Showing papers by "Haiyan Ni published in 2017"

Sources and physicochemical characteristics of black carbon aerosol from the southeastern Tibetan Plateau: internal mixing enhances light absorption

Abstract: . Black carbon (BC) aerosol has important effects on the climate and hydrology of the Tibetan Plateau (TP). An intensive measurement campaign was conducted at Lulang (∼ 3300 m a.s.l. – above sea level), southeastern TP, from September to October 2015, to investigate the sources and physicochemical characteristics of refractory BC (rBC) aerosol. The average rBC mass concentration was 0.31 ± 0.55 µg m−3, which is higher than most prior results for BC on the TP. A clear diurnal cycle in rBC showed high values in the morning and low values in the afternoon. A bivariate polar plot showed that rBC loadings varied with wind speed and direction, which also reflected the dominant transport direction. The estimated net surface rBC transport intensity was +0.05 ± 0.29 µg s−1 m−2, indicating stronger transport from outside the TP compared with its interior. Cluster analysis and a concentration-weighted trajectory model connected emissions from north India to the high rBC loadings, but the effects of internal TP sources should not be overlooked. The average mass median diameter (MMD) of rBC was 160 ± 23 nm, with smaller MMDs on rainy days (145 nm) compared with non-rainy days (164 nm). The average number fraction of thickly coated rBC (FrBC) was 39 ± 8 %, and it increased with the O3 mixing ratios from 10:00 to 14:00 LT, indicating that photochemical oxidation played a role in forming rBC coatings. The average rBC absorption enhancement (Eabs) was estimated to be 1.9, suggesting that light absorption by coated rBC particles was greater than for uncoated ones. The Eabs was strongly positively correlated with the FrBC, indicating an amplification of light absorption for internally mixed rBC. For rBC cores 170 nm. Our study provides insight into the sources and evolution of rBC aerosol on the TP, and the results should be useful for improving models of the radiative effects of carbonaceous aerosols in this area.

Effects of photochemical oxidation on the mixing state and light absorption of black carbon in the urban atmosphere of China

Abstract: The relationship between the refractory black carbon (rBC) aerosol mixing state and the atmospheric oxidation capacity was investigated to assess the possible influence of oxidants on the particles' light absorption effects at two large cities in China. The number fraction of thickly-coated rBC particles (F rBC) was positively correlated with a measure of the oxidant concentrations (OX = O3 + NO2), indicating an enhancement of coated rBC particles under more oxidizing conditions. The slope of a linear regression of F rBC versus OX was 0.58% ppb−1 for Beijing and 0.84% ppb−1 for Xi'an, and these relationships provide some insights into the evolution of rBC mixing state in relation to atmospheric oxidation processes. The mass absorption cross-section of rBC (MACrBC) increased with OX during the daytime at Xi'an, at a rate of 0.26 m2 g−1 ppb−1, suggesting that more oxidizing conditions lead to internal mixing that enhances the light-absorbing capacity of rBC particles. Understanding the dependence of the increasing rates of F rBC and MACrBC as a function of OX may lead to improvements of climate models that deal with the warming effects, but more studies in different cities and seasons are needed to gauge the broader implications of these findings.

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

Abstract: 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 ( 4 m s−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 (6.3–62.6 μg eBC m−2 day−1), followed by the post-monsoon (4.6–45.9 μg eBC m−2 day−1), winter (4.3–43.1 μg eBC m−2 day−1), and monsoon (2.4–24.5 μg 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 58.8–587.6 μg kg−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.