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.

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.

Organic aerosol (OA) constituted a large fraction of aerosol particles during severe haze episodes in winter in northern China, yet our understanding of its physical and chemical processing was limited. Here we investigate the sources and processes of OA during four haze episodes in winter in 2016 using high-resolution aerosol mass spectrometer. The PM 2.5 reached 400 μg/m 3 during the severest episode (Ep1) when Beijing issued a red alert and implemented strict emission controls. Our results showed that secondary OA (SOA) dominated OA during haze episodes on average accounting for 46-66% of OA and was comparable to secondary inorganic aerosol (SIA) with the SOA/SIA ratios being 0.51-0.72. Primary OA from fossil-fuel combustion, biomass burning, and cooking presented very strong diurnal variations during haze episodes and contributed up to 60% in OA at night. Comparatively, the changes in semivolatile and low-volatility SOA were relatively small except a substantial increase in aqueous phase-related oxidized OA (aq-OOA) during Ep1 with high relative humidity and aerosol water content. aq-OOA fell well into a small region in the middle of the triangle plot of f 44 versus f 43 (fraction of m/z 44 and 43 in OA, respectively), which can be used as a diagnostic for the presence of aqueous phase processing of SOA. In addition, the increases of SO 2 + /SO 3 + as a function of relative humidity, the triangle plot of f H2SO þ 4 versus f HSO þ 3 , and high nitrogen-to-carbon ratio in aq-OOA suggest the potential formation of sulfur-and nitrogen-containing organic compounds through aqueous phase processing.

. The CCM calculations are performed with the two ensemble members for REF1 scenario of the chemistry climate model validation (CCMVal) and the one ensemble member for the REF2 scenario. CCM simulates the development of the ozone hole from 1982 to 2000, as observed with a total ozone mapping spectrometer (TOMS), although the year-to-year variation is different from the observation owing to the internal variability of CCM and the ozone decreasing trends of CCM ozone in the two ensemble members of REF1 are underestimated. The trends in temperature and zonal mean zonal wind are analyzed and compared with the observations. There is consistency among the trends in zonal mean temperature, zonal mean zonal wind, and total ozone, but they differ among the ensemble members and observations. The diabatic heating rates and Eliassen-Palm flux fields are investigated in order to explain the differences. A delay trend in the breakup time of the Antarctic polar vortex is obtained for the period of 1980-1999 in the NCEP/ NCAR and ERA40 data. A similar trend is also obtained from the CCM simulations, with statistical significance in one ensemble member of REF1 and REF2. Because the trends of the observations in the EP flux from the troposphere and its deposition in the lower stratosphere are consistent with an advanced breakup date of the polar vortex and because the trends of the CCM simulations are very small, it is likely that the Antarctic ozone depletion had some effect on the delay during the period 1980 . From 2000, the NCEP/NCAR data show a large variation in breakup time, which makes the delay trend much less important. It is likely that the large variation in wave flux masked the effects of the ozone loss during that period. The two ensemble members of the REF1 simulation do not show such a dramatic change in the trend for the period 2000-2004, whereas REF2 shows a change in the trend for that period.

Abstract. We conducted the first real-time continuous vertical measurements of particle extinction (bext), gaseous NO2, and black carbon (BC) from ground level to 260 m during two severe winter haze episodes at an urban site in Beijing, China. Our results illustrated four distinct types of vertical profiles: (1) uniform vertical distributions (37 % of the time) with vertical differences less than 5 %, (2) higher values at lower altitudes (29 %), (3) higher values at higher altitudes (16 %), and (4) significant decreases at the heights of ∼ 100–150 m (14 %). Further analysis demonstrated that vertical convection as indicated by mixing layer height, temperature inversion, and local emissions are three major factors affecting the changes in vertical profiles. Particularly, the formation of type 4 was strongly associated with the stratified layer that was formed due to the interactions of different air masses and temperature inversions. Aerosol composition was substantially different below and above the transition heights with ∼ 20–30 % higher contributions of local sources (e.g., biomass burning and cooking) at lower altitudes. A more detailed evolution of vertical profiles and their relationship with the changes in source emissions, mixing layer height, and aerosol chemistry was illustrated by a case study. BC showed overall similar vertical profiles as those of bext (R2=0.92 and 0.69 in November and January, respectively). While NO2 was correlated with bext for most of the time, the vertical profiles of bext ∕ NO2 varied differently for different profiles, indicating the impact of chemical transformation on vertical profiles. Our results also showed that more comprehensive vertical measurements (e.g., more aerosol and gaseous species) at higher altitudes in the megacities are needed for a better understanding of the formation mechanisms and evolution of severe haze episodes in China.

To better understand the local wind systems in the Himalayas, wind and related atmospheric parameters were observed in the Rongbuk Valley on the northern slope of Mt. Everest, during the HEST2006 campaign, from May 29 to June 29, 2006. Data analysis and a simple numerical simulation show that the dominating down‐valley flow in this valley is mainly formed by the thermally driven winds, “valley wind”, “mountain wind” and “glacier wind”. The vertical air motion is composed of a descending flow from the morning to midnight and an ascending flow for the rest of the day, with important modification from the vertical component of the above down‐valley flow and a compensation flow of the “slope wind”. The analysis also shows that the local wind system is well confined in the Rongbuk Valley due to topographic shielding effects.

[1] High ozone concentrations (70 -80 ppb) were found from late afternoon to midnight at sites at ca. 5000 m above sea level (m.a.s.l.) on Mt. Everest. Observational data suggest that katabatic wind from Mt. Everest was ''pumping down'' ozone-rich air from the upper troposphere. Numerical modelling demonstrates that cooling of glaciers and snow on the northern mountain slopes and heating of the valley surface play important roles in forming katabatic winds and accelerating vertical exchange between the upper atmosphere and surface air. These results suggest that the ''pump-down'' mechanism at high mountains covered with snow/glaciers is an important process in terrestrial intercontinental transport of ozone and atmosphere -land exchanges of masses and energy. Citation: Zhu, T

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