The greenhouse Gas Monitor Instrument (GMI) is a payload of GF-5 satellite. It is mainly used to ascertain the global distribution of carbon dioxide (CO

2

) and methane (CH

4

) and carbon cycling studies. Surface reflection is an important factor for developing a high-precision column-averaged carbon dioxide dry air mole fraction (XCO

2

) retrieval algorithm for the GMI short-wave infrared data. Urban areas are an important source of CO

2

emission

and an apparent directional reflection occurs in the urban underlying surface. However

the aerosol optical depth (AOD 550 nm) in Urban Beijing is relatively large

and combining atmospheric scattering and surface directional reflection makes the retrieval of XCO

2

in Beijing urban area difficult. In this study

we investigate the MODIS Bidirectional Reflectance Distribution Function (BRDF) data in Beijing urban area in 2011–2016. The three parameters (

f

iso

f

geo

and

f

vol

) of the BRDF model vary with season. The ratio of

f

iso

to

f

geo

and of

f

iso

to

f

vol

slightly change. Statistics show that the average value of

f

geo

/

f

iso

is 0.42±0.09 and

f

vol

/

f

iso

is 0.23±0.03. The simplification of the BRDF model allows us to invert the BRDF parameters simultaneously through one observation of the GMI. We present a new algorithm to improve the accuracy of XCO

2

in Beijing urban areas through simultaneous retrieval of BRDF parameters. An a priori value of AOD is provided by using a directional polarimetric camera

which is onboard the GF-5 satellite. The MODIS MCD43A1 product is used to build an a priori surface BRDF database after filtering and resampling. The a priori CO

2

profile is estimated on the basis of the analysis of CO

2

profile distribution in Beijing area. The monthly a priori covariance matrices of CO

2

at a grid cell (2°×3°) on the globe is precalculated as input parameters using the carbon tracker database. The measurement error covariance matrix is (y/SNR)

2

and the signal-to-noise ratio (SNR) used is the result of a laboratory test. This SNR is a tentative value and must be tuned after acquiring in-orbit data. To determine the effect of aerosols and surface BRDF reflection on the retrieval of XCO

2

we simulate GMI measurements in the spectral range of 6310–6380 cm

−1

. We use absolute radiance

ratio radiance

and BRDF methods to retrieve the simulation data separately. The error of the absolute radiance method exceeds 30% when the AOD is 0.05 considering the changes in surface albedo caused by surface direction reflection

and the error of the ratio radiance method is less than 1% in most observations. By contrast

the BRDF method has high accuracy

and its error is less than 0.2%. In the case of high AOD (0.4)

the maximum error of XCO

2

is 4.75% without the inversion of BRDF parameters. The evaluated precisions of the retrieved XCO

2

are less than 0.5% in most cases. To validate the correctness of our algorithm for the measured data

we retrieve the observations of the AOD less than 0.4 from Greenhouse Gases Observing Satellite (GOSAT) L2 data in Beijing urban area in 2016. We find that the correlation is 0.85

and the average error is 1.25 ppm by comparing our retrievals with GOSAT NIES XCO

2

. The retrievals are stable

except the results in summer. The AOD of 64.28% observations in summer is larger than 0.3

and the rest is close to 0.3. The difference in albedo caused by the BRDF characteristics of the urban underlying surface significantly influences the accuracy of XCO

2

retrievals

and the high-precision CO

2

concentration information cannot be obtained when it is not corrected. The aerosol conditions in urban areas are frequently complex. The proposed algorithm for the simultaneous retrieval of surface BRDF parameters and CO

2

content can accurately describe the surface direction reflection. The proposed algorithm improves the retrieval accuracy and data utilization of GMI data in the urban areas of Beijing.