<p>Hydroxyl (OH) play an essential role in atmospheric chemistry. OH radical is an indicator of atmospheric oxidation and self-purification, which determines the removal of most trace gases in the atmosphere, such as CO, SO<sub>2</sub>, NO<sub>2</sub>, CH<sub>4</sub> and other volatile organic compounds (VOCs). A ground-based system for measurement of tropospheric OH radical by Laser Induced Fluorescence technique (AIOFM-LIF) was developed and integrated into a mobile observation platform for field observation. Ambient air expands through a 0.4 mm nozzle to low pressure. OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz, accompanying the release fluorescence of the A<sup>2</sup>&#931;<sup>+</sup>(v&#8217;=0)&#8212;X<sup>2</sup>&#928;<sub>i</sub>(v&#8217;&#8217;=0) transition at 308 nm with the resultant fluorescence being detected by gated photon counting. The detection sensitivity of AIOFM-LIF system was calibrated by a portable standard OH radical source based on water photolysis-ozone actinometry. Following laboratory and field calibrations to characterise the instrument sensitivity, OH radical detection limits were (1.84&#177;0.26) &#215; 10<sup>5</sup> cm<sup>-3</sup> and (3.69&#177;0.52) &#215; 10<sup>5</sup> cm<sup>-3</sup> at night and noon, respectively. During &#8220;A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen&#8221; (STORM) campaign, AIOFM-LIF system was deployed in Shenzhen, China, and OH radical concentration was obtained validly except for the rainy days. Mean diurnal variation of HOx radical concentration was obtained, and the peak was 6.6&#215;10<sup>6</sup> cm<sup>-3</sup> which appeared around 12:00 at noon. A general good agreement of OH radical concentration with j(O<sup>1</sup>D) was observed with a high correlation (R<sup>2</sup> =0.77), which illustrates that photolysis of ozone is an important source of OH radical during this campaign. A box model was applied to simulate the concentrations of OH at this field site, the primary production of OH radical was generally dominated by photolysis of O<sub>3</sub>, HONO, HCHO, while the other production was contributed by calculated species (OVOCs).</p>
An Approach To Single Molecule Detection By Laser-Induced Fluorescence
