Overview of the Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) Field Campaign

Abstract The Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) field project aimed to advance the understanding of coupled meteorological and chemical processes in the atmospheric boundary layer during the seasonal increase in sea ice fracturing in spring. CHACHA sought to understand the interactions between this changing snow-covered surface, surface-coupled clouds, sea spray aerosols, multiphase halogen chemistry, and impacts of emissions from oil and gas extraction on atmospheric chemistry. The project measured greenhouse gases, reactive gases, size-resolved aerosol number concentrations, cloud microphysical properties, and meteorological conditions in real time, while also collecting particles for offline analysis. Two instrumented aircraft were deployed: the Purdue University Airborne Laboratory for Atmospheric Research and the University of Wyoming King Air. Flights were conducted out of Utqiaġvik, Alaska, between 21 February and 16 April 2022, sampling air over snow-covered and newly frozen sea ice in the Beaufort and Chukchi Seas, over open leads, and over the snow-covered tundra of the North Slope of Alaska, including the oil and gas extraction region near Prudhoe Bay. Observations showed that reactive bromine gases generally peaked near the snow-covered surface and decayed rapidly within the lowest few hundred meters where ozone was depleted, with concentrations reduced by nitrogen oxides emitted from oil fields. Cloud microphysical measurements revealed that thin clouds over and downwind of leads grew in vertical extent after contact with open water. Results from dropsondes indicated that convective boundary layers developed over leads, with depths ranging from 250 to 850 m depending on the fetch. Significance Statement Given the rapid pace of physical and chemical changes occurring in the Arctic, and the critical yet insufficiently understood roles of atmospheric turbulence, chemistry, and aerosol-cloud feedbacks, the Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) project aimed to enhance the understanding of the processes governing vertical variability of halogens and aerosols and, in turn, improve predictive capabilities for the Arctic’s physical and chemical state, both now and in the future.