Observations of gas phase hydrochloric acid in the polluted marine boundary layer



Ship-based measurements of gas phase hydrochloric acid (HCl), particulate chloride (pCl), and reactive nitrogen oxides (NOy) were made in the polluted marine boundary layer along the California coastline during spring 2010. These observations are used to assess both the rate of Cl atom production from HCl and the role of direct HCl emissions and subsequent partitioning as a source for pCl. Observations of HCl made in coastal Southern California are broadly correlated with NOz (NOz ≡ NOy – NOx), peaking at 11 A.M. The observed median HCl mixing ratio in Southern California is 1.3 ppb (interquartile range: 0.53–2.7 ppb), as compared to 0.19 ppb (interquartile range: 0.10–0.38 ppb) measured along the Sacramento River between San Francisco and Sacramento. Concurrent measurements of aerosol ion chemistry indicate that aerosol particles sampled in Northern California are heavily depleted in Cl, corresponding to a mean pCl deficit of 0.05 ± 0.03 (1σ) ppb for sub-10 µm aerosol particles. In comparison, aerosols measured in Southern California indicate that over 25% of particles showed an addition of Cl to the particle population. Observations presented here suggest that primary sources of HCl, or gas phase chlorine precursors to HCl, are likely underestimated in the California Air Resource Board emissions inventory. These results highlight the need for future field observations designed to better constrain direct reactive halogen emissions.