Tuesday, 17 May 2011

LIF Instrument Performance

So in the post on the 11th October I told you about my first experience of research flights with the newly installed laser induced fluorescence instrument (LIF). This instrument can measure NO2 and with a specially designed inlet also the sum of peroxynitrates and alkylnitrates. In theory the system can also measure nitric acid (HNO3) but because nitric acid is a 'sticky' molecule it is more difficult to measure and so we will be interested to see the comparison between the LIF HNO3 measurements and those made by the CIMS technique described in the previous post. The instrument draws in air from outside the aircraft and fires a laser pulse through the sample at 532 nm. Any NO2 in the air absorbs this radiation and becomes excited. The excited NO2 then fluoresces, giving off radiation and returning back to the ground state. The radiation can be detected and is proportional to the concentration of NO2 in the sample. Using calibration data we can then calculate the NO2 concentration in the air.

For the RONOCO flights we were very interested to see how the LIF system would compare to the existing chemiluminescence system (operated by FAAM) which measures NO and NO2. The NO2 measurement on this instrument is obtained by passing air through a photolytic converter and then into the chemiluminescence analyser. The photolytic converter contains 2 arrays of blue light emitting diodes (LEDs) which break the NO2 up into NO and an oxygen atom. The NO produced is then passed into the analyser where it is reacted with ozone (O3) to produce excited state NO2 and oxygen. This excited state NO2 emits radiation in a chemiluminescent reaction and the amount of radiation is proportional to the NO that was in the sample. By turning off the LEDs the NO2 is not converted to NO and the signal is due to any NO that was originally in the air sample when it entered the system. If we subtract this signal from that obtained with the LEDs on we get the signal that was due just to the converted NO2. Complicated, but I hope that makes sense!

So to the data.....the plot below shows the flight track for flight B534 off the east coast of the UK. The height of the track shows the level of NO demonstrating that we were sampling both inside and out of a plume of pollution coming off the coast.

The flight track for B534. The height of the track indicates the concentration of NO, a tracer for pollution.

This second plot shows the LIF NO2 data on the same axis as the chemiluminescence NO2 data. You can see that we measured a range of concentrations from below 1 ppbv up to 30 ppbv. The time series traces seem to follow each other well but a better test is to do a correlation plot which is shown on the lower of the two axis. This shows that the relationship between the two measurements is linear and the equation y = 1.1x - 0.22 shows that despite a slight offset (-0.22) changes in the measurements are almost equal (the gradient is close to 1). The R2 value of 0.995 shows a good correlation between the two with few outlying points. All in all a very encouraging comparison! Now we must await the results from a more thorough comparison carried out using these two instruments plus a broadband cavity enhanced absorption spectrometer (operated by University of Cambridge) which will hopefully show that within the instrument errors these two instruments measure the same.

Top axis: Time series of NO2 measurements from the LIF analyser in black and the chemiluminescence analyser in red. Bottom axis: Correlation plot of LIF NO2 against chemiluminescence NO2 giving the equation of the fitted line (black) and the R2 value.

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