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Absolute measurement of radiant power in the X-ray region is essential for many applications in astrophysics, spectroscopy, and X-ray diagnostics. Comparison between different measuring methods is an effective way to check their reliability. In the present work, a comparison of X-ray radiant power absolute measurement between a free-air ionization chamber and a cryogenic electrical substitution radiometer was performed at Beijing Synchrotron Radiation Facility. The absolute radiant power obtained by these two methods were mutually compared via a transfer standard detector's spectral responsivity at a photon energy of 10 keV. The result of the comparison showed that the difference was 0.47%. A conclusion was reached that the free-air ionization chamber and the cryogenic electrical substitution radiometer agreed within the combined relative uncertainty of 3.35%. [ABSTRACT FROM AUTHOR]

Absolute measurement of photon flux under high‐fluence rate X‐rays was required in photon diagnostics for X‐ray lasers. A free‐air ionization chamber used for the reproducibility of air‐kerma in high‐fluence rate X‐ray was designed, and the monoenergetic X‐rays flux of (6–20) keV were absolutely measured on Beijing Synchrotron Radiation Facility. The energy spectrum of the monoenergetic X‐rays was measured using a silicon drift detector, and the proportion of higher harmonics was determined. The saturation curve, air attenuation correction factor, and composite loss correction factor of the free‐air ionization chamber were obtained experimentally. The air‐kerma rate for the (6~20) keV monoenergetic X‐ray was measured accurately, and the relative combined standard uncertainty of the air‐kerma is 0.61%. [ABSTRACT FROM AUTHOR]

Air quality has significantly improved in China; however, new challenges emerge when dust weather is combined with haze pollution during spring in northern China. On March 15, 2021, an extremely severe sandstorm occurred in Beijing, with hourly maximum PM 10 and PM 2.5 concentrations reaching 5267.7 μg m−3 and 963.9 μg m−3, respectively. Continuous sandstorm events usually lead to complicated pollution status in spring. Three pollution types were identified disregarding the time sequence throughout March. The secondary formation type was dominant, with high ratios of PM 2.5 /PM 10 (mean 74%) and PM 1 /PM 2.5 (mean 52%). This suggests that secondary transformations are the primary cause of heavy pollution, even during the dry seasons. Sandstorm type resulted in dramatic PM 10 levels, with a noticeable decrease in PM 2.5 /PM 10 levels (27%), although PM 2.5 levels remain high. The transitional pollution type was distinguished by an independent increase in PM 10 levels, although PM 2.5 and PM 1 levels differed from the PM 10 levels. Throughout March, the sulfur oxidation rate varied considerably, with high levels during most periods (mean 0.52). A strong correlation indicated that relative humidity was the primary variable promoting the formation of secondary sulfate. Sandstorms promote heterogeneous reactions by providing abundant reaction surfaces from mineral particles, therefore aggravating secondary pollution. The sandstorm air mass from the northwest passing through the sand sources of Mongolia carried not only crustal matter but also organic components, such as bioaerosols, resulting in a sharp increase in the organic carbon in PM 2.5. [Display omitted] • Three types of air pollution were identified: secondary formation, sandstorm and transitional pollution. • Haze mixed with sandstorms caused more severe air pollution in Beijing. • Air masses from Mongolia with high bioaerosol burden led to increased organic carbon concentrations. [ABSTRACT FROM AUTHOR]

To control the spread of COVID-19, China implemented a series of lockdowns, limiting various offline interactions. This provided an opportunity to study the response of air quality to emissions control. By comparing the characteristics of pollution in the summers of 2019 and 2020, we found a significant decrease in gaseous pollutants in 2020. However, particle pollution in the summer of 2020 was more severe; PM 2.5 levels increased from 35.8 to 44.7 μg m−3, and PM 10 increased from 51.4 to 69.0 μg m−3 from 2019 to 2020. The higher PM 10 was caused by two sandstorm events on May 11 and June 3, 2020, while the higher PM 2.5 was the result of enhanced secondary formation processes indicated by the higher sulfate oxidation rate (SOR) and nitrate oxidation rate (NOR) in 2020. Higher SOR and NOR were attributed mainly to higher relative humidity and stronger oxidizing capacity. Analysis of PM x distribution showed that severe haze occurred when particles within Bin2 (size ranging 1–2.5 μm) dominated. SO 4 2− (1/2.5) and SO 4 2− (2.5/10) remained stable under different periods at 0.5 and 0.8, respectively, indicating that SO 4 2− existed mainly in smaller particles. Decreases in NO 3 − (1/2.5) and increases in NO 3 − (2.5/10) from clean to polluted conditions, similar to the variations in PM x distribution, suggest that NO 3 − played a role in the worsening of pollution. O 3 concentrations were higher in 2020 (108.6 μg m−3) than in 2019 (96.8 μg m−3). Marked decreases in fresh NO alleviated the titration of O 3. Furthermore, the oxidation reaction of NO 2 that produces NO 3 − was dominant over the photochemical reaction of NO 2 that produces O 3 , making NO 2 less important for O 3 pollution. In comparison, a lower VOC/NO x ratio (less than 10) meant that Beijing is a VOC-limited area; this indicates that in order to alleviate O 3 pollution in Beijing, emissions of VOCs should be controlled. [Display omitted] • Precursors were significantly reduced by COVID-19 lockdown in summer 2020. • Stronger secondary processes caused higher particle pollution in summer 2020. • Beijing is VOC-limited in O 3 pollution. Haze pollution in summer 2020 was more severe because of the stronger secondary processes despite the rapid decrease of precursors caused by the COVID-19 lockdown. [ABSTRACT FROM AUTHOR]