Characteristics and formation of typical winter haze in Handan, one of the most polluted cities in China.

Handan, a city within the North China Plain (NCP) region, is a typical city influenced by regional particulate matter (PM) pollution. One-year hourly semi-continuous observation was carried out in 2015 in Handan with the aim of identifying the chemical composition and variations in PM2.5. Moreover, the concentration of aerosol precursors, meteorological factors, and secondary transformations are considered. The results demonstrate that the annual average PM2.5 concentration in Handan is 122.35µgm-3, approximately 3.5 times higher than the Chinese National Ambient Air Quality Standard (NAAQS) (35µgm-3), and only 12days were below the guideline. As expected, PM concentrations are highest in winter, especially in December. In addition, we measure the concentrations of five species commonly found in PM, nitrate, sulfate, ammonium, inorganic carbon, and organic carbon. Of these, nitrate and sulfate account for the largest proportion of PM2.5; during periods when the PM2.5 concentration was below 400µgm-3, nitrate dominates, while above this concentration, sulfate dominate. This is likely related to the nitrogen and sulfur oxidation ratios, which are in turn, especially the sulfur oxidation ratio, driven by high relative humidity (>60%). In addition, haze events are driven by other meteorological conditions, wind speed and direction, where low wind speeds from the south and southwest enable pollutant accumulation, which are infrequently interspersed with brief periods with high wind speeds that promote pollutant dispersal. Even though Handan is among the ten most polluted cities in China with regard to air pollution, few studies beyond model simulations have analyzed air pollutant concentrations in this city. Therefore, this study makes a significant contribution to understanding air pollution in Handan, which can further be used to improve our understanding of regional pollution in the highly populated North China Plain. These results have implications for the creation of policies and legislation, as well as other pollution control measures.

High molecular weight organic compounds (HMW-OCs) in severe winter haze: Direct observation and insights on the formation mechanism.

High molecular weight organic compounds (HMW-OCs), formed as secondary organic aerosols (SOA), have been reported in many laboratory studies. However, little evidence of HMW-OCs formation, in particular during winter season in the real atmosphere, has been reported. In January 2013, Beijing faced historically severe haze pollution, in which the hourly PM2.5 concentration reached as high as 974 µg m-3. Four typical haze events (HE1 to HE4) were identified, and HE2 (Jan. 9-16) was the most serious of these. Based on the hourly observed chemical composition of PM2.5 and the daily organic composition analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), we found that abundant ion peaks in m/z 200-850 appeared on heavy haze days, whereas these were negligible on a clear day, indicating the existence of HMW-OCs in the wintertime haze. A negative nonlinear correlation between HMW-OCs and O3 suggested that gas oxidation was not likely to be the dominant mechanism for HMW-OCs formation. During the heavy haze events, the relative humidity and mass ratio of H2O/PM2.5 reached as high as 80% and 0.2, respectively. The high water content and its good positive correlation with HMW-OCs indicated that an aqueous-phase process may be a significant pathway in wintertime. The evidence that acidity was much higher during HE2 (0.37 µg m-3) than on other days, as well as its strong correlation with HMW-OCs, indicated that acid-catalyzed reactions likely resulted in HMW-OCs formation during the heavy winter haze in Beijing.

Measurement of semivolatile carbonaceous aerosols and its implications: a review.

Measurement of carbonaceous aerosols is complicated by positive and negative artifacts. An organic denuder with high efficiency for removing gaseous organics is an effective approach to eliminate the positive artifact, and it is a precondition for the accurate determination of SVOC by an adsorbent backup filter. Evaluations of different configurations of the organic denuder, and SVOC determined by different denuder-based samplers, both integrated and semi-continuous, are reviewed. A new equation for determination of the denuder efficiency is estimated, considering the efficiency of removing both the gaseous organics that could be adsorbed by the quartz and the gaseous passing through the quartz that could be subsequently adsorbed by the backup adsorbent filter. The origin of OC on the backup quartz filter, behind either quartz or Teflon filter, is quantitatively evaluated by the denuder-based method based on the data published. The backup-OC is shown to be dominated by either gaseous organics passing through the front filter or the evaporated particulate organic carbon depending on the sampling environment.

Concentration and chemical characteristics of PM2.5 in Beijing, China: 2001-2002.

Weekly PM2.5 samples were simultaneously collected at a semi-residential (Tsinghua University) and a downtown (Chegongzhuang) site in Beijing from August 2001 through September 2002. The ambient mass concentration and chemical composition of PM2.5 were determined. Analyses including elemental composition, water-soluble ions, and organic and elemental carbon were performed. The annual average concentrations of PM2.5 were 96.5 microg m(-3) and 106.9 microg m(-3) at CGZ and HU site, respectively. More than 80% of the PM2.5 mass concentrations were explained by carbonaceous species, secondary particles, crustal matters and trace elements at the two sites. Carbonaceous species were the most abundant components, constituting about 45% and 48% of the total PM2.5 mass concentrations at CGZ and THU site, respectively. SO4(2-), NO3- and NH4+ were three major ions, accounting for 37%, 23% and 20%, respectively, of the total mass of inorganic water-soluble ions.

Atmospheric aerosol concentration level and chemical characteristics of water-soluble ionic species in wintertime in Beijing, China.

Total suspended particulate (TSP) samples were collected during wintertime from November 24, 1998 to February 12, 1999 in Beijing. Ionic species including Cl-, NO3(-), SO4(2-), Na+, NH4(+), K+, Mg2+ and Ca2+ were determined by Ion Chromatography (IC). The sum average concentration of all the determined ions accounted for 18.9% of the TSP concentration, and SO4(2-) appeared the dominant ion with an average concentration of 30.84 microg m(-3); the sum mass concentration of SO4(2-), NO3(-), Ca2+ and NH4(+) accounted for about 83.2% of all the eight ions measured. The study indicated that the chemical form of sulfate and ammonium varies with TSP concentration levels. During heavy pollution periods, the average TSP concentration was 0.66 mg m(-3), and the NH4(+)/SO4(2-) molar ratio was low (0.58). It indicated that sulfate may present as CaSO4 and (NH4)2SO4 x CaSO4 x 2H2O. When TSP concentration (average 0.186 mg m(-3)) was relatively low, the NH4(+)/SO4(2-) molar ratio was 1.94, close to the theoretical ratio of 2 of (NH4)2SO4. Under this condition (NH4)2SO4 is expected to exist as the major form of sulfate. When the TSP concentration level was medium (average 0.35 mg m(-3)), the NH4+/SO4(2-) molar ratio appeared an average value (1.27), (NH4)2SO4, (NH4)2SO4 x CaSO4 x 2H2O and CaSO4 are expected to be present in those aerosol particles. Meteorological conditions including wind speed and wind direction were related to the TSP concentration level.