Indoor aerosols can adversely affect human health as we increasingly spend more time indoors. One of the aerosol research challenges is measuring fine and ultrafine aerosol particles with nanoscale dimensions. Spectroscopic tools, often diffraction-limited, cannot access the intra-particle heterogeneity. In this work, we extend the non-invasive nanoscopy method of peak force infrared (PFIR) microscopy to study indoor aerosols. Laboratory-generated fine bioaerosols were collected after filtration with a surgical face mask to serve as a benchmark sample, followed by a variety of field-collected indoor aerosols with and without the filtration of a facemask. A general heterogeneity is observed in individual aerosol particles, despite their nanoscale dimension. The presence of protein, triglycerides, and salt is detected through chemical and mechanical mapping. The PFIR microscopy is suitable to identify the composition of fine and ultrafine aerosols. Its application is particularly meaningful for understanding the particle structure to reduce aerosol-related transmission of diseases.