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Abstract. Nitrate aerosol makes up a significant fraction of fine particles and plays a key role in regional air quality and climate. The North China Plain (NCP) is one of the most industrialized and polluted regions in China. To obtain a holistic understanding of the nitrate pollution and its formation mechanisms over the NCP region, intensive field observations were conducted at three sites during summertime in 2014–2015. The measurement sites include an urban site in downtown Jinan – the capital city of Shandong Province –, a rural site downwind of Jinan city, and a remote mountain site at Mt. Tai (1534 m a.s.l.). Elevated nitrate concentrations were observed at all three sites despite distinct temporal and spatial variations. Using historical observations, the nitrate ∕ PM2.5 and nitrate ∕ sulfate ratios have statistically significantly increased in Jinan (2005–2015) and at Mt. Tai (from 2007 to 2014), indicating the worsening situation of regional nitrate pollution. A multiphase chemical box model (RACM–CAPRAM) was deployed and constrained by observations to elucidate the nitrate formation mechanisms. The principal formation route is the partitioning of gaseous HNO3 to the aerosol phase during the day, whilst the nocturnal nitrate formation is dominated by the heterogeneous hydrolysis of N2O5. The daytime nitrate production in the NCP region is mainly limited by the availability of NO2 and to a lesser extent by O3 and NH3. In comparison, the nighttime formation is controlled by both NO2 and O3. The presence of NH3 contributes to the formation of nitrate aerosol during the day, while there is slightly decreasing nitrate formation at night. Our analyses suggest that controlling NOx and O3 is an efficient way, at the moment, to mitigate nitrate pollution in the NCP region, where NH3 is usually in excess in summer. This study provides observational evidence of a rising trend of nitrate aerosol as well as scientific support for formulating effective control strategies for regional haze in China.

The wide use and continuous abrasion of rubber-related products appears to be leading to an incredible release of p-phenylenediamine (PPD) antioxidants in the environment. However, no related research has been conducted on the pollution characteristics and potential health risks of PM 2.5 -bound PPDs. We report for the first time the ubiquitous distributions of six emerging PPDs and a quinone derivative, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine quinone (6PPDQ), in PM 2.5 from urban areas of China. Atmospheric contamination levels of PM 2.5 -bound PPDs were found to be mostly in pg m −3 amounts between 2018 and 2019. Urban vehicle rubber tire abrasion was found to probably contribute to the PPDs in PM 2.5 and accounted for their significant spatiotemporaldependent concentration variations. Furthermore, 6PPDQ, an emerging oxidation product of 6PPD in the environment, was first quantified (pg m −3 ) with a total detection rate of 81% in the urban PM 2.5 , demonstrating its broad existence. On the basis of the determined ambient concentrations, the annual intakes of PPDs and 6PPDQ for adults were not low, indicating their possible human health risks induced by long-term exposure. This study confirms the widespread occurrence of PPDs and 6PPDQ in PM 2.5 , showing that the pollution of such compounds in urban air should not be underestimated.

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Abstract. The chemical composition of 39 cloud samples and droplet size distributions in 24 cloud events were investigated at the summit of Mt. Tai from July to October 2014. Inorganic ions, organic acids, metals, HCHO, H 2 O 2 , sulfur(IV), organic carbon, and elemental carbon as well as pH and electrical conductivity were analyzed. The acidity of the cloud water significantly decreased from a reported value of pH 3.86 during (Guo et al., 2012 to pH 5.87 in the present study. The concentrations of nitrate and ammonium were both increased since 2007-2008, but the overcompensation of ammonium led to an increase in the mean pH value. The microphysical properties showed that cloud droplets were smaller than 26.0 µm and most were in the range of 6.0-9.0 µm at Mt. Tai. The maximum droplet number concentration (N d ) was associated with a droplet size of 7.0 µm. High liquid water content (LWC) values could facilitate the formation of larger cloud droplets and broadened the droplet size distribution. Cloud droplets exhibited a strong interaction with atmospheric aerosols. Higher PM 2.5 levels resulted in higher concentrations of water-soluble ions and smaller sizes with increased numbers of cloud droplets. The lower pH values were likely to occur at higher PM 2.5 concentrations. Clouds were an important sink for soluble materials in the atmosphere. The dilution effect of cloud water should be considered when estimating concentrations of soluble components in the cloud phase.

In January 2014, severe haze episodes which sweep across Chinese cities have attracted public concern and interest at home and abroad. In addition to the physicochemical properties of air pollutants, bacteria are thought to be responsible for the spread of respiratory diseases and various allergies. We attempted the bacterial characterization of submicron particles (PM, PM, and PM) under severe haze episodes using high-throughput sequencing and real-time quantitative PCR detecting system based on 21 samples collected from January to March 2014 at Ji'nan, China. The high bacterial concentration in PM (7314cells m), PM (7212cells m), and PM (6982cells m) showed significant negative correlations with SO, NO and O. Under sufficient sequencing depth, 37 phyla, 71 classes, 137 orders, 236 families, and 378 genera were classified, and the bacterial community structure varied significantly in different size fractions. For example, Holophagaceae (Acidobacteria) in PM showed 6-fold higher abundance than that in PM. Moreover, functional categories and bacterial species (Lactococcus piscium, Pseudomonas fragi, Streptococcus agalactiae, and Pseudomonas cichorii) that may potentially be responsible for infections and allergies were also discovered. Source track analysis showed that the ambient bacteria mainly originated from soils, leaf surfaces, and feces. Our results highlighted the importance of airborne microbial communities by understanding the concentration, structure, ecological and health effects, especially those in submicron particles during haze episodes.

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