Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate.

Nitrate (NO3-) has become recognized as the most important water-soluble ion in fine particulate (PM2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO3-concentration, δ18O-NO3- and δ15N-NO3- values in PM2.5 were analyzed. NO3- concentrations increased as PM2.5 levels increased during both polluted and non-polluted days over the entire year. δ18O-NO3- values during cold months (63.5-103‰) were higher than those during warm months (50.3-85.4‰), these results suggested that the nitrate formation pathways shifted from the NO2 + OH (POH) in warm months to the N2O5 + H2O (PN2O5) and NO3 + VOCs (PNO3) pathways in cold months. Especially during cold months, δ18O-NO3- values increased from 65.2-79.9‰ to 80.7-96.2‰ when PM2.5 increased from ∼25 to >100 µg/m3, but when PM2.5 > 100 µg/m3, there were relatively small variations in δ18O-NO3-. These results suggested that nitrate formation pathways changed from POH to PN2O5 and PNO3 pathways when PM2.5 < 100 µg/m3, but that PN2O5 and PNO3 dominated nitrate production when PM2.5 > 100 µg/m3. Higher δ15N-NO3- values in warm months (-11.8-13.8‰) than in cold months (-0.7-22.6‰) may be attributed to differences in NOx emission sources and nitrogen isotopic fractionation among NOx and NO3-. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM2.5 levels.

[Pollution Characteristics and Emission Factors of VOCs from Vehicle Emissions in the Tianjin Tunnel].

The pollution characteristics and emission factors (EFs) of the volatile organic compounds (VOCs) of vehicles were investigated using the tunnel test method on weekdays and weekends in the Wujinglu Tunnel in Tianjin, China. Gas samples in the tunnel were collected with 3.2 L stainless steel canisters and 99 VOCs species were analyzed by gas chromatography-mass spectrometry (GC-MS). The concentration levels, variation characteristics, and EFs of the VOCs were analyzed. The ozone formation potentials (OFPs) and secondary organic aerosol formation potentials (SOAFPs) of the VOCs in the tunnel were calculated. Moreover, a comparison of the study results with current literature was conducted. The total concentrations of VOCs at the inlet and midpoint are (190.85±51.15) µg·m-3 and (257.44±62.02) µg·m-3, respectively. The total EFs are (45.12±10.97) mg·(km·veh)-1 and the EFs for alkanes, alkenes, alkynes, aromatics, halocarbons, and oxygenated volatile organic compounds (OVOCs) are (22.79±7.15), (5.04±1.20), (0.78±0.34), (9.86±2.81), (0.26±0.17), and (6.25±2.27) mg·(km·veh)-1, respectively. They are notably smaller than the values obtained in a previous test in 2009. Isopentane, toluene, ethylene, methyl tert-butyl ether (MTBE), and ethane were the top five species among the VOC EFs. The ratios of methyl tert-butyl ether/benzene (MTBE/B) and methyl tert-butyl ether/toluene (MTBE/T) are 1.07 and 0.77, respectively. This implies that the contribution of evaporative emissions from vehicles to VOCs emissions cannot be ignored. The OFPs and SOAFPs in the tunnel are (145.50±37.85) and (43.87±12.75) mg·(km·veh)-1, respectively. Compared with the test in 2009, the OFPs and SOAFPs are 94.23% and 90.88% smaller, respectively. The sharp decrease of the OFPs and SOAFPs is closely related to stricter emission standards and the upgrade of oil products.

[Association Between Fine Particulate Matter and Asthma Hospital Outpatient Visits in Hangzhou].

To study the short-term effects of air pollution on asthma visits and differences in susceptibility to various groups of people, data for asthma visits from January 1, 2013 to December 31, 2015 were obtained from a Hangzhou hospital. Considering the nonlinear relationships among concentration of air pollutants, respiratory hospital outpatient visits and meteorological factors, Generalized Additive Models (GAM) and stratification analysis were used to explore the lag effects and differences in people stratifications. The natural cubic spline function was used for smoothing the average temperature, the average relative humidity and the long-term trend, using dummy variables to control the effect of the day of the week and of holidays. Correlation of PM2.5, NO2 and SO2 daily mean concentrations were significant (under 0.01) in Spearman correlation analysis, while the correlations of daily mean temperature and 3 pollutants were significantly negative. The lag effects of PM2.5 concentration on outpatient visits of asthma peaked at 3-5 days. The relative risk of asthma reached maximum at lag day 5 as 1.0056 (95% CI:1.0021-1.0091), with per 10 µg·m-3 increment of PM2.5 concentration. The relative risk of asthma outpatient visits of all groups of patients were statistically significant (P<0.05). The relative risk of asthma outpatient visits of males and young and middle-aged group were statistically significant at lag days 3-5, and for females and the elderly, were statistically significant at lag day 5. With the introduction of the effects of NO2, the relative risk of asthma outpatient visits increased at lag 5 day in co-pollutant models. The authors concluded that the increase of PM2.5 may be related to the increase of asthma hospital outpatient visits within 3-5 days in Hangzhou, and the effects on male group and elderly group were more definite.

[Characteristics of Particulate Matter and Carbonaceous Species in Ambient Air at Different Air Quality Levels].

This study describes the characteristics of particulate matter and carbonaceous species at different air quality levels. The concentrations of PM10, PM2.5, PM1, and carbonaceous species in PM2.5 were monitored on-line in Langfang City on March 1-22, 2016. The PM10, PM2.5, and PM1 concentrations were 204.1 µg·m-3, 107.9 µg·m-3, and 87.8 µg·m-3, respectively. Diurnal variations in particulate matter concentrations showed a bimodal distribution. In general, the mass concentrations of particulate matter and carbonaceous species (OC, EC, SOC, and POC) and the ratios of PM1/PM10 and PM2.5/PM10 were lower on better air quality periods. However, the mass concentration of PM10 was the highest on moderately polluted times. The ratios of PM1/PM10 and PM2.5/PM10 reached minimum values on moderately polluted times.The mass concentration of OC was slightly lower in moderately polluted periods than slightly polluted times; it was significantly lower in moderately polluted periods compared to severely polluted time periods. Hourly concentrations of OC and EC were lower between the hours of 13:00 and 23:00 compared to slightly polluted and severely polluted periods. The proportion of PM2.5 and PM1 decreased in moderately polluted time periods, consistent with the corresponding primary pollutants. Besides, the value of OC/EC was larger than 2.0. The concentrations of SOC and POC estimated using the minimum OC/EC ratio were 12.2 µg·m-3 and 5.0 µg·m-3, respectively.