Soot particles strongly absorb sunlight and hence act as a short-lived warming agent. Atmospheric aging of soot particles changes their morphology and mixing state and consequently alter their optical properties. Here we collected soot particles at tunnel, urban, mountaintop, and background sites in Northern China and analyzed their mixing structures and morphology using transmission electron microscopy. Soot particles were further classified into three types: bare-like, partly coated, and embedded. Bare-like soot particles were dominant at the tunnel site, while most soot particles were partly coated or embedded type at other sites. Fractal dimensions (D f) of different types of soot particles ranged from 1.80 to 2.16 and were ordered as: bare-like < partly coated < embedded. Moreover, their average D f changed from 1.8 to 2.0 from the tunnel to the background site. We conclude that the D f can characterize the shape of soot aggregates reasonably well and its variation reflects soot aging processes. Compared with the reported D f of soot particles, we found that D f = 1.8 used in previous optical models primarily represents freshly emitted soot aggregates, rather than the ambient ones.

The radiative forcing of black carbon aerosol (BC) is one of the largest sources of uncertainty in climate change assessments. Contrasting results of BC absorption enhancement ( E) after aging are estimated by field measurements and modeling studies, causing ambiguous parametrizations of BC solar absorption in climate models. Here we quantify E using a theoretical model parametrized by the complex particle morphology of BC in different aging scales. We show that E continuously increases with aging and stabilizes with a maximum of ∼3.5, suggesting that previous seemingly contrast results of E can be explicitly described by BC aging with corresponding particle morphology. We also report that current climate models using Mie Core-Shell model may overestimate E at a certain aging stage with a rapid rise of E, which is commonly observed in the ambient. A correction coefficient for this overestimation is suggested to improve model predictions of BC climate impact.

Elevated ground-level ozone (O3), which is an important aspect of air quality related to public health, has been causing increasing concern. This study investigated the spatiotemporal distribution of ground-level O3 concentrations in China using a dataset from the Chinese national air quality monitoring network during 2013–2015. This research analyzed the diurnal, monthly and yearly variation of O3 concentrations in both sparsely and densely populated regions. In particular, 6 major Chinese cities were selected to allow a discussion of variations in O3 levels in detail, Beijing, Chengdu, Guangzhou, Lanzhou, Shanghai, and Urumchi, located on both sides of the Heihe-Tengchong line. Data showed that the nationwide 3-year MDA8 of ground-level O3 was 80.26 μg/m3. Ground-level O3 concentrations exhibited monthly variability peaking in summer and reaching the lowest levels in winter. The diurnal cycle reached a minimum in morning and peaked in the afternoon. Yearly average O3 MDA8 concentrations in Beijing, Chengdu, Lanzhou, and Shanghai in 2015 increased 12%, 25%, 34%, 22%, respectively, when compared with those in 2013. Compared with World Health Organization O3 guidelines, Beijing, Chengdu, Guangzhou, and Shanghai suffered O3 pollution in excess of the 8-hour O3 standard for more than 30% of the days in 2013 to 2015.