The COVID-19 Pandemic in Atlanta, GA: effects on particle composition as the pandemic progressed

Located in the South-East US, the city of Atlanta, GA has a unique air quality situation. The city has large anthropogenic sources like traffic, combustion, including cooking, and industrial activities. The city also has a large biogenic emissions from surrounding forests. Particulate matter (PM) and ozone (O3) concentrations are usually dictated by a delicate balance between these anthropogenic and biogenic emissions. This balance was disrupted by the COVID-19 pandemic, which resulted in dramatic reductions in traffic and other human activities in the city. We monitored air quality in Atlanta, GA from late-April through the end of 2020. Co-located instruments included gas-phase measurements of O3 and NOX and a PTR-MS for VOC measurements. Particle-phase measurements included a Scanning Mobility Particle Sizer (SMPS), a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (AMS) and a High-Resolution Time-of-Flight Aerosol Chemical Speciation Monitor (ACSM). Source apportionments (positive matrix factorization, PMF) were conducted on the AMS and ACSM data. Resolved aerosol factors were similar to those found previously including biogenic factors like isoprene organic aerosol (ISOP-OA), more-oxygenated oxidized organic aerosol (MO-OOA), less-oxygenated oxidized organic aerosol (LO-OOA) and anthropogenic factors like biomass-burning organic aerosol (BBOA), cooking organic aerosol (COA) and hydrocarbon-like organic aerosol (HOA). Anthropogenic factors like COA had reductions in frequency and amount during the early months of the pandemic (April-May) and increased in later months. In the fall, low wind speeds contributed to large spikes in PM concentrations, primarily composed of HOA, BBOA, and COA. Biogenic factors were affected by traffic emissions this include reductions in the fraction of MO-OOA and the separation LO-OOA into two factors (named LO-OOA1 and LO-OOA2). LO-OOA2 is more abundant in the beginning of the pandemic, has a daytime peak and correlates with nitrate concentrations in April-May. LO-OOA1 behaves as previously observed, peaking at nighttime and correlating with NO3 later in the summer as regular traffic resumes.