First Online Observation of Aerosol Total Organic Nitrogen at an Urban Site: Insights Into the Emission Sources and Formation Pathways of Nitrogenous Organic Aerosols

Nitrogen (N) is a constituent element of ambient organic aerosol; however, we have limited knowledge of the total quantity of organic N (ON) and its relative importance to aerosol N due to a lack of effective analytical methods for ON. Here, we report first online measurements of ON in particulate matter (PM2.5) in urban Shanghai in summer of 2021 using a newly developed aerosol nitrogen analyzer. ON accounted for an average of 25% of total aerosol N and varied in the range of 5%-64% on the hourly scale. Hourly variations identified four distinct types of ON episodes that occurred in different hours of a day. Analysis aided by concurrently measured source indicative pollutants revealed that vehicular emissions, nocturnal chemistry, daytime photochemistry or combined primary sources, and secondary chemistry could drive up ON under different day-to-day atmospheric conditions. Our results help bridge the knowledge gap in the emissions and formation of organic aerosol. Atmospheric particulate matter (PM) exerts profound influences on urban air quality and human health. A large fraction of ambient aerosol is organic matter, in which nitrogen (N)-containing molecules are known constituents. However, our knowledge is extremely limited about the abundance and sources of nitrogenous organics due to a lack of convenient measurement tools to quantify the total aerosol organic nitrogen (ON) in the past. This hurdle has been lifted through our recently developed aerosol nitrogen analyzer that can provide sensitive and simultaneous determinations of aerosol organic and inorganic N. Here, we present a first report of bihourly measured ON at an urban site of Shanghai, a megacity in China during a period of two and a half summer months. ON represented 25% of aerosol N on average and hourly data varied from 5% to 64%. The results indicated importance of ON comparable to that of inorganic N species (i.e., NH4+ ${\text{NH}}_{4}<^>{+}$, NO3- ${\text{NO}}_{3}<^>{-}$) and that the sources/chemistry of ON molecules are considerably dynamic. Past attention has been predominantly placed on inorganic N pollutants or carbonaceous materials in addressing urban PM pollution. This work highlights the need to consider the role of ON, especially as efforts of decarbonization in energy sources unfold. First hourly observation of aerosol total organic nitrogen (ON) at an urban site revealed ON made up 5%-64% of total aerosol nitrogenHourly data identified distinct ON episode types that occurred in different hours of a day linked to dominant sources of varying originsSource indicators revealed vehicle emissions, coal combustion, nocturnal chemistry and daytime photochemistry were major sources of urban ON