Sources and Secondary Production of Organic Aerosols in the Northeastern United States during WINTER

Abstract

Most intensive field studies investigating aerosols have been conducted in summer, and thus, wintertime aerosol sources and chemistry are comparatively poorly understood. An aerosol mass spectrometer was flown on the National Science Foundation/National Center for Atmospheric Research C‐130 during the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) 2015 campaign in the northeast United States. The fraction of boundary layer submicron aerosol that was organic aerosol (OA) was about a factor of 2 smaller than during a 2011 summertime study in a similar region. However, the OA measured in WINTER was almost as oxidized as OA measured in several other studies in warmer months of the year. Fifty‐eight percent of the OA was oxygenated (secondary), and 42% was primary (POA). Biomass burning OA (likely from residential heating) was ubiquitous and accounted for 33% of the OA mass. Using nonvolatile POA, one of two default secondary OA (SOA) formulations in GEOS‐Chem (v10‐01) shows very large underpredictions of SOA and O/C (5×) and overprediction of POA (2×). We strongly recommend against using that formulation in future studies. Semivolatile POA, an alternative default in GEOS‐Chem, or a simplified parameterization (SIMPLE) were closer to the observations, although still with substantial differences. A case study of urban outflow from metropolitan New York City showed a consistent amount and normalized rate of added OA mass (due to SOA formation) compared to summer studies, although proceeding more slowly due to lower OH concentrations. A box model and SIMPLE perform similarly for WINTER as for Los Angeles, with an underprediction at ages <6 hr, suggesting that fast chemistry might be missing from the models.

Rates of Wintertime Atmospheric SO2 Oxidation based on Aircraft Observations during Clear‐Sky Conditions over the Eastern United States

“…This overprediction of OH is not consistent with previous assessments that show OH concentrations calculated by a different model (the Air Quality Forecast Modeling System) are significantly underestimated by a factor of 5 during the winter (Cai et al, ). Previous work on OH approximations during RF 11 showed good agreement between AVOC‐derived OH and GEOS‐Chem (Schroder et al, ). This is not the case in this work, as GEOS‐Chem overestimates OH by approximately an order of magnitude.…”

“…An estimate of the OH mixing ratio can also be produced from simulations conducted with the GEOS‐Chem chemical transport model for the WINTER campaign, as described by Shah et al () and Jaeglé et al (). The GEOS‐Chem and measurement‐estimated OH were consistent for the NYC plume during WINTER RF 3 (Schroder et al, ). The OH mixing ratios were calculated by GEOS‐Chem at 1‐min resolution along the flight track.…”

“…The concentration of OH is equal to the slope of a plot (unweighted least squares), where ln([AVOC]/[toluene])/( k toluene ‐ k AVOC ) for each plume is plotted against their transit time, as shown in Figure S3. Benzene was not used because it was found to have different spatio‐temporal behavior than other AVOCs (Schroder et al, ). OH concentrations from o‐xylene were (‐1.2±9.4)×10 5 molecules/cm 3 and (1.5±3.7)×10 5 molecules/cm 3 for butenes.…”

“…Aerosol sulfate measurements for particles < 1μm in diameter were provided by a high‐resolution time‐of‐flight aerosol mass spectrometer (HR‐ToF‐AMS), and HNO 3 measurements were provided by an iodide time‐of‐flight chemical ionization mass spectrometer (I‐ToF CIMS). The AMS detection limit for sulfate was 137 ng/sm 3 (sm 3 = standard cubic meter at 1 atm and 273.15 K; DeCarlo et al, ; Dunlea et al, ; B. H. Lee et al, ; Schroder et al, ). The AMS collection efficiency, which varied between 0.5 for (NH 4 ) 2 SO 4 and 1.0 for H 2 SO 4 , was determined using the parameterization of Middlebrook et al (), and was applied to the 1‐s data.…”

NOx Lifetime and NOy Partitioning During WINTER

“…OH concentrations during the day are fixed to the average OH concentration estimated by GEOS‐Chem (0.01 ppt in the boundary layer; 0.04 ppt in the free troposphere). Schroder et al () determined the GEOS‐Chem estimates of OH to be reasonable in the NYC plume. Exchange of all species between boxes is allowed to represent mixing between the boundary layer and the free troposphere, and HNO 3 is removed from the bottom box via deposition.…”