Weiqi Xu

“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.

Rapid formation and evolution of an extreme haze episode in Northern China during winter 2015.

We investigate the rapid formation and evolutionary mechanisms of an extremely severe and persistent haze episode that occurred in northern China during winter 2015 using comprehensive ground and vertical measurements, along with receptor and dispersion model analysis. Our results indicate that the life cycle of a severe winter haze episode typically consists of four stages: (1) rapid formation initiated by sudden changes in meteorological parameters and synchronous increases in most aerosol species, (2) persistent evolution with relatively constant variations in secondary inorganic aerosols and secondary organic aerosols, (3) further evolution associated with fog processing and significantly enhanced sulfate levels, and (4) clearing due to dry, cold north-northwesterly winds. Aerosol composition showed substantial changes during the formation and evolution of the haze episode but was generally dominated by regional secondary aerosols (53–67%). Our results demonstrate the important role of regional transport, largely from the southwest but also from the east, and of coal combustion emissions for winter haze formation in Beijing. Also, we observed an important downward mixing pathway during the severe haze in 2015 that can lead to rapid increases in certain aerosol species.

Characteristics of magnetospheric energetics during geomagnetic storms

To investigate the magnetospheric energetics during magnetic storms, we performed a statistical survey of 307 geomagnetic storms between 1995 and 2009. For the purpose of getting a detailed understanding of the energy processes, we conducted our study of storm-time energetics for three time durations: the main phase, the recovery phase, and the total storm period. We found that the partition of the energy dissipation via the ring current injection and high-latitude ionospheric dissipation is controlled by the storm intensity. The proportion of the ring current injection increases linearly as the storm intensity increases for all three time durations. For moderate storms, the high-latitude ionospheric dissipation is dominant, with only similar to 30% energy dissipated via the ring current; whereas for superstorms, the ring current injection becomes dominant, with similar to 70% energy dissipated via the ring current. We also confirmed the essential and crucial role of the total energy input into the magnetosphere during the main phase in controlling the storm intensity. The total energy input during the main phase is directly proportional to the storm intensity. Their correlation efficiency is as high as 0.85. The storm-time energy budget was also quantified in this study. The coupling efficiency indicates an exponential decay as the storm intensity increases, with the coupling efficiency during the main phase less than that during the recovery phase.

Chemical apportionment of aerosol optical properties during the Asia‐Pacific Economic Cooperation summit in Beijing, China

We have investigated the chemical and optical properties of aerosol particles during the 2014 Asia-Pacific Economic Cooperation (APEC) summit in Beijing, China, using the highly time-resolved measurements by a high-resolution aerosol mass spectrometer and a cavity attenuated phase shift extinction monitor. The average (±σ) extinction coefficient (bext) and absorption coefficient (bap) were 186.5 (±184.5) M m−1 and 23.3 (±21.9) M m−1 during APEC, which were decreased by 63% and 56%, respectively, compared to those before APEC primarily due to strict emission controls. The aerosol composition and size distributions showed substantial changes during APEC; as a response, the mass scattering efficiency (MSE) of PM1 was decreased from 4.7 m2 g−1 to 3.5 m2 g−1. Comparatively, the average single-scattering albedo (SSA) remained relatively unchanged, illustrating the synchronous reductions of bext and bap during APEC. MSE and SSA were found to increase as function of the oxidation degree of organic aerosol (OA), indicating a change of aerosol optical properties during the aging processes. The empirical relationships between chemical composition and particle extinction were established using a multiple linear regression model. Our results showed the largest contribution of ammonium nitrate to particle extinction, accounting for 35.1% and 29.3% before and during APEC, respectively. This result highlights the important role of ammonium nitrate in the formation of severe haze pollution during this study period. We also observed very different optical properties of primary and secondary aerosol. Owing to emission controls in Beijing and surrounding regions and also partly the influences of meteorological changes, the average bext of secondary aerosol during APEC was decreased by 71% from 372.3 M m−1 to 108.5 M m−1, whereas that of primary aerosol mainly from cooking, traffic, and biomass burning emissions showed a smaller reduction from 136.7 M m−1 to 71.3 M m−1. As a result, the contribution of primary aerosol to particle extinction increased from 26.8% to 39.6%, elucidating an enhanced role of local primary sources in visibility deterioration during APEC. Further analysis of chemically resolved particle extinction showed that the extinction contributions of aerosol species varied greatly between different air masses but generally with ammonium nitrate, ammonium sulfate, and secondary OA being the three major contributors.

Response of aerosol composition to different emission scenarios in Beijing, China.

Understanding the response of aerosol chemistry to different emission scenarios is of great importance for air pollution mitigating strategies in megacities. Here we investigate the variations in air pollutants under three different emission scenarios, i.e., heating season, spring festival holiday and non-heating season using aerosol composition and gaseous measurements from 2 February to 1 April 2015 along with source apportionment and FLEXPART analysis in Beijing. Our results showed substantially different aerosol composition among three emission scenarios that is primarily caused by different emission sources. All aerosol and gas species showed ubiquitously higher concentrations in heating season than non-heating season with the largest enhancement for fossil OA (FOA) and chloride. On average, the particulate matter (PM) level in winter heating season can be enhanced by 70% due to coal combustion emissions. In contrast, cooking aerosols and traffic related species showed significant reductions as a response of reduced anthropogenic activities during the spring festival holiday, sulfate and secondary organic aerosol (SOA) however even increased due to enhanced aqueous-phase production. Such compensating effects resulted in small changes in PM levels for haze episodes during the holiday period despite reduced anthropogenic emissions. Our results have significant implications that local emission controls during winter severe pollution episodes can reduce primary aerosols substantially, but the mitigating effects can be significantly suppressed by enhanced secondary formation under stagnant meteorological conditions.

Effects of Aqueous-Phase and Photochemical Processing on Secondary Organic Aerosol Formation and Evolution in Beijing, China

Secondary organic aerosol (SOA) constitutes a large fraction of OA, yet remains a source of significant uncertainties in climate models due to incomplete understanding of its formation mechanisms and evolutionary processes. Here we evaluated the effects of photochemical and aqueous-phase processing on SOA composition and oxidation degrees in three seasons in Beijing, China, using high-resolution aerosol mass spectrometer measurements along with positive matrix factorization. Our results show that aqueous-phase processing has a dominant impact on the formation of more oxidized SOA (MO-OOA), and the contribution of MO-OOA to OA increases substantially as a function of relative humidity or liquid water content. In contrast, photochemical processing plays a major role in the formation of less oxidized SOA (LO-OOA), as indicated by the strong correlations between LO-OOA and odd oxygen (Ox = O3 + NO2) during periods of photochemical production (R2 = 0.59-0.80). Higher oxygen-to-carbon ratios of SOA during periods with higher RH were also found indicating a major role of aqueous-phase processing in changing the oxidation degree of SOA in Beijing. Episodes analyses further highlight that LO-OOA plays a more important role during the early stage of the formation of autumn/winter haze episodes while MO-OOA is more significant during the later evolution period.

Aerosol optical properties measurements by a CAPS single scattering albedo monitor: Comparisons between summer and winter in Beijing, China

Aerosol optical properties were measured in Beijing in summer and winter using a state-of-the-art cavity attenuated phase shift single scattering albedo monitor (CAPS PMssa) along with aerosol composition measurements by aerosol mass spectrometers and aethalometers. The SSA directly measured by the CAPS PMssa showed overall agreements with those derived from colocated measurements. However, substantial differences were observed during periods with low SSA values in both summer and winter, suggesting that interpretation of low SSA values needs to be cautious. The average (±σ) extinction coefficient (bext) and absorption coefficient (bap) were 336 (±343) Mm−1 and 44 (±41) Mm−1, respectively, during wintertime, which were approximately twice those observed in summer, while the average SSA was relatively similar, 0.86 (±0.06) and 0.85 (±0.04) in summer and winter, respectively. Further analysis showed that the variations in SSA can be approximately parameterized as a function of mass fraction of secondary particulate matter (fSPM), which is SSA = 0.74 + 0.19 × fSPM (fSPM > 0.3, r2 = 0.85). The contributions of aerosol species to extinction coefficients during the two seasons were also estimated. Our results showed that the light extinction was dominantly contributed by ammonium sulfate (30%) and secondary organic aerosol (22%) in summer, while organic aerosol was the largest contributor (51%) in winter. Consistently, SPM played the major role in visibility degradation in both seasons by contributing 70% of the total extinction.

Using different assumptions of aerosol mixing state and chemical composition to predict CCN concentrations based on filed measurement in Beijing

Understanding the impacts of aerosol chemical composition and mixing state on cloud condensation nuclei (CCN) activity in polluted areas is crucial for accurately predicting CCN number concentrations (NCCN). In this study, we predict NCCN under five assumed schemes of aerosol chemical composition and mixing state based on field measurements in Beijing during the winter of 2016. Our results show that the best closure is achieved with the assumption of size dependent chemical composition for which sulfate, nitrate, secondary organic aerosols, and aged black carbon are internally mixed with each other but externally mixed with primary organic aerosol and fresh black carbon (external–internal size-resolved, abbreviated as EI– SR scheme). The resulting ratios of predicted-to-measured NCCN (RCCN_p/m) were 0.90 – 0.98 under both clean and polluted conditions. Assumption of an internal mixture and bulk chemical composition (INT–BK scheme) shows good closure with RCCN_p/m of 1.0 –1.16 under clean conditions, implying that it is adequate for CCN prediction in continental clean regions. On polluted days, assuming the aerosol is internally mixed and has a chemical composition that is size dependent (INT–SR scheme) achieves better closure than the INT–BK scheme due to the heterogeneity and variation in particle composition at different sizes. The improved closure achieved using the EI–SR and INT–SR assumptions highlight the importance of measuring size-resolved chemical composition for CCN predictions in polluted regions. NCCN is significantly underestimated (withRCCN_p/m of 0.66 – 0.75) when using the schemes of external mixtures with bulk (EXT–BK scheme) or size-resolved composition (EXT– SR scheme), implying that primary particles experience rapid aging and physical mixing processes in urban Beijing. However, our results show that the aerosol mixing state plays a minor role in CCN prediction when the κorg exceeds 0.1.