Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

Aerosol liquid water (ALW) is ubiquitous in ambient aerosol and plays an important role in the formation of both aerosol organics and inorganics. To investigate the interactions between ALW and aerosol organics during haze formation and evolution, ALW was modelled based on long-term measurement of submicron aerosol composition in different seasons in Beijing. ALW contributed by aerosol inorganics (ALW(inorg)) was modelled by ISORROPIA II, and ALW contributed by organics (ALW(org)) was estimated with kappa-Kohler theory, where the real-time hygroscopicity parameter of the organics (kappa(org)) was calculated from the real-time organic oxygen-to-carbon ratio (O/C). Overall particle hygroscopicity (kappa(total)) was computed by weighting component hygroscopicity parameters based on their volume fractions in the mixture. We found that ALW(org), which is often neglected in traditional ALW modelling, contributes a significant fraction (18 %-32 %) to the total ALW in Beijing. The ALW(org) fraction is largest on the cleanest days when both the organic fraction and kappa(org) are relatively high. The large variation in O/C, from 0.2 to 1.3, indicates the wide variety of organic components. This emphasizes the necessity of using real-time kappa(org), instead of fixed kappa(org), to calculate ALW(org) in Beijing. The significant variation in K org (calculated from O/C), together with highly variable organic or inorganic volume fractions, leads to a wide range of kappa(total) (between 0.20 and 0.45), which has a great impact on water uptake. The variation in organic O/C, or derived K org , was found to be influenced by temperature (T), ALW, and aerosol mass concentrations, among which T and ALW both have promoting effects on O/C. During high-ALW haze episodes, although the organic fraction decreases rapidly, O/C and derived K org increase with the increase in ALW, suggesting the formation of more soluble organics via heterogeneous uptake or aqueous processes. A positive feedback loop is thus formed: during high-ALW episodes, increasing kappa(org), together with decreasing particle organic fraction (or increasing particle inorganic fraction), increases kappa(total), and thus further promotes the ability of particles to uptake water.Peer reviewe

Variations of O3O_3 and CO in Summertime at a Rural Site Near Beijing

Large intra-season differences in mixing ratios of CO and $O_3$ were detected at Miyun, a rural site north of Beijing, in summer 2006. Despite an increase in mean daytime mixing ratio of CO from 500 ppbv in June to 700 ppbv in July, mean daytime $O_3$ dropped from 67 ppbv in June to 50 ppbv in July and August. The observed changes in CO and $O_3$ are attributed to the influence of the summer monsoonal circulation that develops over the North China Plain in July. Photochemical production of $O_3$ is reduced as a consequence of increased cloudiness during July and August, as indicated by the strong negative correlation observed between $O_3$ and satellite observations of cloud optical depth, with cloudiness having little effect on CO. The analysis suggests a strategy for emission controls that could be implemented in an economically efficient manner to minimize the frequency of high levels of $O_3$ during summer in Beijing.Engineering and Applied Science

Ozone Air Quality During the 2008 Beijing Olympics: Effectiveness of Emission Restrictions

A series of aggressive measures was launched by the Chinese government to reduce pollutant emissions from Beijing and surrounding areas during the Olympic Games. Observations at Miyun, a rural site 100 km downwind of the Beijing urban center, show significant decreases in concentrations of O3, CO, NOy, and SO2 during August 2008, relative to August 2006–2007. The mean daytime mixing ratio of O3 was lower by about 15 ppbv, reduced to 50 ppbv, in August 2008. The relative reductions in daytime SO2, CO, and NOy were 61%, 25%, and 21%, respectively. Changes in SO2 and in species correlations from 2007 to 2008 indicate that emissions of SO2, CO, and NOx were reduced at least by 60%, 32%, and 36%, respectively, during the Olympics. Analysis of meteorological conditions and interpretation of observations using a chemical transport model suggest that although the day-to-day variability in ozone is driven mostly by meteorology, the reduction in emissions of ozone precursors associated with the Olympic Games had a significant contribution to the observed decrease in O3 during August 2008, accounting for 80% of the O3 reduction for the month as a whole and 45% during the Olympics Period (8–24 August). The model predicts that emission restrictions such as those implemented during the Olympics can affect O3 far beyond the Beijing urban area, resulting in reductions in boundary layer O3 of 2–10 ppbv over a large region of the North China Plain and Northeastern China.Engineering and Applied Science

Characterization of a high-resolution supercritical differential mobility analyzer at reduced flow rates

Classifying sub-3 nm particles effectively with relatively high penetration efficiencies and sizing resolutions is important for atmospheric new particle formation studies. A high-resolution supercritical differential mobility analyzer (half-mini DMA) was recently improved to classify aerosols at a sheath flow rate less than 100 L/min. In this study, we characterized the transfer functions, the penetration efficiencies, and the sizing resolution of the new half-mini DMA at the aerosol flow rate of 2.5-10 L/min and the sheath flow rate of 25-250 L/min using tetra-alkyl ammonium ions and tungsten oxide particles. The transfer functions of the new half-mini DMA at an aerosol flow rate lower than 5 L/min and a sheath flow rate lower than 150 L/min agree well with predictions using a theoretical diffusing transfer function. The penetration efficiencies can be approximated using an empirical formula. When classifying 1.48 nm molecular ions at an aerosol-to-sheath flow ratio of 5/50 L/min, the penetration efficiency, the sizing resolution, and the multiplicative broadening factor of the new half-mini DMA are 0.18, 6.8, and 1.11, respectively. Compared to other sub-3 nm DMAs applied in atmospheric measurements (e.g. the mini-cyDMA, the TSI DMA 3086, the TSI nanoDMA 3085, and the Grimm S-DMA), the new half-mini DMA characterized in this study is able to classify particles at higher aerosol and sheath flow rates, leading to a higher sizing resolution at the same aerosol-to-sheath flow ratio. Accordingly, the new half-mini DMA can reduce the uncertainties in atmospheric new particle formation measurement if coupled with an aerosol detector that could work at the corresponding high aerosol flow rate. (c) 2018 American Association for Aerosol ResearchPeer reviewe