Following the emergence of a novel coronavirus in Wuhan, the People’s Republic of China instituted a number of strict lockdown and more general socio-economic shutdown measures throughout the country starting in late January and continuing into February 2020 in order to arrest the spread of the disease. This resulted in a sharp economic contraction unparalleled in recent Chinese history. Satellite remote sensing shows that nitrogen oxide pollution declined by an unprecedented amount (~50% regionally) from its expected unperturbed value, but regional-scale column aerosol loadings and cloud microphysical properties were not detectably affected. The disparate impact may be tied to differential economic impacts of the shutdown, in which the transportation sector, a disproportionate source of nitrogen oxide emissions, underwent drastic declines (~90% reductions in passenger traffic), whereas industry and power generation, responsible for >90% of particulate emissions, were relatively less affected (~10% reductions in electricity and thermal power generation).

Aerosol-cloud interactions are both uncertain and important in global and regional climate models, and especially in the southeast Atlantic Ocean. This uncertainty in the region is largely due to two correlated factors---the expansive, bright, semi-permanent stratocumulus cloud deck and the fact that southern Africa is the largest source of biomass-burning aerosols in the world. We study this region using the WRF-Chem model with CAM5 aerosols and in situ observations from the ORACLES and LASIC field campaigns in August-October of 2016 through 2018. We compare aerosol and cloud properties to measure and improve model performance and expand upon observational findings of aerosol-cloud effects. Relevant comparison variables include aerosol number concentration, mean particle diameter and spread, CCN activation tendency, hygroscopicity, and cloud droplet number concentrations. Specifically, our approach is to analyze colocated model data along flight tracks to resolve aerosol-cloud interactions. Within and between single-day flights, there is high spatiotemporal variability that can get lost to large-scale averaging analyses. We have found that CCN is substantially under-represented in the model compared to observations. For a given aerosol number concentration, size, supersaturation and hygroscopicity, the model will consider fewer particles as CCN than observations indicate. We plan to explore this result further, diagnosing the model-observation differences more consistently and updating the model with more physically accurate values of aerosol size, concentration, or hygroscopicity based on observations. We will also intercompare multiple instrument platforms involved with the ORACLES and LASIC campaigns. With improved small-scale aerosol-cloud interactions, this work also shows promise to substantially improve that representation in climate models.

The influence of aerosol particles on cloud reflectivity remains one of the largest sources of uncertainty in our understanding anthropogenic climate change. Commercial shipping constitutes a large and concentrated aerosol perturbation in a meteorological regime where clouds have a disproportionally large effect on climate. Yet, to date, studies have been unable to detect climatologically-relevant cloud radiative effects from shipping, despite models indicating that the cloud response should produce a sizable negative radiative forcing (perturbation to Earth’s energy balance). We attribute a significant increase in cloud reflectivity to enhanced cloud droplet number concentrations within a major shipping corridor in the southeast Atlantic. Prevailing winds constrain emissions around the corridor, which cuts through a climatically-important region of expansive low-cloud cover. We use universal kriging, a classic geostatistical method, to estimate what cloud properties would have been in the absence of shipping. In the morning, cloud brightening is consistent with changes in microphysics alone, whereas in the afternoon, increases in cloud brightness from microphysical changes are offset by decreases in the total amount of cloud water. We find a radiative forcing in the southeast Atlantic shipping corridor two orders of magnitude greater than previous observational estimates. Approximately five years of data are required to identify a clear signal. Extrapolating our results globally, we calculate an effective radiative forcing due to aerosol-cloud interactions in low clouds of -0.62 W/m2 (-1.23 to -0.08 W/m2). The unique setup in the southeast Atlantic could be an ideal test for the representation of aerosol-cloud interactions in climate models.