Regional climate models have been widely used to examine the biophysical effects of afforestation, but their performances in this respect have rarely been evaluated. To fill this knowledge gap, an evaluation method based on the “space for time” strategy is proposed here. Using this method, we validate the performances of three regional models, the Regional Climate Model (RegCM), Weather Research and Forecasting (WRF) model and the WRF model run at a convection-permitting resolution (WRF-CP), in representing the local biophysical effects of afforestation over continental China against satellite observations. The results show that WRF and WRF-CP can not accurately describe afforestation-induced changes in surface biophysical properties, e.g. albedo or leaf area index. Second, all models exhibit poor simulations of afforestation-induced changes in latent and sensible heat fluxes. In particular, the observed increase in the summer latent heat due to afforestation is substantially underestimated by all models. Third, the models are basically reasonable in representing the biophysical impact of afforestation on temperature. The cooling of the daily mean surface temperature and 2-meter temperature in summer are reproduced well. Nevertheless, the mechanism driving the cooling effect may be improperly represented by the models. Moreover, the models perform relatively poorly in representing the response of the daily minimum surface temperature to afforestation. This highlights the necessity of evaluating the representation of the biophysical effects by a model before the model is employed to carry out afforestation experiments. This study serves as a test bed for validating regional model performance in this respect.

Abstract. New particle formation processes contribute significantly to the number concentration of ultrafine particles (UFP), and have great impacts on human health and global climate. Iodine oxoacids (HIOx, including iodic acid, HIO3 and iodous acid, HIO2) have been observed in pristine regions and proved to dominate NPF events at some sites. However, the knowledge of HIOx in polluted urban areas is rather limited. Here, we conducted a long-term comprehensive observation of gaseous iodine oxoacids and sulfuric acid in Beijing from January 2019 to October 2021 and also in Nanjing from March 2019 to February 2020, and investigated the contribution of HIOx to UFP number concentration in urban environments. HIO3 concentration is highest in summer, up to 2.85×106 cm-3 and 2.78×106 cm-3 in Beijing and Nanjing, respectively, and is lowest in winter, with a more prominent seasonal variation than H2SO4. HIO3 concentration shows a clear diurnal pattern at both sites with a daily maximum at around noontime, similar to the atmospheric temperature, radiation and ozone (O3) levels. HIO2 concentration has the same diurnal and seasonal trend as HIO3 but is overall about one order of magnitude lower than HIO3 concentration. Back trajectory analysis suggests that the sources for inland iodine species could be a mix of marine and terrestrial origins, both having peak iodine emission in warm seasons. While the contribution of HIO2 to particle growth is marginal in Beijing and Nanjing, our results demonstrate that HIO3 enhances the particle survival probability of sub-3 nm particles by about 40 % (median) and occasionally by more than 100 % in NPF events, suggesting HIOx are non-negligible contributor to UFPs in polluted urban areas. As the growth contribution from HIO3 and H2SO4 is similar on a per-molecule basis, we propose that the sum of HIO3 and H2SO4 could be used to estimate sub-3 nm particle growth of inorganic acid origin, in the polluted atmospheres with a significant amount of HIOx.

TextS1. Field measurements of sulphur dioxide (SO2) at two sites SO2 is measured continuously at the SORPES station using a Thermo TEI 43i. At BUCT/AHL station, SO2 is measured with the same analyser. Due to an instrument malfunction, SO2 concentration is discarded in October and November, 2020 at the BUCT/AHL station. The long-term time traces of daytime (08:00~16:00 LT) mean SO2 and its seasonal and monthly variations at both sites are depicted in Fig. S1. SO2 is primarily emitted through coal combustion in heating seasons in Beijing. The official onset of the heating period in Beijing is 15 th November and the heating ends on 15 th March the following year. The SO2 at BUCT/AHL site is strongly enhanced by the release of SO2 in heating seasons (grey shade area). It is worth noting that the measured concentrations of SO2 at the BUCT/AHL station is higher than that of the SORPES station in winter (January and February), which could partially explain the higher concentration of H2SO4 measured in Beijing. S2. Classification of growth time span at SORPESA Neutral cluster and Air Ion Spectrometer (NAIS, Airel Ltd., Estonia) (Manninen et al., 2016) was deployed to detect the particle number size distribution (PNSD) in the early stages of NPF at the SORPES station. The negatively charged particles in the size range of around 0.8 nm to 42 nm were measured to