Abstract
We report measurements of CH4 (measured
in situ by the Harvard University Picarro Cavity Ringdown Spectrometer
(HUPCRS) and NOAA Unmanned Aircraft System Chromatograph for Atmospheric
Trace Species (UCATS) instruments), O3 (measured in situ by the NOAA
dual-beam ultraviolet (UV) photometer), NO2, BrO (remotely
detected by spectroscopic UV–visible (UV–vis) limb observations; see the
companion paper of Stutz et al., 2016), and of some key brominated source
gases in whole-air samples of the Global Hawk Whole Air Sampler (GWAS)
instrument within the subtropical lowermost stratosphere (LS) and the
tropical upper troposphere (UT) and tropopause layer (TTL). The measurements
were performed within the framework of the NASA-ATTREX (National Aeronautics
and Space Administration – Airborne Tropical Tropopause Experiment) project
from aboard the Global Hawk (GH) during six deployments over the eastern
Pacific in early 2013. These measurements are compared with TOMCAT/SLIMCAT (Toulouse Off-line Model of Chemistry And Transport/Single Layer Isentropic Model of Chemistry And
Transport) 3-D model simulations, aiming at improvements of our understanding of the
bromine budget and photochemistry in the LS, UT, and TTL.Changes in local O3 (and NO2 and BrO) due to transport
processes are separated from photochemical processes in intercomparisons of
measured and modeled CH4 and O3. After excellent agreement is
achieved among measured and simulated CH4 and O3, measured
and modeled [NO2] are found to closely agree with ≤ 15 ppt in
the TTL (which is the detection limit) and within a typical range of 70 to
170 ppt in the subtropical LS during the daytime. Measured [BrO]
ranges between 3 and 9 ppt in the subtropical LS. In the TTL, [BrO]
reaches
0.5 ± 0.5 ppt at the bottom (150 hPa∕355 K∕14 km) and up to about 5 ppt at the
top (70 hPa∕425 K∕18.5 km; see Fueglistaler et al., 2009 for the definition of the TTL used), in overall good agreement with the model
simulations. Depending on the photochemical regime, the TOMCAT∕SLIMCAT
simulations tend to slightly underpredict measured BrO for large
BrO concentrations, i.e., in the upper TTL and LS. The measured
BrO and modeled BrO ∕ Bryinorg ratio is further used
to calculate inorganic bromine, Bryinorg. For the TTL (i.e., when
[CH4] ≥ 1790 ppb), [Bryinorg] is found to increase
from a mean of 2.63 ± 1.04 ppt for potential temperatures
(θ) in the range of 350–360 K to
5.11 ± 1.57 ppt for θ = 390 − 400 K, whereas
in the subtropical LS (i.e., when [CH4] ≤ 1790 ppb), it reaches
7.66 ± 2.95 ppt for θ in the range of 390–400 K. Finally, for the eastern Pacific
(170–90° W), the TOMCAT/SLIMCAT simulations indicate a net loss of ozone of −0.3 ppbv day−1
at the base of the TTL (θ = 355 K) and a net production of
+1.8 ppbv day−1 in the upper part (θ = 383 K).
