Wintertime Gas‐Particle Partitioning and Speciation of Inorganic Chlorine in the Lower Troposphere Over the Northeast United States and Coastal Ocean

Abstract

The formation of photolabile chlorine reservoirs depend on how much chloride is available in the particle to react, which requires the chlorine partitioning to the particle in the troposphere to be well understood. However, limited measurements of gas and particle composition necessary to constrain this chemistry exist. We present measurements from the Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign that show inorganic tropospheric chlorine compounds (Cly) measured in the lower troposphere are dominated by HCl and PM4 particulate chloride (pCl−), with contributions from trace chlorine species like nitryl chloride (ClNO2), hypochlorous acid (HOCl), and molecular chlorine (Cl2). We observed elevated Cly mixing ratios over the ocean (540–625 pptv) compared to over land (178–225 pptv). Observations show 0–20% (0–0.2 μg/m3) of measured chlorine partitions into particles with a diameter less than 1 μm under typical WINTER conditions. The thermodynamic model, ISORROPIA II, overpredicts submicron pCl− by a factor of 2 but is brought into agreement, assuming a small fraction of unmeasured, refractory sea salt <0.1 μg/m3 exists. The model‐measurement disagreement could also be caused by an effective equilibrium constant for HCl that is too large. We derive a lower‐limit equilibrium function (Keq = 2.5 × 106 exp[5,208(1/T − 1/T0)] mol2·kg−2·atm−1) that lowers the model value's temperature dependency. This work provides constraints on Cly in the troposphere, addresses the sensitivity of chlorine partitioning to minor changes in environmental variables, and highlights the remaining questions interfering with our ability to correctly model pCl− concentrations.

Spatial Distributions and Sources of Inorganic Chlorine in PM2.5 across China in Winter

“…5 Overall, the Cl − concentrations in PM2.5 ranged from 0.11 to 26.7 µ g m −3 over the 20 urban sites (Table 1; Figure 2a), which is consistent with previous observations of aerosol Cl − concentrations across China during winter [52][53][54][55]. The concentrations of Cl − in PM2.5 across Chinese cities in our observations were significantly higher than previous studies of aerosol Cl − during wintertime in the northeastern United States (0.06 ~ 0.23 µ g/m 3 ) [14], in Iasi (0.32 ± 0.15 µ g/m 3 ) [30], and in Sao Paulo The water-soluble ions analyzing processes can be referenced to Luo et al [16] and Hsu et al [48]. Briefly, a one-eighth piece of filter was extracted with 50 mL of Milli-Q water (conductivity 18.2 MΩ/cm) in a clear centrifuge tube.…”

“…Figure 2a), which is consistent with previous observations of aerosol Cl − concentrations across China during winter [52][53][54][55]. The concentrations of Cl − in PM 2.5 across Chinese cities in our observations were significantly higher than previous studies of aerosol Cl − during wintertime in the northeastern United States (0.06~0.23 µg/m 3 ) [14], in Iasi (0.32 ± 0.15 µg/m 3 ) [30], and in Sao Paulo (1.4 µg/m 3 ) [56], suggesting intensive inorganic chlorine emissions in China. The aerosol Cl − concentrations in rural sites ranged from 0.07 ± 0.02 µg/m 3 to 0.55 ± 0.19 µg/m 3 across China during winter (Table 2, Figure 2b), which were significantly lower than those in urban sites (Table 1; Figure 2a), suggesting strong anthropogenic emissions of chlorine in urban sites.…”

“…This may be attributed to both sources of atmospheric Cl − and Cl − involved in chemical processes. This study reports the concentrations of aerosol Cl − on a national scale, and provides important information for modeling the global atmospheric reactive chlorine distribution and the effects of chlorine on atmospheric photochemistry.Atmosphere 2019, 10, 505 2 of 16 may disturb the aerosol acidity [11][12][13][14], which further affects the water-soluble ion concentrations aerosol [15,16]. Although the Cl − concentration in aerosol has been incidentally reported in many studies, very few studies have paid attention to Cl − in terrestrial aerosols as compared to marine aerosol, let alone in situ observations on a large spatial scale.The sources of atmospheric Cl − have been identified using several different methods, such as determining stable isotopic compositions of Cl − [17][18][19], monitoring the relationships between Cl − and OC, EC, Na + , and K + [16,[20][21][22], and model simulations [23,24].…”

“…Atmosphere 2019, 10, 505 2 of 16 may disturb the aerosol acidity [11][12][13][14], which further affects the water-soluble ion concentrations aerosol [15,16]. Although the Cl − concentration in aerosol has been incidentally reported in many studies, very few studies have paid attention to Cl − in terrestrial aerosols as compared to marine aerosol, let alone in situ observations on a large spatial scale.…”

Anthropogenic Control Over Wintertime Oxidation of Atmospheric Pollutants

“…During this flight, HONO photolysis was the next largest instantaneous radical source. Contributions from Cl 2 were low, given small (<15 pptv) observed concentrations (Haskins et al, ). ClNO 2 reactive uptake to form Cl 2 , which would amplify the radical source stemming from N 2 O 5 chemistry, was also small (Haskins et al, ).…”