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Knowledge of haze particles in background areas of North China is limited, although they have been studied well in urban settings. Atmospheric aerosol particles were collected at a background site in the North China Plain during 16-31 January, 2011. Water soluble inorganic ions of PM and physicochemical characteristics of individual particles on hazy and clean days were measured by Ion Chromatography (IC) and Transmission Electron Microscopy (TEM), respectively. Average PM mass concentration was 50.4±29.9 μg m with 62.5±26.8 μg m on hazy days and 19.9±11.5 μg m on clean days. SO, NO, and NH with a combined mass concentration of 19.0±11.5 μg m accounted for 69.8%-89.4% of the total water soluble inorganic ions. Size distributions of SO and NH showed one unimodal peak at 0.56-1.8 μm on hazy days, whereas NO appeared as bimodal peaks at 0.56-1.8 and 5.6-10 μm, respectively. Individual particle analyses showed that the dominant aerosols were a mixture of sulfate, nitrate, and carbonaceous species, which together determine their mixing states. 48-h air mass back trajectories on hazy days suggested that air masses crossed the polluted continental areas (such as Jing-jin-ji region and Shandong province) and entrained ground air pollutants 11-19 hours before reaching the background area. During long-range transport particles undergo ageing and tend to be internally mixed mainly due to condensation in the background atmosphere. Our results suggest that hygroscopic and optical properties of these aerosol particles in the background area differ substantially from those in urban areas. [ABSTRACT FROM AUTHOR]

To determine the characteristics of carbonaceous aerosols in rural areas of the North China Plain, field measurements were conducted at Yucheng (YC) in the summers of 2013 and 2014. The concentrations of carbonaceous aerosols at YC exhibited clear diurnal variation, with higher concentrations in the early morning and at night and lower concentrations during the afternoon hours. The mass-balance method designed for particulate matter smaller than 2.5 μm (PM 2.5 ) was used to calculate the organic matter (OM)/organic carbon (OC) ratio. The value obtained, 2.07 ± 0.05, was suggested as a reference to estimate organics in PM 2.5 in rural areas of the North China Plain. Biomass burning was identified to be a significant source of carbonaceous aerosols; approximately half of the samples obtained at YC were affected by biomass burning during summer 2013. Case studies revealed that biomass burning accounted for up to 52.6% of the OC and 51.1% of the elemental carbon in PM 2.5 samples. The organic coatings observed on sulphur-rich and potassium-rich particles indicated the formation of secondary organic aerosols (SOA) from the oxidation of precursor volatile organic compounds (VOCs) during the aging of smoke released from biomass burning. Based on the evolution of the VOCs, the contribution of VOCs oxidation to SOA concentration was 3.21 and 1.07 μg m − 3 ppm − 1 CO under conditions of low nitrogen oxide (NOx) and high NOx, respectively. Aromatics (e.g. benzene, toluene, xylene and ethylbenzene) made the greatest contribution to SOA concentration (88.4% in low-NOx conditions and 80.6% in high-NOx conditions). The results of the study offer novel insights into the effects of biomass burning on the carbonaceous aerosols and SOA formation in polluted rural areas. [ABSTRACT FROM AUTHOR]

To investigate haze-fog (HF) formation mechanisms and transport, trace gases and aerosols in the aged air masses during regional haze episodes were measured at a regional background site in the North China Plain during 4–19 July, 2011. Mixing state of individual particles, soluble ions of PM 2.5 , and particle number concentrations were studied using transmission electron microscope, ambient ion monitoring, and wide-range particle spectrometer, respectively. Average mass concentration of PM 2.5 was 3 times higher on HF days (70 μg/m 3 ) than on clear days (22 μg/m 3 ). The major soluble ionic components (SO 4 2 − , NO 3 − , and NH 4 + ) in PM 2.5 were over 4 times higher on HF days (40.6 μg/m 3 ) than on clear days (9.1 μg/m 3 ). The high sulfur oxidation ratios (SOR) and nitrogen oxidation ratios (NOR) values during HF days suggest that polluted weather favored transformation of SO 2 and NO x into sulfates and nitrates compared to clear days. Particle number fraction of the accumulation mode increases from 11% on clear days up to 26% on HF days. Individual particle analysis shows that secondary inorganic particles (e.g., sulfate and nitrate) as the most abundant species likely determine internal mixing of individual particles and almost half of them mixed refractory particles (e.g., metal, fly ash, soot, and mineral) on HF days. These fine refractory particles were likely emitted from coal fired power plants, heavy industries, and urban city in Shandong and Hebei provinces. Our results suggest that aged air masses mostly contain aged particles of long-range transport and some from new particle formation and growth in the regional background atmosphere. [ABSTRACT FROM AUTHOR]