Free L- and D-amino acids in PM in a megacity in central China, from the perspectives of pollution characteristics, atmospheric processes and sources.

The central region of China faces ongoing challenges in improving and meeting PM standards. While significant progress has been made in controlling anthropogenic sources, natural sources such as amino acids and bioaerosols, which are important components of PM, also require more attention. This study examines the concentrations, seasonal variations, sources, and atmospheric interactions of 15 pairs of free L- and D-amino acids in PM. D-amino acids (1010.8 ± 549.2 pmol m) were more concentrated than L-amino acids (760.4 ± 337.1 pmol m), with L-alanine (Ala), L-serine (Ser), L-arginine (Arg), D-Ala, D-asparagine (Asn), and D-Arg dominant. L- and D-amino acids peaked in autumn (1124.1 pmol m) and spring (1265.4 pmol m), respectively. Correlation analysis of amino acids with atmospheric oxidants NO and O indicated seasonal differences. Source apportionment results revealed that pollen and plant debris were the main sources of L- and D-amino acids, accounting for 40.4 % and 42.6 %, respectively. The main characteristics of the plant pollen debris source are L-Asn, L-Asp, L-glutamic, and L-Arg, as well as D-Tyr, D-Asn, and D-glutamine. The mixed dust source is primarily characterized by L-methionine, L-proline, L-threonine, and L-Arg, along with D-leucine, D-tyrosine (Tyr), and D-isoleucine. Additionally, L-tryptophan (Trp), rather than D-Trp, was identified as a potential biomarker for biomass burning using the Positive Matrix Factorization model, highlighting the importance of distinguishing between these forms in organic aerosol source identification.