Abstract. Sea ice coverage is a key indicator of changes in polar and global climate. Observational estimates of the area and extent of sea ice are primarily derived from passive microwave surface emissions, which are used to develop gridded products of sea ice concentration (SIC). Passive microwave (PM) satellite sensors remain the sole global product for understanding SIC variability. Here, in Part I of a two-part study, we use a dataset of more than 70,000 high-resolution airborne optical classified images from Operation IceBridge, and we first identify biases in commonly used passive microwave products in areas with thin sea ice fractures. We find that passive microwave-derived SIC products overestimate true SIC with biases on average 4.4 % in winter and 3.2 % in summer. We show that ICESat-2, a laser altimeter operational since 2018, has the capacity to sample these thin fractures, with good agreement between ICESat-2 surface-type classifications and near-coincident Worldview and Sentinel-2 data in winter. Using the ICESat-2 surface type classifications, we introduce a new derived product, the linear ice fraction (LIF) and discuss its potential for representing a two-dimensional sea ice concentration field. This paper highlights the biases present in PM-derived SIC and makes a case for considering the integration of ICESat-2 and its high-precision measurements of the sea ice surface to enhance future SIC estimations. In Part II, we identify and evaluate biases associated with the development of a gridded LIF product and compare it to existing PM-SIC data.