Growth in stratospheric chlorine from short-lived chemicals not controlled by the Montreal Protocol

Abstract

We have developed a chemical mechanism describing the tropospheric degradation of chlorine containing very short-lived substances (VSLS). The scheme was included in a global atmospheric model and used to quantify the stratospheric injection of chlorine from anthropogenic VSLS ( ClyVSLS) between 2005 and 2013. By constraining the model with surface measurements of chloroform (CHCl₃), dichloromethane (CH₂Cl₂), tetrachloroethene (C₂Cl₄), trichloroethene (C₂HCl₃), and 1,2-dichloroethane (CH₂ClCH₂Cl), we infer a 2013 ClyVSLS mixing ratio of 123 parts per trillion (ppt). Stratospheric injection of source gases dominates this supply, accounting for ∼83% of the total. The remainder comes from VSLS-derived organic products, phosgene (COCl₂, 7%) and formyl chloride (CHClO, 2%), and also hydrogen chloride (HCl, 8%). Stratospheric ClyVSLS increased by ∼52% between 2005 and 2013, with a mean growth rate of 3.7 ppt Cl/yr. This increase is due to recent and ongoing growth in anthropogenic CH₂Cl₂-the most abundant chlorinated VSLS not controlled by the Montreal Protocol.

Keywords: Montreal Protocol; VSLS; dichloromethane; ozone; phosgene; stratosphere.

Figure 1

Time series of simulated tropical mean (a) CH₂Cl₂, (b) CHCl₃, and (c) C₂Cl₄ mixing ratio (ppt) at the level of zero radiative heating (LZRH) (grated area, ±1 standard deviation) derived from observed surface values. Equivalent observed quantities at the LZRH from the 2011 (114° to 134°W longitude and 7° to ∼20°N latitude, November) and 2013 (92° to 172°W longitude and 0.2° to ∼20°N latitude, February/March) NASA ATTREX missions are also shown. (d) The summed total chlorine in source gases; 3 × CHCl₃+2 × CH₂Cl₂+4 × C₂Cl₄, from model run EXP2 and observed quantity shown with filled stars. The 2011 and 2013 error bars correspond to the total chlorine in source gases when 3 × C₂HCl₃+2 × CH₂ClCH₂Cl is also considered (i.e., EXP3, equivalent ATTREX quantity shown with open stars).

Figure 2

(a) Observed tropical mean profile of COCl₂ volume mixing ratio (ppt) from the ACE‐FTS satellite instrument (2006–2012 average). The horizontal bars denote ±1 standard deviation. Also shown are corresponding model profiles of total COCl₂ (from EXP3, solid line, shading ±1 standard deviation) and VSLS‐derived COCl₂ (from EXP3 minus the control run, dashed line). (b) Simulated tropical mean profiles of organic chlorine from VSLS‐derived COCl₂, CHClO, and their total (shading ±1 standard deviation) in 2013. The location of the tropical tropopause layer (TTL) is indicated.

Figure 3

Observed profiles of HCl volume mixing ratio (ppt) from the NASA (a) 2006 INTEX‐B mission, (b) 2004 Pre‐AVE mission, and (c) 2006 CR‐AVE mission. INTEX‐B measurements obtained on board the DC‐8 aircraft (0–12 km) between ∼40°N and 61°N latitude (Anchorage deployment, Alaska). Pre‐AVE and CR‐AVE measurements obtained on board the WB‐57 aircraft (10–20 km) within the latitude ranges ∼3°S to ∼8°N and ∼1°S to ∼20°N. Measured HCl profiles are campaign means calculated in 1 km altitude bins. Horizontal lines on observed data denote ±1 standard deviation. Also shown are corresponding model mean profiles of HCl derived from all sources (solid profile, ±1 standard deviation) and HCl derived from anthropogenic VSLS only (dashed line).

Figure 4

Time series of simulated annual mean total stratospheric chlorine from VSLS (sum of source and product gas contributions, filled circles, left axis). A linear trend line is applied to EXP2 which considered CH₂Cl₂, CHCl₃, and C₂Cl₄, and the 2005–2013 mean growth rate (ppt Cl⁻¹) is annotated. EXP3 (blue) also considered C₂HCl₃ and CH₂ClCH₂Cl. Also overlaid are annual mean CH₂Cl₂ mixing ratios (ppt) in the Northern Hemisphere calculated from NOAA surface observations (red crosses, right axis).