## 3 Remote Sensing with CM-CLaDS
### Ambient N\({}_{2}\)O Sensing  
To demonstrate capabilities suitable for environmental N\({}_{2}\)O monitoring, the CM-CLaDS prototype setup was deployed in a field environmental measurement site. The system was installed at a measurement site on the University of Maryland Baltimore County (UMBC) campus, about 8 km from Baltimore, MD, USA (Figure 7). The CM-CLaDS sensor was placed inside a non-air-conditioned building and was operated almost continuously for 5 days in October 2011 with only one significant interruption on 26 October due to a power outage. The laser beam was directed through an open window towards a distant retroreflector placed outside the building, yielding a total optical path of 71 m above a neighboring meadow. The laser temperature and current were set to 25 \(\,\mathrm{C}\) and 150 mA, respectively, which allowed targeting the optimal N\({}_{2}\)O transition at 2,207.62 cm\({}^{-1}\). The signal processing was further optimized and enabled acquisition with a higher duty cycle of 12.5% (250 ms of data with 2 s intervals). The CLaDS signal was calibrated using a certified gas mixture composed of 151 \(\,\pm\,\)1 ppmv of N\({}_{2}\)O in dry N\({}_{2}\) at pressure of 760 Torr, which determines the ultimate relative accuracy of concentration measurements to \(\pm\)0.66% (approximately \(\pm\)2.2 ppbv at the expected ambient conditions). The atmospheric N\({}_{2}\)O concentration time series was recorded and is shown in Figure 8.  
Atmospheric temperature and pressure data were measured with a close-by weather station, and both data sets have been used to correct the measured molecular concentration based on the ideal gas law and spectroscopic parameters of the transition. A correction for water vapor in the atmosphere was also applied using the data from a local hygrometer. Each point (black dot) in Figure 8 is an average of data recoded within 5 min (150 points with 2 s interval and effective duty cycle of 12.5%). The RF beat-note power was recorded simultaneously with the CM-CLaDS signal. Due to mechanical drift in the optical arrangement (temperature fluctuations _etc._) during deployment, the beat-note power was slowly decreasing from \(-\)16 dBm on the first day down to slightly below \(-\)20 dBm by 30 October. There were also additional changes in the amount of received optical power caused by changing environmental conditions. The most significant occurred on 10/27 when the beat-note power dropped by more than 20 dB due to atmospheric water condensation on the retroreflector. The signal power was recovered after cleaning the retroreflector. Some other smaller power variations visible in the data were caused primarily by precipitation events. Similar to the laboratory tests shown in Figures 5 and 6, the CM-CLaDS signal measured in the field also shows high immunity to changes in the received optical power. Continuous monitoring of  
Figure 7: Picture of the environmental measurement site on the UMBC campus in Maryland. Locations of the sensor and the retroreflector are labeled. N\({}_{2}\)O concentration was possible even in the event of \(>\)20 dB drop in the received power. Despite an increase in the noise, the mean value of the measured N\({}_{2}\)O concentration was not affected during this event (27 October morning _vs._ afternoon). As shown in Figure 9, the noise dependence on the received power level observed in the field (Figure 8) is in good agreement with the performance tested in the laboratory (based on data from Figure 5). In both cases the noise increases by less than 4 dB per every 10 dB drop in the beat-note power, which is in good agreement with our previous studies on direct CLaDS [18]. The data acquisition in the field was performed by averaging 150 consecutive points (instead of 50 acquired in the laboratory test) and with a higher duty cycle (12.5% _vs._ 5% in the laboratory), which results in an effective acquisition time of 37.5 s (_vs._ 10 s in the laboratory).