G. Luo

Effect of solar variations on particle formation and cloud condensation nuclei

The impact of solar variations on particle formation and cloud condensation nuclei (CCN), a critical step for one of the possible solar indirect climate forcing pathways, is studied here with a global aerosol model optimized for simulating detailed particle formation and growth processes. The effect of temperature change in enhancing the solar cycle CCN signal is investigated for the first time. Our global simulations indicate that a decrease in ionization rate associated with galactic cosmic ray flux change from solar minimum to solar maximum reduces annual mean nucleation rates, number concentration of condensation nuclei larger than 10 nm (CN10), and number concentrations of CCN at water supersaturation ratio of 0.8% (CCN0.8) and 0.2% (CCN0.2) in the lower troposphere by 6.8%, 1.36%, 0.74%, and 0.43%, respectively. The inclusion of 0.2 C temperature increase enhances the CCN solar cycle signals by around 50%. The annual mean solar cycle CCN signals have large spatial and seasonal variations: (1) stronger in the lower troposphere where warm clouds are formed, (2) about 50% larger in the northern hemisphere than in the southern hemisphere, and (3) about a factor of two larger during the corresponding hemispheric summer seasons. The effect of solar cycle perturbation on CCN0.2 based on present study is generally higher than those reported in several previous studies, up to around one order of magnitude.

Long‐Term Trend of Gaseous Ammonia Over the United States: Modeling and Comparison With Observations

Abstract The concentrations of atmospheric ammonia ([NH3]) have been observed to be increasing over the United States in the last decade, especially in Eastern United States. It is important to understand this temporal trend and variation due to the role of NH3 in particle formation and its ecological effects. Here the long‐term trend of [NH3] over the United States is investigated using GEOS‐Chem, a global 3‐D tropospheric chemistry model, and is corroborated with empirical evidence from the Ammonia Monitoring Network. Model simulations, consistent with observations, show increase in [NH3] over the United States from 2001 to 2016, with magnitude largest in the East (~5% to 12%/year) and smallest in the West (~0% to 5%/year). Reasons for this are examined, and evidence for the role of decreasing SO2 and NOx emissions in increasing [NH3] is provided. The contributions of meteorology and NH3 emission changes to the [NH3] increase appear to be small during the period. Our sensitivity study suggests that decreasing SO2 and NOx emissions over the United States owing to stringent regulations explain about 2/3 and 1/3 of the increase in [NH3], respectively. This effect is different for various NH3 and SO2 and NOx regimes. Given the continued reduction of SO2 and NOx emissions due to U.S. regulations mainly aimed at PM2.5 reduction, the present results are important towards better assessing the environmental impact of emission controlling policies.

Ion mediated nucleation and anthropogenic aerosol indirect radiative forcing

A clear understanding of particle formation mechanisms is critical for assessing aerosol indirect radiative forcing (IRF) and associated climate feedback processes. Recent studies reveal the importance of ion-mediated nucleation (IMN) in generating new particles and cloud condensation nuclei (CCN) in the atmosphere. The IMN scheme has been implemented in two widely used community models (CAM5/MAM3 and GEOS-Chem/APM). Here we employ the two models to assess the effect of anthropogenic emissions on the aerosol IRF. Our model simulations, with new particle formation rates calculated by the IMN scheme, show a significant impact of anthropogenic emissions on global CCN abundance, cloud properties, and cloud radiative forcing. The total aerosol indirect radiative forcing based on CAM5/MAM3 is-1.81 W m−2. The first aerosol IRF based on GEOS-Chem/APM is −0.69 W m−2.