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Elemental concentrations of ambient aerosols are commonly sampled over 12–24 h, and the low time resolution puts a great limit on current understanding about the temporal variations and source apportionment based on receptor models. In this work, hourly-resolved concentrations of eighteen elements in PM 2.5 at an urban site in Nanjing, a megacity in Yangtze River Delta of east China, were obtained by using a Xact 625 ambient metals monitor from 12/12/2016 to 12/31/2017. The influence of traffic activities was clearly reflected by the spikes of crustal elements (e.g., Fe, Ca, and Si) in the morning rush hour, and the firework burning and sandstorm events during the sampling periods were tracked by sharp enrichment of Ba, K and Fe, Ca, Si, Ti in PM 2.5 , respectively. To evaluate the advantage of hourly-resolved elements data in identifying impacts from specific emission sources, positive matrix factorization (PMF) analysis was performed with the 1-h data set (PMF 1-h) and 23-h averaged data (PMF 23-h), respectively. The 4- and 6-factor PMF 23-h solutions had similar factor profiles and consistent factor contributions as the corresponding PMF 1-h solutions. However, due to the limit in inter-sample variability, PMF analysis with 23-h average data misclassified some major (e.g., K, Fe, Zn, Ca, and Si) and trace (e.g., Pb) elements in factor profiles, resulting in different absolute factor contributions between PMF 23-h and PMF 1-h solutions. These results suggested the use of high time-resolved data to obtain valid and robust source apportionment results. Image 1 • PM 2.5 elements were measured with 1-h resolution at an urban site in Nanjing city. • Specific local emissions were reflected by the diurnal variations of PM 2.5 elements. • Two pollution episodes were tracked by sharp enrichment of distinct elements. • The 1-h measurements data were preferred over 23-h averages for source apportionment. This study suggests using high time-resolved measurements of PM 2.5 elements for receptor-based source apportionment to obtain robust results. [ABSTRACT FROM AUTHOR]

Abstract: The Denver Aerosol Sources and Health (DASH) study was designed to evaluate associations between PM2.5 species and sources and adverse human health effects. The DASH study generated a five-year (2003–2007) time series of daily speciated PM2.5 concentration measurements from a single, special-purpose monitoring site in Denver, CO. To evaluate the ability of this site to adequately represent the short term temporal variability of PM2.5 concentrations in the five county Denver metropolitan area, a one year supplemental set of PM2.5 samples was collected every sixth day at the original DASH monitoring site and concurrently at three additional sites. Two of the four sites, including the original DASH site, were located in residential areas at least 1.9 km from interstate highways. The other two sites were located within 0.3 km of interstate highways. Concentrations of elemental carbon (EC), organic carbon (OC), and 58 organic molecular markers were measured at each site. To assess spatial variability, site pairs were compared using the Pearson correlation coefficient (r) and coefficient of divergence (COD), a statistic that provides information on the degree of uniformity between monitoring sites. Bi-weekly co-located samples collected from July 2004 to September 2005 were also analyzed and used to estimate the uncertainty associated with sampling and analytical measurement for each species. In general, the two near-highway sites exhibited higher concentrations of EC, OC, polycyclic aromatic hydrocarbons (PAHs), and steranes than did the more residential sites. Lower spatial heterogeneity based on r and COD was inferred for all carbonaceous species after considering their divergence and lack of perfect correlations in co-located samples. Ratio–ratio plots combined with available gasoline- and diesel-powered motor vehicle emissions profiles for the region suggested a greater impact to high molecular weight (HMW) PAHs from diesel-powered vehicles at the near-highway sites and a more uniformly distributed impact to ambient hopanes from gasoline-powered motor vehicles at all four sites. [Copyright &y& Elsevier]

The emergence of the novel corona virus and the resulting lockdowns over various parts of the world have substantially impacted air quality due to reduced anthropogenic activity. The objective of this study is to investigate the impact of COVID-19 lockdown and Spring Festival on air quality of four major cities of Yangtze River Delta (YRD) region, including Shanghai, Nanjing, Hefei, and Hangzhou. In situ measurements were taken for nitrogen dioxide (NO2), particulate matter (PM2.5) and ozone (O3). In situ measurements from 1 January to 25 April were taken two years prior to COVID-19 (2018–19), during COVID-19 lockdown (2020), and one year after the COVID-19 (2021). The results indicated that the concentration of NO2 and PM2.5 dropped considerably during the lockdown days compared to normal days while the O3 concentration showed an upsurge. The NO2 showed reduction of about 54% on average during lockdown level 1 in 2020 whereas, PM 2.5 showed reduction of about 36% through the YRD. A substantial drop was observed in concentration of NO2 during the Spring Festival holidays throughout the YRD from 2019 to 2021. [ABSTRACT FROM AUTHOR]

In this study, collocated samples of particulate matter with aerodynamic diameter less than 2.5 μm (PM 2.5) were collected every sixth day on quartz filters (Q f) at a suburban site in northern Nanjing, China for one year. A backup quartz filter (Q b) was installed behind Q f to estimate positive artifacts. The analysis of gravimetric mass, water-soluble inorganic ions (WSIIs), total organic (OC) and elemental carbon (EC), water-soluble OC (WSOC), total nitrogen (WSTN) and organic nitrogen (WSON) were performed on both Q f and Q b. Due to the high mass loadings on Q f (17.9 ± 7.82 mg filter−1, 4.71–38.5 mg filter−1), the collocated precision of gravimetric mass and dominant species (NH 4 +, NO 3 −, SO 4 2−, OC, and EC) is better than that from previous work. Except K+, EC, and WSON, all other target components were detected on Q b with average Q b /Q f ratios ranging from 3.02 ± 3.48–21.7 ± 22.4%. The final concentrations and uncertainties of PM 2.5 components were determined based on duplicate Q f –Q b data. The results suggest that using an error fraction of 10% will underestimate the uncertainty of less concentrated species (e.g., Ca2+ and Mg2+) in PM 2.5. Due to the evaporation loss of semi-volatile materials from the Q f , the Q f –Q b calculation would lead to an estimate of the lower limit for particulate NH 4 +, NO 3 −, and OC. Synchronized hourly data of PM 2.5 mass and components were obtained at downtown Nanjing. The comparisons of gravimetric versus reconstructed PM 2.5 and filter-based versus continuous measurements of PM 2.5 components indicate that a substantial fraction of the unexplained gravimetric PM 2.5 can be attributed to aerosol water. Image 1 • Collocated precision of PM 2.5 components depends largely on filter loadings. • Besides OC adsorption, quartz filter sampling is also subject to surface reactions. • Assuming a constant error fraction of 10% is appropriate for PM 2.5 major components. • Water content contributes a substantial fraction of the unexplained gravimetric PM 2.5. [ABSTRACT FROM AUTHOR]

In this study, hourly concentrations of PM 2.5 water-soluble inorganic ions, bulk organic carbon (OC), and elemental carbon (EC) were monitored from 1/1/2017 to 12/31/2017 and validated using filter-based offline analysis at an urban site in Nanjing, China. Compared with 2013 or before, the annual average of PM 2.5 concentration (36.5 ± 32.9 μg m−3) in 2017 decreased by more than 40%, NO 3 − (12.8 ± 11.4 μg m−3) became the most abundant water-soluble ion instead of SO 4 2− (9.29 ± 6.07 μg m−3), and the relative contribution of OC (5.92 ± 3.40 μg m−3) and EC (2.95 ± 1.53 μg m−3) to bulk PM 2.5 (24.9 ± 9.31%) increased substantially, indicating the effectiveness of the control policy for reducing gaseous precursor emissions. Based on the diurnal variations of water-soluble ions and gaseous pollutants, NH 4 +, SO 4 2−, and NO 3 − were secondarily formed and NH 4 NO 3 dominated the composition of ammonium salts in PM 2.5. The diurnal changes of OC, EC, and OC/EC ratios reflected prominent influences from local traffic patterns. Positive matrix factorization was performed using hourly data of PM 2.5 components (PMF 1-h), of which the results were justified by comparing to those using 23-h averaged data (PMF 23-h). Given that the secondary ion formation was still the dominant source (68.2%) of PM 2.5 , and the average PM 2.5 concentration in urban Nanjing remained higher than Tier II limit (35 μg m−3) of the Chinese National Ambient Air Quality Standard, controlling emissions of PM 2.5 precursor gases should be continued after the completion of Air Pollution Prevention and Control Action Plan in 2017. Image 1 • NH 4 NO 3 dominated the composition of ammonium salts in PM 2.5 in urban Nanjing. • Diurnal variations of OC and EC reflected influences from local traffic patterns. • Using hourly measurement data for PMF analysis can achieve more robust results. • Gaseous precursors should be continuously controlled to attain air quality standards. [ABSTRACT FROM AUTHOR]