# Influence of Droughs on Mid-Tropospheric CO\({}_{2}\)
## 1 Introduction  
In addition to the trend [1; 2; 3] and the annual cycle [4; 5; 6; 7; 8; 9], atmospheric CO\({}_{2}\) also demonstrates a lot of intra-seasonal [10; 11] and inter-annual variability [12; 13; 14; 15]. During the pre-satellite era, CO\({}_{2}\) analyses mainly utilized in-situ CO\({}_{2}\) measurements to investigate CO\({}_{2}\) variability, which is limited in space. In recent years, CO\({}_{2}\) retrievals from different satellites [16; 17; 18; 19; 20; 21] provide CO\({}_{2}\) concentrations over the global domain, which can be used to explore CO\({}_{2}\) variability at different locations. Utilizing the mid-tropospheric CO\({}_{2}\) from Atmospheric Infrared Sounder (AIRS), it has been found that large-scale processes (e.g., Madden-Julian Oscillation, El Nino-Southern Oscillation, South-Atlantic Walker circulation, and Northern Annular Mode) can modulate CO\({}_{2}\) concentrations in the middle troposphere [10; 14; 22].  
In addition to the influences from large-scale processes, surface CO\({}_{2}\) emissions can also change atmospheric CO\({}_{2}\) concentrations in the middle troposphere [23]. [PERSON] et al. [23] investigated the correlation between mid-tropospheric CO\({}_{2}\) and gross primary production (GPP) from the biosphere in July at 40\({}^{\circ}\)N-50\({}^{\circ}\)N and found a correlation coefficient of \(-0.8\) between mid-tropospheric CO\({}_{2}\) and the GPP. Large GPP means more photosynthesis, thus more CO\({}_{2}\) uptake from biosphere and less CO\({}_{2}\) in the atmosphere.  
Using CO\({}_{2}\) data over Mauna Loa, [PERSON] et al. (2017) found that the decline in the CO\({}_{2}\) seasonal cycle amplitudes since the early 1990s is related to reductions in the carbon sequestration as responses to severe droughts and changes in the atmospheric circulation. [PERSON] et al. (2016) measured soil surface CO\({}_{2}\) fluxes at three sites and found that the soil surface CO\({}_{2}\) fluxes are sensitive to the drought and temperature. By analyzing soil surface CO\({}_{2}\) flux over grassland, [PERSON] et al. (2016) have found that decreasing rain events will lead to less soil surface CO\({}_{2}\) flux and reduced plant growth. [PERSON] et al. (2016) found that the soil surface CO\({}_{2}\) fluxes will increase when temperature and precipitation increase. [PERSON] et al. (2016) analyzed the impact of droughts on net ecosystem carbon exchange in three Mediterranean ecosystems and found that droughts will influence gross primary production more than the ecosystem respiration. [PERSON] et al. (2016) analyzed net ecosystem production on the short grass steppe vegetation and found that net carbon uptake is reduced during low precipitation events. These pervious analyses ([PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016) mainly focus on measurements over a small area. Global distributed CO\({}_{2}\) data from AIRS offer a unique opportunity to explore the impact of droughts on atmospheric CO\({}_{2}\) over large spatial domain.  
In recent years, the drought has become a severe problem over the southwestern USA. The recent drought in California is the most severe in the past 1200 years ([PERSON] et al., 2016). However, investigations of the relationship between the drought events and the concentration of CO\({}_{2}\) over the southwestern USA are lacking. The relatively long-term CO\({}_{2}\) retrievals from AIRS provide a unique opportunity to examine such a relationship. In this paper, we will combine satellite CO\({}_{2}\), vertical pressure velocity, and CO\({}_{2}\) surface emissions to investigate the impact of droughts on CO\({}_{2}\) concentrations in the mid-troposphere over southwest USA.