Apportioned primary and secondary organic aerosol during pollution events of DISCOVER-AQ Houston

Understanding the drivers for high ozone (O-3) and atmospheric particulate matter (PM) concentrations is a pressing issue in urban air quality, as this understanding informs decisions for control and mitigation of these key pollutants. The Houston, TX metropolitan area is an ideal location for studying the intersection between O-3 and atmospheric secondary organic carbon (SOC) production due to the diversity of source types (urban, industrial, and biogenic) and the onand off-shore cycling of air masses over Galveston Bay, TX. Detailed characterization of filter-based samples collected during Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Houston field experiment in September 2013 were used to investigate sources and composition of organic carbon (OC) and potential relationships between daily maximum 8 h average O-3 and PM. The current study employed a novel combination of chemical mass balance modeling defining primary (i.e. POC) versus secondary (i.e. SOC) organic carbon and radiocarbon (C-14) for apportionment of contemporary and fossil carbon. The apportioned sources include contemporary POC (biomass burning [BB], vegetative detritus), fossil POC (motor vehicle exhaust), biogenic SOC and fossil SOC. The filter-based results were then compared with real-time measurements by aerosol mass spectrometry. With these methods, a consistent urban background of contemporary carbon and motor vehicle exhaust was observed in the Houston metropolitan area. Real-time and filter-based characterization both showed that carbonaceous aerosols in Houston was highly impacted by SOC or oxidized OC, with much higher contributions from biogenic than fossil sources. However, fossil SOC concentration and fractional contribution had a stronger correlation with daily maximum 8 h average O-3, peaking during high PM and O-3 events. The results indicate that point source emissions processed by onand off-shore wind cycles likely contribute to peak events for both PM and O-3 in the greater Houston metropolitan area.

Automated summary

Understanding the sources and composition of organic carbon in the Houston metropolitan area and the relationship with O-3 and PM concentrations was investigated using chemical mass balance modeling and radiocarbon analysis. Results showed a consistent urban background of contemporary carbon and motor vehicle exhaust, with fossil SOC having a stronger correlation with daily maximum 8 h average O-3 during high PM and O-3 events. Impacts of point source emissions processed by onand off-shore wind cycles likely contribute to peak events for both PM and O-3 in the greater Houston metropolitan area.