Dear Abdelgadir,

You asked a curious question: could you explain why you even expect a relation between aerosol optical depth (AOD) and atmospheric pressure in the first place? The spatio-temporal distribution of air pressure is intimately linked to atmospheric dynamics, which itself is the result of the differential heating of the atmosphere. The AOD, on the other hand, depends not only on aerosol load (or amount), and hence on the injection processes, but also on the physical and chemical properties of the constituents. These, in turn, vary with the source of the materials: some (like dust) may be related to changes in atmospheric circulation, but others (like industrial pollution) are largely unrelated to weather events. Another factor to take into account is the size (in particular mass) of aerosol particles and their average residence time in the air (which depends on precipitations). Anyway, if you need or want to pursue an investigation along that line, you may find a better relation between AOD and pressure rates of change rather than pressure itself.

Cheers, Michel.

Dear Abdelgadir,

You asked a curious question: could you explain why you even expect a relation between aerosol optical depth (AOD) and atmospheric pressure in the first place? The spatio-temporal distribution of air pressure is intimately linked to atmospheric dynamics, which itself is the result of the differential heating of the atmosphere. The AOD, on the other hand, depends not only on aerosol load (or amount), and hence on the injection processes, but also on the physical and chemical properties of the constituents. These, in turn, vary with the source of the materials: some (like dust) may be related to changes in atmospheric circulation, but others (like industrial pollution) are largely unrelated to weather events. Another factor to take into account is the size (in particular mass) of aerosol particles and their average residence time in the air (which depends on precipitations). Anyway, if you need or want to pursue an investigation along that line, you may find a better relation between AOD and pressure rates of change rather than pressure itself.

Cheers, Michel.

Hello Michel,

Thanks for the very thoughtful comments. What brought up this subject is trying to understand MODIS spatio-temporal variations of aerosol optical depth over the united arab emirates in relation to some weather parameters including air pressure. The preliminary results so far are suggesting an inverse relation contrary to the initial thought. One would imagine that since air pressure is the force exerted by a column of air on a particular area then if this air mass or weight increased ( introduction of lots of dust particles perhaps due to a sand storm) one would observe higher pressure amounts. But the results are quite opposite to these expectations. I couldn't make sense out of this result. Perhaps there is absolutely no relation between the two, not sure though. I will try to see if the rate of change of pressure is related to AOD. Again thanks and best regards

Hi Abdelgadir,

Thanks for your response, which provides useful background information on your query. To proceed further, I suggest limiting the investigation (1) to dust storms (to the exclusion of all other types of aerosols), (2) to the dry regions of the Middle East and similar geographic and climatic areas. In that limited context, it is likely that mineral dust mobilized from soil erosion is the primary component of the aerosol, and some relation between the amount of airborne materials and the atmospheric circulation should be expected. Nevertheless, you may want to use information on the particle size and chemical composition of the aerosols to further screen out any event that does not involve dust.

Please remember (1) that the dust is mobilized in areas where the top soil is dry and where wind gusts are prevalent (i.e., in desert and arid regions), (2) that the mobilization potential is proportional to the cube of the wind speed (see the seminal works of R. A. Bagnold and Dale A. Gillette, and many other more recent works,for instance), and (3) that advection by wind may carry this material over large distances (e.g., from the Sahara to Central America). Hence, the highest correlation between AOD and weather parameters may involve large spatial and temporal lags. Using transport schemes in Lagrangian mode within weather models may provide a useful tool to track the origin of the dust. And again, pressure change, as an indicator of wind, over the source areas, will likely be more useful than the absolute value of pressure itself. Perhaps abrupt changes (linked to the passage of fronts or perturbations) may even be more instructive than smooth, progressive changes.

Good luck in your investigations. Michel.

Hi Michel,

Thanks for the very detailed reply. This is an excellent research plan. Will get started it on it soon as the fasting season is over. Thanks again and best regards

Abdelgadir

Dear Abdelgadir,

I found a good relationship between AOD and PM2.5 over Hong Kong during surface low pressure between 996 and 1010hPa. Generally, surface low pressure is associated with moist and warm air rising slowly above the surface along with pollutants, which gives a degree of vertical mixing of pollutants throughout the atmospheric column. Since PM2.5 is measured near the surface, whereas AOD represents whole column distribution of aerosols, the increased vertical distribution of pollutants under low surface pressure has the potential to increase the correlation between PM2.5 and AOD.

I am not sure this is related to your problem or not.

Good luck!

Regards,

Bilal

Dear Bilal

Thanks for the suggestions perhaps this would be the next stage to try to link AOD in UAE to PM2.5.

Best regards

Abdelgadir