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Abstract. Aerosol effects on low warm clouds over the Yangtze River Delta (YRD, eastern China) are examined using co-located MODIS, CALIOP and CloudSat observations. By taking the vertical locations of aerosol and cloud layers into account, we use simultaneously observed aerosol and cloud data to investigate relationships between cloud properties and the amount of aerosol particles (using aerosol optical depth, AOD, as a proxy). Also, we investigate the impact of aerosol types on the variation of cloud properties with AOD. Finally, we explore how meteorological conditions affect these relationships using ERA-Interim reanalysis data. This study shows that the relation between cloud properties and AOD depends on the aerosol abundance, with a different behaviour for low and high AOD (i.e. AOD< 0.35 and AOD> 0.35). This applies to cloud droplet effective radius (CDR) and cloud fraction (CF), but not to cloud optical thickness (COT) and cloud top pressure (CTP). COT is found to decrease when AOD increases, which may be due to radiative effects and retrieval artefacts caused by absorbing aerosol. Conversely, CTP tends to increase with elevated AOD, indicating that the aerosol is not always prone to expand the vertical extension. It also shows that the COT-CDR and CWP (cloud liquid water path)-CDR relationships are not unique, but affected by atmospheric aerosol loading. Furthermore, separation of cases with either polluted dust or smoke aerosol shows that aerosol-cloud interaction (ACI) is stronger for clouds mixed with smoke aerosol than for clouds mixed with dust, which is ascribed to the higher absorption efficiency of smoke than dust. The variation of cloud properties with AOD is analysed for various relative humidity and boundary layer thermodynamic and dynamic conditions, showing that high relative humidity favours larger cloud droplet particles and increases cloud formation, irrespective of vertical or horizontal level. Stable atmospheric conditions enhance cloud cover horizontally. However, unstable atmospheric conditions favour thicker and higher clouds. Dynamically, upward motion of air parcels can also facilitate the formation of thicker and higher clouds. Overall, the present study provides an understanding of the impact of aerosols on cloud properties over the YRD. In addition to the amount of aerosol particles (or AOD), evidence is provided that aerosol types and ambient environmental conditions need to be considered to understand the observed relationships between cloud properties and AOD. [ABSTRACT FROM AUTHOR]

Aerosol effects on summertime low warm clouds over the Yangtze River Delta (YRD) are examined using co-located MODIS, CALIOP and CloudSat observations. By taking the vertical locations of aerosol and cloud layers into account, we use simultaneously observed aerosol and cloud data to investigate relationships between cloud properties and the amount of aerosol particles (using aerosol optical depth, AOD, as a proxy). Also, we investigate the impact of aerosol types on the variation of cloud properties with AOD. Finally, we explore how meteorological conditions affect these relationships using ERA Interim Reanalysis data. This study shows that the relation between cloud droplet effective radius (CDR) and AOD depends on the aerosol abundance, with a different behaviour for low and high AOD (i.e. AOD <> 0.3). Cloud fraction (CF) is found to be little dependent on the AOD when aerosol and cloud physically interact, but has a positive relation in case of well-separated clouds. Cloud optical Thickness (COT) is found to decrease when AOD increases, which may be due to radiative effects and retrieval artefacts caused by absorbing aerosol. Conversely, cloud top pressure (CTP) tends to increase with elevated AOD, indicating that the aerosol is not always prone to expand the vertical extension. Furthermore, separation of cases with either polluted dust or smoke aerosol shows that COT and CF are smaller for clouds mixed with smoke aerosol which is ascribed to the higher absorption efficiency of smoke than dust. The variation of cloud properties with AOD is analysed for different values of relative humidity (RH) and boundary layer thermodynamic and dynamic conditions, showing that high relative humidity and upward motion of air parcels can enhance the strength of aerosol-cloud interaction, especially pronounced in heavily polluted conditions than in moderately polluted conditions. Meteorological conditions play a weak role in the COT-AOD and CTP-AOD relationships throughout the range of AOD. Overall, the interpretation of the observed relationships between cloud properties and AOD requires that ambient environmental conditions are considered in addition to the aerosol and cloud parameters. [ABSTRACT FROM AUTHOR]

A multi-layer ozone (O3) dry deposition model has been implemented into SOSAA (a model to Simulate the concentrations of Organic vapours, Sulphuric Acid and Aerosols) to improve the representation of O3 within and above the forest canopy in the planetary boundary layer where O3 is a key oxidant agent of biogenic volatile organic compounds (BVOCs) and thus affecting organic aerosol processes. We aim to predict the O3 uptake by a boreal forest canopy under varying environmental conditions and analyse the influence of different factors on total O3 uptake by the canopy as well as the vertical distribution of deposition sinks inside the canopy. We evaluated the newly implemented canopy deposition model by an extensive comparison of simulated and observed O3 fluxes and concentration profiles within and above the boreal forest canopy at SMEAR II (the Station to Measure Ecosystem-Atmosphere Relation II) in Hyytiälä, Finland, in August, 2010. The first half of August showed extremely warm and dry conditions which were probably representative for summer conditions prevailing at this site in future. The simulated O3 turbulent fluxes at the canopy top and the O3 concentration profiles inside the canopy agreed well with the measurement, which indicated that the turbulent transport and the O3 dry deposition onto the canopy and soil surface appeared to be properly represented in the model. In this model, the fraction of wet surface on vegetation leaves was parameterised according to the ambient relative humidity (RH). Model results showed that when RH was larger than 70% the O3 uptake onto wet skin contributed 48.6% to the total deposition during nighttime and 22.0% during daytime. In addition, most of the O3 deposition occurred below 0.8 hc (canopy height) at this site. The contribution of sub-canopy deposition below 4.2 m was modelled to be about 40% of the total O3 deposition during daytime which was similar to previous studies. Whereas for nighttime, the simulated sub-canopy deposition contributed 40-65% to the total O3 deposition which was about two times as that in previous studies (25-30%). The overall contribution of soil uptake was estimated as 36.5%. These results indicated the importance of non-stomatal O3 uptake processes, especially the uptake on wet skin and soil surface. Furthermore, a qualitative evaluation of the chemical removal time scales indicated that the chemical removal rate within canopy was about 5% of the total deposition flux at daytime and 16% at nighttime under current knowledge of air chemistry. The evaluation of the O3 deposition processes provides improved understanding about the mechanisms involved in the removal of O3 for this boreal forest site which are also relevant to the removal of other reactive compounds such as the BVOCs and their oxidation products, which will be focus of a follow-up study. [ABSTRACT FROM AUTHOR]