Abstract. Winter has the worst air pollution of the year in the megacity of Beijing. Despite extensive winter studies in recent years, our knowledge of the sources, formation mechanisms and evolution of aerosol particles is not complete. Here we have a comprehensive characterization of the sources, variations and processes of submicron aerosols that were measured by an Aerodyne high-resolution aerosol mass spectrometer from 17 December 2013 to 17 January 2014 along with offline filter analysis by gas chromatography/mass spectrometry. Our results suggest that submicron aerosols composition was generally similar across the winter of different years and was mainly composed of organics (60 %), sulfate (15 %) and nitrate (11 %). Positive matrix factorization of high- and unit-mass resolution spectra identified four primary organic aerosol (POA) factors from traffic, cooking, biomass burning (BBOA) and coal combustion (CCOA) emissions as well as two secondary OA (SOA) factors. POA dominated OA, on average accounting for 56 %, with CCOA being the largest contributor (20 %). Both CCOA and BBOA showed distinct polycyclic aromatic hydrocarbons (PAHs) spectral signatures, indicating that PAHs in winter were mainly from coal combustion (66 %) and biomass burning emissions (18 %). BBOA was highly correlated with levoglucosan, a tracer compound for biomass burning (r2 = 0.93), and made a considerable contribution to OA in winter (9 %). An aqueous-phase-processed SOA (aq-OOA) that was strongly correlated with particle liquid water content, sulfate and S-containing ions (e.g. CH2SO2+) was identified. On average aq-OOA contributed 12 % to the total OA and played a dominant role in increasing oxidation degrees of OA at high RH levels (> 50 %). Our results illustrate that aqueous-phase processing can enhance SOA production and oxidation states of OA as well in winter. Further episode analyses highlighted the significant impacts of meteorological parameters on aerosol composition, size distributions, oxidation states of OA and evolutionary processes of secondary aerosols.

The main objective of this study is to investigate the formation and evolution mechanism of the regional haze in megacity Beijing by analyzing the process of a severe haze that occurred 20–27 September 2011. Mass concentration and size distribution of aerosol particles as well as aerosol optical properties were concurrently measured at the Beijing urban atmospheric environment monitoring station. Gaseous pollutants (SO₂, NO-NO₂-NO_x, O₃, CO) and meteorological parameters (wind speed, wind direction, and relative humidity) were simultaneously monitored. Meanwhile, aerosol spatial distribution and the height of planetary boundary layer (PBL) were retrieved from the signal of satellite and LIDAR (light detection and ranging). Concentrations of NO, NO₂, SO₂, O₃, and CO observed during 23–27 September had exceeded the national ambient air quality standards for residents. The mass concentration of PM_2.5 gradually accumulated during the measurement and reached at 220 μg m⁻³ on 26 September, and the corresponding atmospheric visibility was only 1.1 km. The daily averaged AOD in Beijing increased from ~ 0.16 at λ = 500 nm on 22 September and reached ~ 3.5 on 26 September. The key factors that affected the formation and evolution of this haze episode were stable anti-cyclone synoptic conditions at the surface, decreasing of the height of PBL, heavy pollution emissions from urban area, number and size evolution of aerosols, and hygroscopic growth for aerosol scattering. This case study may provide valuable information for the public to recognize the formation mechanism of the regional haze event over the megacity, which is also useful for the government to adopt scientific approach to forecast and eliminate the occurrence of regional haze in 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 (O = O + NO) during periods of photochemical production (R = 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.

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.

Abstract. Strict emission controls were implemented in Beijing and adjacent provinces to ensure good air quality during the 2015 China Victory Day parade. Here, we conducted synchronous measurements of submicron aerosols (PM1) at ground level and 260 m on a meteorological tower by using a high-resolution aerosol mass spectrometer and an aerosol chemical speciation monitor, respectively, in Beijing from 22 August to 30 September. Our results showed that the average PM1 concentrations are 19.3 and 14.8 µg m−3 at ground level and 260 m, respectively, during the control period (20 August–3 September), which are 57 and 50 % lower than those after the control period (4–30 September). Organic aerosols (OAs) dominated PM1 during the control period at both ground level and 260 m (55 and 53 %, respectively), while their contribution showed substantial decreases (∼  40 %) associated with an increase in secondary inorganic aerosols (SIAs) after the parade, indicating a larger impact of emission controls on SIA than OA. Positive matrix factorization of OA further illustrated that primary OA (POA) showed similar decreases as secondary OA (SOA) at both ground level (40 % vs. 42 %) and 260 m (35 % vs. 36 %). However, we also observed significant changes in SOA composition at ground level. While the more oxidized SOA showed a large decrease by 75 %, the less oxidized SOA was comparable during (5.6 µg m−3) and after the control periods (6.5 µg m−3). Our results demonstrated that the changes in meteorological conditions and PM loadings have affected SOA formation mechanisms, and the photochemical production of fresh SOA was more important during the control period. By isolating the influences of meteorological conditions and footprint regions in polluted episodes, we found that regional emission controls on average reduced PM levels by 44–45 %, and the reductions were close among SIA, SOA and POA at 260 m, whereas primary species showed relatively more reductions (55–67 %) than secondary aerosol species (33–44 %) at ground level.

No abstract

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.

This study documents recent changes in the characteristics of summer (July-August-September) precipitation in Northeast China (NEC). A significant shift to less precipitation occurred in 1999-2012 as compared with that in 1984-1998. The reduced precipitation in the later period is closely associated with the large-scale anomalous high pressure over East Asia and anomalous descending motion over NEC. Furthermore, the significant reductions in the total cloud cover and moisture content also contribute to the reduced precipitation over NEC. To investigate the possible mechanism for the decadal shift of summer precipitation, a northeast Asian summer monsoon (NEASM) index is defined to describe the monsoon circulation over NEC. The results indicate that the NEASM has weakened since 1999 and is concurrent with the shift of the Pacific Decadal Oscillation (PDO) to the negative phase. Warming sea surface temperature (SST) in the North Pacific can zonally reduce the land-sea thermal contrast and lead to a weak NEASM. Further investigation indicates that the negative phase of the PDO has significant impacts on the atmospheric circulation associated with the NEASM. Additionally, changes in synchronous Arctic sea ice cover (SIC) also likely induce an anomalous sinking movement and weaken water vapour transport; thus, the summer precipitation over NEC decreases.