Abstract
Severe events of wintertime particulate air pollution in Beijing (winter haze) are associated with high relative humidity (RH) and fast production of particulate sulfate from the oxidation ...of sulfur dioxide (SO
2
) emitted by coal combustion. There has been considerable debate regarding the mechanism for SO
2
oxidation. Here we show evidence from field observations of a haze event that rapid oxidation of SO
2
by nitrogen dioxide (NO
2
) and nitrous acid (HONO) takes place, the latter producing nitrous oxide (N
2
O). Sulfate shifts to larger particle sizes during the event, indicative of fog/cloud processing. Fog and cloud readily form under winter haze conditions, leading to high liquid water contents with high pH (>5.5) from elevated ammonia. Such conditions enable fast aqueous-phase oxidation of SO
2
by NO
2
, producing HONO which can in turn oxidize SO
2
to yield N
2
O.This mechanism could provide an explanation for sulfate formation under some winter haze conditions.
Secondary organic aerosol (SOA) produced by atmospheric oxidation of primary emitted precursors is a major contributor to fine particulate matter (PM
) air pollution worldwide. Observations during ...winter haze pollution episodes in urban China show that most of this SOA originates from fossil-fuel combustion but the chemical mechanisms involved are unclear. Here we report field observations in a Beijing winter haze event that reveal fast aqueous-phase conversion of fossil-fuel primary organic aerosol (POA) to SOA at high relative humidity. Analyses of aerosol mass spectra and elemental ratios indicate that ring-breaking oxidation of POA aromatic species, leading to functionalization as carbonyls and carboxylic acids, may serve as the dominant mechanism for this SOA formation. A POA origin for SOA could explain why SOA has been decreasing over the 2013-2018 period in response to POA emission controls even as emissions of volatile organic compounds (VOCs) have remained flat.
Vertical distribution of black carbon (BC) determines the layer where its heating impacts exert. This study presents continuous and simultaneous measurements at surface and on a mountain site above ...the wintertime planetary boundary layer influenced by uplifted surface anthropogenic emissions. BC was observed efficiently transported upwards by daytime convective mixing. However, this vertical transport was less for other particulate masses. An about twofold higher BC mass fraction was thus present at mountain than surface, hereby a lowered single‐scattering albedo (SSA) by 0.06. This may be caused by the evaporative loss of condensed semivolatile materials, prevailing the secondary particulate formation, in a cleaner environment containing less precursors. The elevated BC mass corresponded with the most intensive solar radiation at midday, wielding more heating impacts over the planetary boundary layer (PBL). This phenomenon may apply to other remote regions where a reduced SSA will introduce more positive radiative effects.
Plain Language Summary
Black carbon is strongly light absorbing, and its heating impacts in disturbing the stability of atmosphere depend on its location in the atmospheric column; thus, to understand its vertical distribution and transport mechanism is important. This study conducted simultaneous measurements at both sites on the surface and an elevated mountain site influenced by surface sources. We found that BC can be efficiently and vertically transported to the mountain site but not for the other more volatile substances. This is because a less secondary formation (due to lack of gas precursor) and some repartition process on the particle (back to the gas phase) may have led to a higher BC mass fraction in cleaner environment. This means in some remote places with less gas precursor present, BC may still reach and result in a high mass fraction, leading to more positive radiative effect.
Key Points
Simultaneous measurements at both surface and top of boundary layer were performed for 1 month
BC was transported efficiently to the top of PBL however not for other more volatile aerosols
Higher BC mass fraction was observed over the top of boundary layer with lowered single‐scattering albedo by 0.06
Residential biomass combustion emits a large amount of organic gases into ambient air, resulting in the formation of secondary organic aerosol (SOA) and various environmental and health impacts. In ...this study, we investigated the emission characteristics of non-methane organic compounds (NMOCs) from residential biomass fuels during vigorous combustion (flaming) and stable combustion (smoldering) conditions. We quantified NMOC emission factors based on the CO concentration for different combustion phases and found that NMOC emissions were higher during the smoldering phase and approximately two to four times greater than those during flaming. NMOCs were categorized into volatile organic compounds (VOCs) and intermediate-volatility organic compounds (IVOCs) through the modeling of the organic compound volatility distribution. The photochemical aging of NMOCs revealed furans, phenolics, and certain IVOCs as significant non-traditional SOA precursors, with over half being consumed during a short aging period. A parametric function was established, indicating that accounting for non-traditional SOA precursors and IVOC yields improves the representation of the net enhancement of measured organic aerosol (OA). This study emphasizes the importance of differentiating emissions from various phases of residential biomass combustion and recognizing non-traditional SOA precursors and IVOCs for accurate SOA assessment and prediction.
The role of organic aerosols (OA) as ice nucleating particles (INPs) deserves attention because of their high atmospheric abundance. The low concentration of INPs poses challenges in identifying the ...ice nucleation (IN) of OA among a mix of aerosol types in ambient environment. This study coupled a catalytic stripper system (350°C heating) with a continuous flow diffusion chamber to online investigate the immersion INPs of ambient particles at −30°C at a suburban site. Significant reduction (71 ± 25%) of INP concentrations after evaporation suggested that INPs can be significantly contributed by volatile OA. In addition, nonvolatile OA were more efficient INPs than black carbon. Oxygenated OA by photooxidation and lower ambient promoted the IN activity at noon, when the OA may be more viscous. These results hereby present the first field evidence that OA in anthropogenically influenced regions can be efficient INPs well above the homogeneous IN temperature.
Plain Language Summary
Ice nucleating particles have an important impact on weather and climate by modulating cloud microphysics. Globally, measurements have demonstrated the ubiquitous presence of ice nucleating particles (INPs); however, the contribution of abundant organic aerosols (OA) to INPs remains uncertain owing to their complex compositions and varying phase states. In this study, using concurrent measurements between ambient and heated INPs, we prove the important contribution of volatile OA at 350°C to immersion freezing INPs at −30°C, especially at noon, when OA are more solid‐like because of their oxygenation by photooxidation and lower ambient relative humidity. In addition, nonvolatile OA were found to be a more efficient INPs than black carbon. Therefore, our results provide field evidence that OA in anthropogenically influenced regions could serve as an important atmospheric INPs at 8°C above the homogeneous freezing temperature.
Key Points
Field measurements prove contribution of organic aerosols (OA) in anthropogenically influence region to immersion mode ice nucleation (IN)
OA oxygenated by photooxidation and lower relative humidity showed a correlation with increased at noon
Nonvolatile OA at 350°C heating have a higher IN efficiency than that of black carbon
Abstract The microphysical attributes of black carbon (BC) can determine its absorption and hygroscopic properties. However, long-term information is difficult to obtain from the field. In this ...study, the BC properties including mass concentration, the coating volume ratio (VR) relative to the refractory BC (rBC), the rBC diameter and the fraction of cloud condensation nuclei (CCN), are derived from a number of field experiments using a random forest model. This model effectively derives the long-term BC microphysical properties in the Beijing region from 2013 to 2020 using continuous measurements of particulate matter, gas, BC mass concentration and meteorological parameters. The results reveal notably higher BC coatings (mean VR = 7.2) and a greater fraction of CCN-like BC (51%) in the winter compared to other seasons. Following the implementation of national air pollution control measures in 2017, BC mass exhibited a substantial reduction of 60% (29%) in the winter (summer), and VR decreased by 45% (24%). Apart from the influence of meteorological variations, these can be attributed to the declined primary emissions and the gas precursors which are associated with secondary formation of BC coatings. The reduction of both BC mass loading and coatings leads to its solar absorption decreasing by 50%, and the fraction of CCN-like BC (likely in clouds) decreasing by 23%. Environmental regulation will therefore continue to reduce both direct and indirect radiative impacts of BC in this region.
Particulate organic nitrates (pON) significantly contribute to the mass of organic aerosol and influence the nitrogen oxides cycle in the atmosphere, but their evolution and lifetime remain ...uncertain. This study performed simultaneous measurements on the anthropogenically affected surface site and the mountain site on top of the polluted planetary boundary layer (PBL). After aging in the PBL, organic nitrate was converted from primary sources (decreased from 8.7% to 4.3%) to secondary sources (increased from 6.3% to 36.1%), spanning from the surface to the mountain. The evaporation of more volatile inorganic nitrate and the production of secondary organic nitrate during aging in the PBL contributed to the enhanced pON fraction over the top of PBL. The contribution of light absorption by brown carbon increased by 57% at the top of PBL compared to the surface, consistent with the higher fraction of nitrogenous organic aerosols over the mountain. The results provide field evidence that the nitrogenous organic aerosols (OA) may be preserved by adding into secondary OA and significantly contribute to the enhanced importance of brown carbon after aging the vertical transport in the PBL.
Airborne polycyclic aromatic hydrocarbons (PAHs) are of great concern to human health due to their potential high toxicity. Understanding the characteristics and sources of PAHs, as well as the ...governing factors, is therefore critical. PAHs and refractory black carbon (rBC) are both from combustion sources. This work, for the first time, investigated exclusively the rBC-bound PAH properties by using a laser-only Aerodyne soot-particle aerosol mass spectrometer (SP-AMS). This technique offers highly time-resolved PAH results that a traditional offline measurement is unable to provide. We analyzed two datasets conducted in urban Shanghai during the fall of 2018 and in suburban Nanjing during the winter of 2017, respectively. Results show that the average concentration of PAHs in Nanjing was much higher than that in Shanghai. Nanjing PAHs contained more low molecular weight components while Shanghai PAHs contained more high molecular weight ones. PAHs in Shanghai presented two peaks in early morning and evening, while Nanjing PAHs had only one significant morning peak, but remained high throughout the nighttime. A multi-linear regression algorithm combined with positive matrix factorization (PMF) analyses on sources of PAHs reveals that the industry emissions contributed the majority of PAHs in Nanjing (~80%), while traffic emissions dominated PAHs in Shanghai (~70%). We further investigated the relationships between PAHs with various factors. PAHs in both sites tended to positively correlate with primary pollutants, including primary organic aerosol (OA) factors, and gaseous pollutants of CO, NO2 and SO2, but negatively correlated with secondary OA factors and O3. This result highlights the enhancement of rBC-bound PAHs level due to primary emissions and their oxidation loss upon atmospheric aging reactions. High concentration of PAHs seemed to frequently appear under low temperature and high relative humidity conditions, especially in Shanghai.
Whether the cloud condensation nuclei (CCN) ability of aerosol could be predicted by compositions has been long debated. Measurements of submicron aerosol compositions and size‐resolved CCN ...activation fraction were conducted at a mountain site (1,344 m) near Beijing region during wintertime. The site is influenced in the noon‐afternoon by ground anthropogenic sources through convective mixing (CM) and for certain period received aged pollutants by regional advection (RA). By comparing the measured CCN‐derived hygroscopicity parameter (κCCNc) with that predicted using chemical composition in bulk (κchem), we found for CM period, κchem overpredicted κCCNc by 71 ± 11% (25 ± 13%) at SS = 0.1% (0.3%); whereas for RA‐only period, κchem underpredicted by 13 ± 6% (18 ± 11%) at SS = 0.1% (0.3%). The former, representing fresher aerosols in smaller size, could be explained by not considering the size‐resolved composition, as the bulk measurement mainly reflects the features of larger particle. The latter is proved to result from the depression of droplet surface tension by potential surface‐active organics and the possible liquid‐liquid phase separation occurring at moderate RH, and a use of depressed surface tension (than pure water) of 0.063 ± 0.002 J m−2 would reach an agreement. We propose that a hybrid approach combining size‐resolved composition and reduced surface tension, for fresher and aged sources, respectively, should improve the estimation of aerosol CCN ability.
Plain Language Summary
The ability of cloud condensation nuclei (CCN) of aerosol is important in determining the aerosol‐cloud interaction and this affects cloud formation hereby having environmental and radiative impacts. Previous studies have intensively used the conveniently measured compositions in bulk, and link this with the CCN ability by assuming the surface tension of droplet as water's. In this study, we examined this conversion from bulk composition to CCN ability by direct measurements of both at a mountain site, representing the environment of the top of boundary layer for cloud initialization. The results showed that to reduce the uncertainty of composition‐determined CCN ability, a reduced surface tension should be applied for aged aerosols; and the use of size‐resolved chemical composition will improve the CCN prediction for fresh aerosol. We hereby propose that this hybrid approach for the respective fresher and aged sources should improve the estimation of aerosol CCN ability.
Key Points
Aerosol CCN ability measured at a mountain site more realistically represents the condition for initializing the cloud formation compared to ground measurement
Size‐resolved chemical composition should improve the prediction of CCN ability for fresh aerosol
Reduced surface tension will improve the prediction of CCN ability for aged aerosol
Network function virtualization (NFV) leverages software and general-purpose computation devices to provide various network services through service function chaining (SFC), where data flows through ...a sequence of network functions to obtain the desired services. In this context, the growth of network traffic due to the proliferation of network services presents a challenge. To address this, hybrid optical-electronic networks, combining the advantages of both optical and electronic networks, have been applied to create a more efficient and flexible communication system which also transport traffic through the network functions of SFC. In this paper, we address the problem of embedding SFC in a hybrid optical-electronic network. This problem is even more challenging than the SFC embedding problem in an electronic network, which is already known to be NP-hard. To tackle this problem, we propose a scalable SFC embedding algorithm in hybrid optical-electronic networks that optimizes both optical and electronic bandwidth allocations as well as computation resource allocation. We first provide an integer linear programming formulation for this optimization problem and then propose a scalable heuristic algorithm based on the randomized rounding method to achieve near-optimal solutions. Our numerical results demonstrate that the proposed algorithm achieves both guaranteed performance and scalability through efficient running time.
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