Inland Concentrations of Cl2 and ClNO2 in Southeast Texas Suggest Chlorine Chemistry Significantly Contributes to Atmospheric Reactivity

Measurements of molecular chlorine (Cl-2), nitryl chloride (ClNO2), and dinitrogen pentoxide (N2O5) were taken as part of the DISCOVER-AQ Texas 2013 campaign with a High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS) using iodide (I-) as a reagent ion. ClNO2 concentrations exceeding 50 ppt were regularly detected with peak concentrations typically occurring between 7:00 a.m. and 10:00 am. Hourly averaged Cl-2 concentrations peaked daily between 3:00 p.m. and 4:00 p.m., with a 29-day average of 0.9 +/- 0.3 (1 sigma) ppt. A day-time Cl-2 source of up to 35 ppt.h(-1) is required to explain these observations, corresponding to a maximum chlorine radical (Cl-center dot) production rate of 70 ppt.h(-1). Modeling of the Cl-2 source suggests that it can enhance daily maximum O-3 and RO2 center dot concentrations by 8%-10% and 28%-50%, respectively. Modeling of observed ClNO2 assuming a well-mixed nocturnal boundary layer indicates O-3 and RO2 center dot enhancements of up to 2.1% and 38%, respectively, with a maximum impact in the early morning. These enhancements affect the formation of secondary organic aerosol and compliance with air quality standards for ozone and particulate matter.