Atmospheric ammonia measurements in Houston, TX using an external-cavity quantum cascade laser-based sensor

Abstract

Abstract. In order to improve the current understanding of the dynamics of ammonia (NH 3 ) in a major industrial and urban area, intensive measurements of atmospheric NH 3 were conducted in Houston during two sampling periods (12 February 2010-1 March 2010 and 5 August 2010-25 September 2010). The measurements were performed with a 10.4-µm external cavity quantum cascade laser (EC-QCL)-based sensor employing conventional photo-acoustic spectroscopy. The mixing ratio of NH 3 ranged from 0.1 to 8.7 ppb with a mean of 2.4 ± 1.2 ppb in winter and ranged from 0.2 to 27.1 ppb with a mean of 3.1 ± 2.9 ppb in summer. The larger levels in summer probably are due to higher ambient temperature. A notable morning increase and a mid-day decrease were observed in the diurnal profile of NH 3 mixing ratios. Motor vehicles were found to be major contributors to the elevated levels during morning rush hours in winter. However, changes in vehicular catalytic converter performance and other local or regional emission sources from different wind directions governed the behavior of NH 3 during morning rush hours in summer. There was a large amount of variability, particularly in summer, with several episodes of elevated NH 3 mixing ratios that could be linked to industrial facilities. A considerable discrepancy in NH 3 mixing ratios existed between weekdays and weekends. This study suggests that NH 3 mixing ratios in Houston occasionally exceeded previous modeling predictions when sporadic and substantial enhancements occurred, potentially causing profound effects on particulate matter formation and local air quality.

Open-path, quantum cascade-laser-based sensor for high-resolution atmospheric ammonia measurements

“…The strongest NH 3 absorption transitions occur in the midinfrared spectral region near 10.3 µm and are probed by other QC laser absorption spectrometers (McManus et al, 2008;Gong et al, 2011). However, this spectral region is not optimal for open-path detection, due to the carbon dioxide (CO 2 ) and water vapor (H 2 O) pressure-broadened absorption lines that overlap with the NH 3 absorption lines.…”

“…More recently, advances in room temperature, distributedfeedback quantum cascade (QC) lasers have led to the development of QC-laser-based NH 3 sensors using the strongest NH 3 absorption transitions located in the mid-infrared spectral region (Manne et al, 2006;Whitehead et al, 2007;McManus et al, 2008;Manne et al, 2009;Curl et al, 2010;Ellis et al, 2010;Gong et al, 2011). Although these closedpath systems are highly sensitive (ppbv or sub-ppbv), they suffer from sampling artifacts that limit precision and response time and complicate calibration methods (Whitehead et al, 2008;von Bobrutzki et al, 2010).…”

“…Ammonia surface adsorption and desorption effects in closed-path sensors typically limit the response time (for several hundred ppbv NH 3 changes) to a few seconds, resulting in attenuation of high-frequency concentration fluctuations (Yokelson et al, 2003;Nowak et al, 2007;McManus et al, 2008;Whitehead et al, 2008;Ellis et al, 2010;Gong et al, 2011). The time response of closed-path sensors is often defined by the initial time constant for gas exchange and the slower surface adsorption time constant (e.g., ∼ 0.4 s and ∼ 15 s, respectively, for a QC-TDLAS instrument) .…”

“…Anthropogenic NH 3 emissions have more than doubled since preindustrial times, largely due to agricultural livestock and fertilizer emissions (Galloway et al, 2003;Battye et al, 2003;Anderson et al, 2003;Aneja et al, 2008). Uncertainties as large as 50 % exist in global NH 3 emission budgets due to the spatial and temporal variability of emissions and a lack of in situ measurements for emission inventory validations (Battye et al, 2003;Clarisse et al, 2009).…”

“…Total NH x , the sum of gasphase NH 3 and particulate ammonium (NH + 4 ), is an increasingly important component of the reactive nitrogen budget, with implications for human and ecosystem health (Galloway et al, 2003;Pinder et al, 2008;Bobbink et al, 2010). Ammoniated aerosols are a dominant component of inorganic aerosol mass in both clean and urban regions (Jimenez et al, 2009).…”