Impacts of the aerosol mixing state and new particle formation on CCN in summer at the summit of Mount Tai (1534m) in Central East China.

In this study, the aerosol size distributions, cloud condensation nuclei (CCN) number concentration (NCCN), single-particle chemical composition and meteorological data were collected from May 12 to June 8, 2017, at the summit of Mt. Tai. The effects of new particle formation (NPF) events and aerosol chemical components on CCN at Mt. Tai were analyzed in detail. The results showed that, NPF events significantly enhanced the CCN population, and the enhancement effect increased with increasing supersaturation (SS) value at Mt.Tai. NCCN at SS ranging from 0.1 to 0.9 % on NPF days was 10.9 %, 36.5 %, 44.6 %, 53.5 % and 51.5 % higher than that on non-NPF days from 10:00-13:00 as NPF events progressed. The effect of chemical components on CCN activation under the influence of NPF events was greater than that in the absence of NPF events. The correlation coefficients of EC-Nitrate particles (EC-Sulfate particles) and CCN at all SS levels on NPF days were 1.31-1.59 times (1.17-1.35 times) higher than those on non-NPF days. Nitrate particles promoted CCN activation but sulfate particles inhibited activation at Mt. Tai. There are differences or even opposite effects of the same group of particles on CCN activation under the influence of NPF events in different air masses. EC-Sulfate particles inhibited CCN activation at all SS levels for type I but weakly promoted activation at lower SS ranging from 0.1 to 0.3 % and weakly inhibited it at higher 0.9 % SS for type II. OCEC particles significantly inhibited CCN activation for type II, and this effect decreased with increasing SS. OCEC particles only weakly inhibited activation at SS ranging from 0.5 to 0.7 % for type I. OCEC particles only weakly inhibited this process at 0.1 % SS, while they very weakly promoted activation for SS > 0.1 %. This reveals that the CCN activity is not only related to the chemical composition of the particles, but the mixing state also has an important effect on the CCN activity.

Assessment of factors affecting the diurnal variations of atmospheric PAHs based on a numerical simulation.

Atmospheric polycyclic aromatic hydrocarbons (PAHs) are a type of organic pollutants that seriously endanger human health. Obtaining the diurnal variations of PAHs and clarifying their impact mechanisms are significant for the government to formulate targeted prevention and control measures. However, the influencing factors that dominate the diurnal variations of common PAHs are currently unclear. In order to solve this problem, 16 PAHs selected by the United States Environmental Protection Agency (EPA) as priority-controlled pollutants were simulated with high resolution. The simulation results were validated based on diurnal observations in the vertical direction. Although the model underestimated the particle-phase concentrations of most components, it captured their diurnal variations fairly well. In addition, we assessed the factors affecting the diurnal variations of PAHs with sensitivity tests, including chemical reactions and atmospheric diffusion. The results showed that the transforming ratios of PAHs by oxidants were higher during the day than that at night due to the dominant reactions with OH radical. Atmospheric dispersion affected the vertical distribution of PAHs, which resulted in higher day/night ratios at high altitudes than near the ground. We also compared the strength of atmospheric diffusion and chemical reaction on the diurnal trends of PAHs. Near the ground, atmospheric diffusion was the most dominant factor in determining their diurnal trends. At high altitudes, their diurnal trends were determined by a combination of atmospheric diffusion and chemical reactions. These findings can provide a comprehensive understanding of the diurnal variations of common PAHs, which are informative for the prevention and control of PAHs pollution.

Concentration and atmospheric transport of PM2.5-bound polycyclic aromatic hydrocarbons at Mount Tai, China.

Atmospheric PM2.5-bound polycyclic aromatic hydrocarbons (PAHs) pose a major threat to human health. At present, studies on PAHs in the atmosphere have mostly focused on their concentration levels and source apportionment, whereas studies on the vertical transport of PAHs in the atmosphere are limited. However, the vertical transport of PAHs is important for their diffusion near the ground and their long-range transport at higher altitude. In this study, PM2.5 samples were collected simultaneously at the summit and foot of Mount Tai (MTsummit and MTfoot, respectively) from May to June 2017, and the concentrations of 18 PAHs in the samples were determined. The total concentration of PAHs at MTsummit was 2.406 ng m-3, which was well below the pollution levels of domestic cities, whereas that at MTfoot was as high as 9.068 ng m-3, which was within the range of pollution levels in domestic cities. The total carcinogenic risk for both MTsummit and MTfoot was within the potential risk range. Given the source of PAHs and the diurnal variation of the planetary boundary layer, the PAHs showed opposite diurnal trends at MTsummit and MTfoot. Vertical transport was an important source of daytime PAHs at MTsummit, and the vertical transport efficiency of PAHs decreased with an increasing ring number; this may be due to the combined effects of gas-particle partitioning and chemical reactions. Furthermore, PAHs originating in the surrounding high-emission provinces can affect the Mount Tai area via atmospheric trans-regional transport, and the BaP/BeP ratio is a useful indicator of the transport distance of PAHs.

Characteristics and sources of WSI in North China Plain: A simultaneous measurement at the summit and foot of Mount Tai.

To better understand the characteristics and sources of water soluble ions (WSI) in North China Plain (NCP), fine particles (PM2.5) were simultaneously sampled at the summit (SM) and foot (FT) of Mount Tai during May 12th to June 24th, 2017. Ion chromatography analysis showed that concentration of WSI was lower at SM (22.26 ± 16.53 µg/m3) than that at FT (31.02 ± 21.92 µg/m3). The concentration and proportion of SO42- in total WSI were both lower than the values reported in previous studies. Daytime WSI concentrations were higher than that at nighttime at SM, while the opposite results were obtained at FT, possibly associated with more anthropogenic activities and higher boundary layer height (BLH) during daytimes. A severe pollution event occurred during June 14th - June 16th was documented at both FT and SM. Regional transport and topography-forced vertical transport along the slope of the mountain could explain the higher concentrations of pollutants at SM. The analyses also indicated that NH4+ existed mainly in the form of NH4HSO4 and NH4NO3, but (NH4)2SO4 could also exist, especially when emissions of NH4+ and NH3 were increased during daytime at FT. The results of principal component analysis (PCA) illustrated that secondary aerosols, coal/biomass burnings, sea-salts and crustal/soil dusts were the main sources at SM, and secondary aerosols and crustal/soil dusts contributed most at FT. Backward air-mass trajectories were classified into four clusters, of which air masses with the highest frequency and WSI concentrations were originated from the southwest with secondary ions (SO42-, NO3- and NH4+) as major pollutants.