Ocean acidification is a direct consequence of the CO2 increase and a threat for marine ecosystems. The Arctic Ocean is the most vulnerable region with the strongest pH decrease compared to other regions of the ocean. However, there are very few direct pH measurements due to the lack of a system for continuous measurements although efforts are being made to develop such a system. As a consequence, long-term trends in surface pH are often calculated using two other measured ocean carbon system parameters. The oceanographic community is currently in need of pH sensor technology that will affordably, accurately and efficiently measure ocean chemistry from its shallowest to its deepest waters. MACAO focuses on the development and testing of accurate pH sensors capable of long-term monitoring of the water column while deployed on different platforms (buoys, profilers, ships). The sensor development effort will implement a novel hybrid approach, utilizing two different and complementary measurement techniques (the colorimetric method and the potentiometric method) to generate temporally dense and highly accurate data. The Arctic is a key area for pH monitoring but very hard to access. Autonomous sensors are particularly needed in this region. For testing the hybrid sensors and generating field data, we will focus on profilers in the Arctic to take advantage of on-going scientific projects such as IAOOS Equipex (http://iaoos.ipev.fr) and MOSAIC (www.mosaicobservatory.org). The IAOOS Equipex platforms are installed on the ice to drift with it. They are equipped with a moving depth profiler and autonomous instruments allowing simultaneous observations of key variables in the ocean, atmosphere and ice. Observations are transmitted in real time via a satellite link. For measurements at depth, the IAOOS platform is equipped with a vertical cable guiding the moving profiler. Current profilers are typically equipped with sensors for depth, temperature, conductivity or salinity, and dissolved oxygen. We will add accurate pH measurement capability to the profilers, to study the dynamics of carbon parameters and acidification in the Arctic Ocean. The team at LOCEAN involves biogeochemists working on the ocean carbon cycle and physical oceanographers working on the physics of the Arctic Ocean. Fluidion (http://fluidion.com) is a high-technology company specializing in cutting-edge products based on patented MEMS and microfluidic technology. Fluidion technology addresses markets as diverse as water quality/environmental monitoring, industrial process water, and oceanography/subsea applications. The work is structured in four work packages (WP). WP1 focuses on the development of the pH sensors, WP2 is related to the tests and field measurements and the raw data collected during the project will be analysed and validated in WP3. A dedicated WP4 will focus on communication, website and public outreach as well as valorisation of the results and data obtained during the project. Fluidion expects to generate IP (patents & know-how) from the work related to this project. The economic and scientific benefits are interrelated: the ocean pH sensor market is dominated by academic institutions and research groups, and therefore external academic validation of sensor performance is key for market penetration. The requested funding for LOCEAN corresponds to small equipment, NKE subcontracting, laboratory measurements, travel expenses, publications, a Ph.-D fellowship and engineer contract. For fluidion, the costs are related to personnel, equipment, IP protection, and outsourcing for mechanical design and manufacturing, two surface hybrid and two profiling hybrid sensors.