Emission of ammonia into the atmosphere is well known to be detrimental to the environment due to deposition in natural ecosystems and direct toxicity to sensitive plants. There is new concern that ammonia from agricultural sources contributes to the formation of secondary particulates, which pose a threat to human health. A new inventory of emissions from agricultural sources has been developed for Canada. This inventory accounts for emissions from principal livestock classes (beef and dairy cattle, pigs and poultry) and from fertilizer application. The inventory is based on two new national farm activity surveys, expert opinion and published emission factors. Monthly emissions values were calculated for each of 12 ecoregions. The number of available days for manure application was determined, based on soil trafficability, so that the emission intensity for low and high application days could be calculated. Results to date show generally low emission in winter months and a rapid increase in April or May due to manure application. Mid-summer emissions were generally lower depending on sector, and emissions in fall relative to spring varied with ecoregion. Relative importance of housing, storage and field application varied with livestock sector. For reducing PM risk, it will be necessary to reduce emission in critical periods in areas like southern ON, QC southern AB and coastal BC. The emission data are being used as input for an atmospheric model that predicts chemical transformation, PM formation and long range transport of gaseous ammonia and particulate ammonium.