There has been an increased interest in alternative housing for laying hens in certain parts of the world, including the U.S. Associated with the movement are many questions concerning sustainability of such systems. This study continually quantified concentrations and emissions of ammonia (NH₃), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and particulate matter (PM₁₀ and PM_2.5) for two side-by-side aviary barns each housing 50,000 Hy-Line brown laying hens, located in the Midwestern U.S. The gaseous concentrations were continually monitored using an infrared photoacoustic multi-gas analyzer, while the PM concentrations were measured with tapered element oscillating microbalances (TEOMs). Barn ventilation rate was determined by monitoring the operation time of ventilation fans that had been calibrated in situ. Nineteen consecutive months of monitored data (June 2010 to December 2011) are analyzed and presented. Daily indoor aerial concentrations (mean ±SD) were 8.7 (±8.4) ppm NH₃, 1,636 (±1,022) ppm CO₂, 10.0 (±6.8) ppm CH₄, 2.3 (±1.6) mg m^-3 PM₁₀, and 0.25 (±0.26) mg m^-3 PM_2.5. The aerial emissions are expressed as quantities per hen, per animal unit (AU, 500 kg body weight), and per kg egg output. Daily emission rates (g bird^-1) were 0.15 (±0.08) NH₃, 75 (±15) CO₂, 0.09 (±0.08) CH₄, 0.11 (±0.04) PM₁₀, and 0.008 (±0.006) PM_2.5. The results were compared to reported emission values for conventional (high-rise and manure-belt cage) U.S. laying-hen housing systems. Data from this study provide baseline concentration and emission values for the aviary housing system in the Midwestern U.S., which will also contribute to improvement of the U.S. national air emissions inventory for farm animal operations.