Comment on amt-2022-100

Abstract

Off-line Aerosol Mass Spectrometer (AMS) measurements can provide valuable information about the ambient organic aerosol in areas and periods in which online AMS measurements are not available. However, these offline measurements have low temporal resolution as they are based on filter samples collected usually over 24 hours. In this study we examine whether and how this low time resolution affects source apportionment results. We used a five-month period (November 2016–March 2017) of online measurements in Athens and performed Positive Matrix Factorization (PMF) analysis to both the original dataset, which consists of 30 min measurements, and to time averages from 1 up to 24 h. The 30 min results indicated that five factors were able to represent the ambient organic aerosol (OA): a biomass burning organic aerosol factor (BBOA) contributing 16 % of the total OA, hydrocarbon-like OA (HOA) (29 %), cooking OA (COA) (20 %), more oxygenated OA (MO-OOA) (18 %), and less oxygenated OA (LO-OOA) (17 %). Use of the daily averages resulted in estimated average contributions that were within 8 % of the total OA compared with the high resolution analysis for the five-month period. The most important difference was for the BBOA contribution which was overestimated (25 % for low resolution versus 17 % for high resolution) when daily averages were used. The estimated secondary OA varied from 35 to 28 % when the averaging interval varied between 30 min and 24 h. The error for the low resolution analysis was much higher for individual days and its results especially for high concentration days are quite uncertain. The low resolution analysis introduces errors in the determined AMS profiles for the BBOA and LO-OOA factors but determines the rest relatively accurately (theta angle around 10 degrees or less).