Decadal Carbon Monoxide Trends in the Upper and Lower Troposphere for the Tropics and Midlatitudes
Date
2024-09-16
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0009-0009-0414-3758
Type
Thesis
Degree Level
Masters
Abstract
Carbon monoxide (CO) exists in the troposphere as a result of direct anthropogenic and
natural emissions, and as a byproduct of tropospheric chemistry. It influences air quality and
radiative forcing, and controls the oxidation of methane (CH4) through a feedback relationship
with the hydroxyl radical (OH) and CH4. Emissions inventories estimate that for the
period following 2003, anthropogenic emissions of CO have been decreasing sharply in the
Northern Hemisphere (NH) midlatitudes, while in the tropics and Southern Hemisphere (SH)
midlatitudes there has been a mixture of increasing and decreasing emissions trends. Meanwhile,
studies of tropospheric column CO as observed by satellite instruments find decreasing
trends in CO burden across the tropics and midlatitudes in both hemispheres.
Measurements of CO from the Measurements of Pollution in the Troposphere (MOPITT)
satellite instrument make possible the study of the distribution and long-term changes of
tropospheric CO. To better understand the long-term behaviour of CO over the period of
2003-2023 in the tropics and midlatitudes, MOPITT data is used to analyze CO in the upper
and lower troposphere (UT and LT) for both the June, July, August (JJA) season, as well
as the December, January, February (DJF) season. Measurements from the Atmospheric
Infrared Sounder (AIRS) and Microwave Limb Sounder (MLS) instruments are used to compare
with MOPITT, and the Whole Atmosphere Community Climate Model (WACCM) is
used to simulate CO and examine the possible forcing terms.
From analysis of the MOPITT satellite data, trends are found which differ by atmospheric
layer, region, and season. The MOPITT trends agree in general with the results from AIRS
and MLS, but are not reproduced well by the WACCM model which finds trends that are
consistently weaker and tend toward being more positive. The inconsistencies between the
observational results and WACCM are attributed to uncertainties in the emissions used to
drive the model, which are known to be highly uncertain for many regions, especially those
in the Global South.
For the LT, the CO trends are found to be strongly negative regardless of season. Regionally,
the trends are strongly negative in the NH midlatitudes and weaker at more southern
latitudes. In a zonal mean over the tropics and midlatitudes, these patterns amount to trends of -3.96±0.67%/decade in JJA, and -4.67±0.64%/decade in DJF. The patterns in trends
in the LT resemble trends in emissions, which are estimated to be decreasing strongly for the
NH midlatitudes, and either decreasing or increasing weakly in the tropics and SH midlatitudes.
As a result of the similarities between CO trends and emissions, it is suggested that
the influence of decreasing emissions is the primary cause for the trends in observed CO.
In the UT, negative trends are found in the MOPITT observations for the NH midlatitudes
which weaken and reverse sign towards the SH. The boundary for the transition from negative
to positive trends is found to be seasonally dependent, and is pushed further south in the
DJF season, following the position of the ITCZ. These patterns of positive and negative
trends result in overall trends for the tropics and midlatitudes of -0.17 ± 0.75%/decade
in JJA, and -2.58 ± 1.43%/decade in DJF. From this analysis, the trends observed in the
UT are suggested to be primarily a result of transport and chemical production. These
mechanisms are thought to be dominant as the existence of positive trends in the UT oppose
the decreasing LT trends thought to be driven by emissions; chemical production is found to
be increasing in both the LT and UT, which can drive increases in concentration; and the
trends in the UT are found to be aligned with dynamical features, like the ITCZ.
Description
Keywords
Carbon monoxide, troposphere
Citation
Degree
Master of Science (M.Sc.)
Department
Physics and Engineering Physics
Program
Physics