No abstract

Abstract. The hygroscopicity of organic aerosol (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOA remains poorly characterized, especially in the relatively polluted environment on the North China Plain (NCP). Here we conducted simultaneous wintertime measurements of bulk aerosol chemical compositions of PM2.5 and PM1 and bulk aerosol hygroscopicity of PM10 and PM1 on the NCP using a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM) and a humidified nephelometer system which measures the aerosol light-scattering enhancement factor f(RH). A method for calculating κOA based on f(RH) and bulk aerosol chemical-composition measurements was developed. We found that κOA varied in a wide range with significant diurnal variations. The derived κOA ranged from almost 0.0 to 0.25, with an average (±1σ) of 0.08 (±0.06) for the entire study. The derived κOA was highly correlated with f44 (fraction of m∕z 44 in OA measured by CV-ToF-ACSM), an indicator of the oxidation degree of OA (R=0.79), and the relationship can be parameterized as κOA=1.04×f44-0.02 (κOA=0.3×O:C-0.02, based on the relationship between the f44 and O∕C ratio for CV-ToF-ACSM). On average, κOA reached the minimum (0.02) in the morning near 07:30 local time (LT) and then increased rapidly, reaching the peak value of 0.16 near 14:30 LT. The diurnal variations in κOA were highly and positively correlated with those of mass fractions of oxygenated OA (R=0.95), indicating that photochemical processing played a dominant role in the increase in κOA in winter on the NCP. Results in this study demonstrate the potential wide applications of a humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments and highlight that the parameterization of κOA as a function of OA aging processes needs to be considered in chemical transport models for better evaluating the impacts of OA on cloud formation, atmospheric chemistry, and radiative forcing.

No abstract

Abstract. Carbon (C) release from foliar litter is a primary component in C exchange between the atmosphere and terrestrial ecosystems, but little information is currently related to the effects of freezing and thawing dynamics on C release of foliar litter in cold regions. A two-year field litter decomposition experiment was conducted along an altitudinal gradient (∼2700 m to ∼3600 m) to mimic temperature increases in the eastern Tibetan Plateau. C release was investigated for fresh foliar litter of spruce, fir and birch. The onset of the frozen stage, deep frozen stage, and thawing stage were partitioned according to changes in freezing and thawing dynamics of each winter. High C release was observed in lower altitudes during winter stages, but higher altitudes exhibited high C release during growing season stages. The deep frozen stage showed higher rates of C release than other stages in the second year of decomposition. Negative degree-days showing freezing degree were correlated to C release rates for the deep frozen stages in both years, and this relationship continued for the duration of the experiment, indicating that changes in freezing can directly modify C release from foliar litter. The results suggested that climate warming could delay the onset of C release in fresh litter in this cold region.