Yan Yin

Abstract China experienced severe haze pollution in January 2013. Here we have a detailed characterization of the sources and evolution mechanisms of this haze pollution with a focus on four haze episodes that occurred during 10–14 January in Beijing. The main source of data analyzed is from submicron aerosol measurements by an Aerodyne Aerosol Chemical Speciation Monitor. The average PM1 mass concentration during the four haze episodes ranged from 144 to 300 µg m −3 , which was more than 10 times higher than that observed during clean periods. All submicron aerosol species showed substantial increases during haze episodes with sulfate being the largest. Secondary inorganic species played enhanced roles in the haze formation as suggested by their elevated contributions during haze episodes. Positive matrix factorization analysis resolved six organic aerosol (OA) factors including three primary OA (POA) factors from traffic, cooking, and coal combustion emissions, respectively, and three secondary OA (SOA) factors. Overall, SOA contributed 41–59% of OA with the rest being POA. Coal combustion OA (CCOA) was the largest primary source, on average accounting for 20–32% of OA, and showed the most significant enhancement during haze episodes. A regional SOA (RSOA) was resolved for the first time which showed a pronounced peak only during the record‐breaking haze episode (Ep3) on 12–13 January. The regional contributions estimated based on the steep evolution of air pollutants were found to play dominant roles for the formation of Ep3, on average accounting for 66% of PM1 during the peak of Ep3 with sulfate, CCOA, and RSOA being the largest fractions (> ~ 75%). Our results suggest that stagnant meteorological conditions, coal combustion, secondary production, and regional transport are four main factors driving the formation and evolution of haze pollution in Beijing during wintertime.

Abstract. The absorption Ångström exponent (AAE) is an important aerosol optical parameter used for aerosol characterization and apportionment studies. The AAE of black carbon (BC) particles is widely accepted to be 1.0, although observational estimates give quite a wide range of 0.6–1.3. With considerable uncertainties related to observations, a numerical study is a powerful method, if not the only one, to provide a better and more accurate understanding on BC AAE. This study calculates BC AAE using realistic particle geometries based on fractal aggregate and an accurate numerical optical model (namely the multiple-sphere T-matrix method), and considers bulk properties of an ensemble of BC particles following lognormal size distributions. At odds with the expectations, BC AAE is not 1.0, even when BC is assumed to have small sizes and a wavelength-independent refractive index. With a wavelength-independent refractive index, the AAE of fresh BC is approximately 1.05 and relatively insensitive to particle size. For BC with geometric mean diameters larger than 0.12 µm, BC AAE becomes smaller when BC particles are aged (compact structures or coated by other non-absorptive materials). For coated BC, we prescribe the coating fraction variation based on a laboratory study, where smaller BC cores are shown to develop larger coating fractions than those of bigger BC cores. For both compact and coated BC, the AAE is highly sensitive to particle size distribution, ranging from approximately 0.8 to even over 1.4 with wavelength-independent refractive index. When the refractive index is allowed to vary with wavelength, a feature with observational backing, the BC AAE may show an even wider range. For different BC morphologies, we derive simple empirical equations on BC AAE based on our numerical results, which can serve as a guide for the response of BC AAE to BC size and refractive index. Due to its complex influences, the effects of BC geometry is better to be discussed at certain BC properties, i.e., known size and refractive index.

Abstract. To understand the impact of firework-burning (FW) particles on air quality and human health during the winter haze period, 39 elements, 10 water-soluble ions and 8 fractions of carbonaceous species in atmospheric PM2.5 in Nanjing were investigated during the 2014 Chinese Spring Festival (SF). Serious regional haze pollution persisted throughout the entire sampling period, with PM2.5 averaging at 113 ± 69 μg m−3 and visibility at 4.8 ± 3.2 km. The holiday effect led to almost all the chemical species decreasing during the SF, except for Al, K, Ba and Sr which were related to FW. The source contributions of coal combustion, vehicle emission and road dust decreased dramatically, whereas FW contributed to about half of the PM2.5 during the SF period. The intensive emission of FW particles on New Year's Eve accounted for 60.1% of the PM2.5. Fireworks also obviously modified the chemical compositions of PM2.5, with 39.3% contributed by increased organic matter, followed by steadily increased loadings of secondary inorganic ions. The aging processes of the FW particles lasted for about 4 days reflected by the variations of Ba, Sr, NH4+, NO3−, SO42− and K+, characterized by heterogeneous reactions of SO2 and NOx on crustal materials directly from FW, the replacement of Cl− by NO3− and SO42−, coating of NO3− and SO42− on soot, formation of secondary organic aerosols and metal-catalyzed formation of NO3− and SO42− at higher relative humidity. During aging, the main contributors to the extinction coefficient shifted from elemental carbon and organic matter to ammonium sulfate. The particles raised higher cancer risk of 1.62 × 10−6 by heavy metals (especially for Cd and As). This study provided detailed composition data and first comprehensive analysis of the aging processes of FW particles during the serious haze pollution period and their potential impact on human health.

Numerical simulations were performed to investigate the effect of cloud‐processed mineral dust particles on the subsequent development of cloud and precipitation and possible effects on cloud optical properties. A two‐dimensional (2‐D) nonhydrostatic cloud model with detailed microphysics was used. The initial aerosol spectra used in the 2‐D model consisted of both background cloud condensation nuclei and mineral dust particles. These were taken from the results of three successive runs of a parcel model that simulates the interaction of dust and sulfate particles with cloud drops and trace gases and then evaporates the cloud drops. The results show that insoluble mineral dust particles become effective cloud condensation nuclei (CCN) after passing through a convective cloud. Their effectiveness as CCN increases because of a layer of sulfate that is formed on their surface as they are first captured by growing drops or ice crystals and then released as these hydrometeors evaporate. Upon entering subsequent clouds, these particles increase the concentration of the activated drops and widen the drop size distribution. The present work shows that in continental clouds the effect of cloud‐processed dust particles is to accelerate the formation of precipitation particles, although the amount of precipitation depends on the concentration of the large and giant CCN. In maritime clouds the addition of cloud‐processed aerosol and mineral dust particles has a minimal effect on precipitation because the cloud starts with many large particles already. The addition of more CCN to either maritime or continental clouds increases their optical depth, even for those cases in which the precipitation amount is increased.