribution of elemental concentrations. From the observation, the elemental concentrations, size distribution of elemental concentrations and their variations are analyzed. It shows that concentrations of the most elements in aerosols increase greatly compared with those in the past except that the concentrations of V, K, Sr, and the source of aerosols has changed greatly in the past decade. Fine mode aerosols increase more rapidly in the past decade, which may be due to the contribution of coal combustion and automobile exhaust. Pb content in aerosol is much higher than that at the beginning of 1980s, and has a decreasing trend in recent years because of using non-leaded gasoline.