On the wintertime low bias of Northern Hemisphere carbon monoxide in global model studies

Abstract. The uncertainties in the global budget of carbon monoxide (CO) are assessed to explain causes for the long-standing issue of Northern Hemispheric wintertime underestimation of CO concentrations in global models. With a series of MOZART sensitivity simulations for the year 2008, the impacts from changing a variety of surface sources and sinks were analyzed. The model results were evaluated with monthly averages of surface station observations from the global CO monitoring network as well as with total columns observed from satellites and with vertical profiles from measurements on passenger aircraft. Our basic simulation using MACCity anthropogenic emissions underestimated Northern Hemispheric near-surface CO concentrations on average by more than 20 ppb from December to April with the largest bias over Europe of up to 75 ppb in January. An increase in global biomass burning or biogenic emissions of CO or volatile organic compounds (VOC) is not able to reduce the annual course of the model bias and yields too high concentrations over the Southern Hemisphere. Raising global annual anthropogenic emissions results in overestimations of surface concentrations in most regions all-year-round. Instead, our results indicate that anthropogenic emissions in the MACCity inventory are too low for the industrialized countries during winter and spring. Thus we found it necessary to adjust emissions seasonally with regionally varying scaling factors. Moreover, exchanging the original resistance-type dry deposition scheme with a parameterization for CO uptake by oxidation from soil bacteria and microbes reduced the boreal winter dry deposition fluxes and could partly correct for the model bias. When combining the modified dry deposition scheme with increased wintertime road traffic emissions over Europe and North America (factors up to 4.5 and 2, respectively) we were able to optimize the match to surface observations and to reduce the model bias significantly with respect to the satellite and aircraft observations. A reason for the apparent underestimation of emissions may be an exaggerated downward trend in the RCP8.5 scenario in these regions between 2000 and 2010, as this scenario was used to extrapolate the MACCity emissions from their base year 2000. This factor is potentially amplified by a lack of knowledge about the seasonality of emissions. A methane lifetime of 9.7 yr for our basic model and 9.8 yr for the optimized simulation agrees well with current estimates of global OH, but we cannot exclude a potential effect from errors in the geographical and seasonal distribution of OH concentrations. Finally, underestimated emissions from anthropogenic VOCs can also account for a small part of the missing CO concentrations.

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On the wintertime low bias of Northern Hemisphere carbon monoxide found in global model simulations

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-9295-2014 ◽

2014 ◽

Vol 14 (17) ◽

pp. 9295-9316 ◽

Cited By ~ 67

Author(s):

O. Stein ◽

M. G. Schultz ◽

I. Bouarar ◽

H. Clark ◽

V. Huijnen ◽

...

Keyword(s):

Carbon Monoxide ◽

Northern Hemisphere ◽

Dry Deposition ◽

Atmospheric Composition ◽

Anthropogenic Emissions ◽

Base Case ◽

Boreal Winter ◽

Model Bias ◽

Near Surface ◽

Mixing Ratios

Abstract. Despite the developments in the global modelling of chemistry and of the parameterization of the physical processes, carbon monoxide (CO) concentrations remain underestimated during Northern Hemisphere (NH) winter by most state-of-the-art chemistry transport models. The consequential model bias can in principle originate from either an underestimation of CO sources or an overestimation of its sinks. We address both the role of surface sources and sinks with a series of MOZART (Model for Ozone And Related Tracers) model sensitivity studies for the year 2008 and compare our results to observational data from ground-based stations, satellite observations, and vertical profiles from measurements on passenger aircraft. In our base case simulation using MACCity (Monitoring Atmospheric Composition and Climate project) anthropogenic emissions, the near-surface CO mixing ratios are underestimated in the Northern Hemisphere by more than 20 ppb from December to April, with the largest bias of up to 75 ppb over Europe in January. An increase in global biomass burning or biogenic emissions of CO or volatile organic compounds (VOCs) is not able to reduce the annual course of the model bias and yields concentrations over the Southern Hemisphere which are too high. Raising global annual anthropogenic emissions with a simple scaling factor results in overestimations of surface mixing ratios in most regions all year round. Instead, our results indicate that anthropogenic CO and, possibly, VOC emissions in the MACCity inventory are too low for the industrialized countries only during winter and spring. Reasonable agreement with observations can only be achieved if the CO emissions are adjusted seasonally with regionally varying scaling factors. A part of the model bias could also be eliminated by exchanging the original resistance-type dry deposition scheme with a parameterization for CO uptake by oxidation from soil bacteria and microbes, which reduces the boreal winter dry deposition fluxes. The best match to surface observations, satellite retrievals, and aircraft observations was achieved when the modified dry deposition scheme was combined with increased wintertime road traffic emissions over Europe and North America (factors up to 4.5 and 2, respectively). One reason for the apparent underestimation of emissions may be an exaggerated downward trend in the Representative Concentration Pathway (RCP) 8.5 scenario in these regions between 2000 and 2010, as this scenario was used to extrapolate the MACCity emissions from their base year 2000. This factor is potentially amplified by a lack of knowledge about the seasonality of emissions. A methane lifetime of 9.7 yr for our basic model and 9.8 yr for the optimized simulation agrees well with current estimates of global OH, but we cannot fully exclude a potential effect from errors in the geographical and seasonal distribution of OH concentrations on the modelled CO.

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Impacts of COVID-19 lockdown strategies on NOx, CO and CO2 surface observations on two megacities: focus on the traffic sector in Mexico City (Mexico) and Paris (France)

10.5194/egusphere-egu21-14272 ◽

2021 ◽

Author(s):

Sophie Tran ◽

Michel Ramonet ◽

Thomas Lauvaux ◽

Philippe Ciais ◽

Olivier Laurent ◽

...

Keyword(s):

Mexico City ◽

Road Traffic ◽

Anthropogenic Activities ◽

Atmospheric Dispersion ◽

Monitoring Network ◽

Economic Activities ◽

Capital Cities ◽

Pollutant Concentrations ◽

Travel Restrictions ◽

Co Concentrations

In 2020, the COVID-19 pandemic imposed countries to apply stringent policies to slow down the spread of the SARS-CoV-2 virus. During the Spring time, most countries had announced a national lockdown that had important consequences on many capital cities such as Mexico City and Paris. The shutdown of many of these economic activities had a direct impact on the traffic sector. Travel restrictions led to a drastic decrease of major air pollutants in those two cities. From each local air quality monitoring network, we discriminated background, urban and traffic sites. By looking at the differences between urban sites versus background sites, we observed in Mexico City a decrease of 51%, 58&#160;% and 44&#160;% for &#916;NOx, &#916;CO2 and &#916;CO concentrations, respectively, during the lockdown. Markedly, their concentrations remained below typical levels after the end of the lockdown until September. Then, from September to the end of the year, the pollutants concentrations increased back to the same level as before the lockdown. The same behavior was seen at Paris. During the spring lockdown period, we observed a decrease of 72&#160;%, 70&#160;% and 88&#160;% for &#916;NOx, &#916;CO2 and &#916;CO concentrations, respectively. Until the end of the summer, the concentrations of those pollutants remained at the same level as during the lockdown. From September, we observed an increase of pollutants concentrations to the levels of previous years. Despite road traffic increases by the end of the lockdown in both megacities, the remainly low concentrations seen for those pollutants until September might be an effect of the atmospheric dispersion combined with a slow reactivation of anthropogenic activities. Nevertheless, a second lockdown period imposed in France (from Oct. 30 to Dec. 15) have clearly not shown the same impact on pollutant concentrations as the first one exhibited. On the contrary, no significant changes in pollutant concentrations were observed during the second lockdown, and moreover, peaks of &#916;NOx, &#916;CO2 and &#916;CO concentrations were seen during the last weekends of the lockdown of up to 32&#160;% of increase, compared to the weekday-level during the 2nd lockdown. This can be explained by less stringent travel restrictions combined with pre-Christmas preparations in Paris.

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Assessment of wood burning versus fossil fuel contribution to wintertime black carbon and carbon monoxide concentrations in Athens, Greece

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-10219-2018 ◽

2018 ◽

Vol 18 (14) ◽

pp. 10219-10236 ◽

Cited By ~ 21

Author(s):

Athina-Cerise Kalogridis ◽

Stergios Vratolis ◽

Eleni Liakakou ◽

Evangelos Gerasopoulos ◽

Nikolaos Mihalopoulos ◽

...

Keyword(s):

Carbon Monoxide ◽

Black Carbon ◽

Fossil Fuel ◽

Road Traffic ◽

Monitoring Site ◽

Night Time ◽

Urban Background ◽

Wood Burning ◽

Ambient Concentrations ◽

Co Concentrations

Abstract. The scope of this study was to estimate the contribution of fossil fuel and wood burning combustion to black carbon (BC) and carbon monoxide (CO) during wintertime, in Athens. For that purpose, in situ measurements of equivalent black carbon (eBC) and CO were simultaneously conducted in a suburban and an urban background monitoring site in Athens during the 3 months of winter 2014–2015. For the deconvolution of eBC into eBC emitted from fossil fuel (BCff) and wood burning (BCwb), a method based on the spectral dependency of the absorption of pure black carbon and brown carbon was used. Thereafter, BCwb and BCff estimated fractions were used along with measured CO concentrations in a multiple regression analysis, in order to quantify the contribution of each one of the combustion sources to the ambient CO levels. For a comparative analysis of the results, we additionally estimated the wood burning and fossil fuel contribution to CO, calculated on the basis of their CO ∕ NOx emission ratios. The results indicate that during wintertime BC and CO are mainly emitted by local sources within the Athens Metropolitan Area (AMA). Fossil fuel combustion, mainly from road traffic, is found to be the major contributor to both eBC in PM2.5 and CO ambient concentrations in AMA. However, wintertime wood burning makes a significant contribution to the observed eBC (of about 30 %) and CO concentrations (on average, 11 and 16 % of total CO in the suburban and urban background sites respectively). Both BC and CO from biomass burning (BCwb and COwb, respectively) present a clear diurnal pattern, with the highest concentrations during night-time, supporting the theory of local domestic heating being their main source.

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Assessment of wood burning versus fossil fuel contribution to wintertime black carbon and carbon monoxide concentrations in Athens, Greece

10.5194/acp-2017-854 ◽

2017 ◽

Cited By ~ 3

Author(s):

Athina-Cerise Kalogridis ◽

Stergios Vratolis ◽

Eleni Liakakou ◽

Evangelos Gerasopoulos ◽

Nikolaos Mihalopoulos ◽

...

Keyword(s):

Carbon Monoxide ◽

Black Carbon ◽

Fossil Fuel ◽

Road Traffic ◽

Monitoring Site ◽

Wood Burning ◽

Independent Evaluation ◽

Range Transport ◽

Ambient Concentrations ◽

Co Concentrations

Abstract. The scope of this study was to estimate the contribution of fossil fuel and wood burning combustion to black carbon (BC) and carbon monoxide (CO) during wintertime, in Athens. For that purpose, in-situ measurements of equivalent Black Carbon (eBC) and CO were simultaneously conducted in a suburban and an urban background monitoring site in Athens during three months of winter 2014–2015. For the deconvolution of eBC into eBC emitted from fossil fuel (BCff) and wood burning (BCwb), a method based on the spectral dependency of the absorption of pure black carbon and brown carbon was used. Thereafter, BCwb and BCff estimated fractions were used along with measured CO concentrations in a multiple regression analysis, in order to quantify the contribution of each one of the combustion sources to the ambient CO levels. For an independent evaluation of the results, we additionally estimated the wood-burning and fossil fuel contribution to CO, calculated on the basis of their CO/NOx emission ratios. The results indicate that during wintertime BC and CO are mainly emitted by local sources within the Athens Metropolitan Area (AMA), and are only occasionally affected by long-range transport. Fossil fuel combustion, mainly from road traffic, is found to be the major contributor to both eBC in PM2.5 and CO ambient concentrations in AMA. However, wintertime wood burning makes a significant contribution of about 30 % to the observed eBC and the CO concentrations (on average, 11 % and 16 % of total CO in the two sites, respectively). Both, BC and CO from biomass burning (BCwb and COwb, respectively) present a clear diurnal pattern with highest concentrations during night, supporting the local domestic heating as their main source.

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Source attributions of pollution to the Western Arctic during the NASA ARCTAS field campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-4707-2013 ◽

2013 ◽

Vol 13 (9) ◽

pp. 4707-4721 ◽

Cited By ~ 49

Author(s):

H. Bian ◽

P. R. Colarco ◽

M. Chin ◽

G. Chen ◽

J. M. Rodriguez ◽

...

Keyword(s):

Biomass Burning ◽

Field Experiments ◽

Transport Model ◽

Background Level ◽

Anthropogenic Emissions ◽

Airborne Measurements ◽

Ch4 Production ◽

Regional Sources ◽

Western Arctic ◽

Co Concentrations

Abstract. We use the NASA GEOS-5 transport model with tagged tracers to investigate the contributions of different regional sources of CO and black carbon (BC) to their concentrations in the Western Arctic (i.e., 50–90° N and 190–320° E) in spring and summer 2008. The model is evaluated by comparing the results with airborne measurements of CO and BC from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaigns to demonstrate the strengths and limitations of our simulations. We also examine the reliability of tagged CO tracers in characterizing air mass origins using the measured fossil fuel tracer of dichloromethane and the biomass burning tracer of acetonitrile. Our tagged CO simulations suggest that most of the enhanced CO concentrations (above background level from CH4 production) observed during April originate from Asian anthropogenic emissions. Boreal biomass burning emissions and Asian anthropogenic emissions are of similar importance in July domain wise, although the biomass burning CO fraction is much larger in the area of the ARCTAS field experiments. The fraction of CO from Asian anthropogenic emissions is larger in spring than in summer. European sources make up no more than 10% of CO levels in the campaign domain during either period. Comparisons of CO concentrations along the flight tracks with regional averages from GEOS-5 show that the along-track measurements are representative of the concentrations within the large domain of the Western Arctic in April but not in July.

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Turbulent Three-Dimensional Air Flow and Trace Gas Distribution in an Inhalation Test Chamber

Journal of Fluids Engineering ◽

10.1115/1.483270 ◽

2000 ◽

Vol 122 (2) ◽

pp. 403-411 ◽

Cited By ~ 4

Author(s):

P. W. Longest, ◽

C. Kleinstreuer ◽

J. S. Kinsey

Keyword(s):

Carbon Monoxide ◽

Large Scale ◽

Air Flow ◽

Test Chamber ◽

Secondary Flows ◽

Vortical Flow ◽

Trace Gas ◽

Inlet Section ◽

Height Range ◽

Co Concentrations

Steady incompressible turbulent air flow and transient carbon monoxide transport in an empty Rochester-style human exposure chamber have been numerically simulated and compared with experimental data sets. The system consisted of an inlet duct with a continuous carbon monoxide point source, 45- and 90-degree bends, a round diffuser, a round-to-square transition, a rectangular diffuser, the test chamber, a perforated floor, and again transition pieces from the chamber to an outlet duct. Such a configuration induced highly nonuniform vortical flow patterns in the chamber test area where a pollutant concentration is required to be constant at breathing level for safe and accurate inhalation studies. Presented are validated momentum and mass transfer results for this large-scale system with the main goals of determining the development of tracer gas (CO) distributions in the chamber and analyzing the contributions to CO-mixing. Numerical simulations were conducted employing a k-ε model and the latest available RNG k-ε model for air and CO-mixing. Both models predict similar velocity fields and are in good agreement with measured steady and transient CO-concentrations. It was found that secondary flows in the inlet section and strong vortical flow in the chamber with perforated flooring contributed to effective mixing of the trace gas at breathing levels. Specifically, in the height range of 1.4 m<h<2.0 m above the chamber floor, predicted CO-concentrations rapidly reached a near constant value which agrees well with experimental results. This work can be extended to analyze trace gas mixing as well as aerosol dispersion in occupied test chambers with or without flow redirection devices installed in the upstream section. A complementary application is particle transport and deposition in clean rooms of the electronic, pharmaceutical, and health care industries. [S0098-2202(00)01702-8]

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Temporal and Spatial variation of Contribution from Ship Emissions to the concentration and deposition of air pollutants in the Baltic Sea

10.5194/esd-2016-46 ◽

2016 ◽

Author(s):

Karin Haglund ◽

Björn Claremar ◽

Anna Rutgersson

Keyword(s):

Atmospheric Deposition ◽

Baltic Sea ◽

Dry Deposition ◽

Air Pollutants ◽

Wet Deposition ◽

Sulphur Content ◽

The Baltic Sea ◽

Near Surface ◽

Baltic Sea Region ◽

The Baltic

Abstract. The shipping sector contributes significantly to increasing emissions of air pollutants. In order to achieve sustainable shipping, primarily through new regulations and techniques, greater knowledge of dispersion and deposition of air pollutants is required. Regional model calculations of the dispersion and deposition of sulphur, nitrogen and particulate matter from the international maritime sector in the Baltic Sea and the North Sea have been made for the years 2009 to 2013. In some areas in the Baltic Sea region the contribution of sulphur dioxide, nitrogen oxide and nitrogen dioxide from international shipping represented up to 80 % of the total near surface concentration of the pollutants. Contributions from shipping of PM2,5 and PM10 were calculated to a maximum of 21 % and 13 % respectively. The contribution of wet deposition of sulphur from shipping was maximum 29 % of the total wet deposition, and for dry deposition the contribution from shipping was maximum 84 %. The highest percentage contribution of wet deposition of nitrogen from shipping reached 28 % and for dry deposition 47 %. The highest concentrations and deposition of the pollutants in the study were found near large ports and shipping lanes. High concentrations were also found over larger areas at sea and over land where many people are exposed. With enhanced regulations for sulphur content in maritime fuel, the cleaning of exhausts through scrubbers has become a possible economic solution. Wet scrubbers meet the air quality criteria but their consequences for the marine environment are largely unknown. The resulting potential of future acidification in the Baltic Sea, both from atmospheric deposition and from open-loop scrubber water along the shipping lanes, based on different assumptions about sulphur content in fuel and scrubber usage has been assessed. Shipping is expected to increase globally and in the Baltic Sea region, deposition of sulphur due to shipping will depend on traffic density, emission regulations and technology choices for the emission controls. To evaluate future changes scenarios are developed considering the amount of scrubber technology used. The increase in deposition for the different scenarios differs slightly for the basins in the Baltic Sea. The proportion of ocean acidifying sulphur from ships increases when taking scrubber water into account and the major reason to increasing acidifying nitrogen from ships are due to increasing ship traffic. This study also generates a database of scenarios for atmospheric deposition and scrubber exhaust from the period 2011 to 2050.

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GEOECOLOGICAL MAPPING OF THE PUROVSK AREA IN YAMAL-NENETS AUTONOMOUS REGION

Oil and Gas Studies ◽

10.31660/0445-0108-2016-2-120-125 ◽

2016 ◽

pp. 120-125

Author(s):

N. L. Mamaeva ◽

S. A. Petrov

Keyword(s):

Air Pollution ◽

Carbon Monoxide ◽

Urban Development ◽

Nitrogen Dioxide ◽

Maximum Permissible Concentration ◽

Near Surface ◽

Climate Conditions ◽

Autonomous Region ◽

Atmospheric Air ◽

Nenets Autonomous

In the article there were calculated near-surface concentrations of pollutants in the atmospheric air of Purovsk area, Jamal-Nenets Autonomous region. The purpose was to compare these with the maximum permissible concentration. The geo-ecological maps of dispersion of substances emissions into the air were made up using the unified program for calculating the air pollution taking into account the technical characteristics of sources and emissions themselves, natural, especially climate, conditions, as well as protection urban development activities and the lay of land. The conclusions were made about the excess in the atmosphere of maximum permissible concentration of solids on carbon monoxide in Purovsk, and on nitrogen dioxide in Purovsk, as well as in Ust-Purovsk Tazovskaya Guba permafrost areas.

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Uncertainty Characterization of HOAPS-3.3 Latent Heat Flux Related Parameters

10.5194/amt-2017-176 ◽

2017 ◽

Author(s):

Julian Kinzel ◽

Marc Schröder ◽

Karsten Fennig ◽

Axel Andersson ◽

Rainer Hollmann

Keyword(s):

Latent Heat ◽

Heat Fluxes ◽

Specific Humidity ◽

Boreal Winter ◽

Western Boundary ◽

Near Surface ◽

In Situ Data ◽

Uncertainty Measures ◽

Global Mean

Abstract. Latent heat fluxes (LHF) are one of the main contributors to the global energy budget. As the density of LHF measurements over the global oceans is generally poor, the potential of remotely sensed LHF for meteorological applications is enormous. However, to date none of the available satellite products include estimates of systematic, random retrieval, and sampling uncertainties, all of which are essential for assessing their quality. Here, this challenge is taken on by applying regionally independent multi-dimensional bias analyses to LHF-related parameters (wind speed U, near-surface specific humidity qa, and sea surface saturation specific humidity qs) of the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite (HOAPS) climatology. In connection with multiple triple collocation analyses, it is demonstrated how both instantaneous (gridded) uncertainty measures may be assigned to each pixel (grid box). A high-quality in situ data archive including buoys and selected ships serves as the ground reference. Results show that systematic LHF uncertainties range between 15–50 W m-2 with a global mean of 25 W m-2. Local maxima are mainly found over the subtropical ocean basins as well as along the western boundary currents. Investigations indicate that contributions by qa (U) to the overall LHF uncertainty are in the order of 60 % (25 %). From an instantaneous point of view, random retrieval uncertainties are specifically large over the subtropics with a global average of 37 W m-2. In a climatological sense, their magnitudes become negligible, as do respective sampling uncertainties. Time series analyses show footprints of climate events, such as the strong El Niño during 1997/98. Regional and seasonal analyses suggest that largest total (i.e., systematic + instantaneous random) LHF uncertainties are seen over the Gulf Stream and the Indian monsoon region during boreal winter. In light of the uncertainty measures, the observed continuous global mean LHF increase up to 2009 needs to be treated with caution. First intercomparisons to other LHF climatologies (in situ, satellite) reveal overall resemblance with few, yet distinct exceptions.

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Impact of land–water sensitivity contrast on MOPITT retrievals and trends over a coastal city

Atmospheric Measurement Techniques ◽

10.5194/amt-13-3521-2020 ◽

2020 ◽

Vol 13 (7) ◽

pp. 3521-3542

Author(s):

Ian Ashpole ◽

Aldona Wiacek

Keyword(s):

Transport Model ◽

Weighted Least Squares ◽

Temporal Trends ◽

Lower Troposphere ◽

Near Surface ◽

Surface Level ◽

Coastal City ◽

Water Sensitivity ◽

Land Water ◽

Co Concentrations

Abstract. We compare MOPITT Version 7 (V7) Level 2 (L2) and Level 3 (L3) carbon monoxide (CO) products for the 1∘×1∘ L3 grid box containing the coastal city of Halifax, Canada (longitude −63.58∘, latitude 44.65∘), with a focus on the seasons DJF and JJA, and highlight a limitation in the L3 products that has significant consequences for the temporal trends in near-surface CO identified using those data. Because this grid box straddles the coastline, the MOPITT L3 products are created from the finer spatial resolution L2 products that are retrieved over both land and water, with a greater contribution from retrievals over water because more of the grid box lies over water than land. We create alternative L3 products for this grid box by separately averaging the bounded L2 retrievals over land (L3L) and water (L3W) and demonstrate that profile and total column CO (TCO) concentrations, retrieved at the same time, differ depending on whether the retrieval took place over land or water. These differences (ΔRET) are greatest in the lower troposphere (LT), where mean retrieved volume mixing ratios (VMRs) are greater in L3W than L3L, with maximum mean differences of 11.6 % (14.3 ppbv, p=0.001) at the surface level in JJA. Retrieved CO concentrations are more similar, on average, in the middle and upper troposphere (MT and UT), although large differences (in excess of 40 %) do infrequently occur. TCO is also greater in L3W than L3L in both seasons. By analysing L3L and L3W retrieval averaging kernels and simulations of these retrievals, we demonstrate that, in JJA, ΔRET is strongly influenced by differences in retrieval sensitivity over land and water, especially close to the surface where L3L has significantly greater information content than L3W. In DJF, land–water differences in retrieval sensitivity are much less pronounced and appear to have less of an impact on ΔRET, which analysis of wind directions suggests is more likely to reflect differences in true profile concentrations (i.e. real differences). The original L3 time series for the 1∘×1∘ grid box containing Halifax (L3O) corresponds much more closely to L3W than L3L, owing to the greater contribution from L2 retrievals over water than land. Thus, in JJA, variability in retrieved CO concentrations close to the surface in L3O is suppressed compared to L3L, and a declining trend detected using weighted least squares (WLS) regression analysis is significantly slower in L3O (strongest surface level trend identifiable is −1.35 (±0.35) ppbv yr−1) than L3L (−2.85 (±0.60) ppbv yr−1). This is because contributing L2 retrievals over water are closely tied to a priori CO concentrations used in the retrieval, owing to their lack of near-surface sensitivity in JJA, and these are based on monthly climatological CO profiles from a chemical transport model and therefore have no yearly change (surface level trend in L3W is −0.60 (±0.33) ppbv yr−1). Although our analysis focuses on DJF and JJA, we demonstrate that the findings also apply to MAM and SON. The results that we report here suggest that similar analyses be performed for other coastal cities before using MOPITT surface CO.

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