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<p>Amino acids (AAs) are relevant for nitrogen cycles, climate change and public health. Their size distribution may help to uncover the source, transformation and fate of protein in the atmosphere. This paper explores the use of compound-specific <span class="inline-formula"><i>δ</i>¹⁵</span>N patterns of hydrolyzed amino acid (HAA), <span class="inline-formula"><i>δ</i>¹⁵</span>N values of total hydrolyzed amino acid (<span class="inline-formula"><i>δ</i>¹⁵</span>N<span class="inline-formula">_THAA</span>), degradation index (DI) and the variance within trophic AAs (<span class="inline-formula">∑<i>V</i></span>) as markers to examine the sources and processing history of different sizes of particle in the atmosphere. Two weeks of daily aerosol samples from five sampling sites in the Nanchang area (Jiangxi Province, China) and samples of main emission sources of AAs in aerosols (biomass burning, soil and plants) were collected (Zhu et al., 2020). Here, we measured the concentrations and <span class="inline-formula"><i>δ</i>¹⁵</span>N values of each HAA in two size-segregated aerosol particles (<span class="inline-formula"><i>&gt;</i></span> 2.5 <span class="inline-formula">µ</span>m and PM<span class="inline-formula">_2.5</span>). Our results showed that the average concentrations of THAA in fine particles was nearly 6 times higher than that in coarse particles (<span class="inline-formula"><i>p</i></span> <span class="inline-formula"><i>&lt;</i></span> 0.01) and composition profiles of fine and coarse particles were quite different from each other. The <span class="inline-formula"><i>δ</i>¹⁵</span>N values of hydrolyzed glycine and THAA in both fine and coarse particles were typically in the range of those from biomass burning, soil and plant sources. Moreover, the average difference in the <span class="inline-formula"><i>δ</i>¹⁵</span>N<span class="inline-formula">_THAA</span> value between fine and coarse particles was smaller than 1.5 ‰. These results suggested that the sources of atmospheric HAAs for fine and coarse particles might be similar. Meanwhile, compared to fine particles, significantly lower DI values (<span class="inline-formula"><i>p</i></span> <span class="inline-formula"> </span>&lt;<span class="inline-formula"> </span> 0.05), “scattered” <span class="inline-formula"><i>δ</i>¹⁵</span>N distribution in trophic AA and higher <span class="inline-formula">∑<i>V</i></span> values (<span class="inline-formula"><i>p</i></span> <span class="inline-formula"><i>&lt;</i></span> 0.05) were observed in coarse particles. But the difference in <span class="inline-formula"><i>δ</i>¹⁵</span>N values of source AA (glycine, serine, phenylalanine and lysine) and THAA between coarse particles and fine particles was relatively small. It is likely that AAs in coarse particles have advanced bacterial degradation states compared to fine particles. Besides that, the significant increase in DI values and a decrease in <span class="inline-formula">∑<i>V</i></span> values for coarse particles were observed on days on which precipitation fell (<span class="inline-formula"><i>p</i></span> <span class="inline-formula"> </span>&lt;<span class="inline-formula"> </span> 0.05). This implies that “fresh” AAs in coarse particles were likely released following the precipitation.</p>