The increasing severity of droughts/floods and worsening air quality from increasing aerosols in Asia monsoon regions are the two gravest threats facing over 60% of the world population living in Asian monsoon regions. These dual threats have fueled a large body of research in the last decade on the roles of aerosols in impacting Asian monsoon weather and climate. This paper provides a comprehensive review of studies on Asian aerosols, monsoons, and their interactions. The Asian monsoon region is a primary source of emissions of diverse species of aerosols from both anthropogenic and natural origins. The distributions of aerosol loading are strongly influenced by distinct weather and climatic regimes, which are, in turn, modulated by aerosol effects. On a continental scale, aerosols reduce surface insolation and weaken the land-ocean thermal contrast, thus inhibiting the development of monsoons. Locally, aerosol radiative effects alter the thermodynamic stability and convective potential of the lower atmosphere leading to reduced temperatures, increased atmospheric stability, and weakened wind and atmospheric circulations. The atmospheric thermodynamic state, which determines the formation of clouds, convection, and precipitation, may also be altered by aerosols serving as cloud condensation nuclei or ice nuclei. Absorbing aerosols such as black carbon and desert dust in Asian monsoon regions may also induce dynamical feedback processes, leading to a strengthening of the early monsoon and affecting the subsequent evolution of the monsoon. Many mechanisms have been put forth regarding how aerosols modulate the amplitude, frequency, intensity, and phase of different monsoon climate variables. A wide range of theoretical, observational, and modeling findings on the Asian monsoon, aerosols, and their interactions are synthesized. A new paradigm is proposed on investigating aerosol-monsoon interactions, in which natural aerosols such as desert dust, black carbon from biomass burning, and biogenic aerosols from vegetation are considered integral components of an intrinsic aerosol-monsoon climate system, subject to external forcing of global warming, anthropogenic aerosols, and land use and change. Future research on aerosol-monsoon interactions calls for an integrated approach and international collaborations based on long-term sustained observations, process measurements, and improved models, as well as using observations to constrain model simulations and projections.

Abstract. Four extreme haze episodes occurred in October 2014 in the North China Plain (NCP). To clarify the formation mechanism of hazes in autumn, strengthened observations were conducted in Beijing from 5 October to 2 November. The meteorological parameters, satellite data, chemical compositions and optical properties of aerosols were obtained. The hazes originated from the NCP, developing in the southwest and northeast directions, with the highest concentration of PM2.5 of 469 μg m−3 in Beijing. The NCP was dominated by a weak high pressure system during the haze episode, which resulted in low surface wind speed and relatively stagnant weather. Moreover, the wind slowed down around Beijing city. The secondary aerosols NO3− was always higher than that of SO42−, which indicated the motor vehicles played a more important part in the hazes in October 2014, even though the oxidation rate from SO2 to SO42− was faster than that of NOx to NO3−. Sudden increases of the concentrations of organic matter, Cl− and BC (black carbon) before each haze episode implied that regional transport of pollutants by biomass burning was important for haze formation during autumn. A satellite map of fire points and the backward trajectories of the air masses also indicated this pollution source. The distinct decrease in the PBL (planetary boundary layer) height during four haze episodes restrained the vertical dispersion of the air pollutants. Water vapor also played a vital role in the formation of hazes by accelerating the chemical transformation of secondary pollutants, leading to hygroscopic growth of aerosols and altering the thermal balance of the atmosphere.

Abstract. Aerosol hygroscopicity and cloud condensation nuclei (CCN) activity under background conditions and during pollution events are investigated during the Aerosol-CCN-Cloud Closure Experiment (AC 3 Exp) campaign conducted at Xianghe, China in summer 2013. A gradual increase in size-resolved activation ratio (AR) with particle diameter (D p ) suggests that aerosol particles have different hygroscopicities. During pollution events, the activation diameter (D a ) measured at low supersaturation (SS) was significantly increased compared to background conditions. An increase was not observed when SS was > 0.4 %. The hygroscopicity parameter (κ) was ∼ 0.31-0.38 for particles in accumulation mode under background conditions. This range in magnitude of κ was ∼ 20 %, higher than κ derived under polluted conditions. For particles in nucleation or Aitken mode, κ ranged from 0.20-0.34 for background and polluted cases. Larger particles were on average more hygroscopic than smaller particles. The situation was more complex for heavy pollution particles because of the diversity in particle composition and mixing state. A non-parallel observation CCN closure test showed that uncertainties in CCN number concentration estimates ranged from 30-40 %, which are associated with changes in particle composition as well as measurement uncertainties associated with bulk and size-resolved CCN methods. A case study showed that bulk CCN activation ratios increased as total condensation nuclei (CN) number concentrations (N CN ) increased on background days. The background case also showed that bulk AR correlated well with the hygroscopicity parameter calculated from chemical volume fractions. On the contrary, bulk AR decreased with increasing total N CN during pollution events, but was closely related to the fraction of the total organic mass signal at m/z 44 (f 44 ), which is usually associated with the particle's organic oxidation level. Our study highlights the importance of chemical composition in determining particle activation properties and underlines the significance of long-term observations of CCN under different atmospheric environments, especially regions with heavy pollution.

A B S T R A C T Carbon dioxide (CO 2 ) emissions and sinks in western China are estimated and implied from atmospheric CO 2 measured at Waliguan during the period of 1995Á2008. The observed CO 2 data are first classified as background, elevated, and sequestration, using a modified background identification method. Comparing it with two other methods tests the applicability of the method. By using this method, approximately 17.2%91.2% and 10.1%90.8% of all observed data are identified as elevated and sequestered CO 2 , respectively, the percentages (occurrence rates) for both of which increased during 1995Á2008. CO 2 emissions in western China have enhanced significantly in all seasons during the past 14 yr. Annual mean growth rates of CO 2 emissions in the region increased by Â8.4 Tg C yr(1 (3.9% yr (1 ) during 1995Á2008 but accelerated after 2000 to Â12.6 TgC yr (1 (6.2% yr (1 ). The growth rates of CO 2 emission in western China are lower than the rest of the country. The annual mean emissions in the country during 1995Á1999 and 2000Á2006 are thought to be approximately 5.5 and 6.5 times higher than in western China, respectively. However, the growth rates of CO 2 emissions in western China are higher than global increase rates as reported by other studies. CO 2 sinks in western China varied from 86.0 Tg C yr

Abstract. This study is concerned with the challenges of parameterizing cloud condensation nuclei (CCN) when changes in particle physicochemical properties occur, based on field measurements made at two distinct locations in China. The CCN nucleation efficiency of aerosols produced by local biomass burning was low. This is because the particles were freshly emitted with low oxidation level organics and thus are less hygroscopic. The CCN activation efficiency was enhanced significantly when the site was under the influence of air transported from far away, during which aerosol properties changed with more hygroscopic secondary organic and inorganic components. The influence of the variation in particle number size distribution (PSD) on estimating CCN number concentrations (NCCN) was examined, showing poor correlation (slope = 0.8, R2 = 0.35) of predicted and measured NCCN. While the PSD is found to play a dominant role in predicting (NCCN), a strong dependence of NCCN on the mass fraction of organics (xorg) was also noted. NCCN was underestimated by 52 and 13 % at supersaturation levels of 0.13 and 0.76 %, respectively, when xorg = 66 %. NCCN was slightly overestimated, or in good agreement, with observations when xorg was reduced to 35 % (R2 = 0.94). The applicability of the CCN activation spectrum obtained at Xinzhou to the Xianghe site, about 400 km to the northeast of Xinzhou, was investigated, with the goal of further examining the sensitivity of CCN to aerosol type. Overall, the mean CCN efficiency spectrum derived from Xinzhou performs well at Xianghe when the supersaturation levels are > 0.2 % (overestimation of 2–4 %). However, NCCN was overestimated by ∼ 20 % at supersaturation levels of < 0.1 %. This suggests that the overestimation is mainly due to the smaller proportion of aged and oxidized organic aerosols present at Xianghe compared with Xinzhou.