ALBERT

Description: Traditional "physically-based" hydrological models consider soil to be key in hydrology. According to these models, soil properties determine water movement in both saturated and unsaturated zones, described by matrix-flow formulas known as the Darcy-Richards equations. Soil properties would also determine plant available moisture and thereby control transpiration. These models are data demanding, computationally intensive, parameter rich and founded on a wrong assumption. Instead, we argue it is not the soil that is in control of hydrology, it is the ecosystem. Our assumption is motivated by several arguments. Firstly, in well-developed soils the dominant flow mechanism is preferential, which is not particularly related to soil properties. Secondly, we observe that it is the ecosystem, rather than the soil, that determines the land-surface water balance and hydrological processes. Top-down analysis by large-sample datasets reveal that soil properties are often a poor predictor of hydrological signatures. Bottom-up hydrological models usually do not directly use field measurements of the soil, but "rebalance" the observed soil texture and translate it to soil structure by vegetation indices. Thus, soil-based models may be appropriate at small scale in non-vegetated and agricultural environments, but introduce unnecessary complexity and sub-optimal results in catchments with permanent vegetation. Progress in hydrology largely relies on abandoning the compartmentalized approach, putting ecosystem at the centre of hydrology. This paradigm shift is needed to build more realistic and simpler hydrological models for a changing environment, which is an essential issue in the Decade of Panta Rhei.

Description: New particle formation (NPF) and subsequent particle growth are important sources of condensation nuclei (CN) and cloud condensation nuclei (CCN). While a number of observations have shown positive contributions of NPF to CCN at low supersaturation, negative NPF contributions were often simulated. Using the observations in a typical coastal city of Qingdao, we thoroughly evaluate the simulated number concentrations of CN and CCN using a NPF-explicit parameterization embedded in WRF-Chem model. In terms of CN, the initial simulation shows large biases of particle number concentrations at 10–40 nm (CN10–40) and 40–100 nm (CN40–100). By adjusting the process of gas-particle partitioning, including mass accommodation coefficient of sulfuric acid, the phase changes of primary organic aerosol emissions and the condensational amount of nitric acid, the concomitant improvement of the particle growth process yields a substantial reduction of overestimates of CN10–40 and CN40–100. Regarding CCN, SOA formed from the oxidation of semi-volatile and intermediate volatility organic vapors (SI-SOA) yield is an important contributor. In the original WRF-Chem model with 20 size bins setting, the yield of SI-SOA is too high without considering the differences in oxidation rates of the precursors. Lowering the SI-SOA yield results in much improved simulations of the observed CCN concentrations. On the basis of the bias-corrected model, we find substantial positive contributions of NPF to CCN at low supersaturation (~0.2 %) in Qingdao and over the broad areas of China, primarily due to the competing effects of increasing particle hygroscopicity surpassing that of particle size decrease. This study highlights the potentially much larger NPF contributions to CCN on a regional and even global basis.

Description: New particle formation (NPF) induces a sharp increase in ultrafine particle number concentrations and potentially acts as an important source of cloud condensation nuclei (CCN). As the densely populated area of China, the Yangtze River Delta (YRD) region shows a high frequency of observed NPF events at the ground level, especially in spring. Although recent observational studies suggested a possible connection between NPF at the higher altitudes and ground level, the role played by vertical mixing, particularly in the planetary boundary layer (PBL) is not fully understood. Here we integrate measurements in Nanjing on 15–20 April 2018, and the NPF-explicit Weather Research and Forecast coupled with chemistry (WRF-Chem) model simulations to better understand the governing mechanisms of the NPF and CCN. Our results indicate that newly formed particles at the boundary layer top could be transported downward by vertical mixing as the PBL develops. A numerical sensitivity simulation created by eliminating aerosol vertical mixing suppresses both the downward transport of particles formed at a higher altitude and the dilution of particles at the ground level. The resulting higher Fuchs surface area at the ground level, together with the lack of downward transport, yields a sharp weakening of NPF strength and delayed start of NPF therein. The aerosol vertical mixing, therefore, leads to a more than double increase of surface CN10–40 and a one third decrease of boundary layer top CN10–40. Additionally, the continuous growth of nucleated ultrafine particles at the boundary layer top is strongly steered by the upward transport of condensable gases, with close to half increase of particle number concentrations in Aitken mode and CCN at a supersaturation rate of 0.75%. The findings may bridge the gap in understanding the complex interaction between PBL dynamics and NPF events, reducing the uncertainty in assessing the climate impact of aerosols.