Xiaole Pan

“APEC Blue”: Secondary Aerosol Reductions from Emission Controls in Beijing

China implemented strict emission control measures in Beijing and surrounding regions to ensure good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit. We conducted synchronous aerosol particle measurements with two aerosol mass spectrometers at different heights on a meteorological tower in urban Beijing to investigate the variations in particulate composition, sources and size distributions in response to emission controls. Our results show consistently large reductions in secondary inorganic aerosol (SIA) of 61–67% and 51–57%, and in secondary organic aerosol (SOA) of 55% and 37%, at 260 m and ground level, respectively, during the APEC summit. These changes were mainly caused by large reductions in accumulation mode particles and by suppression of the growth of SIA and SOA by a factor of 2–3, which led to blue sky days during APEC commonly referred to as “APEC Blue”. We propose a conceptual framework for the evolution of primary and secondary species and highlight the importance of regional atmospheric transport in the formation of severe pollution episodes in Beijing. Our results indicate that reducing the precursors of secondary aerosol over regional scales is crucial and effective in suppressing the formation of secondary particulates and mitigating PM pollution.

Emissions of nonmethane volatile organic compounds from open crop residue burning in the Yangtze River Delta region, China

Open crop residue burning is one of the major sources of air pollutants including the precursors of photooxidants like ozone and secondary organic aerosol. We made measurements of trace gases including nonmethane volatile organic compounds (NMVOCs) in a rural area in central East China in June 2010. During the campaign, we identified six biomass burning events in total through the simultaneous enhancement of carbon monoxide and acetonitrile. Four cases represented fresh plumes ( 3 h after emission), as determined by photochemical age. While we were not able to quantify formic acid, we identified an enhancement of major oxygenated volatile organic compounds (OVOCs) as well as low molecular alkanes and alkenes, and aromatic hydrocarbons in these plumes. The observed normalized excess mixing ratios (NEMRs) of OVOCs and alkenes showed dependence on air mass age, even in fresh smoke plumes, supporting the view that these species are rapidly produced and destructed, respectively, during plume evolution. Based on the NEMR data in the fresh plumes, we calculated the emission factors (EFs) of individual NMVOC. The comparison to previous reports suggests that the EFs of formaldehyde and acetic acid have been overestimated, while those of alkenes have been underestimated. Finally, we suggest that open burning of wheat residue in China releases about 0.34 Tg NMVOCs annually. If we applied the same EFs to all crops, the annual NMVOC emissions would be 2.33 Tg. The EFs of speciated NMVOCs can be used to improve the existing inventories.

Observation of the simultaneous transport of Asian mineral dust aerosols with anthropogenic pollutants using a POPC during a long‐lasting dust event in late spring 2014

We observed a long-lasting dust event from 25 May to 2 June 2014, using a polarization optical particle counter (POPC). The transport of dust plumes over East Asia was verified on the basis of observations of Moderate Resolution Imaging Spectroradiometer, a lidar network, and surface synoptic observation stations. Mixing of dust and anthropogenic pollutants was investigated according to the variation in the depolarization ratio as a function of particle size. The nonsphericity of dust particles varied due to the impact of anthropogenic pollutants on their pathway. In the coarse mode, dust particles always had a clear nonspherical configuration, although large amounts of nitrate were also present. Supermicron particles are occasionally present in a spherical configuration, possibly due to the complex mixing of natural dust and anthropogenic particles. Statistically, ~64% of the total nitrate mass was deemed to be transported from outside of Japan due to a trapping effect in the dust plume.

Laboratory measurements of emission factors of nonmethane volatile organic compounds from burning of Chinese crop residues

The emission factors (EFs) of nonmethane volatile organic compounds (NMVOCs) emitted during the burning of Chinese crop residue were investigated as a function of modified combustion efficiency in laboratory experiments. NMVOCs, including acetonitrile, aldehydes/ketones, furan, and aromatic hydrocarbons, were monitored by proton-transfer-reaction mass spectrometry. Rape plant was burned in dry conditions and wheat straw was burned in both wet and dry conditions to simulate the possible burning of damp crop residue in regions of high temperature and humidity. We compared the present data to field data reported by Kudo et al. (2014). Good agreement between field and laboratory data was obtained for aromatics under relatively more smoldering combustion of dry samples, but laboratory data were slightly overestimated compared to field data for oxygenated VOC (OVOC). When EFs from the burning of wet samples were investigated, the consistency between the field and laboratory data for OVOCs was stronger than for dry samples. This may be caused by residual moisture in crop residue that has been stockpiled in humid regions. Comparison of the wet laboratory data with field data suggests that Kudo et al. (2014) observed the biomass burning plumes under relatively more smoldering conditions in which approximately a few tens of percentages of burned fuel materials were wet.

Real-time observational evidence of changing Asian dust morphology with the mixing of heavy anthropogenic pollution

Natural mineral dust and heavy anthropogenic pollution and its complex interactions cause significant environmental problems in East Asia. Due to restrictions of observing technique, real-time morphological change in Asian dust particles owing to coating process of anthropogenic pollutants is still statistically unclear. Here, we first used a newly developed, single-particle polarization detector and quantitatively investigate the evolution of the polarization property of backscattering light reflected from dust particle as they were mixing with anthropogenic pollutants in North China. The decrease in observed depolarization ratio is mainly attributed to the decrease of aspect ratio of the dust particles as a result of continuous coating processes. Hygroscopic growth of Calcium nitrate (Ca(NO3)2) on the surface of the dust particles played a vital role, particularly when they are stagnant in the polluted region with high RH conditions. Reliable statistics highlight the significant importance of internally mixed, ‘quasi-spherical’ Asian dust particles, which markedly act as cloud condensation nuclei and exert regional climate change.

Importance of coarse‐mode nitrate produced via sea salt as atmospheric input to East Asian oceans

The atmospheric input of anthropogenic total reactive oxygenated nitrogen (NOy) to ocean regions in East Asia during 2002–2004 was revisited with an updated regional chemical transport model and the latest emissions inventory. The updated model treats both fine- and coarse-mode nitrate (NO3−). Coarse-mode NO3− is produced by the reaction of nitric acid (HNO3) and sea salt particles. The modeling system reproduced the atmospheric concentration and wet deposition amount of NO3− quantitatively compared with observations. The fraction of coarse-mode NO3− was also well captured. NOy deposition amounts over marginal seas and open oceans were 733 and 730 Gg N/yr, which are increases of 1.6- and 2.2-fold, respectively, by including coarse-mode NO3−. Anthropogenic NOx emissions from China were 5377 Gg N/yr, and 3060 Gg N/yr was exported from China; therefore, the NOy deposition amount over ocean regions in East Asia (1463 Gg N/yr) corresponded to almost half (48%) of the export amounts.

Shipborne observations of atmospheric black carbon aerosol particles over the Arctic Ocean, Bering Sea, and North Pacific Ocean during September 2014

Measurements of refractory black carbon (rBC) aerosol particles using a highly sensitive online single particle soot photometer were performed on board the R/V Mirai during a cruise across the Arctic Ocean, Bering Sea, and North Pacific Ocean (31 August to 9 October 2014). The measured rBC mass concentrations over the Arctic Ocean in the latitudinal region > 70°N were in the range 0–66 ng/m3 for 1 min averages, with an overall mean value of 1.0 ± 1.2 ng/m3. Single-particle-based observations enabled the measurement of such low rBC mass concentrations. The effects of long-range transport from continents to the Arctic Ocean were limited during the observed period, which suggests that the low rBC concentration levels would prevail over the Arctic Ocean. An analysis of rBC mixing states showed that particles with a nonshell/noncore structure made a significant contribution to the rBC particles detected over the Arctic Ocean.

The organic molecular composition, diurnal variation, and stable carbon isotope ratios of PM2.5 in Beijing during the 2014 APEC summit

Organic tracers are useful for investigating the sources of carbonaceous aerosols but there are still no adequate studies in China. To obtain insights into the diurnal variations, properties, and the influence of regional emission controls on carbonaceous aerosols in Beijing, day-/nighttime PM2.5 samples were collected before (Oct. 15th - Nov. 2nd) and during (Nov. 3rd - Nov. 12th) the 2014 Asia-Pacific Economic Cooperation (APEC) summit. Eleven organic compound classes were analysed using gas chromatography/mass spectrometry (GC/MS). In addition, the stable carbon isotope ratios (δ13CTC) of total carbon (TC) were detected using an elemental analyser/isotope ratio mass spectrometry (EA/irMS). Most of the organic compounds were more abundant during the night than in the daytime, and their concentrations generally decreased during the APEC. These features were associated with the strict regional emission controls and meteorological conditions. The day/night variations of δ13CTC were smaller during the APEC than those before the APEC the summit, suggesting that regionally transported aerosols are potentially played an important role in the loading of organic aerosols in Beijing before the APEC summit. The source apportionment based on the organic tracers suggested that biomass burning, plastic and microbial emissions, and fossil fuel combustion were important sources of organic aerosols in Beijing. Furthermore, a similar contribution of biomass burning to OC before and during the APEC suggests biomass burning was a persistent contributor to PM2.5 in Beijing and its surroundings.

Seasonal variabilities in chemical compounds and acidity of aerosol particles at urban site in the west Pacific

Mass concentrations of chemical compounds in both PM2.5 (particle aerodynamic diameter, Dp < 2.5 μm) and PM2.5-10 (2.5 < Dp < 10 μm), and acidity of aerosol particles were measured at an urban site in western Japan using a continuous dichotomous Aerosol Chemical Speciation Analyzer (ACSA-12) throughout 2014. Mass concentrations of both PM2.5 and sulfate had distinct seasonal variabilities with maxima in spring and winter, mostly due to long-range transport with the prevailing westerly wind. Mass concentration of nitrate in PM2.5 (fNO3) showed an obvious warm-season-low and cold-season-high pattern as a result of both gas-aerosol phase equilibrium processes under high temperature conditions as well as transport. Nitrate in PM2.5-10 (cNO3) increased during long-range transport of dust, implying the great importance of heterogeneous processes at the surface of coarse mode particles. In this study, Δ[H+] (derived from the difference in pH of extract liquid with/without sampling) was used to indicate the acidity of particles. We found that acidity of particles in PM2.5 (fΔH) was mostly positive with a maximum in August because of the large fraction of nitrate and sulfate. Acidity of particles in PM2.5-10 (cΔH) was negative in winter and spring due to presence of alkaline matter from crustal sources. This study highlights the great importance of anthropogenic pollutants on the acidity of particles in the western Pacific Ocean and further impact on the marine environment and climate.

Synergistic effect of water-soluble species and relative humidity on morphological changes of aerosol particles in Beijing mega-city during severe pollution episodes

Depolarization ratio (δ) of backscattered light from aerosol particle is an applicable parameter for real-time 18 distinguishing spherical and non-spherical particles, which has been widely adopted by ground-based Lidar observation and 19 satellite remote sensing. From November 2016 to February of 2017, it consecutively suffered from numbers of severe air 20 pollution at Beijing with daily averaged mass concentration of PM2.5 (aerodynamic diameter less than 2.5μm) larger than 150 21 μg/m. We preformed concurrent measurements of water-soluble chemical species and depolarization properties of aerosol 22 particles on the basis of a continuous dichotomous Aerosol Chemical Speciation Analyzer (ACSA-14) and a bench-top optical 23 particle counter with a polarization detection module (POPC). We found that δ value of ambient particles generally decrease 24 as mass concentration of PM2.5 increased at unfavorable meteorological condition. Ratio of mass concentration of nitrate 25 (NO3) to that of sulfate (SO4) in PM2.5 was 1.5 ± 0.6, indicating of great importance of NOx in the formation of heavy 26 pollution. Mass concentration of NO3 in PM2.5 (fNO3) was generally an order of magnitude higher than that in coarse mode 27 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-623 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 2 August 2018 c