MAX-DOAS observations of aerosols, formaldehyde and nitrogen dioxide in the Beijing area: comparison of two profile retrieval approaches

[摘要] A 4-year data set of MAX-DOAS observations in the Beijing area(2008–2012) is analysed with a focus on NO₂,HCHOand aerosols. Two very different retrievalmethods are applied. Method A describes the tropospheric profile with13 layers and makes use of the optimal estimation method. Method Buses 2–4 parameters to describe the tropospheric profile and aninversion based on a least-squares fit. For each constituent(NO₂, HCHO and aerosols) the retrievaloutcomes are compared in terms of tropospheric column densities, surfaceconcentrationsand "characteristic profile heights" (i.e. theheight below which 75% of the vertically integratedtropospheric column density resides).

We find best agreement between the two methods for troposphericNO₂ column densities, with a standard deviation of relativedifferences below 10%, a correlation of 0.99 and a linearregression with a slope of 1.03. For tropospheric HCHO column densities we find a similar slope, but alsoa systematic bias of almost 10% which is likely related todifferences in profile height. Aerosol optical depths (AODs) retrievedwith method B are 20% high compared to method A. They aremore in agreement with AERONET measurements, which are on average only5% lower, however with considerable relative differences(standard deviation ~ 25%). With respect to near-surfacevolume mixing ratios and aerosol extinction we find considerablylarger relative differences: 10 ± 30,−23 ± 28 and −8 ± 33% for aerosols,HCHO and NO₂ respectively. The frequencydistributions of these near-surface concentrations show howevera quite good agreement, and this indicates that near-surfaceconcentrations derived from MAX-DOAS are certainly useful ina climatological sense. A major difference between the two methods isthe dynamic range of retrieved characteristic profile heights which islarger for method B than for method A. This effect is most pronouncedfor HCHO, where retrieved profile shapes with methodA are very close to the a priori, and moderate for NO₂and aerosol extinction which on average show quite good agreement forcharacteristic profile heights below 1.5 km.

One of the main advantages of method A is the stability, even undersuboptimal conditions (e.g. in the presence of clouds). Method B isgenerally more unstable and this explains probably a substantial partof the quite large relative differences between the two methods.However, despite a relatively low precision for individual profileretrievals it appears as if seasonally averaged profile heightsretrieved with method B are less biased towards a priori assumptionsthan those retrieved with method A. This gives confidence in theresult obtained with method B, namely that aerosol extinction profiles tend onaverage to be higher than NO₂ profiles in spring andsummer, whereas they seem on average to be of the same height inwinter, a result which is especially relevant in relation to thevalidation of satellite retrievals.

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