Ningning Zhang

Dynamics of the larch taiga?permafrost coupled system in Siberia under climate change

Larch taiga, also known as Siberian boreal forest, plays an important role in global and regional water–energy–carbon (WEC) cycles and in the climate system. Recent in situ observations have suggested that larch-dominated taiga and permafrost behave as a coupled eco-climate system across a broad boreal zone of Siberia. However, neither field-based observations nor modeling experiments have clarified the synthesized dynamics of this system. Here, using a new dynamic vegetation model coupled with a permafrost model, we reveal the processes of interaction between the taiga and permafrost. The model demonstrates that under the present climate conditions in eastern Siberia, larch trees maintain permafrost by controlling the seasonal thawing of permafrost, which in turn maintains the taiga by providing sufficient water to the larch trees. The experiment without permafrost processes showed that larch would decrease in biomass and be replaced by a dominance of pine and other species that suffer drier hydroclimatic conditions. In the coupled system, fire not only plays a destructive role in the forest, but also, in some cases, preserves larch domination in forests. Climate warming sensitivity experiments show that this coupled system cannot be maintained under warming of about 2 ◦ C or more. Under such conditions, a forest with typical boreal tree species (dark conifer and deciduous species) would become dominant, decoupled from the permafrost processes. This study thus suggests that future global warming could drastically alter the larch-dominated taiga–permafrost coupled system in Siberia, with associated changes of WEC processes and feedback to climate.

The rural carbonaceous aerosols in coarse, fine, and ultrafine particles during haze pollution in northwestern China.

The carbonaceous aerosol concentrations in coarse particle (PM10: Dpu2009≤u200910xa0μm, particulate matter with an aerodynamic diameter less than 10xa0μm), fine particle (PM2.5: Dpu2009≤u20092.5xa0μm), and ultrafine particle (PM0.133: Dpu2009≤u20090.133xa0μm) carbon fractions in a rural area were investigated during haze events in northwestern China. The results indicated that PM2.5 contributed a large fraction in PM10. OC (organic carbon) accounted for 33, 41, and 62xa0% of PM10, PM2.5, and PM0.133, and those were 2, 2.4, and 0.4xa0% for EC (elemental carbon) in a rural area, respectively. OC3 was more abundant than other organic carbon fractions in three PMs, and char dominated EC in PM10 and PM2.5 while soot dominated EC in PM0.133. The present study inferred that K+, OP, and OC3 are good biomass burning tracers for rural PM10 and PM2.5, but not for PM0.133 during haze pollution. Our results suggest that biomass burning is likely to be an important contributor to rural PMs in northwestern China. It is necessary to establish biomass burning control policies for the mitigation of severe haze pollution in a rural area.

Physicochemical characteristics of black carbon aerosol and its radiative impact in a polluted urban area of China

Black carbon (BC) aerosol plays an important role in the Earths radiative balance. An intensive measurement campaign was conducted at Xian, China from Dec. 2012 to Jan. 2013 to investigate the sources and physicochemical characteristics of refractory BC (rBC) and its direct radiative forcing at the surface. The overall average rBC concentration for the campaign was 8.0u2009±u20097.1u2009µgu2009m-3. Source apportionment based on positive matrix factorization showed that traffic was the dominant rBC source (46.0%), followed by coal burning (33.9%) and biomass burning (20.1%). The rBC mass size distributions were mono-modal and lognormal with larger mass median diameters for coal burning source (215u2009nm) compared with the traffic source (189u2009nm). Coal-burning rBC was more strongly associated with sulfate than traffic rBC, suggesting a higher cloud-condensation-nuclei activity. The slope of a robust linear regression between rBC and carbon monoxide (CO) for all samples was 5.9u2009µgu2009m-3 ppm-1, and slope for the coal burning source (4.5u2009µgu2009m-3 ppm-1) was larger than that for the traffic source (2.7u2009µgu2009m-3 ppm-1). The net rBC emission during winter of 2009 was estimated to be 4.5 Gg based on the relationship between rBC and CO. A Tropospheric Ultraviolet and Visible radiation model showed that the average daytime value for the clear-sky direct radiative forcing due to rBC from 23 Dec. 2012 to 31 Jan. 2013 was -47.7u2009±u200928.9u2009Wu2009m-2, which amounted to an average of 45.7% of the total surface atmospheric aerosol forcing.

Chemical composition of rainwater at Lijiang on the Southeast Tibetan Plateau: influences from various air mass sources

Daily rainwater samples collected at Lijiang in 2009 were analyzed for pH, electrical conductivity, major ion (SO42−, Cl−, NO3−, Na+, Ca2+, Mg2+, and NH4+) concentrations, and δ18O. The rainwater was alkaline with the volume-weighted mean pH of 6.34 (range: 5.71 to 7.11). Ion concentrations and δ18O during the pre-monsoon period were higher than in the monsoon. Air mass trajectories indicated that water vapor from South Asia was polluted with biomass burning emissions during the pre-monsoon. Precipitation during the monsoon was mainly transported by flow from the Bay of Bengal, and it showed high sea salt ion concentrations. Some precipitation brought by southwest monsoon originated from Burma; it was characterized by low δ18O and low sea salt, indicating that the water vapor from the region was mainly recycled monsoon precipitation. Water vapor from South China contained large quantities of SO42−, NO3−, and NH4+. Throughout the study, Ca2+ was the main neutralizing agent. Positive matrix factorization analysis indicated that crustal dust sources contributed the following percentages of the ions Ca2+ 85xa0%, Mg2+ 75xa0%, K+ 61xa0%, NO3− 32xa0% and SO42− 21xa0%. Anthropogenic sources accounted for 79xa0%, 68xa0%, and 76xa0% of the SO42−, NO3− and NH4+, respectively; and approximately 93xa0%, 99xa0%, and 37xa0% of the Cl−, Na+, and K+ were from a sea salt source.

Development of source profiles and their application in source apportionment of PM 2.5 in Xiamen, China

Ambient PM2.5 samples were collected at four sites in Xiamen, including Gulangyu (GLY), Hongwen (HW), Huli (HL) and Jimei (JM) during January, April, July and October 2013. Local source samples were obtained from coal burning power plants, industries, motor vehicles, biomass burning, fugitive dust, and sea salt for the source apportionment studies. The highest value of PM2.5 mass concentration and species related to human activities (SO42–, NO3–, Pb, Ni, V, Cu, Cd, organic carbon (OC) and elemental carbon (EC)) were found in the ambient samples from HL, and the highest and lowest loadings of PM2.5 and its components occurred in winter and summer, respectively. The reconstructed mass balance indicated that ambient PM2.5 consisted of 24% OM (organic matter), 23% sulfate, 14% nitrate, 9% ammonium, 9% geological material, 6% sea salt, 5% EC and 10% others. For the source profiles, the dominant components were OC for coal burning, motor vehicle, biomass burning and sea salt; SO42– for industry; and crustal elements for fugitive dust. Source contributions were calculated using a chemical mass balance (CMB) model based on ambient PM2.5 concentrations and the source profiles. GLY was characterized by high contributions from secondary sulfate and cooking, while HL and JM were most strongly affected by motor vehicle emissions, and biomass burning and fugitive dust, respectively. The CMB results indicated that PM2.5 from Xiamen is composed of 27.4% secondary inorganic components, 20.8% motor vehicle emissions, 11.7% fugitive dust, 9.9% sea salt, 9.3% coal burning, 5.0% biomass burning, 3.1% industry and 6.8% others.

A 10-year observation of PM 2.5 -bound nickel in Xi’an, China: Effects of source control on its trend and associated health risks

This study presents the first long term (10-year period, 2004–2013) datasets of PM2.5-bound nickel (Ni) concentration obtained from the daily sample in urban of Xi’an, Northwestern China. The Ni concentration trend, pollution sources, and the potential health risks associated to Ni were investigated. The Ni concentrations increased from 2004 to 2008, but then decreased due to coal consumption reduction, energy structure reconstruction, tighter emission rules and the improvement of the industrial and motor vehicle waste control techniques. With the comparison of distributions between workday and non-workday periods, the effectiveness of local and regional air pollution control policies and contributions of hypothetical Ni sources (industrial and automobile exhausts) were evaluated, demonstrating the health benefits to the populations during the ten years. Mean Ni cancer risk was higher than the threshold value of 10−6, suggesting that carcinogenic Ni still was a concern to the residents. Our findings conclude that there are still needs to establish more strict strategies and guidelines for atmospheric Ni in our living area, assisting to balance the relationship between economic growth and environmental conservation in China.

Concentration and sources of atmospheric nitrous acid (HONO) at an urban site in Western China

Highly time-resolved measurements of nitrous acid (HONO) were carried out with a highly sensitive long path absorption photometer (LOPAP) at an urban site of Xian in Western China from 24 July to 6 August 2015 to investigate the atmospheric variations, sources, and formation pathways of HONO. The concentrations of HONO vary from 0.02 to 4.3ppbv with an average of 1.12ppbv for the entire measurement period. The variation trends of HONO and NO2 are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO2 ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO2. The nocturnal HONO level is governed by heterogeneous formation from NO2, followed by homogeneous formation of NO with OH and then by direct emissions. Further, it is found that the heterogeneous formation of HONO is largely affected by relative humidity and aerosol surface. Daytime HONO budget analysis indicates that an additional unknown source with HONO production rate of 0.75ppbvh-1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO.

Light absorption properties of brown carbon over the southeastern Tibetan Plateau

We present a study of the light-absorbing properties of water-soluble brown carbon (WS-BrC) and methanol-soluble brown carbon (MeS-BrC) at a remote site (Lulang, 3326m above sea level) in the southeastern Tibetan Plateau during the period 2015-2016. The light absorption coefficients at 365nm (babs365) of WS-BrC and MeS-BrC were the highest during winter and the lowest during monsoon season. MeS-BrC absorbs about 1.5 times higher at 365nm compared to WS-BrC. The absorption at 550nm appears lower compared to that of 365nm for WS-BrC and MeS-BrC, respectively. Higher average value of the absorption Ångström exponent (AAE, 365-550nm) was obtained for MeS-BrC (8.2) than that for WS-BrC (6.9). The values of the mass absorption cross section at 365nm (MAC365) indicated that BrC in winter absorbs UV-visible light more efficiently than in monsoon. The results confirm the importance of BrC in contributing to light-absorbing aerosols in this region. The understanding of the light absorption properties of BrC is of great importance, especially in modeling studies for the climate effects and transport of BrC in the Tibetan Plateau.

Chemical source profiles of urban fugitive dust PM2.5 samples from 21 cities across China

Urban fugitive (road and construction) dust PM2.5 samples were collected in 21 cities of seven regions in China. Seven water-soluble ions, eight sub-fractions of carbonaceous components, and 19 elements were determined to investigate the chemical profiles of urban fugitive dust. Among the analyzed chemical compositions and on regional average, the elemental compositions showed the highest proportion (12.5-28.9% in road dust (RD) and 13.1-38.0% in construction dust (CD)), followed by water-soluble ions (5.1-19.0% in RD and 4.2-16.4% in CD) and carbonaceous fractions (5.4-9.6% in RD and 4.9-9.3% in CD). Chemical compositions measured in CD were all slightly lower than those in RD although statistically insignificant (pu202f>u202f0.05). Soil dust, which was estimated from Fe concentration, was proved to be the biggest contributor to urban fugitive dust PM2.5 mass. While, it showed a higher contribution in Northern China (71.5%) than in Southern China (52.1%). Higher enrichment factors were found for elemental S, Zn and Pb in RD than CD, reflecting stronger anthropogenic sources (i.e. vehicle exhaust) in RD. Low NO3-/SO42- and high SO42-/K+ ratios both indicated that fugitive dust was strongly influenced by stationary sources (e.g. coal combustion), and this influence was especially strong in Northern China. Coefficients of divergence proved that dust profiles within the same region were more similar than across regions, reflecting that urban fugitive dust was influenced more by local sources than long-range transport.

Seasonal Transport and Dry Deposition of Black Carbon Aerosol in the Southeastern Tibetan Plateau

To investigate the regional transport and dry deposition of black carbon (BC) aerosol in the southeastern Tibetan Plateau, continuous equivalent BC (eBC) mass concentrations were measured at a high-altitude remote site of Lulang from July 2008 to July 2009. The bivariate polar plots for eBC mass concentrations showed that large eBC values were often associated with low winds (<xa02xa0mxa0s−1) during the pre-monsoon, post-monsoon, and winter seasons. Moreover, strong winds (>xa04xa0mxa0s−1) from southeast or northeast also contribute to the large eBC loadings during the pre-monsoon, monsoon, and post-monsoon seasons. The concentration-weighted trajectory analysis showed that emissions from the eastern Kingdom of Bhutan, Assam of India, and southern Shannan Prefecture of Tibet had the most important contributions to the eBC pollution at Lulang during the pre-monsoon and monsoon seasons. In contrast, the eBC potential source region shifted to the east and southeast of Lulang during the post-monsoon and to the north India and northwest Nepal during the winter. The estimated eBC deposition rate was the highest for the pre-monsoon (6.3–62.6xa0μgxa0eBCxa0m−2 day−1), followed by the post-monsoon (4.6–45.9xa0μg eBC m−2 day−1), winter (4.3–43.1xa0μg eBC m−2 day−1), and monsoon (2.4–24.5xa0μg eBC m−2 day−1). Further calculations of eBC concentrations in the snow surface were 33.3–333.2, 61.5–614.7, 27.0–269.9, and 58.8–587.6xa0μgxa0kg−1 during the pre-monsoon, monsoon, post-monsoon, and winter seasons, respectively, which resulted in the snow albedos being reduced by 2.6–25.3, 4.7–46.6, 2.1–20.5, and 4.5–44.5% accordingly.