Hydrogen Chloride (HCl) at Ground Sites During CalNex 2010 and Insight Into Its Thermodynamic Properties.

Gas phase hydrogen chloride (HCl) was measured at Pasadena and San Joaquin Valley (SJV) ground sites in California during May and June 2010 as part of the CalNex study. Observed mixing ratios were on average 0.83 ppbv at Pasadena, ranging from below detection limit (0.055 ppbv) to 5.95 ppbv, and were on average 0.084 ppbv at SJV with a maximum value of 0.776 ppbv. At both sites, HCl levels were highest during midday and shared similar diurnal variations with HNO3. Coupled phase partitioning behavior was found between HCl/Cl− and HNO3/NO3− using thermodynamic modeling and observations. Regional modeling of Cl− and HCl using Community Multiscale Air Quality captures some of the observed relationships but underestimates measurements by a factor of 5 or more. Chloride in the 2.5–10 μm size range in Pasadena was sometimes higher than sea salt abundances, based on co‐measured Na+, implying that sources other than sea salt are important. The acid‐displacement of HCl/Cl− by HNO3/NO3− (phase partitioning of semi‐volatile acids) observed at the SJV site can only be explained by aqueous phase reaction despite low RH conditions and suggests the temperature dependence of HCl phase partitioning behavior was strongly impacted by the activity coefficient changes under relevant aerosol conditions (e.g., high ionic strength). Despite the influence from activity coefficients, the gas‐particle system was found to be well constrained by other stronger buffers and charge balance so that HCl and Cl− concentrations were reproduced well by thermodynamic models. Plain Language Summary: Despite the known importance of gas‐particle partitioning in chlorine activation chemistry, the sources and atmospheric fate of hydrogen chloride (HCl) and particulate Cl− require further study. HCl concentrations and the corresponding phase partitioning mechanisms with Cl− in PM2.5 were studied through field measurements and modeling as part of the CalNex field campaign. The measurements confirmed that gas phase HCl was the dominant component of atmospheric chlorine and suggested that even though the current modeling can explain the diurnal variation of HCl, the mixing ratios were severely underestimated. Temperature dependence of Cl− replacement reaction by HNO3 was observed at one site. This study suggests further model development and field observations of HCl, as well as more reliable understanding of HCl phase partitioning behavior are needed to better predict the atmospheric fate of HCl and Cl−. Key Points: Hydrogen chloride (HCl) was measured at two CalNex sites. At both, the diurnal variation of HCl was similar and HCl was correlated with HNO3 on a given dayThe Community Multiscale Air Quality model captures HCl diurnal variation but underestimates the atmospheric chlorine concentrations by at least a factor of 5The aqueous phase exchange reaction drove coupled partitioning behavior between HCl/Cl− and HNO3/NO3− at one site despite low humidity [ABSTRACT FROM AUTHOR]