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

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 ( Δ RET) 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, Δ RET 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 Δ RET, 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 .