We investigate and report here the air stability of air-processed mixed halide CH3NH3PbI3-xClx perovskite solar cells. The solar cells were prepared in planar heterojunction as well as mesoscopic structures using four perovskite precursors having different wt% concentrations, viz. 13.7, 17.5, 24.1 and 38.9 wt%. Compact TiO2 layer (TiO2-bl) was used as electron transport layer, whereas poly(3-hexylthiophene) (P3HT) was used as hole transport layer. All the solar cells were processed in identical conditions. Irrespective of device structures, the higher precursor concentration resulted in larger crystalline grains, which led to higher power conversion efficiency in the solar cells. The solar cells were stored in dark and tested for their stability as per the International Summit on Organic Photovoltaic Stability (ISOS) protocol ISOS-D-1. The degradation profiles of solar cells have also been studied in natural outdoor conditions under direct sunlight as per the ISOS-O-1 protocols. Interestingly, the solar cells having larger perovskite grains exhibited higher efficiency but inferior air stability compared to those having smaller grains. Poorer stability of solar cells having larger perovskite grains has been attributed to poorer morphological and compositional stability of perovskite films.